BIOMIMETIC CHEMISTRY

Carbonyl catalysis enables abiomimetic asymmetricMannich reactionJianfeng Chen1,2*, Xing Gong1*, Jianyu Li1, Yingkun Li1, Jiguo Ma1, Chengkang Hou1,Guoqing Zhao1, Weicheng Yuan2†, Baoguo Zhao1†

Chiral amines are widely used as catalysts in asymmetric synthesis to activate carbonylgroups for a-functionalization. Carbonyl catalysis reverses that strategy by using acarbonyl group to activate a primary amine. Inspired by biological carbonyl catalysis,which is exemplified by reactions of pyridoxal-dependent enzymes, we developedan N-quaternized pyridoxal catalyst for the asymmetric Mannich reaction of glycinatewith aryl N-diphenylphosphinyl imines. The catalyst exhibits high activity andstereoselectivity, likely enabled by enzyme-like cooperative bifunctional activation ofthe substrates. Our work demonstrates the catalytic utility of the pyridoxal moietyin asymmetric catalysis.

Enamine catalysis is a powerful activationmode in organocatalysis (Fig. 1A) (1–3). Theprocess involves conversion of carbonylcompound 1 into enamine4 through imineintermediate 3 (an iminium intermediate

if a secondary amine is applied) and is catalyzedby amine 2 (4). Nucleophilic addition of theactivated enamine to an electrophile can proceedto yield substituted carbonyl 6. On the otherhand, the formation of imine 3 also increasesthe a-H acidity of amine 2 (5–7) to facilitate for-mation of the a-amino carbanion 7 (8, 9), whichcan also react with an electrophile to produce 8(10). If the product 8 can be hydrolyzed underthe reaction conditions to regenerate 1, it wouldbe possible to use the carbonyl compound 1 as acatalyst (11) to promote a-functionalization ofamine 2.a-Functionalization of primary amines has

been used extensively to make pharmaceutical-ly relevant compounds (12, 13). This transfor-mation usually involves three steps, includingprotection of the NH2 group with stoichiomet-ric aldehydes or ketones before reaction anddeprotection to release the desired free aminesafter reaction. In contrast, the carbonyl-catalyzedprocess eliminates the protecting manipula-tions and is atom economical (14, 15). An idealreaction should meet the following require-ments: (i) Carbonyl catalyst 1 must be electron-withdrawing enough to promote deprotonationto form a-amino anion 7 for further reactionwith an electrophile; (ii) both the carbonylcatalyst 1 and the imine intermediate 3 shouldbe much less reactive than the electrophile in

competition for the active a-amino anion 7under the reaction conditions; and (iii) for anasymmetric version, carbonyl catalyst 1 shouldcontrol positioning of the incoming electro-phile. Development of such carbonyl cata-lysts is thus a formidable challenge in organicchemistry.In biological systems, pyridoxal-dependent

aldolases can act as carbonyl catalysts for di-rect a-functionalization, as in the aldol reac-tion of glycine (16–18). The enzymatic processcan provide a straightforward way to synthesizepharmaceutically important chiral b-hydroxy-a-amino acids and derivatives (16), such as Parkinsondrug L-threo-3,4-dihydroxyphenylserine (19) anddrug candidate (2R,3S)-2-amino-3-hydroxy-3-(pyridin-4-yl)-1-(pyrrolidin-1-yl)propan-1-one (20),without protecting manipulations. The mech-anistic pathway is exemplified by the widely in-vestigated threonine aldolase–catalyzed aldolreaction of glycine depicted in Fig. 1B (21). Theenzymatic condensation of glycine and pyridoxal-5′-phosphate (PLP) forms aldimine 13. Depro-tonation of 13 produces resonance-stabilizedcarbanion 14, which undergoes addition toaldehyde 11 and subsequent hydrolysis to formb-hydroxy-a-amino acid 12 and regeneratePLP. Studies on imitating the biological process,contributed by Kuzuhara (22, 23) and Breslow(24) and coworkers, have shown that stoichi-ometric chiral pyridoxals can promote aldoladdition of glycine to aldehydes in the pres-ence of metal salts, producing b-hydroxy-a-amino acids with moderate enantioselectivityand low diastereoselectivity. Recently, Richardand coworkers found that 5′-deoxypyridoxalpromotes Mannich addition of glycine to glycine-5′-deoxypyridoxal imine to form the correspond-ing Mannich adduct (25, 26). These studies,together with those on the enzymatic aldol re-action of glycine (16–21), have laid a foundationfor the development of asymmetric carbonylcatalysts.

The Mannich reaction of glycinate yieldsa,b-diamino acid derivatives that have syntheticutility and biological activity (27, 28) [for selectedpharmaceutically active molecules containingan a,b-diamino acid moiety, see fig. S1 in sup-plementary materials (SM)]. We have developedchiral pyridoxal andpyridoxamine catalysts capableof asymmetric transamination (29, 30) of a-ketoacids (31, 32). These chiral pyridoxals were ourstarting point for the design of carbonyl catalystsfor a biomimetic Mannich reaction. During theenzymatic aldol reaction, the coenzyme pyridoxalis protonated at the pyridine nitrogen (Fig. 1B)(21), increasing the electron-withdrawing propertyof the pyridoxal and thus greatly improving itscapability to activate the a-H of glycine. However,outside of the context of an enzyme, this protonwill be readily removed by bases in a reactionmixture. To circumvent this problem,wedesignedN-quaternized biaryl axially chiral pyridoxal 16b(Fig. 1C). Chiral acid 20 (32) was condensed withamino alcohols to afford compounds 21 in 80to 97% yields (Fig. 2) (for the synthesis of 20,see SM). We propose that the b-hydroxy amideside chainwill activate the imine substrate duringcatalysis (33, 34). Treatment of 21 with methyliodide (MeI) followed by deprotection with HClyielded N-quaternized pyridoxals 16a to 16f asdark brown salts.In the presence of 1 mole % (mol %) pyridoxal

(R,S)-16b, reaction of tert-butyl glycinate (17)with N-diphenylphosphinyl imine 18a occurredsuccessfully, generating diamino acid ester 19ain a 90% yield with a >20:1 diastereomeric ratio(dr) and 99% enantiomeric excess (ee) (SM andtable S1, entry 2). The structure and absoluteconfiguration of the major diastereomer anti-19awere confirmed by x-ray analysis of its acetylderivative (2R,3R)-19a-acetyl (SM and fig. S3).No reaction was observed without 16 present(table S1, entry 7). Catalyst (R,S)-16b is the mostefficient in terms of yield and selectivities amongthe pyridoxals (16a to 16f) examined (table S1,entry 2 versus entries 1 and 3 to 6). A mixed sys-tem of CHCl3-H2O (1:1) was the choice of solvent.Further studies showed that ethyl and benzylglycinates both produced the corresponding di-amino acid esters with 99% ee but with some-what lower yields than tert-butyl glycinate (65and 64% versus 90%) (SM and table S2).Under the optimized conditions, we exam-

ined the substrate scope of the imine fragment(Fig. 3). Monosubstituted (for 19a to 19m)and polysubstituted (for 19n to 19p) phenylimines as well as fused-aromatic imines (for19q to 19t) reacted smoothly with glycinate 17in the presence of 1 mol % pyridoxal (R,S)-16b,generating the corresponding a,b-diamino acidesters 19a to 19t in good to excellent yieldswith excellent diastereo- and enantioselectiv-ities. With 0.2 mol % of 16b, reaction of imine18a and 17 can still be completed in 11 hoursto yield product 19a without any loss of selec-tivities. Imines containing electron-withdrawingsubstituents such as CN (for 19f), CF3 (for 19l),and pyrazole (for 19h) exhibit somewhat lowerenantioselectivity. Heteroaromatic imines (for

RESEARCH

Chen et al., Science 360, 1438–1442 (2018) 29 June 2018 1 of 5

1The Education Ministry Key Lab of Resource Chemistry andShanghai Key Laboratory of Rare Earth Functional Materials,Shanghai Normal University, Shanghai 200234, China.2Chengdu Institute of Organic Chemistry, Chinese Academyof Sciences, Chengdu 610041, China.*These authors contributed equally to this work.†Corresponding author. Email: yuanwc@cioc.ac.cn (W.Y.);zhaobg2006@shnu.edu.cn (B.Z.)

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19u to 19ab) are also effective for the reac-tion. Imines (for 19ac to 19ae) bearing bio-logically active chiral moieties produced thecorresponding a,b-diamino acid esters withexcellent enantiopurities. A double asymmetricMannich reaction was demonstrated by sub-strate 22. Tetraamino diacid ester 23 was ob-tained in a 69% yield as a single enantiomer.Alkyl imines are not effective for the reaction,probably because of the rapid decompositionof the imines under the reaction conditions(fig. S4) (35).The a,b-diamino acid ester products are

synthetically useful and biologically important(Fig. 4A) (27). Treatment of compound 19a withHCl in MeOH led to selective removal of thediphenylphosphinyl group to produce NH2-freediamino acid ester 24. After treatment with3.0 M aqueous HCl, enantiopure diamino acid25 and tetraamino diacid 26 were obtained ina 90% yield from 19a and in an 81% yield from23, respectively. Mannich adduct 19e can beconverted to chiral diamino alcohol 27 in a 73%yield in two steps.A plausible catalytic mechanismwas proposed

for the reaction (Fig. 4B). Condensation of pyri-doxal catalyst (R,S)-16b with glycinate 17 formsaldimine 28. Deprotonation of a–C–H of 28generates active carbanion 29, which then under-goes asymmetric addition to imine 18 and sub-sequent hydrolysis to form a,b-diamino acidester 19 and regenerate catalyst 16b, completinga catalytic cycle. This proposed catalytic path-way imitates the enzymatic glycine aldol pro-cess (Fig. 1B) (21). The formation of imine 28between catalyst 16b and glycinate 17 not onlytemporarily protects the NH2 group of 17 but alsogreatly activates its a proton. N-quaternizationofpyridoxal16b increases its electron-withdrawingcapability to stabilize delocalized carbanion 29,resulting in marked increase of catalytic ac-tivity. This is supported by the inactivity of pyri-doxal 32 without N-methylation for the reaction(Fig. 4C).The side chain of catalysts 16a to 16f influences

activity and selectivity (SM and table S1). Toexplain the side-chain effect as well as the originof chirality, we propose the orientation for theaddition of carbanion 29 to imine 18, as shownin structure 30 (Fig. 4B). As glycinate 17 is de-protonated to yield the delocalized carbanion,imine 18 is also activated by the side chainthrough hydrogen bonds with the N–H and O–Hmoieties. We thus propose that the catalyst notonly activates both of the substrates but, similarto an enzyme, also orients the addition by bring-ing the two reactants together with a specificspatial arrangement. This cooperative bifunctionalactivation mode (36) leads to product (2R,3R)-19with excellent selectivities. The proposed transi-tion state is further supported by control experi-ments (Fig. 4, C and D, 16g and 16h versus 16b).Methylation of theN–HorO–Hgroup of the sidechain led to decreases in activity and in diastereo-and enantioselectivities, likely because the meth-ylation weakens or eliminates the hydrogen bondwith the imine 18 (33, 34).

Chen et al., Science 360, 1438–1442 (2018) 29 June 2018 2 of 5

Fig. 1. Carbonyl catalysis. (A) Enamine catalysis versus carbonyl catalysis. E+, electrophile.(B) Biological carbonyl catalysis: threonine aldolase–catalyzed aldol reaction of glycine.(C) Bioinspired carbonyl catalysis: N-quaternized chiral pyridoxal–catalyzed biomimetic asymmetricMannich reaction. Ar, aryl; tBu, tert-butyl; Ph, phenyl.

Fig. 2. Synthesis of chiral pyridoxals 16a to 16f. MOM, methoxymethyl acetal; EDCl,N-(3-dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride; THF, tetrahydrofuran; Et, ethyl;iPr, isopropyl; Bn, benzyl; Cy, cyclohexyl.

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Chen et al., Science 360, 1438–1442 (2018) 29 June 2018 3 of 5

Fig. 3. Substrate scope of the biomimetic Mannich reaction. Reactionswere carried out with 18 (0.20 mmol), 17 (0.30 mmol), 16b (0.0020 mmol),and NaHCO3 (0.50 mmol) in CHCl3-H2O (0.3 ml/0.3 ml) at 10°C for 4 to39 hours unless otherwise stated. For 19p, the reaction was carried out in halfscale. Isolated yields were based on imine 18 or 22. The dr (anti/syn) valueswere determined by 1H nuclear magnetic resonance (NMR) analysis of crudereaction mixtures.The ee values were determined by high-performance liquid

chromatography (HPLC) analysis.The absolute configuration of 19a wasassigned as (2R,3R) by x-ray analysis of its acetyl derivative (2R,3R)-19a-acetyl(SM and fig. S3).The absolute configurations of 19b to 19ae and 23 areproposed by analogy. † indicates the ratios of the four theoretic diastereomersdetermined by 1H NMR and HPLC analyses; ‡ indicates that only oneisomer was observed, as judged by 1H NMR and HPLC analyses.TMS,trimethylsilyl; TBS, tert-butyldimethylsilyl.

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Fig. 4. Applicationsand mechanism.(A) Synthetic trans-formations. (B) Pro-posed mechanism.(C) Comparison ofcatalytic selectivities.(D) Comparison ofcatalytic activities.

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ACKNOWLEDGMENTS

This work is in memory of R. Breslow for his outstandingcontributions to the field of biomimetic chemistry and enzymology.Funding: We are grateful for the generous financial supportfrom the NSFC (21672148 and 21472125) and the Ministry ofEducation of China (PCSIRT_IRT_16R49). Author contributions:B.Z. conceived of and directed the project. J.C. and X.G. developedboth the chiral N-quaternized pyridoxal catalysts and the biomimeticasymmetric Mannich reaction and conducted most of the

experiments. J.L., Y.L., J.M., and C.H. synthesized intermediates forcatalyst development and also performed parts of substratescreening experiments. G.Z. searched for reaction conditions forselective removal of the diphenylphosphinyl group of 19a.W.Y. and B.Z. cowrote the manuscript. Competinginterests: The authors declare no competing interests.Data and materials availability: Crystallographic data forcompounds (S,S)-38-oxime in fig. S2 and (2R,3R)-19a-acetyl in fig.S3 are available from the Cambridge Crystallographic Data Centreunder reference numbers CCDC 1837630 and CCDC 1837631,respectively. All other data to support the conclusions are includedin the main text or supplementary materials.

SUPPLEMENTARY MATERIALS

www.sciencemag.org/content/360/6396/1438/suppl/DC1Materials and MethodsFigs. S1 to S4Tables S1 to S4References (37–52)

24 February 2018; accepted 2 May 201810.1126/science.aat4210

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Carbonyl catalysis enables a biomimetic asymmetric Mannich reaction

ZhaoJianfeng Chen, Xing Gong, Jianyu Li, Yingkun Li, Jiguo Ma, Chengkang Hou, Guoqing Zhao, Weicheng Yuan and Baoguo

DOI: 10.1126/science.aat4210 (6396), 1438-1442.360Science 

, this issue p. 1438Sciencethe subsequent reactions, allowing for stereoselective formation of a product with two adjacent amines.amine, a derivative of the amino acid glycine, to create an activated species. An appendage on the catalyst coordinateselectron-withdrawing pyridine ring adjacent to an aldehyde group. This catalyst works like the vitamin by reacting with an

developed an organic catalyst, modeled on vitamin B6, which contains anet al.chiral catalyst. Chen Organic synthesis of molecules with defined stereochemistry requires a chiral center, which in turn may involve a

Inspiration from a vitamin

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