Dynamic kinetic resolution of 2-methyl-2-nitrocyclohexanol: Combining the intramolecular nitroaldol (Henry) reaction & lipase-catalysed resolution

Article abstract of DOI:10.1016/j.tet.2018.01.055

Efforts to combine the intramolecular nitroaldol reaction with lipase-catalysed resolution of the resulting nitroaldol adduct in a one-pot dynamic kinetic resolution (DKR) are described. Significant challenges were encountered in the combination of the two systems. trans-2-Methyl-2-nitrocyclohexyl acetate (±)-3b was isolated in excellent enantiopurity (>98% ee) via a sequential DKR sequence where the lipase-mediated resolution and base-mediated interconversion of 2-methyl-2-nitrocyclohexanol 2 were effected alternately, demonstrating the feasibility of this approach initially. Further work showed, for the first time, evidence that a DKR-type system is possible for 2. Reaction engineering allowed the design of a sequential one-pot reaction system which furnished the products with excellent enantioselectivity, and good diastereoselectivity.

Full text of DOI:10.1016/j.tet.2018.01.055

Accepted Manuscript
Dynamic kinetic resolution of 2-methyl-2-nitrocyclohexanol: Combining the
intramolecular nitroaldol (Henry) reaction & lipase catalysed resolution
Aoife M. Foley, Declan P. Gavin, Rebecca E. Deasy, Sinéad E. Milner, Thomas S.
Moody, Kevin S. Eccles, Simon E. Lawrence, Anita R. Maguire
PII:
S0040-4020(18)30115-7
10.1016/j.tet.2018.01.055
TET 29270
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 20 December 2017
Revised Date: 29 January 2018
Accepted Date: 30 January 2018
Please cite this article as: Foley AM, Gavin DP, Deasy RE, Milner SinéE, Moody TS, Eccles KS,
Lawrence SE, Maguire AR, Dynamic kinetic resolution of 2-methyl-2-nitrocyclohexanol: Combining
the intramolecular nitroaldol (Henry) reaction & lipase catalysed resolution, Tetrahedron (2018), doi:
10.1016/j.tet.2018.01.055.
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ACCEPTED MANUSCRIPT
Graphic Abstract
Herein, we describe the development of a reversible intramolecular base catalysed Henry reaction coupled
with the hydrolase mediated kinetic resolution with the view to selective acetylation of a single stereoisomer is
described.
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Dynamic
Kinetic
Resolution
of
2-Methyl-2-
nitrocyclohexanol: Combining the Intramolecular
Nitroaldol (Henry) Reaction & Lipase Catalysed
Resolution
Aoife M. Foley,a Declan P. Gavin,a Rebecca E. Deasy,b Sinéad E. Milner,b
Thomas S. Moody,c Kevin S. Eccles,b Simon E. Lawrence,b and Anita R.
Maguire*d
aSchool of Chemistry, Analytical and Biological Chemistry Research Facility, Synthesis and Solid-State
Pharmaceutical Centre, University College Cork, Cork, Ireland
bSchool of Chemistry, Analytical and Biological Chemistry Research Facility, University College Cork, Cork,
Ireland
cDepartment of Biocatalysis & Isotope Chemistry, Almac, Craigavon, BT63 5QD, Northern Ireland, United
Kingdom; Arran Chemical Company Limited, Unit 1, Monksland Industrial Estate, Athlone, Co. Roscommon,
Ireland.
dSchool of Chemistry & School of Pharmacy, Analytical and Biological Chemistry Research Facility, Synthesis
and Solid-State Pharmaceutical Centre, University College Cork, Cork, Ireland
E-mail: a.maguire@ucc.ie; Fax: +353 21 4901770; Tel: +353 21 490 1694/2125/3501
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Abstract
Efforts to combine the intramolecular nitroaldol reaction with lipase-catalysed resolution of the
resulting nitroaldol adduct in a one-pot dynamic kinetic resolution (DKR) are described. Significant
challenges were encountered in the combination of the two systems. trans-2-Methyl-2-
nitrocyclohexyl acetate (±)-3b was isolated in excellent enantiopurity (>98% ee) via a sequential
DKR sequence where the lipase-mediated resolution and base-mediated interconversion of 2-methyl-
2-nitrocyclohexanol 2 were effected alternately, demonstrating the feasibility of this approach
initially. Further work showed, for the first time, evidence that a DKR-type system is possible for 2.
Reaction engineering allowed the design of a sequential one-pot reaction system which furnished the
products with excellent enantioselectivity, and good diastereoselectivity.
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Keywords:
Biocatalysis
Kinetic Resolution
Henry reaction
Dynamic Kinetic Resolution
Lipases
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1. Introduction
The Henry reaction is an important base-mediated transformation in organic chemistry leading to
vicinal nitroalcohols (Scheme 1), which can be converted into a wide variety of synthetic
intermediates, such as 1,2-aminoalcohols and α-hydroxycarboxylic acids.1-3 Although the reaction is
known for decades, stereo- and diastereoselective approaches leading to enantiomerically pure
nitroalcohols are still challenging. Principal approaches to the catalytic asymmetric nitroaldol
reaction, including transition metal- and organo-catalysed methods, have been reviewed in detail.2, 4, 5
In the past decade, there has been an emergence of biocatalytic protocols to resolve the products of
the Henry reaction due to their mild reaction conditions and high selectivity.6 There are two distinct
biocatalytic methods; direct enzyme-catalysed (hydroxynitrile lyases) asymmetric nitroaldol reaction7
or initial chemical formation of the β-nitroalcohol product followed by enzymatic kinetic resolution of
the resulting stereoisomers. The latter suffers from the limitation of attaining a maximum theoretical
yield of 50%. In recent years, we have developed an effective protocol for kinetic resolution of 2-
nitrocyclohexanol 5.8 In this study, we attempted to develop a dynamic kinetic resolution for
compound 5, and expanded it to 2-methyl-2-nitrocyclohexanol 2.
Scheme 1 DKR of the nitroaldol through lipase catalysis
Previously, Vongvilai et al have developed a procedure for the intermolecular dynamic kinetic
resolution of β-nitroalcohols, but this was limited by the need to use a large excess of the nitroalkane
in order to shift the equilibrium towards the formation of the product.9, 10 In our current study, we have
the added challenge of a second stereocentre, leading to potential complexity in stereocontrol.
Herein, we report our studies, combining the reversible intramolecular base-catalysed nitroaldol
reaction of 6-nitroheptanal 1 with a one-pot lipase-mediated acetylation and kinetic resolution of the
subsequent β-nitroalcohol 2 leading to 2-methyl-2-nitrocyclohexyl acetate 3 with moderate
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diasteroselectivity but excellent enantiopurity (Scheme 2). Ideally, this DKR would provide exclusive
access to a single stereoisomer of 3. To date limited research has been carried out in this area, and
while the viability of a one-pot reaction combining the Henry reaction with enzyme-mediated
dynamic resolution has previously been described, it is evident that there are significant limitations to
be overcome before this protocol has a broad synthetic utility.9-11
Scheme 2 Stepwise investigation of the DKR of the intramolecular nitroaldol reaction through lipase catalysis
2. Results and Discussion
2.1 Diastereoselective lipase-mediated transesterification
We have previously reported efficient kinetic bioresolution for both the cis- and trans-2-
nitrocyclohexanols (±)-5a and (±)-5b via enzyme-mediated transesterification and ester hydrolysis.8
While base-mediated interconversion of cis- and trans-2-nitrocyclohexanol (±)-5a and (±)-5b is
readily established, initially believed to indicate reversible nitroaldol reaction, detailed investigation
demonstrated that the interconversion was complicated due to competing epimerisation via
deprotonation geminal to the nitro group (Scheme 3). Critically, when enantiopure cis- and trans-2-
methyl-2-nitrocyclohexanol (±)-5a and (±)-5b are individually exposed to base, interconversion of
(±)-5a and (±)-5b is seen but without stereochemical scrambling at C(1)OH centre. This can only be
rationalised by deprotonation at the C(2)NO2 centre, rather than a retro-Henry reaction. The
interconversion of (±)-5a and (±)-5b via the reversible nitroaldol reaction is essential for a DKR
approach as the alternative deprotonation pathway does not enable racemisation at the cyclohexanol
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stereogenic centre but only affects the centre bearing the nitro substituent. The complication of the
competing pathway prevented the development of a one-pot intramolecular nitroaldol reaction of 6-
nitrohexanal 4 with dynamic kinetic lipase-mediated resolution of 2-nitrocyclohexanol 5 (Scheme 2).
Scheme 3 Epimerisation pathway versus the reversible nitroaldol reactions
Therefore, use of a modified substrate, 2-methyl-2-nitrocyclohexanol 2, was investigated, as it was
envisaged that this would avoid base-mediated epimerisation at C2 due to the presence of the methyl
moiety and, consequently, interconversion of the cis- and trans-2-methyl-2-nitrocyclohexanol (±)-2a
and (±)-2b can only occur through ring opening and closing of the aldehyde 6-nitroheptanal 1.
The bioresolution of an intramolecular nitroaldol reaction of 6-nitroheptanal 1 was investigated in a
stepwise manner (Scheme 2). The first step in this study involved independent examination of the
lipase-mediated kinetic resolution of the racemic cis- and trans-2-methyl-2-nitrocyclohexanols (±)-2a
and (±)-2b identifying the most efficient lipase to perform this biotransformation enantioselectively
and diastereoselectively. Ideally, for an efficient dynamic process, one enantiomer of either (±)-2a or
(±)-2b would be efficiently and selectively acetylated. The alcohol substrates (±)-2a and (±)-2b were
obtained by sodium borohydride reduction of 2-methyl-2-nitrocyclohexanone,12, 13 while subsequent
acetylation with acetic anhydride in DCM with pyridine & catalytic DMAP gave the novel racemic
acetates (±)-3a and (±)-3b. The relative stereochemistry of (±)-3a was confirmed by X-ray
crystallography. The optimum reaction conditions reported for the kinetic resolution of cis- and trans-
2-nitrocyclohexanol (±)-5a and (±)-5b employing vinyl acetate as both solvent and acyl donor were
applied in this study for the lipase-mediated transesterification of 2.8
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Following a screen using a targeted panel of lipases, several potential lead enzymes were identified
(full detail included in SI). Candida antartica lipase B (immob) (CALB immob) was chosen as the
lipase to continue the investigation as it gave moderate selectivity (5:1, after 12 hours) for acetylation
of (±)-2a over (±)-2b as well as excellent enantioselectivity (>98% ee) for both diastereomers. Table
1 shows the selectivity over time, with acetate (±)-3b being the preferred product initially, but with
longer reaction times a relative increase in (±)-3a is observed as (±)-2b is depleated.
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Table 1 Diastereoselective CAL-B (immob) mediated transesterification of cis- and trans-2-methyl-2-nitrocyclohexanol (±)-
2a and (±)-2b in vinyl acetate
Alcohol 2
Acetate 3
Enzyme
Source
Reaction Time
cis-2a (%)[a]
trans-2b (%)[a]
cis-3a (%)[a]
trans-3b (%)[a]
12 h
44
38
36
30
30
3
15
17
23
32
18.5 h
40.5 h
62.5 h
43
41
4
6
CAL-B
(immob)
27
11
(15% ee[b] [c]
)
(74% ee[b] [d]
)
(>98% ee[b] [e]
)
(>98% ee[b] [f]
)
,
,
,
,
1
1
[a]Conversions were estimated by H NMR spectroscopy and were derived from integration of the H NMR spectrum of the mixture
of the crude material. Starting material was an equimolar mixture of cis-2a and trans-2b [b]Enantiomeric excess [ee (%)] was
determined by chiral HPLC analysis [Daicel Chiralcel OJ-H, hexane/i-PrOH = 97:3, flow rate 0.75 mL/min, ambient temperature,
injection volume 10 ꢀL, λ = 209.8 nm]. [c]The principal enantiomer was (1S,2R)-cis-2-methyl-2-nitrocyclohexanol (1S,2R)-2a. [d]The
principal enantiomer was (1S,2S)-trans-2-methyl-2-nitrocyclohexanol (1S,2S)-2b. [e]The principal enantiomer was (1R,2S)-cis-2-
methyl-2-nitrocyclohexyl acetate (1R,2S)-3a. [f]The principal enantiomer was (1R,2R)-trans-2-methyl-2-nitrocyclohexyl acetate
(1R,2R)-3b.
We also explored the use of acylating agents other than vinyl acetate (details included in SI) in
conjunction with CAL-B (immob). While vinyl acetate and phenyl acetate performed equally well,
including over longer reaction times (48 h), use of the higher boiling phenyl acetate did not offer any
synthetic advantages and was difficult to separate from the product.
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2.2 Dynamic interconversion process
Crucial to the success of the lipase-mediated DKR protocol is selection of the correct base. The
fundamental requirement of the selected base is to mediate reversible ring closure of 6-nitroheptanal 1
and thus catalyse the dynamic interconversion process between the diastereomeric alcohols (±)-2a and
(±)-2b. It was then envisaged that combination with the diastereoselective lipase-mediated
transesterification (as discussed above) would lead to a one-pot DKR of the intramolecular nitroaldol
reaction through lipase catalysis.
The novel aldehyde 1 was synthesised via a base-catalysed nitroaldol reaction with nitroethane and
methyl 5-oxopentanoate. Subsequent reductive elimination of the β-nitroalcohol generated the
nitroalkane.14 Finally the aldehyde 1 was readily accessible via reduction of the ester (Error!
Reference source not found.).
Scheme 4 Synthesis of 6-nitroheptanal 1
As vinyl acetate had been utilised for the lipase-mediated transesterification in both the analytical
and synthetic-scale, the base-mediated investigation was effected in the same solvent to ultimately
enable combination of the two processes in a single pot. Therefore, a series of bases was screened
with 6-nitroheptanal 1 in vinyl acetate, (details in SI) and although there was evidence of a small
amount of background chemical acylation, DBU was chosen as the base for the intramolecular Henry
reaction.
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2.3 Two-pot kinetic resolution process
A two-pot reaction was carried out, where the reaction mixture was alternately exposed to the
cyclisation/interconversion conditions and the resolution conditions, with a work up in between to
remove the previous catalyst (Scheme 5). This overcame the problem of competing chemical
acylation, and base-lipase inhibition by physically separating the two reactions. This ultimately
returned the preferred product (±)-3a with excellent enantiopurity (>98% ee) which made up 57% of
the reaction mixture (see SI, Table 9).
Scheme 5 Two-pot process
2.4 Two-Pot System: Influence of Solvents and Immobilized Base
Details of early attempts to develop a one-pot process are described in the SI. Several problems
were evident and needed to be overcome to effectively combine the two systems. The background
chemical acylation was problematic in the presence of DBU, even at low concentrations. Accordingly,
as use of vinyl acetate as the solvent was not viable in this process, reduced loading and the use of an
organic solvent would need to be explored. Additionally, in one pot processes containing both the
base and the lipase, neither the interconversion nor the resolution were effective due to interactions
between the base and the lipase, leading to base-lipase inhibition.
It was decided to look again at the two separate transformations (the base-mediated interconversion
and the resolution) and focus on optimising each part, building on insight generated at this point to
address the unresolved problems. As one of the best bases for the interconversion was DBU,
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immobilised DBU was explored to determine if a heterogenous base would overcome the problem of
base-lipase inhibition, and to facilitate separation. Similarly, CAL-B (immob) was chosen as the
lipase.
Solvent screens were carried out for both the interconversion, and the resolution processes
separately. In addition to this, the vinyl acetate loading was initially reduced to 50 equivalents. We
found that the resolution, mediated by CAL-B (immob), performed well in non-polar solvents
(toluene & hexane) (See SI, Table 10, entries 1 & 2) while the interconversion is also conveniently
favoured by these non-polar solvents (See SI, Table 11). Sampling was more reliable using toluene as
solvent than using hexane, in which the substrates and products were poorly soluble. In addition, the
rate of interconversion of (±)-2a to (±)-2b in toluene was increased relative to the rate in neat vinyl
acetate. Interestingly, while the interconversion catalysed by DBU (immob) was sluggish in ethyl
acetate (See SI, Table 11, Entry 6), this solvent furnished us with excellent diastereomeric
discrimination (1:11) in the resolution step (See SI, Table 10, Entry 6).
Further decreasing concentration of vinyl acetate, as a solution in toluene, as anticipated,
significantly reduced the issue of background chemical acylation (5 eq. of vinyl acetate gave <5%
chemical acylation after 12 hours). Notably, the kinetic resolution was equally effective with vinyl
acetate loading as low as 3 eq. of vinyl acetate in toluene as the earlier resolution with 50 eq. vinyl
acetate in toluene.
It was found that 30 °C was the optimum temperature for the resolution step. Although at higher
temperature (50 °C) the enantioselectivity was still excellent, the diastereoselectivity was not
maintained. The interconversion was more efficient at 50 °C than at the lower temperature, as
anticipated.
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2.5 Cycling System
Following this, a series of one-pot and two-pot reactions were attempted with the same solvent
conditions for both the interconversion and the resolution. (See SI, Table 14).
The first reactions attempted examined the two processes at 50°C, without and with removal of the
base before addition of the lipase by a simple filtration through Celite® (See SI, Table 14, Entry 1 &
2). This allowed us to carry out the interconversion first, to enrich a predominately (±)-2a sample to
give us more of the enzymatically favoured (±)-2b diastereomer. A reaction was also carried out in
which both catalysts were introduced at the same time (see SI, Table 14, Entry 3); inhibition of the
base-mediated interconversion in the presence of the lipase was observed and this was overcome in
entry 1 & 2 by delayed addition of the lipase. These experiments showed, for the first time, evidence
of a dynamic system when both the base and the lipase were present together, albeit this required a
time-delayed addition of the biocatalyst, key to the success of this system.
Further attempts to design the one-pot reaction focussed on a model circular flow system (Scheme
6
), where each reaction was carried out in a separate vessel with the previous catalyst [DBU (immob)
or CALB (immob)] removed by filtration before exposure to the next. This allowed us to carry out the
two individual reactions while physically separating the two catalysts. The reaction mixture was
cycled through each chamber in turn. Evaporation of the solvent and vinyl acetate from the reaction
mixture prior to reexposure to the base was undertaken to avoid chemical acylation. Nevertheless, it
was shown that when the evaporation step was omitted i.e. vinyl acetate was carried over to the DBU
step, only a modest decrease in enantioselectivity was observed. Details of these reactions are
included in the SI.
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Scheme 6 Cycling system, separating the catalysts
Table 2 shows the optimised one-pot procedure (similar experiments shown in SI, Table 16) where
the reaction material was alternately exposed to the interconversion conditions and the resolution
conditions, and cycled through three times. It was found that the addition of the catalysts needed to be
staggered, as DBU (immob) is less effective in the presence of CAL-B (immob) (see SI Table 16,
Entry 3) . DBU (immob) was added first, in this case interconversion was carried out for 32 h at 50 °C
for step 1A, although it was subsequently shown that 32 hours was not required and that the same (±)-
2a: (±)-2b ratio (~36:64) was reached after only 18 hours; critically, due to staggered addition of
reagents, it was possible to conduct the interconversion at 50 °C and then cool the reaction mixture to
30 °C for the lipase-mediated resolution, carried out in the same reaction vessel, with the vinyl acetate
and the CAL-B (immob) added after the interconversion and cooling to 30 °C.
Table 2 One-pot reaction
Step[a]
Alcohol 2[b]
Acetate 3[b]
Entry
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cis-2a
trans-2b
cis-3a
trans-3b
1A
1B
2A
2B
3A
3B
36 (4)
18 (39)
16 (16)
17 (16)
9 (7)
64 (0)
27 (>98)
27 (7)
25 (62)
21 (13)
17 (87)
0 (-)
0 (-)
14 (>98)
17 (92)
16 (97)
19 (94)
18 (97)
41 (>98)
40 (97)
43 (98)
51 (97)
53 (96)
1
11 (55)
[
a]Reaction conditions: Each step was 12 hours except 1a, which was 32 hours. Step A was carried out at 50 °C, 500 rpm, Step B
was carried out at 30 °C, 400 rpm. The reaction mixture was filtered through a plug of Celite® to remove the immobilised
catalysts after the resolution (step B). 3 equivalents of vinyl acetate were added with the CAL-B (immob). [b]Expressed as a %
1
estimated by integrating of the relevant H NMR signals. Numbers in parenthesis are % ee values, which were determined by
chiral HPLC analysis [Daicel OJ-H, hexane/i-PrOH = 98.5:1.5, flow rate 0.5 ml/min, ambient temperature, injection volume = 10
µL, λ = 209.8 nm].
The final ratio of the products and substrates show, that, although not a traditional one-pot
reaction, a modular circular system has been engineered, through extensive optimisation of the
individual steps.
The starting material used was a mixture of (±)-2a and (±)-2b (87:13) instead of aldehyde 1. In the
presence of DBU only, aldehyde 1 will cyclise, and in this case, the mixture of nitroalcohols 2 or
aldehyde 1 would be expected to give the same outcome under the conditions for step 1A.
These results are promising, showing, for the first time, a one-pot, dynamic system for the
resolution of alcohol 2. While this is not the desired one-pot reaction, it represents a significant move
towards a one-pot procedure. In order to achieve this, several parameters were examined and their role
evaluated. Filtration (after step B) is necessary to remove the immobilised catalysts, but does not need
to be carried out after each step (A&B), only the end of each cycle (after step B). Sequential addition
of the catalysts was found to be key to this one-pot procedure, where the immobilised base was added
to the reaction mixture first, to allow interconversion to be carried out, followed by the introduction of
the immobilised lipase, to carry out the resolution. While the evaporation step (after B) was shown to
give better enantiopurity after three cycles as well as more product (~60% vs ~50%) than without
vacuum step, it could be removed if necessary to simplify the procedure.
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3. Conclusion
Significant progress has been made in the individual elements of the dynamic resolution process
for trans-3b. Efficient kinetic resolution has been effected for both the cis-2-methyl-2-
nitrocyclohexanol (±)-2a and trans-2-methyl-2-nitrocyclohexanol (±)-2b. CAL-B (immob) acelated
both (±)-2a & (±)-2b in the ratio of ca. 10:30, with excellent enantioselectivity. Furthermore, the
lipase-mediated transesterification of both (±)-2a and trans-2b have been performed on a preparative-
scale yielding (1S,2S)-2b, (1R,2S)-3a and (1R,2R)-3b in excellent enantiopurity (≥98% ee). A series
of bases has been screened and DBU was identified to mediate the intramolecular nitroaldol reaction
and associated dynamic interconversion process. However, background chemical acetylation in the
presence of vinyl acetate and lipase-mediated base inhibition presented significant barriers to the
development of a one-pot DKR process. Clearly, screening a larger range of enzymes, both wild type
and engineered enzymes, offers potential for an enhanced DKR protocol.
A sequential two-pot DKR process was developed whereby the dynamic interconversion process
was performed independently of the lipase-mediated kinetic resolution. Good diastereoselectivity was
observed with CAL-B (immob) and the transesterification of the trans-2-methyl-2-nitrocyclohexanol
(±)-2b was the dominant kinetic resolution process. The trans–acetate (1R,2R)-3b was the major
component (57%) of the reaction mixture and and isolated in excellent enantiomeric excess (≥98%
ee).
Following this, the conditions were further explored, focussing on conditions which work for the
individual parts. Conditions were developed which furnished us with a dynamic system albeit with
poor efficiency. Key to this was the timing of the reagent addition, by first adding the base, to carry
out the interconversion, then the lipase for the resolution, we successfully demonstrated a dynamic
system was possible (Table 2).
Achieving DKR in the intramolecular nitroaldol is extremely challenging due to the number of
competing processes arising. By careful exploration of the process conditions including variation of
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biocatalyst, base and solvent we have demonstrated for the first time the feasibility of this process. In
addition to careful analysis of the reaction conditions for each individual step in this process,
introduction of the methyl group in (±)-2a and (±)-2b to avoid epimerisation via deprotonation as seen
in (±)-5a and (±)-5b was critical to the success of this outcome. Investigation of the potential to
enhance this through continuous flow is currently underway.
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4. Experimental Section
4.1 General procedures
Dichloromethane was distilled from phosphorus pentoxide. Ethanol was distilled from magnesium
ethoxide. Infrared spectra were recorded as films on NaCl plates or as KBr discs. Melting points are
uncorrected. NMR spectra were recorded on a 300 MHz or 600 MHz NMR spectrometer. All spectra
were recorded at room temperature (~20 °C) in deuterated chloroform (CDCl3) using
1
1
tetramethylsilane (TMS) as an internal standard. H-1H and H-13C correlations were used to confirm
the NMR peak assignments. HRMS was performed on a TOF instrument in electrospray ionization
(ESI) mode; samples were made up in acetonitrile. Elemental analysis was performed using a Perkin-
Elmer 240 and Exeter Analytical CE440 elemental analysers. Enantiomeric excess were measured by
HPLC, using a Chiralcel® OJ-H column (5 × 250 mm) from Daicel Chemical Industries Limited.
Mobile phase, flow rate, detection wavelength and temperature are included in the appropriate tables.
HPLC analysis was performed on a Waters alliance 2690 separations module with a PDA detector.
When only one single enantiomer could be detected, the enantiomeric excess is quoted as >98%.
Optical rotations were measured on a Perkin-Elmer 141 polarimeter at 589 nm in a 10 cm cell;
concentrations (c) are expressed in g/100 mL. [ꢀ]ꢂꢁꢃis the specific rotation of a compound and is
expressed in units of 10−1 deg cm2 g−1. All lipases were kindly donated by Almac Sciences. Single
crystal X-ray diffraction measurements were made on a Bruker APEX II DUO diffractometer fitted
with an Oxford Cryosystems COBRA for controlling the sample temperature. Mo Kα radiation (λ =
0.7107 Å, graphite monochromator) was used for (±)-3a and Cu Kα radiation (λ = 1.5418 Å, doubly
curved silicon crystal monochromator) was used for (1R,2S)-3a. Analysis was undertaken with the
SHELX suite of programs15 and diagrams prepared with Mercury 3.0.16 All non-hydrogen atoms were
located and refined with anisotropic thermal parameters. Hydrogen atoms were included in calculated
positions or they were located and refined with isotropic thermal parameters. Crystallographic data
(excluding structure factors) for the structures in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication numbers CCDC 969530 and 969531.
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These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
4.2 Synthesis of racemic substrates
4.2.1 2-Methyl-2-nitrocyclohexanone13
2-Nitrocyclohexanone (2.50 g, 17.47 mmol) in dichloromethane (19 mL) was added in one portion
to a stirred solution of tetrabutylammonium hydroxide (11.33 g, of a 40% aq. solution, 17.47 mmol)
in water (17 mL) under nitrogen. The reaction mixture was stirred for 10 min, and then methyl iodide
(4.4 mL, 69.87 mmol) was added in one portion. The reaction mixture was stirred vigorously for 36 h
at room temperature. The reaction mixture was then transferred to a separating funnel and layers
separated. The organic layer was washed with water (20 mL). The aqueous phase was extracted with
dichloromethane (2 × 30 mL) and the combined organic fractions were dried, filtered and
concentrated under reduced pressure in an ice cold water bath. Diethyl ether (60 mL) was added to
precipitate the tetrabutylammonium iodide salt, the solution was filtered and concentrated under
reduced pressure in an ice cold water bath to give a crude mixture (1.54 g) of 2-methyl-2-
nitrocyclohexanone and ring cleavage products, methyl 6-nitrohexanoate and methyl 6-
nitroheptanoate (76:13:11 respectively) as an orange oil. Purification by column chromatography on
silica gel using hexane/diethyl ether 97/3 as eluent gave the pure α-nitro ketone (0.78 g, 36%) as a
colourless oil; vmax/cm−1 (film) 2950, 2873, 1732, 1549; δH (600 MHz) 1.67 [3H, s, C(2)CH3], 1.69-
1.85 (4H, m, ring protons), 1.98-2.10 (1H, m, ring protons), 2.55-2.64 (2H, m, ring protons), 2.86-
2.90 (1H, m, ring protons).
4.2.2 2-Methyl-2-nitrocyclohexanol 212
A solution of 2-methyl-2-nitrocyclohexanone (0.98 g, 6.24 mmol) in distilled ethanol (10 mL) was
added dropwise over 10 min to a stirred suspension of NaBH4 (0.24 g, 6.24 mmol) in distilled ethanol
(25 mL) at 0 °C under nitrogen and stirring was continued for 5 h at 0 °C. The ice bath was then
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removed and aqueous hydrochloric acid (10%) was added to adjust to pH 1. The solution was
concentrated under reduced pressure and the resulting residue was partitioned between water (10 mL)
and dichloromethane (10 mL). The aqueous phase was extracted with dichloromethane (3 × 10 mL)
and the combined organic extracts were washed with brine (30 mL), dried, filtered and concentrated
under reduced pressure to give a crude mixture (0.72 g) of nitroalcohols 2a and 2b (41:59
respectively) as a yellow oil. Purification by column chromatography on silica gel using hexane/ethyl
acetate 97/3 as eluent gave cis-2-methyl-2-nitrocyclohexanol 2a (186.6 mg, 19%) as a light yellow
low melting solid, m.p. 33-35 °C; vmax/cm−1 (film) 3445, 2945, 1539, 1352; δH (600 MHz) 1.39-1.46
[1H, m, one of C(5)H2], 1.48-1.52 [2H, m, C(4)H2], 1.63 [3H, s, C(2)CH3], 1.68-1.77 [3H, m, one of
C(5)H2, one of C(3)H2 and one of C(6)H2], 1.82-1.87 [1H, m, one of C(6)H2], 2.43-2.48 [1H, m, one
of C(3)H2], 2.84 [1H, d, J 8.4, OH], 3.91 [1H, br s, C(1)H]; δC (150 MHz) 21.5 [CH2, br, C(5)H2],
21.9 [CH2, C(4)H2], 24.2 [CH3, C(2)CH3], 30.6 [CH2, C(6)H2], 32.9 [CH2, br, C(3)H2], 73.1 [CH, br,
C(1)H], 91.5 [C, C(2)]; HRMS (ES+): Exact mass calculated for C7H14NO3 [M+H]+ 160.0974 Found
160.0974. (more polar) trans-2-Methyl-2-nitrocyclohexanol (±)-2b (274.6 mg, 28%) was isolated as a
white crystalline solid, m.p. 38-41 °C; (Found C, 53.06; H 8.17; N 8.62. C7H13NO3 requires C, 52.82;
H, 8.23; N, 8.80%); vmax/cm−1 (KBr) 3376, 2947, 1538, 1340; δH (600 MHz) 1.33-1.47 [3H, m, one of
C(4)H2, one of C(5)H2 and one of C(6)H2], 1.61 [3H, s, C(2)CH3], 1.70-1.73 [1H, m, one of C(4)H2]
1.76-1.79 [1H, m, one of C(5)H2], 1.86-1.91 [1H, m, one of C(3)H2], 1.95-1.99 [1H, m, one of
C(6)H2], 2.13-2.16 [1H, m, one of C(3)H2], 2.74 [1H, s, OH], 4.28-4.29 [1H, m, C(1)H]; δC (150
MHz) 16.3 [CH3, br, C(2)CH3], 22.1 [CH2, C(4)H2], 23.5 [CH2, C(5)H2], 30.6 [CH2, C(6)H2], 35.9
[CH2, br, C(3)H2], 72.4 [CH, C(1)H], 93.0 [C, C(2)].
4.2.3 cis-2-Methyl-2-nitrocyclohexyl acetate (±)-3a
N,N-Dimethylaminopyridine (DMAP) (2.0 mg, 0.02 mmol) was added to a stirring solution of cis-
2-methyl-2-nitrocyclohexanol (±)-2a (131.8 mg, 0.83 mmol), acetic anhydride (0.5 mL, 5.80 mmol)
and pyridine (0.3 mL, 3.39 mmol) in dichloromethane (5 mL). The reaction mixture was stirred at
room temperature for 21 h under nitrogen. A saturated aqueous solution of sodium bicarbonate (10
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mL) was added and the mixture stirred for 30 min. The solution was transferred to a separating funnel
and washed with a saturated aqueous solution of CuSO4 (20 mL), water (20 mL), a saturated aqueous
solution of sodium bicarbonate (20 mL) and brine (20 mL). The organic extract was dried, filtered and
concentrated under reduced pressure to give crude acetate (±)-3a (145.8 mg, 88%) as a clear oil which
solidified on cooling to a white crystalline solid which was sufficiently pure to use without further
purification, m.p. 46-48 °C; vmax/cm−1 (KBr) 2960, 1738, 1544, 1362; δH (300 MHz) 1.26–1.48 [3H,
m, C(5)H2 and one of C(4)H2], 1.54-1.67 {4H, m containing 1.54 [3H, s, C(2)CH3] and [1H, m, one of
C(6)H2]}, 1.70-1.82 [1H, m, one of C(4)H2], 1.88-2.05 {5H, m containing 1.95 [3H, s, COCH3] and
[2H, m, one of C(6)H2 and one of C(3)H2]}, 2.22-2.32 [1H, m, one of C(3)H2], 5.26-5.29 [1H, m,
C(1)H]; δC (75.5 MHz) 18.6 [CH2, C(5)H2], 19.9 [CH3, C(2)CH3], 20.9 [CH2, C(4)H2], 22.1 [CH3,
COCH3], 25.8 [CH2, C(6)H2], 29.7 [CH2, C(3)H2], 72.4 [CH, C(1)H], 87.5 [C, C(2)], 168.6 [C,
COCH3]; HRMS (ES+): Exact mass calculated for C9H15O2 [M-NO2]+ 155.1072 Found 155.1065.
A sample of (±)-3a suitable for single crystal X-ray diffraction was recrystallised from slow
evaporation of deuterated solvent.
4.2.4 trans-2-Methyl-2-nitrocyclohexyl acetate (±)-3b
This was prepared following the procedure for 3a, from DMAP (2 mg, 0.02 mmol), trans-2-
methyl-2-nitrocyclohexanol (±)-2b (152.8 mg, 0.96 mmol), acetic anhydride (0.6 mL, 6.68 mmol) and
pyridine (0.3 ꢀL, 3.97 mmol) in dichloromethane (5 mL) to give the crude acetate (±)-3b (137.5 mg,
71%) as a light yellow oil which was sufficiently pure to use without further purification; vmax/cm−1
(film) 2950, 1748, 1538, 1361; δH (300 MHz) 1.33–1.52 [3H, m, one of C(4)H2, one of C(5)H2 and
one of C(6)H2], 1.62-1.81 {5H, m containing 1.65 [3H, s, C(2)CH3] and [2H, m, one of C(5)H2 and
one of C(4)H2]}, 1.95-2.19 {6H, m containing 2.03 (3H, s, COCH3) and [3H, m, C(3)H2 and one of
C(6)H2]}, 5.50-5.55 [1H, m, C(1)H]; δC (75.5 MHz) 17.6 [CH3, C(2)CH3], 20.9 [CH3, COCH3], 21.9,
22.7 [CH2, C(5)H2, and C(4)H2], 28.1 [CH2, C(6)H2], 36.5 [CH2, C(3)H2], 73.9 [CH, C(1)H], 90.4 [C,
C(2)], 169.5 (C, COCH3); HRMS (ES+): Exact mass calculated for C9H15O2 [M-NO2]+ 155.1072
Found 155.1072.
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4.3 Lipase-mediated kinetic resolution
4.3.1 General procedure for the development of one-pot procedures: lipase-mediated
transesterification of 2-methyl-2-nitrocyclohexanol (±)-2a or (±)-2b with vinyl acetate as acetylating
agent and dynamic interconversion process (analytical scale)
In a typical experiment, the following were added as appropriate to a solution of nitroalcohols (±)-
2a and (±)-2b (typically 15 mg) in the appropriate solvent (10 mg/mL), DBU (immob) (0.5 eq.) (18
mg), vinyl acetate (50, 5 or 3 eq. as appropriate) and CAL-B (immob) (approx. 15 mg). The small test
tube was sealed and agitated as appropriate for the specified length of time (generally 12 hours). The
solutions were filtered through Celite® and concentrated under reduced pressure. The sample was
analysed by 1H NMR spectroscopy, reconcentrated and dissolved in a mixture of isopropanol/hexane
[10:90 (HPLC grade)] and enantioselectivity determined by chiral HPLC.
4.3.2 General procedure for the development of two-pot procedures: dynamic interconcverion
process and lipase-mediated transesterification of 2-methyl-2-nitrocyclohexanol (±)-2a or (±)-2b with
vinyl acetate as acetylating agent (analytical scale)
In a typical experiment, DBU (immob) (0.5 eq., ~90 mg) was added to a solution of nitroalcohols
(±)-2a & (±)-2b in toluene (5 mL, 10 mg/mL) and the test tube was sealed and shaken at 500 rpm for
the required amount of time (generally 12 hours) at the specified temperature (usually 50°C). The
reaction solution was filtered through Celite®, and concentrated under reduced pressure and
redissolved in the appropriate amount of toluene.1 CAL-B (immob) (18 mg/ 1 mL solvent) and vinyl
acetate (3 eq) were added and the solution shaken at 400 rpm and 30°C) for 12h in a sealed test tube.
1
Before concentration, a 0.5 mL sample (approx. 5 mg material) is removed, and the volume of toluene
added after the concentration is equal to the volume of the reaction mixture after the sample is taken. e.g. after
12 hours the total volume will be 4.5 mL, because 1x 0.5 mL samples were taken.
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The solution was filtered through a plug of Celite® and concentrated under reduced pressure if
applicable. The samples were taken before each filtration and filtered through Celite®, concentrated
1
under reduced pressure. The samples were analysed by H NMR spectroscopy, reconcentrated and
dissolved in a mixture of isopropanol/hexane [10:90 (HPLC grade)] and enantioselectivity determined
by chiral HPLC.
4.3.3 General procedure for the development of one-pot procedures: dynamic interconverion
process and lipase-mediated transesterification of 2-methyl-2-nitrocyclohexanol (±)-2a or (±)-2b with
vinyl acetate as acetylating agent (analytical scale) – cycling procedure
In a typical experiment, immobilised DBU (0.5 eq., ~90 mg) was added to a solution of
nitroalcohols (±)-2a & (±)-2b in toluene (5 mL, 10 mg/mL) and the test tube was sealed and shaken at
50°C, 500 rpm for the required amount of time. CAL-B (immob) (app 100 % w/w) and vinyl acetate
(3 eq.) was added and the test tube was sealed, shaken at 400 rpm for 12 h at 30 °C. At the end of the
cycle the solution was filtered through a plug of Celite® if appropriate and the filtrate was recycled
through the process again. Before the addition of each reagent an aliquot (0.5 mL) was taken, filtered
1
through Celite®, concentrated under reduced pressure. The samples were analysed by H NMR
spectroscopy, reconcentrated and dissolved in a mixture of isopropanol/hexane [10:90 (HPLC grade)]
and enantioselectivity determined by chiral HPLC.
4.3.4 Diastereoselective lipase-mediated transesterification of 2-methyl-2-nitrocyclohexanol 2
CAL-B (immob) (88.6 mg) was added to a equimolar mixture of cis- and trans-2-methyl-2-
nitrocyclohexanol (±)-2a and (±)-2b (105.4 mg, 0.66 mmol) dissolved in vinyl acetate (5 mL). The
reaction mixture was shaken at 750 rpm at room temperature. Reaction monitoring was conducted as
follows; an aliquot (0.5 - 1 mL) of reaction mixture was isolated and filtered through Celite®, washed
1
with ethyl acetate and concentrated under reduced pressure. The sample was analysed by H NMR
1
spectroscopy. The final extraction following H NMR spectroscopy was dissolved in a mixture of
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isopropanol/hexane [10:90 (HPLC grade)] and enantioselectivity determined by chiral HPLC. The
results of the screen are summarised in Table 1.
4.4 Synthesis of 6-nitroheptanal 1
4.4.1 Methyl 5-hydroxypentanoate17
Sulfuric acid (conc. 95-97%, 1 mL, 18.76 mmol) was added to a stirred solution of δ-valerolactone
(15.00 g, 149.82 mmol) in methanol (269 mL). The reaction mixture was maintained at reflux while
stirring for 48 h and then cooled to 0 °C and NaHCO3 (1.90 g) added. The mixture was stirred for 10
min at 0 °C, then it was filtered through Celite® and the solvent removed under reduced pressure to
give the crude methyl ester (19.12 g, 97%) as a milky white oil which was used without further
purification; vmax/cm−1 (film) 3418, 2954, 1732, 1441; δH (300 MHz) 1.53-1.62 [2H, m, C(3)H2], 1.64-
1.76 [2H, m, C(4)H2], 2.36 [2H, t, J 7.4, C(2)H2], 3.52 (1H, br s, OH), 3.61 [2H, t, J 6.3, C(5)H2],
3.67 (3H, s, OCH3).
4.4.2 Methyl 5-oxopentanoate17, 18
Methyl 5-hydroxypentanoate (13.00 g, 98.36 mmol) was added to a solution of pyridinium
chlorochromate (PCC) (31.50 g, 146.39 mmol) and crushed 4Å molecular sieves (4.80 g) in
dichloromethane (473 mL) at 0 °C under nitrogen and allowed to warm slowly to room temperature.
The mixture was stirred for 2 h and then diluted with diethyl ether (250 mL). The mixture was filtered
through a sintered glass funnel containing a layer each of silica gel and Celite®. The filtrate was
concentrated under reduced pressure and the residue redissolved in diethyl ether and the filtration
process repeated to remove the remaining chromium residues. The resultant solution was concentrated
under reduced pressure to produce the crude aldehyde (9.96 g) as a light yellow oil. Purification by
vacuum distillation gave the aldehyde (6.44 g, 50%) as a clear oil; b.p. 34-49 °C at 0.1 mmHg (Lit., 18
80 °C at 0.2 mmHg); vmax/cm−1 (film) 2936, 1733, 1198; δH (300 MHz) 1.91-2.01 [2H, quintet, J 7.2,
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C(3)H2], 2.38 [2H, t, J 7.2, C(2)H2], 2.54 [2H, dt, J 1.2, 7.2, C(4)H2], 3.68 (3H, s, OCH3), 9.78 [1H, t,
J 1.2, C(1)H].
4.4.3 Methyl 5-hydroxy-6-nitroheptanoate19
Powdered potassium tert-butoxide (0.92 g, 8.21 mmol) was added to a solution of methyl 5-
oxopentanoate (5.36 g, 41.22 mmol) and nitroethane (8.8 mL, 123.83 mmol) in tert-
butanol:tetrahydrofuran (1:1, 50 mL) at 0 °C under nitrogen. The reaction mixture was stirred at room
temperature overnight, and then diluted with diethyl ether (100 mL) and water (100 mL). The solution
was transferred to a separating funnel and the layers were separated. The organic layer was washed
with a saturated aqueous solution of sodium bicarbonate (100 mL) and brine (100 mL). The combined
aqueous layers were extracted with diethyl ether (2 × 200 mL) and the combined organic layers were
dried, filtered, and concentrated under reduced pressure to give a crude equimolar mixture of nitro
alcohol diastereomers (8.44 g) as a yellow oil. Purification by column chromatography on silica gel
using hexane/ethyl acetate 90/10 as eluent gave a equimolar mixture of pure nitro alcohol
diastereomers (5.25 g, 62%) as a light yellow oil; (Found C, 46.89; H 7.37; N 6.56. C8H15NO5
requires C, 46.82; H, 7.37; N, 6.83%); vmax/cm−1 (film) 3444, 2955, 1728, 1557, 1393; δH (300 MHz)
1.38-1.62 {5H, m containing 2 × d, 1.54 [1.5H, d, J 6.9, C(7)H3 of 1 diastereomer], 1.55 [1.5H, d, J
6.9, C(7)H3 of 1 diastereomer] and [2H, m, C(4)H2 of 2 diastereomers]}, 1.63-1.96 [2H, m, C(3)H2 of
2 diastereomer], 2.39 [2H, t, J 7.1, C(2)H2 of 2 diastereomers], 2.91-2.95 (1H, OH, of 2
diastereomers), 3.68 (3H, s, OCH3, of 2 diastereomers), 3.90-3.98 [0.5H, m, C(5)H, of 1
diastereomer], 4.14-4.21 [0.5H, m, C(5)H, of 1 diastereomer], 4.47-4.59 [1H, m, C(6)H of 2
diasteromers]. δC (75.5 MHz) 12.5, 16.1 [2 x CH3, C(7)H3 of 2 diastereomer], 20.3, 21.0 [2 x CH2,
C(3)H2 of 2 diastereomer], 32.1, 32.3 [2 x CH2, C(4)H2 of 2 diastereomer], 33.3 [CH2, C(2)H2 of 2
diastereomers], 51.7 (CH3, OCH3 of 2 diastereomers), 71.7, 72.4 [2 x CH, C(5)H, of 2 diastereomers],
86.4, 87.7 [2 x CH, C(6)H, of 2 diastereomers], 174.0 [C, C(1) of 2 diastereomers].
4.4.4 Methyl 5-acetoxy-6-nitroheptanoate19 and methyl 6-nitroheptanoate 13
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A solution of an equimolar mixture of methyl 5-hydroxy-6-nitroheptanoate diastereomers (5.00 g,
24.37 mmol), DMAP (150 mg, 1.23 mmol), acetic anhydride (2.5 mL, 26.71 mmol) and diethyl ether
(62 mL) was stirred for 4 h at room temperature under nitrogen and concentrated under reduced
pressure to yield the crude equimolar mixture of nitro acetate diastereomers as a bright green oil; δH
(300 MHz) 1.548 [1.5H, d, J 6.9, C(7)H3 of 1 diastereomer] 1.554 [1.5H, d, J 6.9, C(7)H3 of 1
diastereomer], 1.58-1.80 [4H, m, C(3)H2 and C(4)H2 of 2 diastereomers], 2.06 (1.5H, s, COCH3 of 1
diastereomer) 2.09 (1.5H, s, COCH3 of 1 diastereomer), 2.25-2.47 [2H, m, C(2)H2 of 2
diastereomers], 3.68 (3H, s, OCH3 of 2 diastereomers), 4.65-4.77 [1H, m, C(5)H of 2 diasteromers],
5.29-5.35 [1H, m, C(6)H of 2 diasteromers]. A suspension of NaBH4 (0.68 g, 17.98 mmol) in
dimethylsulfoxide (63 mL) was added dropwise to the crude nitro acetates at 0 °C under nitrogen and
the solution was stirred overnight at room temperature. The reaction mixture was then acidified with
aqueous hydrochloric acid (1M), extracted with diethyl ether (3 × 100mL), dried, filtered and
concentrated under reduced pressure to yield the crude methyl ester (3.42 g, 74%) as a clear oil which
was used without further purification; vmax/cm−1 (film) 2953, 1738, 1552, 1361; δH (300 MHz) 1.26-
1.44 [2H, m, C(4)H2], 1.53 [3H, d, J 6.9, C(7)H3], 1.61-1.80 [3H, m. C(3)H2 and one of C(5)H2],
1.92-2.13 [1H, m, one of C(5)H2], 2.32 [2H, t, J 7.4, C(2)H2], 3.67 (3H, s, OCH3), 4.51-4.62 [1H, sym
m, C(6)H].
4.4.5 6-Nitroheptan-1-ol2
DIBAL-H (38 mL, 38.00 mmol, 1M solution in hexanes) was added dropwise to a solution of
methyl 6-nitroheptanoate (2.54 g, 13.45 mmol) in doubly distilled dichloromethane (100 mL) at −78
°C under nitrogen. The reaction mixture was stirred at −78 °C for 1 h, then warmed to −40 °C and
stirred for a further 1 h. The temperature was then increased to −10 °C and the reaction carefully
2
DIBAL-H selective reduction of methyl 6-nitroheptanoate to 6-nitroheptanal 1 lead to a mixture of products including the starting
material, desired aldehyde 1 and 6-nitroheptan-1-ol. Due to the lack of selectivity methyl 6-nitroheptanoate was reduced completely to 6-
nitroheptan-1-ol and subsequently oxidised to 1.
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quenched with aqueous hydrochloric acid (1M, 20 mL). Vigorous stirring and further addition of
aqueous hydrochloric acid (1M, 20 mL) was required to degrade the gelatinous nature of the
aluminium salt precipitate. The solution was transferred to a separating funnel and the layers were
separated. The aqueous layer was extracted with dichloromethane (2 × 50 mL). The combined organic
layer was washed with aqueous hydrochloric acid (1M, 100 mL), brine (100 mL), dried, filtered and
concentrated under reduced pressure in an ice cold water bath to give the crude nitro alcohol (1.67 g)
as a cloudy yellow oil. Purification by chromatography on silica gel using hexane/ethyl acetate 80/20
as eluent gave the pure nitro alcohol (1.26 g, 58%) as a clear oil; vmax/cm−1 (film) 3365, 2940, 1553,
1391; δH (300 MHz) 1.26-1.48 [4H, m, C(3)H2 and C(4)H2], 1.52-1.61 {5H, m containing 1.53 [3H, d,
J 6.6, C(7)H3] and [2H, m, C(2)H2]}, 1.64-1.79 [1H, m, one of C(5)H2], 1.86 (1H, br s, OH), 1.96-
2.08 [1H, m, one of C(5)H2], 3.63 [2H, t, J 6.5, C(1)H2], 4.52-4.63 [1H, sym m, C(6)H]; δC (75.5
MHz) 19.2 [CH3, C(7)H3], 25.2, 25.5 [2 x CH2, C(3)H2 and C(4)H2], 32.2 [CH2, C(2)H2], 35.1 [CH2,
C(5)H2], 62.5 [CH2, C(1)H2], 83.5 [CH, C(6)H]; HRMS (ES+): Exact mass calculated for C7H16NO3
[M+H]+ 162.1130 Found 162.1122.
4.4.6 6-Nitroheptanal 1
PCC (2.00 g, 9.28 mmol) and crushed 3Å molecular sieves (0.42 g) were added to a solution of 6-
nitroheptan-1-ol (0.68 g, 4.19 mmol) in doubly distilled dichloromethane (24 mL) under nitrogen at
room temperature. Stirring was continued for 4 h. The reaction mixture was diluted with diethyl ether
(100 mL) and filtered through a sintered glass funnel containing a layer each of Celite® and silica gel.
The filtrate was concentrated under reduced pressure. The resulting oil was diluted with diethyl ether
(40 mL), washed with water (2 × 50 mL), brine (50 mL), dried, filtered and concentrated under
reduced pressure to give the crude aldehyde 1 (0.58 g) as a light brown oil. Purification by column
chromatography on silica gel using hexane/ethyl acetate 90/10 as eluent gave the pure aldehyde 1
(0.44 g, 65%) as a clear oil; vmax/cm−1 (film) 2943, 1724, 1549, 1392; δH (300 MHz) 1.27-1.47 [2H, m,
C(4)H2], 1.53 [3H, d, J 6.6, C(7)H3], 1.59-1.82 [3H, m. C(3)H2 and one of C(5)H2], 1.94-2.12 [1H, m,
one of C(5)H2], 2.44-2.50 [2H, dt, J 1.2, 7.2, C(2)H2], 4.51-4.63 [1H, m, C(6)H], 9.77 [1H, t, J 1.4
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C(1)H]; δC (75.5 MHz) 19.2 [CH3, C(7)H3], 21.3 [CH2, C(3)H2], 25.2 [CH2, C(4)H2], 34.8 [CH2,
C(5)H2], 43.4 [CH2, C(2)H2], 83.2 [CH, C(6)H], 201.8 [CH, C(1)H]; HRMS (ES+): Exact mass
calculated for C7H14NO3 [M+H]+ 160.0974 Found 160.0972.
Supporting Information
Chiral HPLC conditions and spectra for the separation of all four enantiomeric pairs (±)-2a and
1
(±)-2b and (±)-3a and (±)-3b and copies of H and 13C NMR spectra of all novel compounds are
available as Supporting Information. Details of supplementary experiments and results, as indicated
throughout the manuscript, are also included in the Supporting Information.
Acknowledgements
The Irish Research Council for Science, Engineering & Technology (IRCSET), Eli Lilly and
Pfizer are gratefully acknowledged for financial support of R. E. Deasy, K. S. Eccles and S. E. Milner.
Synthesis and Solid State Pharmaceutical Centre (Science Foundation Ireland, grant number
12/RC/2275) are gratefully acknowledged for finiancial support of A. M. Foley and D. P. Gavin.
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Supplementary Information
NMR Spectra and HPLC chromatograms are included in supplementary data, as well as additional
results, as mentioned in the Results and Discussion
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