synthesis of 1,4-disubstituted pyrazolo[3,4-d]pyrimidines from...

PAPER ▌1791

paperSynthesis of 1,4-Disubstituted Pyrazolo[3,4-d]pyrimidines from 4,6-Dichloro-pyrimidine-5-carboxaldehyde: Insights into Selectivity and ReactivitySelective Synthesis of 4-Chloropyrazolo[3,4-d]pyrimidinesChristie Morrill,* Suresh Babu, Neil G. Almstead, Young-Choon Moon*PTC Therapeutics, Inc., 100 Corporate Court, South Plainfield, NJ 07080, USAFax +1(908)2220567; E-mail: [email protected]; E-mail: [email protected]

Received: 28.03.2013; Accepted: 06.05.2013

Dedicated to Prof. Scott E. Denmark on the occasion of his 60th birthday

Abstract: Strategies for carrying out the reaction of 4,6-dichloro-pyrimidine-5-carboxaldehyde with both aromatic and aliphatic hy-drazines to generate 1-substituted 4-chloropyrazolo[3,4-d]pyrimidines in a selective, high-yielding, and operationally sim-ple manner are presented. For aromatic hydrazines, the reaction isperformed at a high temperature in the absence of an external base.For aliphatic hydrazines, the reaction proceeds at room temperaturein the presence of an external base. The observed selectivity andreactivity trends are rationalized through consideration of theproposed reaction mechanism. The 1-substituted 4-chloropyrazo-lo[3,4-d]pyrimidine products serve as versatile synthetic intermedi-ates, through further functionalization of the 4-chloride moiety,enabling the rapid generation of a structurally diverse array of 1,4-disubstituted pyrazolo[3,4-d]pyrimidines.

Key words: condensation, hydrazines, hydrazones, nucleophilicaromatic substitution, pyrazolo[3,4-d]pyrimidines

Pyrazolo[3,4-d]pyrimidines have attracted much attentionin drug discovery programs. Because of their structural re-semblance to purine nucleobases, they are ideally suitedto selectively interact with a diverse array of pharmaceu-tically relevant targets. For example, pyrazolo[3,4-d]py-rimidine substrates have been established as inhibitors ofPDE9 (Alzheimer’s disease, diabetes),1 Src (osteoporo-sis),2 p38α (inflammatory and autoimmune diseases),3

MRP4 (resistance to anticancer drugs),4 and ADA (isch-emia);5 as well as antagonists of adenosine A2A (Parkin-son’s disease)6 and adenosine A3 (inflammation,regulation of cell growth).7 They have demonstrated bothanticonvulsant8 and antibacterial9 activities. They havealso been extensively exploited as inhibitors of oncogenickinases, including Bcr,10 Abl,10,11 mTOR,12 PI3K,12e–f,13

Src,2,11b,14 EGF-R,15 and CDK.16 Pyrazolo[3,4-d]pyrimi-dines serve as excellent core structures for drug analoguesbecause they offer multiple options for diversification,through functionalization of the 1-, 2-, 3-, 4-, or 6-position(Figure 1).

As pyrazolo[3,4-d]pyrimidines are ubiquitous in the phar-maceutical industry, a number of different strategies havebeen employed for their synthesis. Most of these strate-gies, however, can be categorized into two disconnections(Scheme 1). The most commonly cited synthetic route in-volves generation of the pyrazolo[3,4-d]pyrimidine ringsystem through the annulation of a carbonyl compoundonto a 5-amino-4-cyanopyrazole, which in turn is synthe-sized from 2-(ethoxymethylene)malononitrile and a hy-drazine (route A),1a,3,5,7–9,12a or some variationthereof.1c,2,11a,14–16 The second most common syntheticroute reverses the construction order of the two rings byadding a hydrazine onto an existing 4-chloropyrimidine-5-carboxaldehyde through a combined condensation/nucleophilic aromatic substitution reaction (routeB).3,4,6,7,10,12,13,17 In these cases the pyrimidine starting ma-terial is generally purchased or synthesized from anothercommercially available pyrimidine. Less common syn-thetic routes include a Diels–Alder reaction between 5-amino-1-phenyl-4-pyrazolecarboxylic acid and 1,3,5-tri-azines,18 the cyclization of 4-aminopyrimidine-5-carbox-aldehyde oximes,19 and the reaction of 5-(benzoyl-amino)pyrazoles with nitriles.20

Scheme 1 Common methods to form pyrazolo[3,4-d]pyrimidines

Recent interest in our laboratories has focused on the gen-eration of a structurally diverse library of 1,4-disubstitut-ed pyrazolo[3,4-d]pyrimidines. The most efficientstrategy to obtain these compounds would be to utilizeroute B (Scheme 1), performing the direct condensation ofcommercially available 4,6-dichloropyrimidine-5-car-boxaldehyde (1) with various substituted hydrazines(Scheme 2). An array of 1-substituted 4-chloropyrazo-lo[3,4-d]pyrimidines (4) would thus be generated, whichcould undergo subsequent diversification at the 4-position

Figure 1 1-Substituted pyrazolo[3,4-d]pyrimidines

N

N NN

R

1

4

2

6

3

NN

R

NC

H2N

N

N Cl

H

O

N

N NN

R

route A

route B

NC

NCOEt

HNNH2

R

HNNH2

R

carbonylcompound

SYNTHESIS 2013, 45, 1791–1806Advanced online publication: 06.06.20130 0 3 9 - 7 8 8 1 1 4 3 7 - 2 1 0 XDOI: 10.1055/s-0033-1338862; Art ID: SS-2013-C0248-OP© Georg Thieme Verlag Stuttgart · New York

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1792 C. Morrill et al. PAPER

Synthesis 2013, 45, 1791–1806 © Georg Thieme Verlag Stuttgart · New York

through displacement of the 4-chloro substituent. Where-as application of route A (Scheme 1) toward the formationof key intermediate 4 would necessitate multiple syntheticsteps, usage of route B could potentially generate 4 in asingle step.

This proposed synthetic route is advantageous in terms ofits simplicity. However, the requisite condensation reac-tion faces selectivity issues. Desired product 4 is generallyobserved to be the major product of this reac-tion,3,4,10,12,13,17a–c but several other products are possible,namely hydrazone 3 and 2-substituted pyrazolo[3,4-d]py-rimidine 5 (Scheme 2). Indeed all three products were ob-served in our early attempts to condense carboxaldehyde1 with various arylhydrazines (vide infra). Althoughproduct mixtures have been previously reported in the lit-erature for condensation reactions involving 117a or relat-ed pyrimidine substrates,10,17c the development ofmethods for achieving selectivity remains challenging.Our efforts in this area led us to establish strategies to se-lectively generate 4 in high yield, which we reported in arecent communication.21 Herein, we present additional in-sights into both the scope of this reaction and the factorsthat influence its selectivity. We also describe the further

functionalization of 4 to generate a variety of 1,4-disubsti-tuted pyrazolo[3,4-d]pyrimidines.

Reactions Involving Aromatic Hydrazines

Our initial efforts were directed toward the reaction ofcarboxaldehyde 1 with arylhydrazines 2a–d. We appliedthe same reaction conditions that had been reported in theliterature for the direct condensation of 1 with arylhydra-zines.17a None of these experiments yielded selective reac-tions (Table 1, entries 1–4). The major products werepyrazolo[3,4-d]pyrimidines 4 and 5, with the formation ofsmall quantities of hydrazone 3. A clear electronic trendwas observed. Electron-rich hydrazines like 2a stronglyfavored the formation of 4 (entry 1), whereas electron-deficient hydrazines like 2d favored the formation of 5(entry 4). Performing these same reactions in the absenceof triethylamine led to dramatically different results: hy-drazone 3 was the predominant product in all cases (en-tries 5–8).

These observations prompted us to consider the reactionmechanism, suspecting that it would provide insight intodeveloping a strategy to achieve product selectivity. The

Scheme 2 Direct condensation of 1 with 2

N

N

Cl

Cl

O

H HNNH2

R

+

1 2

N

N NN

R

Cl

4

N

N

Cl

Cl

N

H

3

HN

RN

N NN

Cl

5

R+ +

Table 1 Initial Reactions of 1 with Arylhydrazinesa

Entry R Et3N (equiv) 1 (%)c 3 (%)c 4 (%)c 5 (%)c

1 4-MeOC6H4 (a) 2.1 ND ND 91 9

2 Ph (b) 2.1 ND 2 63 35

3 4-ClC6H4 (c) 2.1 2 7 40 51

4 4-F3CC6H4 (d) 2.1 9 9 8 74

5 4-MeOC6H4 (a) 0 3 92 5 ND

6 Ph (b) 0 4 96 ND ND

7 4-ClC6H4 (c) 0 2 98 ND ND

8 4-F3CC6H4 (d) 0 21 79 ND ND

a Reaction conditions: 0.2 M, 1.05 equiv of 2, 0.3–0.6 mmol scale.b HCl salt.c Determined from relative 1H NMR ratios in the crude reaction mixture. ND = not detected.

N

N

Cl

Cl

O

H HNNH2

R

+

1 2b

N

N NN

R

Cl

4

N

N

Cl

Cl

N

H

3

HN

RN

N NN

Cl

5

R+ +Et3N

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PAPER Selective Synthesis of 4-Chloropyrazolo[3,4-d]pyrimidines 1793

© Georg Thieme Verlag Stuttgart · New York Synthesis 2013, 45, 1791–1806

presence of two different electrophilic sites on 1 and twodifferent nucleophilic sites on 2 results in three possiblereaction pathways (Scheme 3). Initial condensation withthe aldehyde moiety of 1 can only proceed with the exter-nal nitrogen of 2, reversibly generating tetrahedral inter-mediate i, which then eliminates water to form hydrazone3 (path A). Hydrazone 3 can be isolated as such, or it cancyclize to form 4. Initial displacement of the chloro sub-stituent of 1, on the other hand, can occur with either ni-trogen of 2, reversibly generating either ii (path B) or iv(path C), which in turn form iii or v, followed by 4 or 5,respectively.

Product 5 can only form if 2 displaces the chloro substit-uent of 1 prior to the condensation process (Scheme 3,path C), whereas product 4 can be generated through ei-ther the initial condensation of 2 with the aldehyde (pathA) or initial chloride displacement (path B). We thereforeneeded to determine which pathway was generating 4 inthe examples shown in entries 1–4 of Table 1. Was 2 ini-tially condensing with the aldehyde or displacing the chlo-ride? To answer this question, hydrazones 3a–d wereisolated and resubjected to the original reaction conditions(Table 2). Within the original reaction time of one hour,cyclization to form 4 was not observed with any of thehydrazones (Table 2, entries 1–4). Even with a prolongedreaction time, only small quantities of 4 were observed(entries 5–8). These results indicate that at 65 °C in thepresence of an external base, 4 does not readily form viacyclization of hydrazone 3 (Scheme 3, path A). Although3 itself does form to a small extent under these reactionconditions (Table 1, entries 2–4), it does not cyclize. Prod-uct 4 must therefore arise primarily through initial chlo-ride displacement by 2 (Scheme 3, path B).

The results given in Table 1 suggest that the preferred re-action pathway is dictated by the presence or absence ofan external base. We believe that this observation is con-sistent with the reaction mechanisms illustrated in

Scheme 3. For each possible pathway, product formationis dependent upon the productive collapse of a tetrahedralintermediate, either i, ii, or iv, whose formation is revers-ible. One would expect the productive collapse of i to befacilitated by acidic conditions, therefore advancing pathA. Basic conditions, on the other hand, should instead ac-celerate the productive collapse of ii and iv, promotingboth paths B and C.

Returning to our goal of developing strategies to selec-tively synthesize 4, we postulated that our observationsregarding the effect of an external base on the preferred

Scheme 3 Possible reaction pathways

N

N NN

R

Cl

N

N NN

Cl

R

4

5

N

N

Cl

Cl

N

H

3

HN

R

N

N

Cl

NH

O

H

HNR

N

N

Cl

N

O

H

R

NH2

iii

v

+1 2N

N

Cl O

H

iiN

Cl

NH2

RH

N

N

Cl O

H

ivN

Cl

NH

HH

R

N

N

Cl

Cl

H

NO

HN

R

H

H

i

path

A

path B

path C

Table 2 Resubjection of 3 to Original Reaction Conditionsa

Entry R Time (h) 3 (%)b 4 (%)b

1 4-MeOC6H4 (a) 1 >99 ND

2 Ph (b) 1 >99 ND

3 4-ClC6H4 (c) 1 >99 ND

4 4-F3CC6H4 (d) 1 >99 ND

5 4-MeOC6H4 (a) 24 88 12

6 Ph (b) 24 95 5

7 4-ClC6H4 (c) 24 96 4

8 4-F3CC6H4 (d) 24 99 1

a Reaction conditions: 0.2 M, 1.05 equiv of HCl, 1 equiv of H2O, 0.3 mmol scale.b Determined from relative 1H NMR ratios in the crude reaction mix-ture. ND = not detected.

N

N

Cl

Cl

N

H

3

HN

RN

N NN

R

Cl

4

Et3N (2.1 equiv)

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reaction pathway could be exploited. Performing the con-densation in the presence of an external base inherentlyleads to product mixtures, as it facilitates reaction throughboth paths B and C (Scheme 3). However, carrying out thereaction in the absence of an external base promotes onlypath A and thus generates a single product, hydrazone 3.Although 3 did not readily cyclize to form 4 in the exam-ples presented in Tables 1 and 2, a subsequent cyclizationreaction, utilizing alternative reaction conditions, mightallow conversion of 3 into 4. While this strategy would re-sult in a two-step procedure, selectivity issues would becompletely precluded.

Our next task was to optimize the synthesis and isolationof hydrazone 3. Having established reaction conditions toselectively generate 3 from 1 and 2 at 65 °C (Table 1, en-tries 5–8), we screened additional solvents at room tem-perature. Nucleophilic solvents such as isopropyl alcoholwere not compatible with 1, as they readily displaced thechloro substituents. The conversion of 1 into 3 did pro-ceed cleanly in THF, 1,4-dioxane, acetonitrile, acetic acid,and DMF. The reaction was the most efficient in DMF,exhibiting essentially quantitative conversion within twohours. Either the free base or the HCl salt of 2 was a viablestarting material, but for electron-deficient hydrazinessuch as 2d, reactions involving the HCl salt progressedmore slowly. Adding cold aqueous sodium bicarbonate tothe crude reaction mixture and filtering the resultant pre-cipitate led to the isolation of 3. Purification via columnchromatography was not necessary. Table 3 shows theconversion of 1 into aromatic hydrazones 3a–o using ouroptimized reaction conditions.21 The reaction worked ef-ficiently for electron-rich substrates (entries 1–3), elec-tron-deficient substrates (entries 6–10), stericallyhindered substrates (entries 1, 6, and 11), and heteroaro-matic substrates (entries 12–15).

With hydrazones 3a–o in hand, we needed to identify re-action conditions under which they would efficiently cy-clize to form 4a–o. Hydrazones like 3 tend to resistcyclization, presumably because they exist in the E-con-figuration.17a,22 Heating at elevated temperatures cansometimes facilitate the process.17a Even prolonged heat-ing of 3 at 65 °C in the presence of triethylamine was in-sufficient to substantially effect cyclization to form 4(Table 2, entries 5–8). Performing the cyclization of 3a inthe absence of triethylamine, however, led to vastly im-proved conversion (Table 4, entry 1). Unfortunately wealso observed a small amount of side product 6a, whichpresumably resulted from a reaction between 4a and anequivalent of unreacted 3a. Performing the reaction inacetonitrile led to increased consumption of 3a, but a sig-nificantly greater amount of 6a formed as well (entry 2).A similar scenario was observed at 80 °C (entry 3). It wasthus clear that, although this cyclization occurred at tem-peratures as low as 65 °C, higher temperatures were nec-essary in order to increase the cyclization rate such that allof 3a would cyclize before reacting with 4a. At 140 °C, 3awas completely consumed within 20 minutes, and only asmall amount of 6a formed (entry 4). Formation of 6a

could be further suppressed by either reducing the concen-tration (entry 5) or further increasing the temperature (en-tries 6, 7).23

The reactions of hydrazones 3b–d were evaluated next.The cyclization proceeded more slowly as the phenyl sub-stituent of 3 became less electron-rich. For example, after20 minutes at 140 °C, all of 3a had been consumed (Table4, entry 4), 19% of 3b remained (entry 8), 52% of 3c re-mained (entry 11), and 86% of 3d remained (entry 14).Complete conversion of 3 into 4, without formation of 6,was again achieved by increasing the temperature (entries8–16). A temperature of 180 °C was sufficient to effectthe quantitative conversion of 3 into 4 within 20 minutesfor every substrate except 3d, which required a tempera-ture of 200 °C (entries 15, 16).

After establishing the optimal conditions to convert 3a–dinto 4a–d, this cyclization reaction was performed with avariety of other aromatic hydrazones (Table 5).21 For thesake of simplicity, these reactions were performed at 200°C, since most substrates cyclized within 20 minutes at

Table 3 Optimized Conversion of 1 into 3a

Entry R Isolated yield (%)

1 2-MeOC6H4 (e) 93

2 4-MeOC6H4 (a) 87

3 4-MeC6H4 (f) 92

4 Ph (b) 97

5 4-FC6H4 (g) 94

6 2-ClC6H4 (h) 92

7 3-ClC6H4 (i) 89

8 4-ClC6H4 (c) 93

9 4-BrC6H4 (j) 96

10c 4-F3CC6H4 (d) 99

11 1-naphthyl (k) 92

12d 2-pyridyl (l) 85

13d 2-quinoxalyl (m) 88

14d 2-benzo[d]oxazolyl (n) 91

15d 4-(5-methylthieno[2,3-d]pyrimidyl) (o) 98

a Reaction conditions: 0.3–1 M, 1.05 equiv of 1, 0.5–3 mmol scale.b HCl salt, unless otherwise noted.c Free base of 2 was used.d Bis-HCl salt of 2 was generated in situ using 2 equiv of 4 M HCl in 1,4-dioxane.

N

N

Cl

Cl

O

H HNNH2

R

+

1 2b

N

N

Cl

Cl

N

H

3

HN

R

r.t., 1–2 h

DMF

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this temperature, regardless of their electronic properties.Acceptable solvents for this process included THF, 1,4-dioxane, and acetonitrile. Both microwave and conven-tional heating (sealed tube) were evaluated, and no differ-ence was observed between the two methods in terms ofthe reaction efficiency. 1,4-Dioxane was utilized for reac-tions involving conventional heating, due to its higherboiling point. Acetonitrile, on the other hand, was the pre-ferred solvent for the microwave reactions, because itreached 200 °C more readily in our microwave reactorcompared to THF or 1,4-dioxane. Pyrazolo[3,4-d]pyrimi-dine products 4a–o were isolated by adding cold aqueoussodium bicarbonate and filtering the resultant precipitate.Purification via column chromatography was not neces-sary.

Reactions Involving Aliphatic Hydrazines

Having developed an efficient two-step procedure to se-lectively synthesize 4 from 1 and various aromatic hydra-zines, the reaction of 1 with aliphatic hydrazines wasinvestigated. Unlike the aromatic hydrazines (Table 1, en-tries 1–4), aliphatic substrates selectively generated 4 inhigh yield in the presence of an external base (Table6).21,24 Essentially quantitative conversion to 4 was ob-served within one hour at room temperature, and isomericproduct 5 was not detected. Products 4p–t were cleanlyisolated by removing the solvent in vacuo, following anaqueous workup, without the use of column chromatogra-phy. We observed significantly higher yields than thosereported in the literature for similar reactions.17a This im-provement in yield may exist because our reactions werecarried out at room temperature instead of 65 °C, as someproduct decomposition could occur at elevated tempera-tures.25

Table 4 Optimization of the Cyclization of 3 to Form 4a

Entry R Temp (°C)b

Time (h)

3 (%)c

4 (%)c

6 (%)c

1d 4-MeOC6H4 (a) 65 24 47 48 5

2 4-MeOC6H4 (a) 65 24 16 16 68

3 4-MeOC6H4 (a) 80 16 ND 33 67

4 4-MeOC6H4 (a) 140 0.3 ND 95 5

5e 4-MeOC6H4 (a) 140 0.3 ND >99 ND

6 4-MeOC6H4 (a) 160 0.3 ND 98 2

7 4-MeOC6H4 (a) 180 0.3 ND >99 ND

8 Ph (b) 140 0.3 19 76 5

9 Ph (b) 160 0.3 ND 97 3

10 Ph (b) 180 0.3 ND >99 ND

11 4-ClC6H4 (c) 140 0.3 52 47 1

12 4-ClC6H4 (c) 160 0.3 7 91 2

13 4-ClC6H4 (c) 180 0.3 ND >99 ND

14 4-F3CC6H4 (d) 140 0.3 86 10 4

15 4-F3CC6H4 (d) 180 0.3 10 90 ND

16 4-F3CC6H4 (d) 200 0.3 ND >99 ND

a Reaction conditions: 0.2 M in MeCN unless otherwise noted, 0.1–0.9 mmol scale.b Microwave heating was used in all cases except for entries 1–3.c Determined from relative 1H NMR ratios in the crude reaction mix-ture. ND = not detected.d 0.2 M in THF.e 0.02 M in MeCN.

N

N NN

Cl

R

4

N

N

Cl

Cl

N

H

3

HN

R

+

N

N

Cl

Cl

N

H

6

NR

N

NN

N

R

Table 5 Optimized Cyclization of 3 to Form 4a

Entry R Isolated yield (%)

1 2-MeOC6H4 (e) 85

2b 4-MeOC6H4 (a) 97

3 4-MeC6H4 (f) 98

4b Ph (b) 91

5 4-FC6H4 (g) 99

6c 2-ClC6H4 (h) 94

7 3-ClC6H4 (i) 92

8b 4-ClC6H4 (c) 97

9 4-BrC6H4 (j) 86

10b 4-F3CC6H4 (d) 96

11 1-naphthyl (k) 92

12d 2-pyridyl (l) 90

13 2-quinoxalyl (m) 92

14 2-benzo[d]oxazolyl (n) 92

15 4-(5-methylthieno[2,3-d]pyrimidyl) (o) 90

a Reaction conditions: 0.5 M in 1,4-dioxane using conventional heat-ing (sealed tube) unless otherwise noted, 0.5 mmol scale.b Reaction conditions: 0.5 M in MeCN using microwave heating, 0.5 mmol scale.c Reaction time: 40 min.d Product 4 was isolated as the HCl salt.

N

N

Cl

Cl

N

H

3

HN

RN

N

Cl

4

NN

R

200 °C

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The reactions described in Table 6 could proceed via ei-ther path A or path B (Scheme 3). To determine whetheror not path A was operative in these cases, isolated hydra-zones 3s and 3t were stirred with two equivalents of trieth-ylamine in THF at room temperature, and subsequentexamination of the crude reaction mixture by 1H NMRanalysis revealed no formation of 4s or 4t. These observa-tions suggest that the reactions shown in Table 6 must pro-ceed through path B, analogous to the reactions shown inentries 1–4 of Table 1. This interpretation is consistentwith our earlier postulation that reactions between 1 and 2initially undergo chloride displacement by 2 when an ex-ternal base is present.

Discussion of the Reaction Mechanism

The difference in product selectivity between aliphaticand aromatic hydrazines in the presence of an externalbase is striking. Aliphatic substrates selectively generate4 (Table 6), proceeding exclusively through path B(Scheme 3). Aromatic substrates, on the other hand, gen-erate mixtures of 3, 4, and 5 (Table 1, entries 1–4); with 3resulting from path A, 4 resulting from path B, and 5 re-sulting from path C. The high selectivity with which ali-phatic hydrazines 2q–t react with 1 could be attributed tothe reactivity difference between the two nitrogen atomsof 2q–t. It has been previously observed that the internalnitrogen of methylhydrazine (2q) is more nucleophilicthan the external nitrogen.26 A similar situation likely ex-ists for hydrazines 2r–t. If the internal nitrogen atom of 2serves as the predominant nucleophile in the reaction with1, then intermediate ii (Scheme 3) will form preferentiallyover iv. Assuming that productive collapse of ii is rapid inthe presence of an external base, then path B will be thesole reaction pathway, ensuring the selective formation of4. With aromatic hydrazines the situation becomes more

complicated, as it appears that all reaction pathwaysshown in Scheme 3 are operative. We suggest that this re-duced selectivity exists because the two nitrogen atoms ofaromatic hydrazines are competitive nucleophiles. Initialreaction at the internal nitrogen of 2 initiates path B, whileinitial reaction at the external nitrogen leads to both pathsA and C.

Despite the inherent lack of selectivity that exists for aro-matic hydrazines in the presence of an external base, aclear electronic trend is evident in entries 1–4 of Table 1and warrants an explanation. Electron-rich hydrazineslike 2a favor reaction through path B (Scheme 3). Elec-tron-deficient hydrazines like 2d show the opposite selec-tivity, with path C being preferred. Path B and path Cproceed through tetrahedral intermediates ii and iv, re-spectively, which presumably exist as an equilibrium mix-ture. In this scenario, the product distribution would bedictated by the position of the equilibrium. Consideringthe structures of ii and iv, one would expect ii to be fa-vored if R is electron-donating and disfavored if R is elec-tron-withdrawing, whereas iv should not be significantlyaffected by the electronic nature of the R group. This in-terpretation supports the results presented in entries 1–4 ofTable 1. With electron-rich 2a, ii is favored and thus 4forms selectively (Table 1, entry 1). With electron-defi-cient 2d, ii is disfavored, and thus 5 is the major product(Table 1, entry 4). The above discussions provide a ratio-nale to explain our observations, which can serve as a pre-dictive model for other systems. However, furthermechanistic studies are required in order to more fully de-termine the operative reaction pathways.

Some additional studies were performed to determine ifthe selectivity that was inherent in reactions involving al-iphatic hydrazines would hold if the internal hydrazine ni-trogen atom was less reactive than those of hydrazines2q–t. The internal nitrogen atom of aliphatic hydrazine 2uis sterically hindered with a bulky tert-butyl group. At 65°C, the reaction of 2u with 1 resulted in a nearly 1:1 mix-ture of 4u and 5u (Table 7, entry 1). Selectivity toward 4uimproved somewhat at lower temperatures (entries 2, 3),but 5u still formed. In the case of hydrazine 2v, the inter-nal nitrogen is sterically unencumbered, but it is renderedelectron-deficient through an electron-withdrawing triflu-oroethyl substituent. At 65 °C, the reaction of 2v with 1led to a mixture of 4v and 5v, with 5v being the majorproduct (entry 4). This result is similar to that obtainedwith electron-deficient arylhydrazines 2c and 2d (Table 1,entries 3, 4) and is consistent with our hypothesis regard-ing intermediate ii being disfavored by an electron-with-drawing R group (vide supra). Lowering the reactiontemperature did not significantly alter the product selec-tivity in this case (Table 7, entry 5).27 As with the aromatichydrazines, product selectivity toward 4 could beachieved with hydrazines 2u and 2v by instead perform-ing their reactions with 1 as two-step procedures carriedout in the absence of an external base (Scheme 4).

Table 6 Reactions of 1 with Aliphatic Hydrazinesa

Entry R Et3N (equiv)

Isolated yield (%)

1 H (p) 1 74

2b Me (q) 1 92

3 CH2CH2OH (r) 1 81

4c cyclohexyl (s) 2 84

5d Bn (t) 3 95

a Reaction conditions: 0.3–0.8 M, 1.05 equiv of 2, 0.5–0.8 mmol scale.b i-Pr2NEt was used instead of Et3N.c HCl salt of 2 was used.d Bis-HCl salt of 2 was used.

N

N

Cl

Cl

O

H HNNH2

R

+

1

N

N NN

R

Cl

4

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2

Et3N

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One-Pot Reactions Involving Arylhydrazines

Thus far we had established efficient procedures to selec-tively react 1 with 2 to obtain 1-substituted 4-chloropyr-azolo[3,4-d]pyrimidines (4), using either aromatic oraliphatic hydrazines. Reactions with most aliphatic hydra-zines could achieve selectivity through a single-step reac-tion. Aromatic hydrazines, however, required a two-stepprocedure. We wondered if this transformation could in-stead be carried out as a one-pot reaction. Several litera-ture procedures describe single-step, high temperaturereactions of other heteroaromatic chlorocarboxaldehydeswith substituted hydrazines, performed in the absence ofan external base. These procedures selectively generatedthe desired pyrazolo-quinoline,28 -naphthyridine,29 or-pyrazole28a isomers, analogous to 4, in a single step.However, none of these products contained a sensitivesubstituent comparable to the 4-chloride of 4.

Arylhydrazines 2a–d were used to evaluate the efficiencyof a one-pot, high temperature reaction with 1 in the ab-sence of an external base (Table 8). As expected, these re-actions demonstrated high selectivity towardpyrazolo[3,4-d]pyrimidine 4 over 5. A slight excess of 1was employed in these reactions, as excess 2 could dis-place the 4-chloro substituent of 4 and thus generate by-products. The primary issue for these reactions was thatthe in situ generated water partially hydrolyzed the 4-chloro substituent of 4, generating by-product 7. When 1and 2a were subjected to the previously optimized cycli-zation conditions (Table 5), 35% conversion to by-prod-uct 7a was observed (Table 8, entry 1). Production of 7acould be significantly reduced by decreasing the concen-tration (entry 2). Further suppression of 7a was accom-plished by shortening the reaction time (entries 3–5). Byemploying a reaction time of only one minute, chloridehydrolysis could be reduced to <5%, while maintainingquantitative cyclization of 3, with both 2a and 2b (entries5, 6).

Electron-deficient products 4c and 4d were both slower toform and more sensitive to hydrolysis. These reactionsneeded more time to reach completion and required ahigher level of dilution in order to suppress chloride hy-drolysis (Table 8, entries 7–13). For these more challeng-ing substrates, we investigated the options of minimizingthe presence of HCl or water. For example, using 2d in itsfree base form, as opposed to the HCl salt, significantlysuppressed the hydrolysis process (entries 14, 15). Theseresults suggest that chloride hydrolysis is promoted by ac-id. Alternatively, it was found that performing the reactionof 1 with 2c in the presence of magnesium sulfate couldminimize the formation of 7c (entries 16, 17).30 The opti-mized one-pot reactions of 1 with 2a–d were performedon a preparative scale as well (Table 9). Products 4a–dwere isolated in good yield after adding cold aqueous so-dium bicarbonate to the crude reaction mixture and filter-ing the resultant precipitate. This method of purificationwas adequate to remove the small quantities of by-product7 from the reactions involving 2a and 2b, but not fromthose of 2c and 2d.

Table 7 Reactions of 1 with 2u and 2va

Entry R Temp (°C)

Time (h)

1 (%)b

4 (%)b

5 (%)b

1 t-Bu (u) 65 1 ND 54 46

2 t-Bu (u) r.t. 1 2 70 28

3 t-Bu (u) 0 7 4 85 11

4 CH2CF3 (v) 65 1 ND 36 64

5 CH2CF3 (v) r.t. 1 4 33 63

a Reaction conditions: 0.2 M, 1.05 equiv of 2, 2.1 equiv of Et3N, 0.2 mmol scale. HCl salt of 2u was used. 2v was used as a 70 wt% solu-tion in H2O.b Determined from relative 1H NMR ratios in the crude reaction mix-ture. ND = not detected.

N

N

Cl

Cl

O

H HNNH2

R+

1

N

N NN

R

Cl

42

Et3N

THF

+N

N NN

Cl

5

R

Scheme 4 Two-step reactions of 1 with 2u and 2v

N

N

Cl

Cl

O

H HNNH2

+

1

N

N NN

Cl

4u2u

r.t., 2 hN

N

Cl

Cl

N

H

3u

HN

(microwave)

THF 100 °C, 1 h

91%isolated yield

98%isolated yield

N

N

Cl

Cl

O

H HNNH2

+

1

N

N NN

Cl

4v2v

CF3

CF3

N

N

Cl

Cl

N

H

3v

HN CF3

HCl

DMF

r.t., 2 h

66%isolated yield

DMF

(microwave)

MeCN200 °C, 5 min

90%isolated yield

(1)

(2)

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Comparing the one-pot procedure (Table 9) to the two-step procedure (Tables 3 and 5) for reactions involvingaromatic hydrazines, there are advantages and disadvan-tages to each. The one-pot procedure is operationally sim-pler, but it suffers from substrate-dependent sensitivity toboth concentration and reaction time. These issues wouldbecome especially important if larger scale reactions weredesired. The two-step procedure requires an additionalisolation step, but it is clearly more robust and is still quitefacile. We suggest that for small scale reactions, in which4 will undergo a subsequent transformation, the one-potprocedure is the method of choice. If large quantities of 4are desired in highly pure form, then we recommend theusage of the two-step synthesis.

Further Functionalization of the 4-Position

Finally, we demonstrated the utility of 4-chloropyrazo-lo[3,4-d]pyrimidine 4 as a key intermediate to generate avariety of 1,4-disubstituted pyrazolo[3,4-d]pyrimidines.Due to the highly reactive nature of the 4-chloro substitu-ent, it is possible to further functionalize the 4-position of4 through nucleophilic aromatic substitution reactions.The one-pot procedures presented in Table 9 were com-bined with the subsequent addition of a nucleophile, gen-erating pyrazolo[3,4-d]pyrimidines 8 and 9 using two-step, one-pot reactions (Scheme 5).31 This procedure wasnot only successful with amines (equation 1), but unacti-vated 1-methylindole was also able to displace the chlo-ride (equation 2).

Additional examples of functionalization at the 4-positionof 4 can be readily demonstrated using isolated 4 as the

Table 8 Optimization of One-Pot Reactions of 1 with 2a–da

Entry R Time (min) Conc. (M) 3 (%)c 4 (%)c 7 (%)c

1 4-MeOC6H4 (a) 20 0.5 ND 65 35

2 4-MeOC6H4 (a) 20 0.2 ND 83 17

3 4-MeOC6H4 (a) 10 0.2 ND 92 8

4 4-MeOC6H4 (a) 5 0.2 ND 94 6

5 4-MeOC6H4 (a) 1 0.2 ND 97 3

6 Ph (b) 1 0.2 ND 98 2

7 4-ClC6H4 (c) 1 0.2 9 77 14

8 4-ClC6H4 (c) 3 0.2 ND 66 34

9 4-ClC6H4 (c) 3 0.1 ND 76 24

10 4-ClC6H4 (c) 3 0.01 ND 98 2

11 4-F3CC6H4 (d) 3 0.2 25 65 10

12 4-F3CC6H4 (d) 15 0.2 ND 53 47

13 4-F3CC6H4 (d) 15 0.01 ND 97 3

14d 4-F3CC6H4 (d) 15 0.2 ND 96 4

15d 4-F3CC6H4 (d) 15 0.1 ND 98 2

16e 4-ClC6H4 (c) 3 0.1 ND 95 5

17e,f 4-ClC6H4 (c) 5 0.1 ND 97 3

a Reactions were performed in MeCN unless otherwise noted; heating was performed using a microwave reactor; 1.05 equiv of 1, 0.2 mmol scale.b HCl salt unless otherwise noted.c Determined from relative 1H NMR ratios in the crude reaction mixture. ND = not detected.d Free base of 2 was used.e MgSO4 (3 equiv) was used.f Reaction was performed in THF with 1 equiv of i-Pr2NEt.

N

N

Cl

Cl

O

H HNNH2

R

+

1

N

N NN

R

Cl

42b

200 °CN

N

Cl

Cl

N

H

3

HN

RN

N NN

R

OH

7

+

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starting material. As illustrated in Scheme 6,31 the 4-chlo-ro substituent of 4c was directly displaced with alkoxides(equations 1 and 2) or Grignard reagents (equation 3). 4calso underwent Suzuki cross-coupling reactions (equation4). In all of these cases, the 4-chloride of 4c was selective-ly displaced over the chloro substituent on the 1-phenylmoiety, providing an additional opportunity for selectivefunctionalization of these molecules. Many other exam-ples of further functionalization at the 4-position of 4 canbe found in the literature. The 4-chloride of 4b has beentransformed into various sulfur moieties.32 The 4-chloride

of 4t has been phosphonated.33 Compounds 4b and 4qhave undergone both cyanation (with potassium cya-nide)34 and aroylation (with aromatic aldehydes)35 at the4-position. Despite the highly electrophilic nature of the4-chloride of 4, this position can even be rendered nucleo-philic. For example, the 4-position of 4b has been lithiatedand subsequently added to aldehydes and ketones.36

In summary, we have developed efficient procedures toselectively convert 4,6-dichloropyrimidine-5-carboxalde-hyde (1) and various hydrazines into 1-substituted 4-chlo-ropyrazolo[3,4-d]pyrimidines (4), using either aromaticor aliphatic hydrazines. Each hydrazine class has a dis-tinct set of requisite reaction conditions to achieve selec-tivity. For aromatic substrates, the key is to carry out thereaction in the absence of an external base. This reactioncan be performed as either a two-step procedure or a one-pot process. Aliphatic hydrazines, on the other hand, gen-erally exhibit high selectivity toward 4 in the presence ofan external base. The observations that we have docu-mented have been rationalized within the proposed reac-tion mechanism, establishing a model that can be utilizedto predict the optimal conditions needed to effect selectivereactions between other combinations of 4-chloropyrimi-dine-5-carboxaldehydes and substituted hydrazines. Fi-nally, all of the protocols reported herein are operationallysimple, high-yielding, and involve reaction conditionsthat are mild enough to preserve the 4-chloro substituentof 4. The highly reactive nature of this 4-chloride can beexploited through further functionalization at the 4-posi-tion. Thus 4 serves as a highly versatile synthetic interme-diate, capable of rapidly generating a structurally diversearray of 1,4-disubstituted pyrazolo[3,4-d]pyrimidines.

Table 9 Optimized One-Pot Reactions of 1 with 2a–da

Entry R Conditions Isolated yield (%)

1 4-MeOC6H4 (a) MeCN (0.2 M), 1 min 87

2 Ph (b) MeCN (0.2 M), 1 min 95

3c 4-ClC6H4 (c) THF (0.1 M), 5 min 86d

4e 4-F3CC6H4 (d) MeCN (0.1 M), 15 min 95d

a Heating was performed using a microwave reactor, 1.05 equiv of 1, 0.5 mmol scale.b HCl salt unless otherwise noted.c MgSO4 (3 equiv), i-Pr2NEt (1 equiv).d 1H NMR analysis of 4 shows the presence of 3 mol% of 7.e Free base of 2 was used.

N

N

Cl

Cl

O

H HNNH2

R

+

1

N

N NN

R

Cl

42b

200 °C

Scheme 5 Further functionalization of 4 via one-pot reactions

N

N

Cl

Cl

O

H HNNH2

+

1

N

N NN

N

8

2a78%

isolated yield

HCl

(1)

1. MeCN, 200 °C, 1 min (microwave)

2. morpholine (1.2 equiv) i-Pr2NEt (3.2 equiv) r.t., 30 min

OMe

OMe

O

N

N

Cl

Cl

O

H

HNNH2

+

1

N

N NN

92d 40%isolated yield

(2)

1. MeCN, 200 °C, 15 min (microwave)

2. 1-methylindole (3 equiv) 200 °C, 2 h (microwave)

CF3

CF3

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1H and 13C NMR spectra were recorded on a Bruker 500 MHz NMRspectrometer. Flash column chromatography was performed on aCombiFlash Companion system (Isco, Inc.) with Silicycle pre-packed silica gel cartridges. HRMS (ESI positive) analyses wereperformed on an LTQ Orbitrap Discovery mass spectrometer. An-hydrous solvents were purchased from Acros and used without fur-ther purification. All commercially available reagents (Aldrich,Acros, Matrix Scientific) were purchased and used without furtherpurification unless otherwise noted.

Microwave Irradiation Experiments: Microwave irradiation exper-iments were performed in sealed vessels using a Biotage Initiator 60microwave reactor, which utilized a continuous focused microwavepower delivery system with operator-selectable power output from0 to 400 W. The reaction temperature was continuously monitoredby measuring the outer temperature of the reaction vessel with a cal-ibrated infrared temperature control mounted on the side of the re-action vessel. Reactions generally reached their target temperaturewithin 2 min upon initiation of the irradiation process, and they re-mained within five degrees of that temperature throughout the des-ignated reaction time. All reactions were performed using a stirringoption, which was accomplished through use of a Teflon-coated

magnetic stir bar located inside of the reaction vessel and a rotatingmagnetic plate located below the floor of the microwave cavity.

Purification of 1: 4,6-Dichloropyrimidine-5-carboxaldehyde (1)was not stable at room temperature for extended periods of time.Carboxaldehyde 1 obtained from commercial sources was a yellow-orange solid that was contaminated with its monohydrolyzed deriv-ative, 4-chloro-6-hydroxypyrimidine-5-carboxaldehyde (5–10mol% by 1H NMR, DMSO-d6), along with other minor impurities.Within days, the amount of this impurity had increased, and overtime the bis-hydrolyzed derivative, 4,6-dihydroxypyrimidine-5-car-boxaldehyde, was also detected. For the studies described herein,commercially purchased 1 (Aldrich or Matrix Scientific) was ini-tially purified by silica gel chromatography (0 to 10% EtOAc gra-dient in hexanes) to obtain a white solid (which usually containedfaint yellow portions) whose sole impurity was 4-chloro-6-hy-droxypyrimidine-5-carboxaldehyde in <5 mol% by 1H NMR(DMSO-d6). This material was stored in the refrigerator when not inuse and maintained its integrity for several months.

Conversion of 1 into 3a–o and 3u–v; 4,6-Dichloro-5-{[2-(4-methoxyphenyl)hydrazono]methyl}pyrimidine (3a); Typical Procedure ATo a 50 mL round-bottomed flask was added 1 (97%, 500 mg, 2.74mmol), 4-methoxyphenylhydrazine hydrochloride (2a; 456 mg,2.61 mmol), and DMF (10 mL). The reaction was allowed to stir un-der a N2 atmosphere at r.t. for 1 h. Ice (5 g) was added with stirring,followed by a solution of NaHCO3 (330 mg, 3.9 mmol) in H2O (5mL). The resultant precipitate was filtered, washed with H2O, anddried at 40–50 °C under vacuum overnight to obtain 3a; yield: 678mg (87%); yellow powder; mp 114–115 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.01 (s, 1 H), 8.70 (s, 1 H),7.96 (s, 1 H), 7.01–7.10 (m, 2 H), 6.84–6.94 (m, 2 H), 3.70 (s, 3 H). 13C NMR (126 MHz, DMSO-d6): δ = 157.5, 153.9, 153.7, 137.9,126.5, 125.4, 114.7, 113.6, 55.2.

HRMS (ESI): m/z [M + H]+ calcd for C12H10Cl2N4O: 297.0304;found: 297.0301 (100), 301.0240 (10), 299.0270 (64), 261 (60).

4,6-Dichloro-5-[(2-phenylhydrazono)methyl]pyrimidine (3b)Yield (Procedure A): 678 mg (97%); yellow powder; mp 152–153 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.15 (s, 1 H), 8.75 (s, 1 H),8.04 (s, 1 H), 7.23–7.32 (m, 2 H), 7.11 (d, J = 7.57 Hz, 2 H), 6.86(t, J = 7.25 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 157.9, 154.5, 144.1, 129.3,127.0, 126.4, 120.4, 112.5.

HRMS (ESI): m/z [M + H]+ calcd for C11H8Cl2N4: 267.0199;found: 267.0194 (100), 271.0137 (10), 269.0165 (60), 231 (98).

4,6-Dichloro-5-{[2-(4-chlorophenyl)hydrazono]methyl}pyrimi-dine (3c)Yield (Procedure A): 734 mg (93%); yellow powder; mp 167–168 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.24 (s, 1 H), 8.76 (s, 1 H),8.04 (s, 1 H), 7.27–7.35 (m, 2 H), 7.04–7.15 (m, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 158.1, 154.7, 143.1, 129.2,128.0, 126.2, 123.7, 114.0.

HRMS (ESI): m/z [M + H]+ calcd for C11H7Cl3N4: 300.9809; found:300.9804 (100), 306.9709 (3), 304.9746 (31), 302.9775 (92), 265(64).

4,6-Dichloro-5-({2-[4-(trifluoromethyl)phenyl]hydrazo-no}methyl)pyrimidine (3d)Yield (Procedure A): 862 mg (99%); yellow powder; mp 159–160 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.48 (s, 1 H), 8.80 (s, 1 H),8.12 (s, 1 H), 7.61 (d, J = 8.51 Hz, 2 H), 7.23 (d, J = 8.51 Hz, 2 H).

Scheme 6 Further functionalization of 4c

N

N NN

O

1083%

isolated yield

(1)

i-PrOH (2 equiv)t-AmONa (2 equiv)

Cl

N

N NN

Cl

4c

Cl

THF, 0 °C to r.t., 16 h

N

N NN

O

1187%isolated yield

(2)

phenol (2 equiv)t-AmONa (2 equiv)

Cl

N

N NN

Cl

4c

Cl

THF, 0 °C to r.t., 16 h

N

N NN

Me

1282%

isolated yield

(3)MeMgBr (2 equiv)

Cl

N

N NN

Cl

4c

Cl

THF, 0 °C to r.t., 17 h

N

N NN

1363%isolated yield

(4)

PhB(OH)2 (1.1 equiv)K2CO3 (3 equiv)PdCl2(dppf) (9%)

Cl

N

N NN

Cl

4c

Cl

THF, 170 °C, 10 min(microwave)

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13C NMR (126 MHz, DMSO-d6): δ = 158.4, 155.1, 147.2, 129.8,126.6 (q, J = 3.78 Hz), 126.0, 124.8 (q, J = 271.90 Hz), 120.1 (q,J = 32.34 Hz), 112.4.

HRMS (ESI): m/z [M + H]+ calcd for C12H7Cl2F3N4: 335.0073;found: 335.0066 (100), 338.9996 (10), 337.0036 (68), 299 (40).

4,6-Dichloro-5-{[2-(2-methoxyphenyl)hydrazono]methyl}py-rimidine (3e)Yield (Procedure A): 413 mg (93%); yellow powder; mp 113–114 °C.1H NMR (500 MHz, DMSO-d6): δ = 10.67 (s, 1 H), 8.74 (s, 1 H),8.41 (d, J = 1.26 Hz, 1 H), 7.39 (dd, J = 1.58, 7.88 Hz, 1 H), 6.99(dd, J = 1.42, 8.04 Hz, 1 H), 6.91 (dt, J = 1.10, 7.65 Hz, 1 H), 6.81–6.88 (m, 1 H), 3.86 (s, 3 H). 13C NMR (126 MHz, DMSO-d6): δ = 157.9, 154.4, 145.6, 133.3,128.7, 126.5, 121.3, 120.4, 112.4, 111.2, 55.6.


4,6-Dichloro-5-[(2-p-tolylhydrazono)methyl]pyrimidine (3f)Yield (Procedure A): 387 mg (92%); yellow powder; mp 145–146°C.1H NMR (500 MHz, DMSO-d6): δ = 11.05 (s, 1 H), 8.72 (s, 1 H),7.99 (s, 1 H), 7.05–7.12 (m, 2 H), 6.98–7.04 (m, 2 H), 2.23 (s, 3 H). 13C NMR (126 MHz, DMSO-d6): δ = 157.7, 154.2, 141.8, 129.7,129.1, 126.4, 126.1, 112.6, 20.3.


4,6-Dichloro-5-{[2-(4-fluorophenyl)hydrazono]methyl}pyrimi-dine (3g)Yield (Procedure A): 400 mg (94%); yellow powder; mp 150–151 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.12 (s, 1 H), 8.73 (s, 1 H),8.01 (d, J = 0.95 Hz, 1 H), 7.06–7.16 (m, 4 H). 13C NMR (126 MHz, DMSO-d6): δ = 157.9, 156.7 (d, J = 235.62Hz), 154.4, 140.7 (d, J = 1.26 Hz), 126.9, 126.3, 115.9 (d, J = 23.94Hz), 113.6 (d, J = 7.56 Hz).

HRMS (ESI): m/z [M + H]+ calcd for C11H7Cl2FN4: 285.0105;found: 285.0097 (100), 289.0042 (11), 287.0068 (68), 249 (76).

4,6-Dichloro-5-{[2-(2-chlorophenyl)hydrazono]methyl}pyrimi-dine (3h)Yield (Procedure A): 415 mg (92%); tan powder; mp 161–163 °C.1H NMR (500 MHz, DMSO-d6): δ = 10.76 (s, 1 H), 8.79 (s, 1 H),8.56 (d, J = 0.95 Hz, 1 H), 7.55 (dd, J = 1.26, 8.20 Hz, 1 H), 7.38(dd, J = 1.26, 7.88 Hz, 1 H), 7.26–7.33 (m, 1 H), 6.89 (ddd,J = 1.42, 7.25, 8.04 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 158.5, 155.1, 140.5, 131.2,129.6, 128.2, 126.2, 121.2, 116.8, 114.6.

HRMS (ESI): m/z [M + H]+ calcd for C11H7Cl3N4: 300.9809;found: 300.9803 (92), 306.9713 (3), 304.9746 (27), 302.9775(100), 265 (79).

4,6-Dichloro-5-{[2-(3-chlorophenyl)hydrazono]methyl}pyrimi-dine (3i)Yield (Procedure A): 400 mg (89%); bright yellow powder; mp172–174 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.28 (s, 1 H), 8.78 (s, 1 H),8.05 (d, J = 0.95 Hz, 1 H), 7.29 (t, J = 8.04 Hz, 1 H), 7.10 (t,J = 2.05 Hz, 1 H), 6.98–7.06 (m, 1 H), 6.88 (ddd, J = 0.79, 2.05,7.88 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 158.2, 154.9, 145.6, 133.9,131.0, 128.7, 126.1, 119.8, 111.8, 111.2.

HRMS (ESI): m/z [M + H]+ calcd for C11H7Cl3N4: 300.9809;found: 300.9803 (100), 306.9716 (3), 304.9744 (31), 302.9774(94), 265 (73).

5-{[2-(4-Bromophenyl)hydrazono]methyl}-4,6-dichloropyrimi-dine (3j)Yield (Procedure A): 495 (96%); yellow powder; mp 161–162 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.23 (s, 1 H), 8.76 (s, 1 H),8.05 (d, J = 0.95 Hz, 1 H), 7.40–7.48 (m, 2 H), 7.01–7.09 (m, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 158.1, 154.7, 143.4, 132.0,128.1, 126.2, 114.5, 111.4.

HRMS (ESI): m/z [M + H]+ calcd for C11H7BrCl2N4: 344.9304;found: 344.9301 (60), 350.9214 (4), 348.9250 (47), 346.9275 (100),311 (45).

4,6-Dichloro-5-{[2-(naphthalen-1-yl)hydrazono]methyl}pyrim-idine (3k)Yield (Procedure A): 290 mg (92%); brown powder; mp 159–160°C.1H NMR (500 MHz, DMSO-d6): δ = 11.33 (s, 1 H), 8.79 (s, 1 H),8.53 (d, J = 0.63 Hz, 1 H), 8.27–8.37 (m, 1 H), 7.85–7.94 (m, 1 H),7.50–7.59 (m, 3 H), 7.41–7.49 (m, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 158.2, 154.8, 139.3, 133.9,129.9, 128.2, 126.6, 126.3, 126.0, 125.0, 121.5, 121.2, 120.2, 107.7.


4,6-Dichloro-5-{[2-(pyridin-2-yl)hydrazono]methyl}pyrimi-dine (3l)Yield (Procedure A): 455 mg (85%); tan powder; mp 187–188 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.54 (br s, 1 H), 8.79 (s, 1 H),8.24 (s, 1 H), 8.14–8.20 (m, 1 H), 7.70 (ddd, J = 1.73, 7.09, 8.51 Hz,1 H), 7.26 (d, J = 8.20 Hz, 1 H), 6.87 (ddd, J = 0.95, 5.04, 7.25 Hz,1 H). 13C NMR (126 MHz, DMSO-d6): δ = 158.5, 156.2, 155.2, 147.7,138.4, 129.8, 126.2, 116.3, 106.8.


2-{2-[(4,6-Dichloropyrimidin-5-yl)methylene]hydrazinyl}qui-noxaline (3m)Yield (Procedure A): 282 mg (88%); orange powder; mp 222–223°C.1H NMR (500 MHz, DMSO-d6): δ = 12.27 (s, 1 H), 9.07 (s, 1 H),8.86 (s, 1 H), 8.32–8.37 (m, 1 H), 7.96 (d, J = 8.20 Hz, 1 H), 7.69–7.80 (m, 2 H), 7.53–7.63 (m, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 159.0, 155.9, 149.9, 140.5,138.3, 136.0, 132.9, 130.6, 128.8, 126.5, 126.0, 125.8.


2-{2-[(4,6-Dichloropyrimidin-5-yl)methylene]hydrazinyl}ben-zo[d]oxazole (3n)Yield (Procedure A): 279 mg (91%); yellow powder; mp 207–208 °C.1H NMR (500 MHz, DMSO-d6): δ = 12.63 (br s, 1 H), 8.90 (s, 1 H),8.37 (s, 1 H), 7.55 (d, J = 7.57 Hz, 1 H), 7.45 (br m, 1 H), 7.24 (t,J = 7.57 Hz, 1 H), 7.15 (t, J = 7.57 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 159.5, 159.2, 156.7, 147.5,137.4, 126.1, 124.3, 122.0, 116.0, 109.6. Note that not all 13C peaksare visible, and several are broad (see spectrum in the SupportingInformation). Product 3n likely exists as a mixture of rotamers.


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4-{2-[(4,6-Dichloropyrimidin-5-yl)methylene]hydrazinyl}-5-methylthieno[2,3-d]pyrimidine (3o)Yield (Procedure A): 330 mg (98%); yellow powder; mp 182–184 °C.1H NMR (500 MHz, DMSO-d6): δ = 11.69 (br s, 1 H), 8.91 (s, 1 H),8.48 (s, 1 H), 7.90 (d, J = 3.47 Hz, 1 H), 7.21 (s, 1 H), 2.58 (s, 3 H).13C NMR spectrum for 3o could not be obtained due to the highlyinsoluble nature of this compound in DMSO-d6 and all other com-mon NMR solvents.

HRMS (ESI): m/z [M + H]+ calcd for C12H8Cl2N6S: 338.9981;found: 338.9997 (100), 342.99 (13), 340.997 (34), 165 (24).

5-[(2-tert-Butylhydrazono)methyl]-4,6-dichloropyrimidine (3u)Yield (Procedure A): 590 mg (91%); white powder; mp 114–115 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.65 (s, 1 H), 8.25 (br s, 1 H),7.71 (s, 1 H), 1.20 (s, 9 H).13C NMR (126 MHz, DMSO-d6): δ = 157.3, 153.6, 127.1, 123.4,53.6, 28.4.


4,6-Dichloro-5-{[2-(2,2,2-trifluoroethyl)hydrazono]methyl}py-rimidine (3v)Compound 3v was synthesized according to Procedure A, from 1(97%, 400 mg, 2.19 mmol) and 2,2,2-trifluoroethylhydrazine (2v;70 wt% in H2O, 535 mg, 3.28 mmol) in DMF (2.2 mL) to obtain 396mg of 3v as a yellow powder, which contained 1 wt% DMF by 1HNMR (66% yield); mp 113–114 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.79 (br s, 1 H), 8.73 (s, 1 H),7.80 (s, 1 H), 4.04 (q, J = 9.46 Hz, 2 H).13C NMR (126 MHz, DMSO-d6): δ = 158.5, 155.2, 126.9, 126.8,125.0 (q, J = 281.82 Hz), 50.0 (q, J = 31.50 Hz).

HRMS (ESI): m/z [M + H]+ calcd for C7H5Cl2F3N4: 272.9916;found: 272.9924 (100), 105 (22).

Conversion of 3a–o into 4a–o; 4-Chloro-1-(4-methoxyphenyl)-1H-pyrazolo[3,4-d]pyrimidine (4a); Typical Procedure BTo a microwave vial was added 3a (149 mg, 0.5 mmol) and MeCN(1 mL). The reaction mixture was heated with stirring in a micro-wave reactor at 200 °C for 20 min; then cooled to r.t. A cold solutionof NaHCO3 (63 mg, 0.75 mmol) in H2O (4 mL) was added with stir-ring. The resultant precipitate was filtered, washed with H2O, anddried at r.t. under vacuum overnight to obtain 4a (Note: Isolation ofequally pure 4 could alternatively be accomplished by simply con-centrating the crude reaction mixture in vacuo. The in situ generatedHCl, which was present in the crude reaction mixture, was generallynot observed to form a salt with 4 upon concentration and thus pre-sumably evaporated); yield: 126 mg (97%); tan powder; mp 122–123 °C.

Alternative One-Pot Synthesis of 4a: To a microwave vial was add-ed 1 (97%, 95 mg, 0.52 mmol), 4-methoxyphenylhydrazine hydro-chloride (2a; 87 mg, 0.5 mmol), and MeCN (2.5 mL). The reactionmixture was heated with stirring in a microwave reactor at 200 °Cfor 1 min; then cooled to r.t. The crude reaction mixture was cooledin an ice bath. A cold solution of NaHCO3 (130 mg, 1.5 mmol) inH2O (8 mL) was added, with stirring. The resultant precipitate wasfiltered, washed with H2O, and dried at r.t. under vacuum overnightto obtain 113 mg (87%) of 4a as a tan powder.1H NMR (500 MHz, DMSO-d6): δ = 8.95 (s, 1 H), 8.72 (s, 1 H),7.96–8.02 (m, 2 H), 7.13–7.20 (m, 2 H), 3.84 (s, 3 H). 13C NMR (126 MHz, DMSO-d6): δ = 158.4, 155.2, 154.1, 152.2,133.4, 130.9, 123.3, 114.5, 114.3, 55.5.

HRMS (ESI): m/z [M + H]+ calcd for C12H9ClN4O: 261.0538;found: 261.0541 (100), 263.0511 (33), 234 (4).

4-Chloro-1-phenyl-1H-pyrazolo[3,4-d]pyrimidine (4b)Yield (Procedure B): 105 mg (91%); tan powder; mp 111–112 °C.

Alternative One-Pot Synthesis of 4b: To a microwave vial was add-ed 1 (97%, 95 mg, 0.52 mmol), phenylhydrazine hydrochloride (2b;72 mg, 0.5 mmol), and MeCN (2.5 mL). The reaction mixture washeated with stirring in a microwave reactor at 200 °C for 1 min; thencooled to r.t. The crude mixture was cooled in an ice bath. A coldsolution of NaHCO3 (130 mg, 1.5 mmol) in H2O (8 mL) was added,with stirring. The resultant precipitate was filtered, washed withH2O, and dried at r.t. under a N2 flow to obtain 110 mg (95%) of 4bas a yellow powder.1H NMR (500 MHz, DMSO-d6): δ = 8.99 (s, 1 H), 8.77 (s, 1 H),8.11–8.18 (m, 2 H), 7.60–7.66 (m, 2 H), 7.42–7.48 (m, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.4, 154.2, 152.6, 137.8,134.0, 129.4, 127.4, 121.4, 114.7.

HRMS (ESI): m/z [M + H]+ calcd for C11H7ClN4: 231.0432;found: 231.0432 (100), 233.0402 (33), 204 (3).

4-Chloro-1-(4-chlorophenyl)-1H-pyrazolo[3,4-d]pyrimidine (4c)Yield (Procedure B): 129 mg (97%); tan powder; mp 130–131 °C.

Alternative One-Pot Synthesis of 4c: To a microwave vial was add-ed 4-chlorophenylhydrazine hydrochloride (2c; 90 mg, 0.5 mmol),i-Pr2NEt (87 μL, 0.5 mmol), MgSO4 (181 mg, 1.5 mmol), and THF(5 mL). The reaction mixture was stirred at r.t. for 10 min, and 1(97%, 95 mg, 0.52 mmol) was then added. The reaction mixture washeated with stirring in a microwave reactor at 200 °C for 5 min; thencooled to r.t. The crude mixture was filtered through filter paper,eluting with CH2Cl2. The solvent was removed in vacuo. MeCN (2mL) was added, and the solution was cooled in an ice bath. A coldsolution of NaHCO3 (130 mg, 1.5 mmol) in H2O (8 mL) was addedwith stirring. The resultant precipitate was filtered, washed withH2O, and dried at r.t. under a N2 flow to obtain 117 mg (86%) of 4cas a yellow powder, which also contained 3 mol% 7c by 1H NMRanalysis.1H NMR (500 MHz, DMSO-d6): δ = 8.99 (s, 1 H), 8.76 (s, 1 H),8.19 (d, J = 8.51 Hz, 2 H), 7.67 (d, J = 8.83 Hz, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.5, 154.2, 152.6, 136.6,134.3, 131.4, 129.4, 122.5, 114.8.


4-Chloro-1-(4-(trifluoromethyl)phenyl)-1H-pyrazolo[3,4-d]py-rimidine (4d)Yield (Procedure B): 129 mg (96%); orange powder; mp 88–89 °C.

Alternative One-Pot Synthesis of 4d: To a microwave vial was add-ed 1 (97%, 95 mg, 0.52 mmol), 4-(trifluoromethyl)phenylhydrazine(2d; 88 mg, 0.5 mmol), and MeCN (5 mL). The reaction mixturewas heated with stirring in a microwave reactor at 200 °C for 15min; then cooled to r.t. The crude mixture was cooled in an ice bath.A cold solution of NaHCO3 (130 mg, 1.5 mmol) in H2O (8 mL) wasadded, with stirring. The resultant precipitate was filtered, washedwith H2O, and dried at r.t. under a N2 flow to obtain 146 mg (95%)of 4d as a yellow powder, which also contained 3 mol% 7d by 1HNMR analysis.1H NMR (500 MHz, DMSO-d6): δ = 9.02 (s, 1 H), 8.80 (s, 1 H),8.43 (d, J = 8.51 Hz, 2 H), 7.96 (d, J = 8.51 Hz, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.7, 154.3, 153.1, 140.9,134.9, 127.0 (q, J = 32.13 Hz), 126.7 (q, J = 3.78 Hz), 123.9 (q,J = 272.16 Hz), 120.9, 115.1.

HRMS (ESI): m/z [M + H]+ calcd for C12H6ClF3N4: 299.0306;found: 299.0304 (100), 301.0275 (30), 189 (6).

4-Chloro-1-(2-methoxyphenyl)-1H-pyrazolo[3,4-d]pyrimidine (4e)Yield (Procedure B): 110 mg (85%); tan powder; mp 85–87 °C.

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1H NMR (500 MHz, DMSO-d6): δ = 8.85 (s, 1 H), 8.70 (s, 1 H),7.56–7.66 (m, 1 H), 7.49 (dd, J = 1.58, 7.57 Hz, 1 H), 7.32 (dd,J = 0.95, 8.20 Hz, 1 H), 7.16 (dt, J = 1.26, 7.57 Hz, 1 H), 3.72 (s, 3H). 13C NMR (126 MHz, DMSO-d6): δ = 155.2, 154.7, 153.9, 153.8,133.4, 131.4, 128.9, 125.3, 120.6, 113.2, 112.9, 55.9.


4-Chloro-1-p-tolyl-1H-pyrazolo[3,4-d]pyrimidine (4f)Yield (Procedure B): 120 mg (98%); yellow powder; mp 122–124 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.98 (s, 1 H), 8.75 (s, 1 H),7.98–8.07 (m, 2 H), 7.43 (d, J = 7.88 Hz, 2 H), 2.40 (s, 3 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.4, 154.1, 152.4, 136.9,135.5, 133.8, 129.8, 121.3, 114.5, 20.6.


4-Chloro-1-(4-fluorophenyl)-1H-pyrazolo[3,4-d]pyrimidine (4g)Yield (Procedure B): 123 mg (99%); tan powder; mp 156–158 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.99 (s, 1 H), 8.78 (s, 1 H),8.17 (dd, J = 4.89, 8.67 Hz, 2 H), 7.48 (t, J = 8.83 Hz, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 160.7 (d, J = 245.7 Hz),155.4, 154.2, 152.5, 134.2 (d, J = 2.52 Hz), 134.0, 123.6 (d, J = 8.82Hz), 116.3 (d, J = 23.94 Hz), 114.6.

HRMS (ESI): m/z [M + H]+ calcd for C11H6ClFN4: 249.0338;found: 249.0336 (100), 251.0307 (34), 197 (3).

4-Chloro-1-(2-chlorophenyl)-1H-pyrazolo[3,4-d]pyrimidine (4h)Yield (Procedure B): 125 mg (94%); tan powder; mp 107–110 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.91 (s, 1 H), 8.81 (s, 1 H),7.80 (dd, J = 1.42, 8.04 Hz, 1 H), 7.73 (dd, J = 1.73, 7.72 Hz, 1 H),7.68 (dt, J = 1.73, 7.80 Hz, 1 H), 7.59–7.64 (m, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.6, 154.1, 154.0, 134.2,134.1, 131.9, 131.0, 130.4, 130.2, 128.4, 113.4.


4-Chloro-1-(3-chlorophenyl)-1H-pyrazolo[3,4-d]pyrimidine (4i)Yield (Procedure B): 121 mg (92%); tan powder; mp 112–113 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.05 (s, 1 H), 8.82 (s, 1 H),8.32 (t, J = 2.05 Hz, 1 H), 8.18 (ddd, J = 0.79, 1.97, 8.28 Hz, 1 H),7.67 (t, J = 8.20 Hz, 1 H), 7.53 (ddd, J = 0.95, 2.05, 8.04 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.7, 154.3, 152.9, 139.0,134.6, 133.7, 131.3, 127.1, 120.6, 119.5, 115.0.


1-(4-Bromophenyl)-4-chloro-1H-pyrazolo[3,4-d]pyrimidine (4j)Yield (Procedure B): 210 mg (86%); tan powder; mp 166–168 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.02 (s, 1 H), 8.81 (s, 1 H),8.15–8.19 (m, 2 H), 7.82–7.86 (m, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.6, 154.3, 152.7, 137.1,134.5, 132.4, 122.9, 119.8, 114.9.

HRMS (ESI): m/z [M + H]+ calcd for C11H6BrClN4: 308.9537;found: 308.9533 (77), 312.9483 (24), 310.9510 (100), 197 (4).

4-Chloro-1-(naphthalen-1-yl)-1H-pyrazolo[3,4-d]pyrimidine (4k)Yield (Procedure B): 128 mg (92%); brown powder; mp 97–100 °C.

1H NMR (500 MHz, DMSO-d6): δ = 8.86 (s, 1 H), 8.85 (s, 1 H),8.21 (d, J = 8.20 Hz, 1 H), 8.13 (d, J = 8.20 Hz, 1 H), 7.76–7.81 (m,1 H), 7.70–7.75 (m, 1 H), 7.63 (ddd, J = 0.95, 6.94, 8.20 Hz, 1 H),7.52 (ddd, J = 1.10, 6.94, 8.35 Hz, 1 H), 7.41 (dd, J = 0.63, 8.51 Hz,1 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.4, 154.5, 154.1, 134.0,133.8, 132.9, 130.1, 129.1, 128.3, 127.5, 126.9, 125.9, 125.4, 122.6,113.6.


4-Chloro-1-(pyridin-2-yl)-1H-pyrazolo[3,4-d]pyrimidine (4l)Yield (Procedure B): 121 mg (90%); tan powder; mp 175–177 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.02 (s, 1 H), 8.81 (s, 1 H),8.68 (d, J = 3.78 Hz, 1 H), 8.11–8.18 (m, 1 H), 8.06–8.11 (m, 1 H),7.56 (dd, J = 6.78, 5.20 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 155.7, 154.1, 153.1, 149.8,148.9, 139.2, 134.5, 123.4, 117.2, 114.7.


2-(4-Chloro-1H-pyrazolo[3,4-d]pyrimidin-1-yl)quinoxaline (4m)Yield (Procedure B): 141 mg (92%); rose-colored powder; mp 220–223 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.79 (s, 1 H), 9.15 (s, 1 H),8.98 (s, 1 H), 8.23 (dd, J = 1.42, 8.35 Hz, 1 H), 8.16–8.21 (m, 1 H),7.92–8.02 (m, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 156.3, 155.7, 154.3, 144.7,141.6, 140.5, 139.5, 135.9, 131.3, 130.2, 129.32, 129.25. Note thatnot all 13C peaks are visible (see spectrum in the Supporting Infor-mation). Product 4m was highly insoluble in DMSO-d6 and all othercommon NMR solvents; thus the 13C spectrum could not be record-ed at a concentration adequate to visualize all 13C peaks.


2-(4-Chloro-1H-pyrazolo[3,4-d]pyrimidin-1-yl)benzo[d]oxa-zole (4n)Yield (Procedure B): 124 mg (92%); yellow powder; mp 236–239 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.18 (s, 1 H), 9.03 (s, 1 H),7.85–7.94 (m, 2 H), 7.47–7.53 (m, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 157.0, 154.61, 154.58, 151.6,148.7, 140.1, 137.9, 125.61, 125.58, 119.9, 115.4, 111.1.


4-(4-Chloro-1H-pyrazolo[3,4-d]pyrimidin-1-yl)-5-methylthie-no[2,3-d]pyrimidine (4o)Yield (Procedure B): 137 mg (90%); yellow powder; mp 185–190 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.24 (s, 1 H), 8.96 (s, 1 H),8.95 (s, 1 H), 7.84 (d, J = 1.26 Hz, 1 H), 1.84 (d, J = 1.26 Hz, 3 H).13C NMR (126 MHz, DMSO-d6): δ = 172.1, 156.2, 155.0, 154.6,152.7, 150.0, 135.2, 128.7, 126.7, 125.6, 113.9, 15.2.

HRMS (ESI): m/z [M + H]+ calcd for C12H7ClN6S: 303.0214;found: 303.0224 (100), 305.019 (34), 105 (26).

Conversion of 1 into 4p–t; 4-Chloro-1H-pyrazolo[3,4-d]pyrimi-dine (4p); Typical Procedure C To a 20 mL vial was added 1 (97%, 88 mg, 0.5 mmol), Et3N (0.070mL, 0.5 mmol), and THF (1 mL). The reaction mixture was allowedto stir at 0 °C for 10 min under a N2 atmosphere. To the solution wasadded hydrazine monohydrate (2p; 0.026 mL, 0.525 mmol) in THF

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(1 mL) dropwise. The reaction mixture was allowed to warm to r.t.and stirred for 1 h. The solvent was removed in vacuo, and the res-idue was partitioned between CH2Cl2 (6 mL) and H2O (10 mL). Thelayers were separated, and the aqueous layer was extracted withCH2Cl2 (2 × 6 mL). The combined organic layers were dried(MgSO4) and concentrated in vacuo to obtain 4p; yield: 56 mg(74%); dark yellow powder; mp 156–158 °C.1H NMR (500 MHz, DMSO-d6): δ = 14.51 (br s, 1 H), 8.83 (s, 1 H),8.45 (d, J = 0.95 Hz, 1 H). 13C NMR (126 MHz, CDCl3): δ = 155.4, 155.1, 154.7, 133.8, 113.5.


4-Chloro-1-methyl-1H-pyrazolo[3,4-d]pyrimidine (4q)Yield (Procedure C): 78 mg (92%); yellow powder; mp 94–96 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.88 (s, 1 H), 8.48 (s, 1 H),4.08 (s, 3 H). 13C NMR (126 MHz, DMSO-d6): δ = 154.6, 153.5, 152.7, 131.8,112.9, 34.3.


2-(4-Chloro-1H-pyrazolo[3,4-d]pyrimidin-1-yl)ethanol (4r)Yield (Procedure C): 120 mg (81%); yellow powder; mp 91–93 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.86 (s, 1 H), 8.49 (s, 1 H),4.82 (br s, 1 H), 4.51 (t, J = 5.67 Hz, 2 H), 3.86 (t, J = 5.52 Hz, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 154.5, 153.5, 153.3, 132.0,113.0, 59.1, 50.1.


4-Chloro-1-cyclohexyl-1H-pyrazolo[3,4-d]pyrimidine (4s)Yield (Procedure C): 100 mg (84%); tan powder; mp 56–57 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.85 (s, 1 H), 8.47 (s, 1 H),4.72–4.88 (m, 1 H), 1.91–2.00 (m, 4 H), 1.87 (d, J = 13.24 Hz, 2 H),1.64–1.75 (m, 1 H), 1.40–1.56 (m, 2 H), 1.27 (d, J = 12.93 Hz, 1 H). 13C NMR (126 MHz, DMSO-d6): δ = 154.3, 153.5, 151.9, 131.7,113.0, 56.6, 31.8, 24.82, 24.80.


1-Benzyl-4-chloro-1H-pyrazolo[3,4-d]pyrimidine (4t)Yield (Procedure C): 175 mg (95%); tan powder; mp 72–74 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.91 (s, 1 H), 8.53 (s, 1 H),7.23–7.36 (m, 5 H), 5.70 (s, 2 H). 13C NMR (126 MHz, DMSO-d6): δ = 154.9, 153.8, 152.8, 136.2,132.6, 128.6, 127.8, 127.7, 113.1, 50.7.


1-tert-Butyl-4-chloro-1H-pyrazolo[3,4-d]pyrimidine (4u)To a microwave vial was added 3u (124 mg, 0.5 mmol) and THF (3mL). The reaction mixture was heated with stirring in a microwavereactor at 100 °C for 1 h; then cooled to r.t. The solvent was re-moved in vacuo to obtain 103 mg (98%) of 4u as an orange oil,which solidified upon storage in the refrigerator; mp 36–37 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.84 (s, 1 H), 8.40 (s, 1 H),1.75 (s, 9 H).13C NMR (126 MHz, DMSO-d6): δ = 153.6, 153.4, 152.3, 130.5,114.1, 61.0, 28.7.


4-Chloro-1-(2,2,2-trifluoroethyl)-1H-pyrazolo[3,4-d]pyrimi-dine (4v)To a microwave vial was added 3v (110 mg, 0.4 mmol) and MeCN(4 mL). The reaction mixture was heated with stirring in a micro-wave reactor at 200 °C for 5 min; then cooled to r.t. The solvent wasremoved in vacuo to obtain 85 mg (90%) of 4v as an orange oil,which solidified upon storage in the refrigerator; mp 57–58 °C.1H NMR (500 MHz, CDCl3): δ = 8.85 (s, 1 H), 8.28 (s, 1 H), 5.10(q, J = 8.20 Hz, 2 H).13C NMR (126 MHz, CDCl3): δ = 155.4, 155.3, 154.6, 134.0, 122.7(q, J = 280.56 Hz), 114.0, 48.3 (q, J = 36.54 Hz).

HRMS (ESI): m/z [M + H]+ calcd for C7H4ClF3N4: 237.0149;found: 237.0156 (43), 239.0126 (14), 102 (48).

4-[1-(4-Methoxyphenyl)-1H-pyrazolo[3,4-d]pyrimidin-4-yl]morpholine (8)To a microwave vial was added 1 (97%, 95 mg, 0.52 mmol), 4-me-thoxyphenylhydrazine hydrochloride (2a; 87 mg, 0.5 mmol), andMeCN (2.5 mL). The reaction mixture was heated with stirring in amicrowave reactor at 200 °C for 1 min; then cooled to r.t. Morpho-line (53 μL, 0.61 mmol) and i-Pr2NEt (280 μL, 1.6 mmol) were add-ed, and the mixture was allowed to stir, open to air, at r.t. for 30 min.H2O (6 mL) was added with stirring. The resultant precipitate wasfiltered, washed with H2O, and dried at r.t. under vacuum overnightto obtain 122 mg (78%) of 8 as a tan powder; mp 147–148 °C.1H NMR (500 MHz, CDCl3): δ = 8.47 (s, 1 H), 8.11 (s, 1 H), 7.92–7.98 (m, 2 H), 7.02–7.09 (m, 2 H), 4.05 (t, J = 4.57 Hz, 4 H), 3.89–3.93 (m, 4 H), 3.87 (s, 3 H).13C NMR (126 MHz, CDCl3): δ = 158.4, 157.2, 155.3, 153.9, 133.1,131.9, 123.8, 114.3, 101.4, 66.5, 55.5, 45.6.

HRMS (ESI): m/z [M + H]+ calcd for C16H17N5O2: 312.1455;found: 312.1465 (100), 102 (6).

4-(1-Methyl-1H-indol-3-yl)-1-[4-(trifluoromethyl)phenyl]-1H-pyrazolo[3,4-d]pyrimidine (9)To a microwave vial was added 1 (97%, 95 mg, 0.52 mmol), 4-(tri-fluoromethyl)phenylhydrazine (2d; 88 mg, 0.5 mmol), and MeCN(5 mL). The reaction mixture was heated with stirring in a micro-wave reactor at 200 °C for 15 min; then cooled to r.t. 1-Methylin-dole (187 μL, 1.5 mmol) was added, and the mixture was heatedwith stirring in a microwave reactor at 200 °C for 2 h. The crudeproduct mixture was dry-loaded directly onto silica gel and purifiedtwice by silica gel chromatography (12 g then 4 g SiO2, 0 to 30%EtOAc gradient in hexanes) to obtain 81 mg (40%) of 9 as a yellowpowder; mp 198–199 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.09 (s, 1 H), 9.06 (s, 1 H),8.82 (s, 1 H), 8.73–8.78 (m, 1 H), 8.56 (d, J = 8.51 Hz, 2 H), 7.95(d, J = 8.83 Hz, 2 H), 7.55–7.61 (m, 1 H), 7.26–7.36 (m, 2 H), 3.96(s, 3 H).13C NMR (126 MHz, DMSO-d6): δ = 157.4, 156.0, 153.2, 141.6,137.4, 136.01, 135.97, 126.5 (q, J = 3.78 Hz), 126.2 (q, J = 32.76Hz), 126.1, 124.1 (q, J = 272.16 Hz), 123.0, 122.9, 121.8, 120.6,111.1, 110.6, 110.3, 33.3.

HRMS (ESI): m/z [M + H]+ calcd for C21H14F3N5: 394.1274;found: 394.1285 (100), 102 (2).

1-(4-Chlorophenyl)-4-isopropoxy-1H-pyrazolo[3,4-d]pyrimi-dine (10)To a conical vial was added 4c (40 mg, 0.15 mmol), i-PrOH (23 μL,0.3 mmol), and THF (0.75 mL). The reaction mixture was allowedto stir, open to air, in an ice bath for 5 min, after which t-AmONa(2.5 M in THF, 120 μL, 0.3 mmol) was added. The reaction was al-lowed to reach r.t. and stirred for 16 h. The crude reaction mixturewas transferred to a separatory funnel, and CH2Cl2 (10 mL), sat. aqNH4Cl (10 mL) and brine (5 mL) were added. The layers were sep-arated, and the aqueous layer was extracted with CH2Cl2 (2 × 10mL). The combined organic layers were dried (Na2SO4), and the

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crude material was purified by silica gel chromatography (4 g SiO2,0 to 10% EtOAc gradient in hexanes) to obtain 36 mg (83%) of 10as a white powder; mp 145–146 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.71 (s, 1 H), 8.49 (s, 1 H),8.21–8.27 (m, 2 H), 7.62–7.68 (m, 2 H), 5.63 (sept, J = 6.20 Hz, 1H), 1.43 (d, J = 6.31 Hz, 6 H).13C NMR (126 MHz, DMSO-d6): δ = 163.0, 156.1, 154.3, 137.3,133.5, 130.7, 129.3, 122.2, 103.9, 70.6, 21.6.


1-(4-Chlorophenyl)-4-phenoxy-1H-pyrazolo[3,4-d]pyrimidine (11)A procedure analogous to that employed for 10 using sodium phen-oxide as the nucleophile was carried out. The crude material was pu-rified by silica gel chromatography (4 g SiO2, 0 to 10% EtOAcgradient in hexanes) to obtain 42 mg (87%) of 11 as a pale yellowpowder; mp 188–189 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.69 (s, 1 H), 8.50 (s, 1 H),8.23–8.29 (m, 2 H), 7.65–7.71 (m, 2 H), 7.53 (t, J = 7.88 Hz, 2 H),7.34–7.41 (m, 3 H).13C NMR (126 MHz, DMSO-d6): δ = 163.3, 156.0, 154.8, 151.8,137.1, 133.6, 131.0, 129.9, 129.4, 126.3, 122.5, 122.0, 103.8.


1-(4-Chlorophenyl)-4-methyl-1H-pyrazolo[3,4-d]pyrimidine (12)To a conical vial was added 4c (40 mg, 0.15 mmol) and THF (0.75mL). The solution was allowed to stir, open to air, in an ice bath for5 min, after which MeMgBr (1 M in THF, 300 μL, 0.3 mmol) wasadded. The reaction mixture was allowed to reach r.t. and stirred for17 h. The crude reaction mixture was transferred to a separatoryfunnel, and CH2Cl2 (10 mL), sat. aq NH4Cl (10 mL), and brine (5mL) were added. The layers were separated, and the aqueous layerwas extracted with CH2Cl2 (2 × 10 mL). The combined organic lay-ers were dried (Na2SO4), and the crude material was purified by sil-ica gel chromatography (4 g SiO2, 0 to 30% EtOAc gradient inhexanes) to obtain 30 mg (82%) of 12 as a yellow powder; mp 145–146 °C.1H NMR (500 MHz, DMSO-d6): δ = 8.98 (s, 1 H), 8.77 (s, 1 H),8.23–8.29 (m, 2 H), 7.61–7.68 (m, 2 H), 2.82 (s, 3 H).13C NMR (126 MHz, DMSO-d6): δ = 164.1, 155.5, 151.8, 137.2,135.4, 130.6, 129.3, 122.0, 115.2, 21.8.


1-(4-Chlorophenyl)-4-phenyl-1H-pyrazolo[3,4-d]pyrimidine (13)To a microwave vial was added 4c (40 mg, 0.15 mmol), phenylbo-ronic acid (20 mg, 0.16 mmol), K2CO3 (62 mg, 0.45 mmol),PdCl2(dppf) (10 mg, 0.014 mmol), THF (2.2 mL), and H2O (10 μL).The reaction mixture was heated with stirring in a microwave reac-tor at 170 °C for 10 min. The crude reaction mixture was passedthrough a small silica gel plug eluting with EtOAc, and the crudematerial was purified by silica gel chromatography (4 g SiO2, 0 to10% EtOAc gradient in hexanes) to obtain 29 mg (63%) of 13 as awhite powder; mp 165–166 °C.1H NMR (500 MHz, DMSO-d6): δ = 9.21 (s, 1 H), 9.03 (s, 1 H),8.35 (dd, J = 1.73, 7.72 Hz, 2 H), 8.31 (d, J = 9.14 Hz, 2 H), 7.63–7.73 (m, 5 H).13C NMR (126 MHz, DMSO-d6): δ = 160.1, 155.7, 153.2, 137.1,135.8, 135.6, 131.9, 130.9, 129.4, 129.3, 129.1, 122.5, 112.3.


Acknowledgment

The authors wish to thank Dr. Ramil Baiazitov (PTC Therapeutics,Inc.) and Prof. Scott E. Denmark (University of Illinois, Urbana-Champaign) for helpful discussions and Jane Yang (PTC Therapeu-tics, Inc.) for analytical support.

Supporting Information for this article is available online athttp://www.thieme-connect.com/ejournals/toc/synthesis.Supporting InformationSupporting Information

References

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synthesis of 1,4-disubstituted pyrazolo[3,4-d]pyrimidines from...

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