A lactate-derived chiral aldehyde for determining the enantiopurity of enantioenriched primary amines

Article abstract of DOI:10.1039/c5ob01398d

In this paper we describe the use of a chiral aldehyde derived from lactate esters for determining the enantiopurity of primary amines, via the formation of diastereomeric imines. The method was shown to be suitable for reproducibly determining the enantiopurity of a diverse set of chiral amines. Both enantiomers of the aldehyde can be prepared in two steps from commercially available materials.

Full text of DOI:10.1039/c5ob01398d

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A lactate-derived chiral aldehyde for determining
the enantiopurity of enantioenriched
primary amines†
Cite this: DOI: 10.1039/c5ob01398d
Samantha M. Gibson, Rachel M. Lanigan, Laure Benhamou, Abil E. Aliev and
Tom D. Sheppard*
Received 9th July 2015,
Accepted 21st July 2015
In this paper we describe the use of a chiral aldehyde derived from lactate esters for determining the
enantiopurity of primary amines, via the formation of diastereomeric imines. The method was shown to
be suitable for reproducibly determining the enantiopurity of a diverse set of chiral amines. Both enantio-
mers of the aldehyde can be prepared in two steps from commercially available materials.
DOI: 10.1039/c5ob01398d
www.rsc.org/obc
Introduction
During the course of many synthetic organic chemistry
research projects, it is essential to be able to determine the
enantiopurity of chiral intermediates or products in order to
ascertain whether any racemisation has taken place. This is
typically achieved either via chiral HPLC analysis (which gener-
ally requires a sample of the opposite enantiomer or of the
racemic material), or via the formation of a derivative with a
chiral reagent which enables the resulting diastereomeric
species to be observed via standard spectroscopic
techniques.1–6 Ideally, the chiral derivatising reagent should
be readily accessible in either enantiomeric form, and should
undergo rapid and efficient reaction with the analyte in order
to minimise errors which can occur due to the inherent
diastereoselectivity of the reactions. For chiral alcohols, the
formation of an ester using the chiral acid 1 (Fig. 1) originally
developed by Mosher has become the standard method,
enabling the determination of not only the enantiopurity, but
also the absolute stereochemistry of the major isomer.1 Acid 1
is available in both enantiomeric forms, and undergoes
efficient reaction with secondary alcohols either via the corres-
ponding acid chloride or via direct coupling reactions.
Fig. 1 Chiral derivatising agents.
alternative strategy is to use a chiral aldehyde to generate an
imine.3–5 This strategy is potentially useful as imine formation
is often rapid. However, the aldehyde used must be fairly
hindered in order to give the E-imine selectively as a single
geometrical isomer, and it must also be stable towards racemi-
sation under the conditions used for imine formation.
There have been a few reports in this area, including the use of
myrtenal 22,3 as well as o-formylbenzene boronic acid 3 in
combination with enantiomerically enriched BINOL.4 Myrtenal
2, has not been widely employed, with the original reports
simply demonstrating that the diastereomeric imines derived
from racemic amines could easily be distinguished by NMR.
This is, however, very different from the measurement of the
enantiopurity of an amine derived from a chemical reaction in
which small quantities of the minor diastereoisomer must
typically be quantified as accurately as possible. The enantio-
mer of 2 is also not readily available. Aldehyde 3, in combi-
nation with chiral bis-phenols, has been used to measure the
enantiopurity of a range of amines synthesised by various
methods.5 In our hands it has proved suitable for the measure-
ment of the enantiopurity of simple branched amines (e.g.
α-alkylbenzylamines), but we were not able to apply it to deter-
mine the enantiopurity of a series of α-aminoamides prepared
in our amidation chemistry.7
For chiral amines, however, derivatisation methods are
somewhat more difficult. If amides are prepared using a chiral
acid such as 1, then the resulting compounds are frequently
difficult to analyse via NMR due to the presence of rotational
isomers caused by slow rotation around the C–N bond.1,2 An
Department of Chemistry, University College London, 20 Gordon St, London, WC1H
0AJ, UK. E-mail: tom.sheppard@ucl.ac.uk; Tel: +44 (0)2076 792467
†Electronic supplementary information (ESI) available: Experimental pro-
cedures, spectroscopic data and 1H and 13C NMR spectra. See DOI: 10.1039/
c5ob01398d
In this paper, we describe the use of chiral aldehyde 4 as a
new method for the determination of the enantiopurity of
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chiral amines. We demonstrate its application to a diverse set
of commercially available chiral amines including samples
covering a wide range of enantiopurities as well as samples of
α-aminoamides obtained directly from amidation reactions.
Results and discussion
Scheme 2 Imine formation with aldehyde
4 in a range of NMR
During the course of our work on borate-mediated amidation solvents.
reactions,7 we frequently needed to determine the enantio-
purity of α-aminoamides in order to determine whether race-
misation had taken place during the coupling reactions. In
many cases, the enantiopurity could be determined via chiral Table 1 Separation of imine 13C signals in diastereoisomers 8a/8b
HPLC analysis of the Boc derivative. However, in several cases,
Solvent
Peak separation (ppm)
it was not possible to separate the enantiomers using column
systems available in our laboratory. The requirement to
prepare an authentic sample of the opposite enantiomer (or
the racemate) also makes this approach extremely tedious
when many samples need to be analysed. We therefore
decided to explore the use of chiral aldehydes to see whether it
would be possible to determine the enantiomeric purity of the
corresponding free amine through imine formation and sub-
sequent NMR analysis. Unfortunately, none of the reported
amine derivatisation methods proved suitable for our needs,3,4
with very slow imine formation reactions that did not readily
go to completion, sometimes giving complex mixtures by NMR
that were difficult to analyse accurately. We therefore elected to
investigate an alternative chiral aldehyde for this purpose.
Both enantiomers of aldehyde 4 can be prepared in two steps
from commercially available lactate esters (Scheme 1) and have
been widely used as chiral pool building blocks for a range of
synthetic applications.8 Importantly, no detectable racemisa-
tion occurs during the synthesis of 4,† and it does not race-
mise readily on storage for up to one month,† despite the
presence of an alpha proton.
CDCl3
0.11
0.09
0.21
0.13
DMSO-d6
MeOH-d4
C6D6
Unfortunately, the 1H NMR did not offer sufficient resolution
between the two diastereoisomers to enable accurate quantifi-
cation. However, the 13C NMR clearly showed two separate peaks
for the imine carbons around 170 ppm which were readily
resolved in all of the solvents used (Table 1). Fortunately, using
modern NMR machines it is relatively straightforward to obtain
accurate quantification from 13C NMR, provided the carbon
signals under scrutiny have similar relaxation times. Typically, a
15-minute 13C NMR experiment was sufficient to obtain spectra
with a strong enough signal to noise ratio, enabling efficient ana-
lysis of multiple samples in a short period of time.
With samples of both enantiomers of aldehyde 4 in hand,
we explored the derivatisation of a selection of commercially
available chiral amines 9–20 (Table 2). In the majority of cases,
excellent resolution of the diastereomers was observed,
enabling the enantiopurity9 of the chiral amines to be deter-
mined. There was good agreement between the measurements
obtained with the two enantiomeric aldehydes, and the two
values were averaged to give a more accurate measurement.
The method could be used to derivatise a selection of benzyl-
amines (10–12, 16, 19) as well as amines containing only ali-
phatic substituents at the adjacent carbon atom (9, 13–15).
Bicyclic amines such as 17, 18 and 20 could also be analysed,
although in the latter case the two imine carbon signals were
very close together so the ratio of the two diastereoisomers
could only be quantified by peak height and not by integration.
A key test of any enantiomeric determination method is the
ability to accurately and reproducibly measure the enantio-
meric purity of a set of unknown samples. To this end, we
challenged the method with a set of ‘blind’ tests (Table 3).
Three ‘unknown’ scalemic samples of chiral amine were pre-
pared by each of three researchers by weighing out appropriate
quantities of commercially available amines 9–11. The three
samples were then analysed ‘blind’ by the other two research-
ers using both enantiomers of aldehyde 4. The measured er in
each case was calculated by taking the average of the values
To determine the suitability of 4 for the derivatisation of
chiral amines we examined the imine formation reaction
between racemic amino amide 7 and (S)-4 in a range of deuter-
ated NMR solvents (Scheme 2 and Table 1). Pleasingly, the corres-
ponding diastereomeric mixture of imines 8a/8b was formed
rapidly at room temperature in a variety of different solvents.
Scheme 1 Synthesis of both enantiomers of aldehyde 4. aThe enantio-
purity was determined by imine formation with L-phenylalanine methyl
ester.†
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Table 2 Determination of the enantiomeric purity of commercial chiral
amines 9–20 using aldehyde 4
Table 2 (Contd.)
Enantiomeric ratio9
Enantiomeric ratio9
Δ(δC)/ppm
(solvent)
Amine
(S)-4
(R)-4
Average
Δ(δC)/ppm
(solvent)
Amine
(S)-4
(R)-4
Average
0.116 (d4-MeOH)
97 : 3
96 : 4
96.5 : 3.5
98 : 2
99 : 1
98.5 : 1.5
0.173 (CDCl3)
95 : 5
96 : 4
95.5 : 4.5
0.070 (d4-MeOH)
98 : 2
98 : 2
98 : 2
97 : 3
98 : 2
99 : 1
99 : 1
99 : 1
98 : 2
99 : 1
98.5 : 1.5
98.5 : 1.5
98.5 : 1.5
97.5 : 2.5
98.5 : 1.5
0.200 (d4-MeOH)
51 : 49
50 : 50
50.5 : 49.5
0.036 (C6D6)
50 : 50
50 : 50
50 : 50a
0.093 (d4-MeOH)
a The ratio of imines was determined using peak height rather than
integration.
Table 3 Preparation and analysis of amine samples of ‘unknown’ enan-
tiopurity using amines 9–11
0.139 (d4-MeOH)
Entry Amine Prepared by Actual era (R : S) Measured erb (R : S)
1
2
3
4
5
6
7
8
9
11
10
9
RML
RML
RML
LB
69 : 31
34 : 66
77 : 23
6 : 94
70 : 30 (LB)
69 : 31 (SMG)
34 : 66 (LB)
35 : 65 (SMG)
77 : 23 (LB)
0.163 (d4-MeOH)
99 : 1
99 : 1
98 : 2
98 : 2
98.5 : 1.5
98.5 : 1.5
78 : 22 (SMG)
8 : 92 (RML)
9 : 91 (SMG)
87 : 13 (RML)
85 : 15 (SMG)
25 : 75 (RML)
25 : 75 (SMG)
88 : 12 (RML)
87 : 13 (LB)
53 : 47 (RML)
53 : 47 (LB)
17 : 83 (RML)
18 : 82 (LB)
11
10
9
0.264 (CDCl3)
LB
86 : 14
24 : 76
89 : 11
53 : 47
16 : 84
LB
11
10
9
SMG
SMG
SMG
0.792 (CDCl3)
99 : 1
99 : 1
99 : 1
a Calculated using weights of amines used and the measured er values
for amines 9–11 from Table 2. b Average of values obtained with (S)-4
and (R)-4, see supporting information for original data and the
corresponding NMR spectra.
0.164 (d4-MeOH)
0.186 (d4-MeOH)
99 : 1
98 : 2
99 : 1
98 : 2
99 : 1
98 : 2
obtained with (S)-4 and (R)-4. As can be seen from the results,
extremely consistent measurements were obtained by the three
researchers, and in all cases these were in very close agreement
with the actual enantiopurity of the sample based on the orig-
inal weighings. There is a small divergence in the measured
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Scheme 4 Determination of the enantiopurity of amidation products
using aldehydes 4 and chiral HPLC.
increases over time demonstrating that in this particular case
racemisation of imine 22a via the enamine 23 takes place. In
the corresponding reaction between (S)-21 and (R)-4, a similar
reaction profile is observed, although the initial imine for-
Scheme 3 Reaction of quaternary amino acid ester 21 with aldehyde 4.
enantiopurities at high er (entries 4 and 7), but this is reason- mation did not quite go to completion in this case. This
ably expected as the errors in measuring the small integral for nevertheless enables us to conclude that the original amine
the minor isomer in these cases is likely to be relatively large.
(S)-21 is of high enantiopurity (∼99 : 1 er) as no minor imine
Pleasingly, an amine ( )-21 bearing an adjacent quaternary 22b is observed in the reaction of (S)-21 and (S)-4 until
centre reacted with aldehyde 4 to give two easily resolved dia- consumption of the amine 21 is complete. This demonstrates
stereomeric imines (Scheme 3). However, the formation of that either: (i) no detectable imine 22b is formed (there is no
enamine 23 (as a single geometrical isomer) also took place, significant quantity of (R)-21 present); or (ii) imine 22b is
and significant amounts of 23 were observed before all of the formed but is rapidly converted into the enamine 23. The
starting amine 21 had been consumed. The formation of latter is unlikely to be the case as no significant quantity of the
imine 22 was relatively slow between the enantioenriched minor imine (in this case ent-22a) is observed when (S)-21 is
amine (S)-21 and (S)-4. As can be seen in Fig. 2, the proportion reacted with (R)-4.
of enamine present in the sample is already ∼35% by the time
In the case of all other amines analysed (Table 2), imine for-
the starting amine is completely consumed (∼1.5 hours). Even- mation was observed to be much more rapid than enamine
tually, after a total reaction time of three hours the diastereo- formation, enabling accurate determination of the enantio-
meric imine 22b is observed and its concentration gradually purity in each case. However, enamine formation was seen to
take place in some cases if samples of the imine were stored
for prolonged periods of time. It is therefore recommended
that NMR analysis should be carried out immediately after
sample preparation in order to obtain accurate results.
We next went on to examine some compound samples
obtained from our amidation chemistry. Amides 25a–25c were
prepared via direct borate-mediated amidation of the corres-
ponding Boc-amino acids 24a–24c (Scheme 4).7 Acidic de-
protection of the Boc group gave the corresponding amines as
their trifluoroacetate salts which were converted to the free
amine by filtration through a basic ion exchange resin. The
enantiopurity of the amines 26 was then measured using both
enantiomers of aldehyde 4, and the enantiopurity of the Boc-
protected compounds 25 was measured by chiral HPLC. In all
three cases, good agreement was seen between the two
methods, demonstrating that the amidation reactions and sub-
sequent deprotection occur with very little loss in enantio-
purity under these conditions.
Conclusions
In summary, we have demonstrated that aldehyde 4 is a suit-
able derivatising agent for the determination of the enantio-
Fig. 2 Imine/enamine formation from amine 21 over time: (a) reaction
of (S)-21 and (S)-4; (b) reaction of (S)-21 and (R)-4.
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purity of chiral primary amines by NMR spectroscopy. Both
enantiomers can be prepared on multigram scale in two steps
from commercially available lactate esters, and they react
rapidly with primary amines in a range of NMR solvents to
enable straightforward analysis of the diastereomeric imines
generated.
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Acknowledgements
6 For a reagent developed for determination of the enantio-
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We would like to acknowledge the Department of Chemistry at
University College London (studentship funding for RML and
SMG) and the Engineering and Physical Sciences Research
Council (postdoctoral funding for LB, grant reference EP/
K001183/1) for financial support.
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