differentiation of stereoisomeric diols by using ch 3ob +och 3 in a small fourier transform ion...

and Ion Processes ELSEVIER International Journal of Mass Spectrometry and Ion Processes 141 (1995) 229-240

Differentiation of stereoisomeric diols by using CH3OB+OCH3 in a small Fourier transform ion cyclotron resonance mass spectrometer

D.T. Leeck a, T.D. Rana tunga a, R.L. Smith a, T. Partanen b, P. Vainiotalo b'*, H.I. Kentt~imaa a'*

aDepartment of Chemistry, Purdue University, West Lafayette, IN 47907-1393, USA bDepartment of Chemistry, University of Joensuu. P.O. Box 111, SF-80101 Joensuu, Finland

Received 8 July 1994; accepted 10 November 1994

Abstract

Gas-phase reactions of stereoisomeric cyclic diols with CH3OB+OCH3 were examined in a small Fourier transform ion cyclotron resonance mass spectrometer. CH3OB+OCH3 is a strong electrophile and rapidly abstracts an OH group from the diols studied. This very exothermic reaction is followed by spontaneous fragmentation of the resulting ion. In addition to this reaction, cis-diols also react with CH3OB+OCH3 by an intramolecular displacement of CH3OH in the initially formed, short-lived adduct ion. The product distributions allow distinction between the cis- and trans-isomers of 1,2-cyclopentanediol, and between the cis- (diendo- and diexo-) and trans-isomers of 2,3-trinorbornanediol.

Keywords: Borocations; Diols; FT-ICR; Ion-molecule reactions; Stereoisomers

1. Introduction

Determination of the stereochemistry of biological molecules is an area of considerable current interest. Electron ionization mass spectrometry is relatively insensitive to the stereochemistry of molecules. However, chemical ionization mass spectrometry has shown great promise for differentiation of stereoisomers in producing fewer but more informative product ions [1-4].

The hydroxyl group is among the most general and important functional groups in biological compounds. Cyclic diols are often used as model compounds when developing

* Corresponding authors.

methods for stereochemical analysis of structurally more complex molecules [1-4]. For example, cis- and trans-l,2-cyclopentanediols have frequently been employed to test new chemical ionization methods. The differentiation of these diols is usually based on their different basicities [5-8]. cis-Diols typi- cally yield more abundant protonated molecules (or adduct ions) than trans-diols because of the formation of intramolecular hydrogen bonds [2,9,10]. An interesting recent study utilized trimethyl borate as the chemical ionization reagent gas [11]. This reagent was found to yield distinctly different product distributions for the isomeric 1,2- cyclopentanediols. However, the exact nature of the reactions leading to the structurally

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230 D. T. Leeck et al./International Journal of Mass Spectrometry and Ion Processes 141 (1995) 229-240

informative products is unknown, as many different ions are generated from trimethyl borate upon chemical ionization in the ion source of a mass spectrometer.

Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry is a powerful technique for the study of ion/molecule reactions of long-lived ions. These mass spectro- meters are capable of isolating interesting ions and trapping the ions for variable periods of time, thereby making it possible to carry out a detailed examination of the reaction sequence. Isolation of the reactant ion elimi- nates the difficulty of differentiating the reaction products of different ions present in the reaction region. Thus, FT-ICR is well suited for attempts to develop ion/molecule reactions for stereoisomer analysis.

A preliminary report on the performance of a small Fourier transform ion cyclotron resonance mass spectrometer based on a 0.4 T permanent magnet appeared recently [12]. The promising preliminary results obtained using this instrument [12], together with the potential for further miniaturization, makes this device a good candidate for benchtop FT-ICR. We discuss here the results obtained by using this small FT-ICR spectrometer to distinguish between the cis- and trans-stereo-

isomers of 1,2-cyclopentanediol and 2,3- trinorbornanediol. A novel chemical approach was utilized to carry out the distinction: a mass-selected fragment ion of trimethyl borate, CH3OB+OCH3, was employed as the chemical ionization reagent. This boron ion is known to form a strong boron-oxygen bond with oxygen-containing compounds (D(B-O) = 39.5 4- 3.5 kcal mo1-1 for methanol) [13(a)]. The exothermicity of the B-O bond formation is thought to be the driving force for the reactions of this ion [13], e.g. for the rapid abstraction of water as well as a hydroxyl group from various simple alcohols [13]. It was of interest to find out whether diols react with CH3OB+OCH3 by the same

pathways as alcohols, and whether stereoisomeric diols show different relative reaction rates and/or relative abundances of product ions.

2. Experimental

All experiments were carried out in a proto- type Fourier transform ion cyclotron resonance mass spectrometer described in detail previously [12]. This instrument comprises a single cubic 3 cm cell housed within the bore of a 0.4 T permanent magnet. The vacuum chamber is pumped with a 300 1 s -1 diffusion pump to a nominal base pressure of <~ 2 x 10 .9 Torr. The instrument is controlled by an Extrel FTMS 2001 data station (with Nicolet 1280 computer) equipped with an Extrel FTMS SWIFT module.

The overall performance of the instrument is greatly improved by the use of grounded screens in front of the trapping plates. The screens [12,14] consist of 0.001 gauge tungsten wire woven to make a mesh of three wires per centimeter in both the x- and the y-directions (the x , y plane is defined as the plane that is perpendicular to the magnetic field lines). The screens decrease the electric field gradients [14] caused by the trapping plates held at 1-2 V. As a result, the space-charge and the magnetron motion effects are decreased.

Samples were introduced into the cell by using either a pulsed valve set-up consisting of a reservoir attached to two General Valve Corporation pulsed valves or one of two Varian leak valves. Before introduction into the instrument, each sample was degassed several times by freezing with liquid N2, pump- ing away the air, and allowing the sample to warm up to room temperature. Trimethyl borate was pulsed into the instrument at a maximum nominal pressure of 1.2 x 10 . 7

Torr (300 mTorr in the pulsed valve reservoir). The diols were introduced into the cell

D. T. Leeck et al./International Journal of Mass Spectrometry and Ion Processes 141 (1995) 229-240 231

(# e- l :

C

1.,

Q >

¢} O :

A

o-~ CP U C

" 0

C

e~ < e. o 0 >

CP er

a

C

A

r , , i - • , • i " • " "

5 0

B

7 3

1 0 4 1 2 8 - H 2 0 / /

. . . . I . . . . ~ . . . . ] . . . . a . . . . I 1 0 0 1 5 0 2 0 0

7 3

• , . . . . I . , I J , • ' , " . . - - , . . . . . . . . . . . . . . . . i - . .

I . . . . . . . . . I . . . . ' . . . . I . . . . ' . . . . 5 0 1 0 0 1 5 0 2 0 0

C 7 3

6 7 . 111

• ; " "' " ~ : I ; . ~ . . , ~ . ~ . l h I . . . . " nr- "---" " I - " " ' - -~ " ' " : "" - : '1 5 0 1 0 0 1 5 0 2 0 0

m/z

Fig. 1. A typical multiple-stage MS experiment in a miniature FT-ICR mass spectrometer: A, ionization of trimethyl borate (MW 104) and trans-2,3-trinorbornanediol (MW 128) by 50 eV electron ionization; B, isolation of the ion CH3OB+OCH3 (m/z 73); C, reaction of the ion of m/z 73 with trans-2,3-trinorbornanediol for 500 ms (nominal pressure of 5 x 10 -8 Torr).

at a nominal pressure of 5.0 x 10 -8 Torr by using a leak valve.

A typical experiment sequence is shown in Fig. 1. CH3OB+OCH3 was generated by electron ionization of trimethyl borate pulsed into the cell (Fig. 1A). The filament current (5- 8 #A), electron kinetic energy (20-50 eV) and electron beam duration (10-30 ms) were opti- mized for each experiment. A delay of at least 400 ms preceded the beam event, ensuring that the concentration of trimethyl borate in the

cell had reached the maximum. After ionization, the neutral trimethyl borate was pumped away. The reactant CH3OB+OCH3 ions were cooled for 600 ms by allowing them to collide with the neutral diol present in the cell. The reactant ions were then isolated by ejecting all other ions from the cell (Fig. IB). This was accomplished through the application of one or several stored waveform inverse Fourier transform waveforms [15] and/ or a sequence of r.f. excitation sweeps to the


B

0

¢J u t "

0 "10 C - t O .

0

(B >

~.o-'

0 K'~_-

I , ) o r - I 0

" I 0

:o<=) .

I 0

:> , ~

-.g

? OH OH

67 +

] 7 3 . . . . . . . i . . . . . . d i l . . . . . . . = - " ~_ L . , . . t - -U . . . . . . . ~J¢ i t . . a . . | . . . . ~ | I L = , J t J~ d I I ~ -. - a . I , . =JLA

. . . . . . . ' . . . . . . . . . ' . . . . . . . . . 1Jo 5 . . . . . . . . 20 '- -I0" 5 0 100

67 OH OH

+

m / z

OH OH

'H '+

50

103

÷ , / ]143 . . . ' r , , • . .=" ' := .==. ' L . . . . .

100 2 0 0 m/z

Fig. 2. Reaction of CH3OB+OCH3 with (A) trans-l,2-cyclopentanediol (5 × 10 -8 Torr) and (B) cis-l,2-cyclopentanediol (5 × 10 -8 Torr).

excitation plates of the cell. The isolated ions were allowed to react with the diol for a variable period of time (Fig. 1C). Each reaction spectrum was background corrected by using a previously described procedure [16]. Each ion/molecule reaction was examined on at least three separate days to verify the repro- ducibility of the data, which was found to be good. Plots of In (relative ion abundance) as a function of the reaction time were constructed for each ion/molecule reaction.

All the spectra shown are the average of at least 100 acquisitions obtained using chirp detection with an excitation sweep of 54 Vp_p amplitude, 381 kHz bandwidth, and 2 kHz #s -~ sweep rate. The spectra were recorded as 32k datapoints and subjected to one zero fill before Fourier transformation.

The diols were synthesized using common laboratory procedures [17-19]. All the other reagents were commercially available and

were used as received from the manufacturer. The purity of all reagents was checked by GC and by MS.

3. Results and discussion

The reactions of two different types of stereoisomeric diols with known structures, 1,2-cyclopentanediols (Figs. 2 and 3) and 2,3- trinorbornanediols (Figs. 4 and 5), were examined in a small low-field FT-ICR instrument. This mass spectrometer allows the inves- tigation of complex ion/molecule reactions without the complication of interfering reactions. The type of neutral molecules undergoing ion/molecule reactions can be controlled by pulsing into the cell those reagents that are needed to generate the reactant ions but whose ion/molecule reactions should be avoided. This capability was important in the

D. T. Leeck et al./International Journal of Mass Spectrometry and Ion Processes 141 (1995) 229-240

A B

233

0 .0

- 0 . 5

- 1 . 0

i -1.5 - 2 . 0

- 2 . 5

- 3 . 0

- - 3 . 5 m

- 4 . 0 _5

- 4 . 5

- 5 . 0

- 5 . 5 0 . 0

r

; ~ e 5

I I i I

0 . 2 0 . 4 0 . 6 0 . 8 1 .0

T i m e (s)

0 .0

- 1 . 0

o" - 1 . 5 103 -~ - 2 . 0 8 5

-~ - 2 . 5

~ -3 .o

-:" - 3 . 5 m -4.0 _5

- 4 . 5

- 5 . 0

- 5 . 5 i ~ ~ = = 1 .2 0 . 0 0 . 2 0 . 4 0 . 6 0 . 8 1 .0 1 .2

T i m e (s)

Fig. 3. Relative ion abundances (normalized to the total measured ion current) as a function of time for the reaction of CH3OB+OCH3 (m/z 73) with (A) trans-l,2-cyclopentanediol (5 x 10 8 Torr) and (B) cis-l,2-cyclopentanediol (5 x 10 8 Tort).

0.0 A - 0 . 5

-1 .0

-1 .5 .<

- 2 . 0

- 2 , S t.J ,..,, z - 3 . 0 ._1

- 3 . 5

9 5

o . o 0 . 2 0 . 4 o . 6 0 . 8 1 . 0 1 .2

REACTION TIME (SEC) l

a o . o

~-~ - o . 5 z

< - 1 .o z

- 1 . s .<

-~ - 2 . 0

- 2 . 5

z --~ - 3 . 0

- 3 . 5 0 . 0

9

1

, i J

0 . 2 0 . 4 0 . 6 0 . 8 1.0

REACTION TIME ( S r C )

1.2

..(

5 w c~ v z _J

C

-1

- 2

- 3

- 4 . 0

~ 2 9

1

i o . 5 1 .o 1 . s 2 . o

REACTION TIME(SIt.C)

2 . 5

Fig. 4. Relative ion abundances (normalized to the total measured ion current) as a function of time for the reaction of CH 30B+OCH3 (m/z 73) with (A) trans-2,3-trinorbornanediol (5 x 10 -8 Torr), (B) endo-2,3-trinorbornanediol (5 x 10 -8 Torr), and (C) exo-2,3-trinorbornanediol (5 x 10 -s Torr).


ul ¢) 0

i <. C o > ,

re"

A 73

- - , . | . . . . . . .

"51o

93

. . . . - ~ | . . . . . . .

. . . . l d 0 . . . . i ~ 0 . . . .

< e -

o,

73

~ . ~ . O H +

1,1 .1 j j - J = . . . . . . . . i _

. . . . 1~o . . . . . . . . . ~o . . . . . .

u ~

f -

e -

. Q

¢ -

o q ) > ,

67 I

• ! . . , ,

. . . . 5~

93

111 .'H+ ~

. . . . 1do ' ' ~ = . . . . "~,4o . . . . . . .

Fig. 5. Reaction of CH3OB+OCH3 for 500 ms with (A) trans-2,3-trinorbornanediol (5 x 10 ̀ 8 Torr), (B) exo-2,3-trinorbornanediol (5 x 10 8 Torr), and (C) endo-2,3-trinorbornanediol (5 x 10 -8 Torr).

present work, as CH3OB+OCH3 reacts quickly with neutral trimethyl borate [13]. In order to avoid interference from this reaction, trimethyl borate was pulsed into the cell before the electron ionization event (Fig. 1A) and pumped away prior to examination of the reactions of CH3OB+OCH3 with the cyclic diols. The ionic reactant was selected by ejecting all other ions besides CH3OB+OCH3 out of the cell prior to the reaction period (Fig. 1B).

Information about the reaction rate con- stants and the identity of primary and secon-

dary reaction products was obtained by observing the reactions as a function of time. The linear decay of CH3OB+OCH3 shown in the time plots (Figs. 3 and 4) indicates that the ion was properly cooled prior to reaction and that the decay follows the expected pseudo- first-order kinetics. Upon further examination of these time plots, primary and secondary reaction products are clearly distinguishable. For example, the ion of m/z 103 in Fig. 3B and the ion of m/z 129 in Fig. 4B are identified as secondary product ions because they are observed only at longer


+ CH3OBOCH 3 > OH OH H--

H3CO OCH3 H ~ ) / \OCH,

m/z 175

A

1,6-H-Shill

+OHB/O - CH30H OH /0 L.~B\-~ I H3C~ HOCH3 H3CO +

m/z 143

Scheme

reaction times. The slopes of the linear decay of CH3OB+OCH3 (Figs. 3 and 4) show that all these reactions are facile and not sensitive to differences in the stereochemistry of the reactant ions. The branching ratios of the different product channels are discussed below for the two different types of diol studied.

3.1.1,2- Cyclopen tanediols

The most important difference in the reac- tivity of CH3OB+OCH3 toward the stereoisomeric 1,2-cyclopentanediols is the formation of a primary product ion of m/z 143 for only the cis-isomer (Figs. 2 and 3; the ion of m/z 103 in Figs. 2B and 3B is a secondary product that is formed by proton transfer from primary product ions to the diol). The product ion of m/z 143 arises from elimination of CH3OH from the initially generated adduct ion (A, Scheme 1). This adduct is a short-lived intermediate which is not

observed in the reaction spectrum. Elimi- nation of CH3OH probably occurs [20] by rearrangement of the adduct A by consecutive intramolecular 1,4-H and 1,6-H shifts followed by intramolecular displacement of CH3OH (Scheme 1).

The fact that elimination of CH3OH does not occur for the trans-l,2-cyclopentanediol supports the mechanism shown in Scheme 1: loss of CH3OH from the trans-isomer would generate a highly strained product ion. This finding is in agreement with the results obtained in the earlier study, in which trimethyl borate was used as the chemical ionization reagent in an ion source [11]. Also under these conditions (albeit upon reactions with unspecified reactant ions), the cis-l,2-cyclopentanediol yields a product ion of rn/z 143 but the trans-isomer does not [11].

All the other primary product ions generated upon reaction of the cis- and trans-l,2- cyclopentanediols with CH3OB+OCH3 (Figs.

236 D. T. Leeek et al./International Journal of Mass Spectrometry and Ion Processes 141 (1995) 229-240

OH OH + OH~O~H ~+ + (CH30)2BOH + CH3OBOCH 3 ~

/ OH B nVz 85

H3C/ ~OCH3 B m/z 175

A / [ •

+ H20 O ~ ]

m/z 67 m/z 85

Scheme 2.

2 and 3) are believed to be formed via hydroxyl group abstraction by the boron ion. Previous work has shown that hydroxyl abstraction from alcohols by this boron ion is common and facile [13]. For the diols, hydroxyl abstraction probably initially generates the short- lived ion B (m/z 85; Scheme 2). Some of the ions B most likely isomerize by a 1,2-hydride shift to yield a stable protonated ketone, one of the observed final products (m/z 85; Scheme 2), while others dissociate by loss of water. This proposal is supported by previous studies on the dissociation reactions of protonated cyclopentanone and cyclohexanone. The decomposition of protonated cyclohexanone by the loss of water has been demonstrated to occur via the higher homolog of the ion B [7,21]. Further, the product ion generated by loss of water from protonated cyclopentanone has been demonstrated to have the structure shown in Scheme 2 (m/z 67) [22].

The cis-1,2-cyclopentanediol yields an abundant protonated diol (m/z 103) as a secondary product upon reaction of some of the primary product ions with the diol. The protonated diol corresponds to the most abundant ion at longer reaction times. This ion is probably

stabilized by intramolecular hydrogen bond- ing [5]. Protonated trans-diol is not formed upon reaction of the trans-1,2-pentanediol.

3.2. 2,3- Trinorbornanediols

Analogous to the 1,2-cyclopentanediols, the cis- (diendo- and diexo-) 2,3-trinorbornanediols react with CH3OB+OCH3 to give product ions differing from those observed for the trans-isomer (Figs. 4 and 5). All three isomers of 2,3-trinorbornanediol react with CH3OB+OCH3 to form a short-lived intermediate A (m/z 201; Scheme 3) which is not observed in the reaction spectrum. This ion can react by several pathways. Loss of CH3OH from the adduct formed from the cis-isomers probably occurs via consecutive 1,4-H and 1,6-H shifts, followed by an intramolecular displacement (Scheme 3), just as was proposed for the cis-l,2-cyclopentanediols (Scheme 1). An analogous product ion (m/z 169) was not observed for the trans- stereoisomer (Fig. 5A). Hence, the formation of this product ion requires that the hydroxyl groups be located on the same side of the ring. This reaction can be used to distinguish between the cis- and trans-stereoisomers


~ O H

OH

OCH3

+ 1 A-H-Shift + CH3OBOCH3 ~ OCH3 !=

m/z 73 OH~

m/z 201

A

1,6-H-Shift

/oc.3 B

OH +

~z169

-CHsOH

OCH3

OH

Scheme 3.

of 2,3-trinorbornanediol. The spectra of the diendo- and diexo-stereoisomers are essen- tially the same.

All the other primary product ions from the reaction of CH3OB+OCH3 with the 2,3- trinorbornanediols are believed to be formed via hydroxyl abstraction by the borocation (Schemes 4 and 5). This reaction initially

yields the transient intermediate B. Once the hydroxyl group has been abstracted, the stereochemistry of the ion is lost. Hence the product ions arising from this reaction channel are identical for the three stereoisomeric diols (m/z 67, 93 and 111; Fig. 5), and are formed with similar relative abundances (however, these ions undergo

~ O H OH

+ CH3OBOCH 3

m/z 73

OCH3

H OCH3

OH

nYz 201 A

OH m/z 111

B

+ (CH30);tBOH

Scheme 4.

OH +

nYz 111

238

OH

rn/z 111

B

D. T. Leeck et al./International Journal of Mass Spectrometry and Ion Processes 141 (1995) 229-240

H•CH=CHOH +

m/z 111 C

( 1,4 H - shift

i

+

H CH-~OH2 I, 5 H" shift - H20

a - C H = C H

+

D rdz 67

1, 4 H - shift

Scheme 5.

~ H2

+

- N 0 ~oh

m/z 93

I

secondary reactions at different rates, since the neutral reagent,the diol, is different).

The intermediate ion B most likely reacts by two different pathways [7]. Much as for the 1,2-pentanediols, a 1,2-hydride shift in this ion B yields a stable protonated ketone as one of the final reaction products (m/z 111; Scheme 4). Alternatively, a heterolytic bond cleavage may occur, resulting in a new intermediate ion C (Scheme 5). Some of the ions C may undergo a 1,5-H shift (pathway a, Scheme 5), followed by loss of water and ethyne (m/z 67; Scheme 5). However, the majority of the ions C react by a 1,4-H shift (pathway b, Scheme 5) and then eliminate water, generat- ing a stable allylic cation (m/z 93; Scheme 5). Support for this mechanism is provided by a careful earlier study on a reaction analogous to the pathway b (Scheme 5) [7]. Protonated 1,2- cyclohexanediols were demonstrated to elimi-

nate water and eventually yield an intermediate ion analogous to that formed by a 1,4-H shift in the ion C (pathway b, Scheme 5). This intermediate was shown to isomerize via a 1,4-H shift and then to eliminate water in a similar manner, as illustrated in Scheme 5 (pathway b) [7].

Similarly to the cis-l,2-cyclopentanediols, the diexo- and diendo-stereoisomers of 2,3- trinorbornanediol react with the primary product ions (predominantly that of m/z 67; Figs. 4B and 4C) by proton abstraction to give the protonated diol as a secondary product (rn/z 129; Figs. 5B and 5C). These reactions were not observed for the trans-isomer.

4. Conclusion

A promising, novel approach for stereo-


isomer analysis was examined by using a small low-field FT-ICR mass spectrometer. Investi- gation of the ion/molecule reactions was sim- plified by employing a pulsed valve set-up for reagent introduction. Pulsing the reagent into the cell allowed the highly reactive ion CH3OB+OCH3 to be studied without the pre- sence of the corresponding neutral reagent. Ejection of the unwanted ions from the cell allowed the examination of the ion/molecule reactions of the isolated reactant ion. Hence this instrument provides great control over the reactions occurring in the cell. The primary and secondary product ions were read- ily distinguished by examination of the kinetic data. The results presented here suggest that complicated chemical problems, including stereoisomer analyses, can be studied by using this type of small FT-ICR mass spectrometer.

The cis- and trans-stereoisomers of 1,2- cyclopentanediols and 2,3-trinorbornanediols can be distinguished on the basis of their reaction with CH3OB+OCH3 . Only the adduct formed from the cis-isomers reacts further by an intramolecular displacement of methanol. Other reaction pathways available to both the cis- and the trans-stereoisomers include hydroxyl abstraction from the diol by the reactant ion CH3OB+OCH3. The products originating from the hydroxyl abstraction were observed for all the diols, suggesting that this reaction does not require the pres- ence of two hydroxyl groups on the same side of the ring.

Although the reactions of CH3OB+OCH3 allow the differentiation of the cis- (diexo- and diendo-) isomers of 2,3-trinorbornanediol from the trans-isomer, the diendo- and diexo- isomers yield identical product distributions. The lack of selectivity of CH3OB+OCH3 towards the cis-diols probably arises from the great exothermicity of B-O bond formation. The excess energy is apparently great enough to overcome any steric barriers associated with

CH3OB+OCH3 interacting with the hydroxyl groups of the cis-isomers. These isomers might be distinguishable if the hydroxyl abstraction channel were to be made less favorable, e.g. less exothermic, as possible differences in the reaction rates for the stereoselective CH3OH elimination channel may then become more obvious. The electrophilicity of dicoordinated borocations is controlled by the substituents on the boron [13]. In order to enhance the efficiency for the structurally informative CH3OH-elimination channel, and to carry out the distinction between the cis-stereoisomers, reactions of less electrophilic borocations such as (CH3)zNB+N(CH3)2 are currently being examined.

Acknowledgments

Extrel FTMS, the National Science Foundation (CHE-9409644), and the Lubrizol Corporation are thanked for their financial support of this research. D.T.L. thanks Eli Lilly and Company for an Eli Lilly Fellowship. T.P. and P.V. are grateful for the financial support provided by the Academy of Finland. Lori Castro is thanked for her assistance with the preparation of the schemes and help with a few experiments.

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differentiation of stereoisomeric diols by using ch 3ob +och 3 in a small fourier transform ion...

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