Determination of absolute configuration of secondary alcohols using thin-layer chromatography

Article abstract of DOI:10.1021/jo400432q

A new implementation of the competing enantioselective conversion (CEC) method was developed to qualitatively determine the absolute configuration of enantioenriched secondary alcohols using thin-layer chromatography. The entire process for the method requires approximately 60 min and utilizes micromole quantities of the secondary alcohol being tested. A number of synthetically relevant secondary alcohols are presented. Additionally, ¹H NMR spectroscopy was conducted on all samples to provide evidence of reaction conversion that supports the qualitative method presented herein.

Full text of DOI:10.1021/jo400432q

Note
pubs.acs.org/joc
Determination of Absolute Configuration of Secondary Alcohols
Using Thin-Layer Chromatography
Alexander J. Wagner and Scott D. Rychnovsky*
Department of Chemistry, 1102 Natural Sciences II, University of CaliforniaIrvine, Irvine, California 92627, United States
S
* Supporting Information
ABSTRACT: A new implementation of the competing
enantioselective conversion (CEC) method was developed to
qualitatively determine the absolute configuration of enan-
tioenriched secondary alcohols using thin-layer chromatog-
raphy. The entire process for the method requires approx-
imately 60 min and utilizes micromole quantities of the
secondary alcohol being tested. A number of synthetically
1
relevant secondary alcohols are presented. Additionally, H
NMR spectroscopy was conducted on all samples to provide
evidence of reaction conversion that supports the qualitative method presented herein.
he determination of absolute configuration of enantioen-
riched stereogenic centers is an important step in the
T
process of characterizing isolated natural products and novel
synthetic compounds.1−3 There are a number of methods used
to determine absolute configuration including the advanced
Mosher method,4−6 chiral derivatization reagents,7 vibrational
circular dichroism,8 exciton chirality,9−11 NMR spectroscopic
chiral shift reagents,12−15 lipase-catalyzed resolutions,16 and X-
ray crystallographic analysis.17
Figure 1. Birman’s HBTM catalyst.26 The S enantiomer is shown.
in this study. The HBTM catalyst and subsequent analogs have
been shown to be quite versatile, with kinetic resolutions
reported for a variety of different substrates.27−38 The HBTM
catalyst can be stored under air at room temperature in a
desiccator without decomposition, and it was prepared in a two
step procedure from commercially available starting materi-
als.26,34 The mnemonic (Figure 2) previously reported for
secondary alcohols in our group was used to determine
absolute configuration.
In order to test the use of thin-layer chromatography as a
characterization method, parallel reactions were set up for all of
the secondary alcohols studied (Table 1). The checkmark
denotes the reaction that was qualitatively determined to be the
We recently reported the competing enantioselective
conversion (CEC) method for determining absolute config-
uration which utilizes each enantiomer of a kinetic resolution
reagent (catalytic or stoichiometric) in parallel reactions where
the determination is guided by a difference in the rate between
the parallel reactions.18−20 A mnemonic then confirms the
absolute configuration based on the fast reacting enantiomer of
the kinetic resolution reagent. The method is a modern
implementation of the Horeau method.21−25 Our method has
been reported for secondary alcohols as well as oxazolidinones,
lactams, and thiolactams where the parallel reactions were
analyzed by 1H NMR spectroscopy to determine reaction
conversion at a specified time. A similar strategy was developed
for primary amines where the parallel reactions of proto and
deutero chiral acylating reagents were analyzed by mass
spectrometry.20
To showcase the versatility of this general method, we
decided to investigate the use of thin-layer chromatography
(TLC) as an additional characterization technique that could be
utilized in assisting the analysis of the fast and slow parallel
reactions to determine absolute configuration. Secondary
alcohols are one of the most common functional groups
incorporated in polyketide, terpene, and saccharide natural
products and thus were chosen as test substrates. Our previous
report for determining the absolute configuration of secondary
alcohols used Birman’s homobenzotetramisole (HBTM)
kinetic resolution catalyst (Figure 1),26 which was also used
Figure 2. Predictive mnemonic used in determining the absolute
configuration of secondary alcohols using each enantiomer of the
HBTM catalyst.
Received: February 27, 2013
© XXXX American Chemical Society
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Table 1. Determination of Absolute Configuration of Secondary Alcohol Substrates by TLC and Confirmation of Reaction
1
Conversion by H NMR Spectroscopy
a
b
c
Reactions were quenched at 30 min unless otherwise noted. Enantiomeric ratios measured by chiral SFC or chiral HPLC. Reactions quenched
after 20 min.
1
fast reaction with TLC analysis. The circled conversion number
in the adjacent column denotes the fast reaction determined by
a higher conversion using H NMR spectroscopy. The parallel
reactions used micromole quantities of substrate and an average
of 0.17 mg of HBTM catalyst per reaction. Deuterated
chloroform was used as the solvent in order to allow a
comparative quantitative analysis of reaction conversion via H
NMR spectroscopy after the qualitative TLC analysis was
completed. The reactions were quenched with 50 μL of
methanol-d4 after 20 or 30 min, followed by the addition of
CDCl3 to give a total additive volume of 500 μL. Then an
aliquot of 2 μL from each reaction was removed and spotted on
1
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a TLC plate. The amount applied to each TLC spot ranged
from 29 to 130 nmol of the combined alcohol and ester
reaction mixture. The TLC plate was run, dried, and stained
with an ethanolic solution of phosphomolybdic acid
(PMA).39,40
An example from entry 1 is provided (Figure 3a). Analysis of
the PMA-stained TLC plate showed a noticeable difference
In entries 2 and 3, the fast reaction is determined for both
electron-donating and electron-withdrawing benzyl alcohols.
Note that in entry 2, with an enantiomeric ratio of 85:15, the
qualitative analysis still determines the appropriate fast reaction.
In entries 4 and 6, the fast reaction is determined with naphthyl
and electron-donating naphthyl systems. The method is also
compatible with a number of functional groups including
acetals (entry 5), esters (entry 8), phthalimide (entry 9),
alkenes (entry 10), silyl ethers (entry 11), protected amines
(entry 12), and silyl-protected alkynes (entry 13). Entries 10−
13 additionally feature heterocyclic benzofuran and benzothio-
phene derivatives. The entries in this table provide a set of
secondary alcohols complementary to those previously
reported and further expand the substrate scope. The limits
of this method depend upon the selectivity of the catalyst with
each substrate and the optical purity of the sample. In any case
where the fast-reacting substrate is not clearly identified by
TLC analysis, the conversions should be evaluated by NMR
analysis.
We investigated the feasibility of determining the absolute
configuration of secondary alcohols using the competing
enantioselective conversion method coupled with a qualitative
TLC analysis. The method was found to be successful for a
variety of alcohols as presented in Table 1. The assay is simple
to run, can be completed in 30−60 min, and requires no
sophisticated equipment. The TLC analysis shows nanomolar
sensitivity, and the method provides a practical micromolar-
scale determination of absolute configuration for secondary
alcohols.
EXPERIMENTAL SECTION
■
General Experimental Methods. All reactions were carried out
capped under air with CDCl3 as the reaction solvent. CDCl3 was
treated with Na2SO4 prior to use. Propionic anhydride was distilled
over P2O5 prior to use. N,N-Diisopropylethylamine was distilled over
CaH2 prior to use. Entries 1−3 were synthesized according to
literature procedures.41−43 Entries 6 was synthesized by the Jarvo
group.44 Entries 4, 5, and 7−13 were synthesized by the Jarvo group.45
Chiral analytical traces of entries 1−3, 6, and 7 are included in the
Supporting Information. Chiral analytical traces of entries 4, 5, and 8−
13, as well as relevant characterization data, are included in the cited
publication.45 All microliter quantities used in the reactions were
added using micropipets. The temperature was measured via
thermometer in a separate 1 dram vial filled with deionized water.
The temperatures were recorded between 21.0 and 24.1 °C over the
course of all parallel reactions reported herein. Thin-layer chromatog-
raphy was performed using TLC glass plates (silica gel 60 F254). All
TLC plates were eluted with 30% ethyl acetate in hexanes and were
stained with phosphomolybdic acid (PMA) TLC stain, 20 wt % in
ethanol. Images of the TLC plates were recorded with an iPad 2 from
approximately 12 in. above the surface of the plate. The zoom feature
on the iPad 2 camera was used to acquire the close-up pictures that are
Figure 3. (a) Image of the stained TLC plate from entry 1 of Table 1
after 30 min. (b) H NMR spectra of each reaction after the TLC
analysis.
1
1
between the fast and slow reactions. The fast reaction (S-
HBTM) has a larger spot density of the higher Rf spot
corresponding to the ester product formed from the reaction.
The slow reaction (R-HBTM) has a larger spot density of the
lower Rf spot corresponding to the alcohol starting material.
included. H NMR spectra were obtained using standard acquisition
parameters and were referenced to chloroform (∂ = 7.26).
Preparation of Reagent Stock Solutions. All stock solutions
were prepared fresh on the day that they were used in the reactions.
(1) R-HBTM (10.4 mg, 0.0390 mmol) was added to a 1 mL
volumetric flask. CDCl3 was added to give a total volume of 1.00 mL.
The solution was transferred to a 1 dram vial. A 500.0 μL portion was
removed via micropipet and transferred to a 2 mL volumetric flask.
CDCl3 was added to give a total volume of 2.00 mL and a resultant
solution molarity of 0.00975 M. The solution was transferred to a 1
dram vial and capped under air.
1
This qualitative analysis correlates with the subsequent H
NMR spectra (Figure 3b), which reveal the S-HBTM reaction
has reached 80% conversion and the R-HBTM reaction has
reached 12% conversion during the 30 min period. According
to the mnemonic, the fast reaction with S-HBTM results in the
alcohol behind the plane of the molecule as drawn (R1 = Ph, R2
= Me), resulting in an assignment of absolute configuration for
entry 1 as the R enantiomer.
(2) S-HBTM (10.4 mg, 0.0390 mmol) was added to a 1 mL
volumetric flask. CDCl3 was added to give a total volume of 1.00 mL.
The solution was transferred to a 1 dram vial. A 500.0 μL portion was
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removed via micropipet and transferred to a 2 mL volumetric flask.
CDCl3 was added to give a total volume of 2.00 mL and a resultant
solution molarity of 0.00975 M. The solution was transferred to a 1
dram vial and capped under air.
(3) Propionic anhydride (369.3 μL, 2.880 mmol) was added to a 2
mL volumetric flask. CDCl3 was added to give a total volume of 2.00
mL and a resultant solution molarity of 1.44 M. The solution was
transferred to a 1 dram vial and capped under air.
(4) N,N-Diisopropylethylamine (463.3 μL, 2.660 mmol) was added
to a 2 mL volumetric flask. CDCl3 was added to give a total volume of
2.00 mL and a resultant solution molarity of 1.33 M. The solution was
transferred to a 1 dram vial and capped under air.
Procedure for Determining Absolute Configuration.
(1) Parallel Reactions. This procedure will be illustrated with a
specific example of entry 9 from Table 1. Separate reactions were run
with both the R-HBTM and S-HBTM stock solutions, respectively, but
the procedure for both reactions is otherwise identical. The R-HBTM
reaction is shown below with entry 9. All reactions were run in CDCl3
in order to also record 1H NMR spectra of the crude mixtures;
however, if only the TLC analysis is intended, the reactions can also be
carried out in toluene. Conditions for all entries can be found in the
Supporting Information.
Department of Defense (DoD) through the National Defense
Science & Engineering Graduate Fellowship (NDSEG)
Program (A.J.W.) and the National Science Foundation
Graduate Fellowship Program (A.J.W.).
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ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR spectra of all parallel reactions, pictures of all TLC
plates, and a table with specific conditions for all reactions
carried out. This material is available free of charge via the
Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: srychnov@uci.edu.
Notes
■
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Prof. Elizabeth R. Jarvo, Hanna M. Wisniewska, Iva
M. Yonova, and Steven Nguyen for generously providing
alcohols used in this report. Support was provided by the
National Science Foundation (CHE 1152449), as well as by the
D
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(39) PMA stain: The PMA stain was chosen from a number of tested
TLC stains because it displayed comparable sensitivity for secondary
alcohols and their corresponding propionate esters with a set of
calibrated equimolar serial dilutions.
(40) Images of the TLC plates are included in the Supporting
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was tested in 30% ethyl acetate in hexanes prior to running the
reactions.
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