Chemoenzymatic formal synthesis of (-)- and (+)-epibatidine

Article abstract of DOI:10.1039/c2ob06904k

The cis-dihydrocatechol, derived from enzymatic cis-dihydroxylation of bromobenzene using the microorganism Pseudomonas putida UV4, was converted into (-)-epibatidine in eleven steps with complete stereocontrol. In addition, an unprecedented palladium-catalysed disproportionation reaction gave the (+)-enantiomer of an advanced key intermediate employed in a previous synthesis of epibatidine.

Full text of DOI:10.1039/c2ob06904k

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PAPER
Chemoenzymatic formal synthesis of (−)- and (+)-epibatidine†
Derek R. Boyd,a Narain D. Sharma,a Magdalena Kaik,a Peter B. A. McIntyre,a Paul J. Stevenson*a and
Christopher C. R. Allenb
Received 11th November 2011, Accepted 17th January 2012
DOI: 10.1039/c2ob06904k
The cis-dihydrocatechol, derived from enzymatic cis-dihydroxylation of bromobenzene using the
microorganism Pseudomonas putida UV4, was converted into (−)-epibatidine in eleven steps with
complete stereocontrol. In addition, an unprecedented palladium-catalysed disproportionation reaction
gave the (+)-enantiomer of an advanced key intermediate employed in a previous synthesis of epibatidine.
monohydroxylation,16 1,4-addition to chiral enones derived from
1
Introduction
(−)-quinic acid,17 organocatalytic 1,4-addition to nitroalkenes,18
and addition of allyl zinc reagents to chiral imines.19 Although
in most of these syntheses the stereoselectivity for installation of
all three chiral centres was very high it could be further improved
by recrystallisation, or recycling the unwanted stereoisomers, in
none of the synthetic approaches was the innate stereoselectivity
for generating all three chiral centres greater than 95% de/ee.
To date, over eight hundred alkaloids have been isolated from
the defensive skin secretions of frogs of the Dendrobates
family.1 Of these compounds, undoubtedly, the most important
is (−)-epibatidine (Fig. 1) a potent nicotinic receptor antagonist
isolated from the Ecuadorian poison frog Epipedobates tricolor.2
As an analgesic, (−)-epibatidine is much more potent than mor-
phine. Since its mode of action is through the nicotinic receptor,
and not the opiate receptor, it has provided a stimulus in the
search of new analgesics based around structural elements of the
(−)-epibatidine structure.3 Due to the scarcity of material, avail-
able from the natural source, (−)-epibatidine could only be
assigned a tentative structure which was confirmed later by its
synthesis.4 Subsequently, keen interest was generated in the syn-
thesis of epibatidine and within two years of its structure elucida-
tion eleven racemic syntheses were reported.5,6 To date, over
fifty syntheses and formal syntheses of epibatidine have appeared
in the literature and the work has also been reviewed.7 All the
very early routes relied on separation of enantiomers to get enan-
tioenriched material. These samples were subsequently used to:
(a) confirm the absolute configuration of (−)-epibatidine and (b)
investigate its much awaited detailed biological evaluation.
2
Results and discussion
Dioxygenase-catalysed oxidation of arene substrates provides a
direct route to a wide range of enantiopure mono- and poly-
hydroxylated bioproducts. Earlier studies of aromatic substrates,
in these and other laboratories, using mutant strains (e.g. UV4,
39D) of the soil bacterium Pseudomonas putida and Escherichia
coli recombinant strains, each containing toluene dioxygenase,
have given access to an extensive range of over 400 metab-
olites.20 These enantiopure cis-dihydrodiols are ideal synthetic
precursors for cyclohexane based natural products, as every
carbon in the ring is fully functionalised and the biotransform-
ations can be easily scaled up. Therefore it is not surprising that
these new chiral pool derivatives are being extensively used in
the synthesis of natural products,21 carbasugars,22 active pharma-
ceutical ingredients23 and as ligands in asymmetric catalysis.24
Starting from dihydrodiol 1 our chemoenzymatic route to
(−)-epibatidine is outlined (Scheme 1). Chemoselective hydro-
genation25,26 of the unsubstituted alkene bond gave the tetra-
hydrodiol 2. Previous X-ray studies have shown that in the
Nearly one quarter of the reported syntheses of epibatidine
gave enantioenriched material without resolution. A wide variety
of strategies have been employed to obtain optically active epi-
batidine including asymmetric desymmetrisation,8,9 asymmetric
protonation,10 Diels–Alder reaction with chiral allenes,11 oxazo-
lidinones12 and nitroso dienophiles,13,14 1,3-dipolar cyclo-
addition with sultam dienophiles,15 biocatalytic asymmetric
aSchool of Chemistry and Chemical Engineering, Queen’s University,
Belfast BT9 5AG, UK. E-mail: p.stevenson@qub.ac.uk; Fax: +44 (0)-
28-9097-4687; Tel: +44 (0) 28 9097 4426
bSchool of Biological Sciences, Queen’s University, Belfast BT9 5AG,
UK
†Electronic supplementary information (ESI) available: Copies of 1H
and 13C NMR spectra. See DOI: 10.1039/c2ob06904k
Fig. 1 Structures of (+) and (−)-epibatidine.
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Scheme 1 Reagents and conditions: (i) H2 (1.5 bar), Rh–G, MeOH, 2 h, rt, (90%); (ii) TBSCl, C6H5N, 12 h, rt (86%); (iii) trichloroacetyl isocya-
nate, CH2Cl2, 0.5 h, 0 °C then Na2CO3, Et2O : MeOH : H2O 4 : 4 : 1, 5 h, rt (90%); (iv) (a) Ph3P, E t 3N, CBr4, CH2Cl2 −10 °C 1.5 h. (b) [3,3] sigma-
tropic rearrangement. (c) 1% MoCl2O2, CH2Cl2, t-BuOH, 16 h, rt (76% from 3; (v) 6-chloropyridin-3-ylboronic acid, 2% PdCl2(dppf),
PhMe : EtOH : H2O 1 : 1 : 1, Na2CO3, 5 h, 100 °C (93%); (vi) TBAF, THF, 1 h (95%); (vii) PtO2, H2, 1 bar, EtOH, 5 h, rt (60%); (viii) Et3N, MsCl,
CH2Cl2, 0 °C, 20 minutes (91%); (ix) LiBr, THF, 60 °C, 36 h (78%); (x) TFA, CH2Cl2 1.5 h, rt (94%); (xi) CHCl3, 55 °C, 72 h (95%).
crystalline state the hydroxyl group in cyclic allylic alcohols
prefers to occupy a pseudo-axial position, to minimise allylic
1,2-strain with the substituent on the alkene.27 Analysis of the
proton NMR spectrum of diol (−)-2 revealed proton H-1 to be a
doublet of triplets (J 9.3, 3.9 Hz). The large diaxial coupling to
H-6 also indicated that the C-1 hydroxyl group was equatorial
and that the C-2 hydroxyl group was pseudo-axial in solution.
As axial, or pseudo-axial alcohols occupy a more crowded pos-
ition; they are much less reactive than equatorial alcohols in reac-
tions that lead to an increase in steric bulk i.e. reactions
involving the replacement of hydrogen attached to the oxygen
with a larger group.28 Thus silylation, of diol (−)-2 was chemo-
selective with the homoallylic equatorial hydroxyl group
forming the silyl ether 3. The silyl group in ether (−)-3 was
found to be prone to scrambling, on storage or attempted purifi-
cation, and as such was used immediately for the next step.
syn-Replacement of the alcohol oxygen by nitrogen with
allylic rearrangement was efficiently achieved, with complete
stereocontrol, using the Ichikawa allyl cyanate to isocyanate
[3,3]-sigmatropic rearrangement.29 An attractive feature of this
protocol is that it proceeds at low temperature and, in principle,
the isocyanate intermediate 6 could be directly converted into
any urethane N-protecting group by reaction with an appropriate
alcohol. Employing Ichikawa’s conditions, alcohol (−)-3 was
converted to urethane 4 in high yield. Urethane 4 was then de-
hydrated to furnish intermediate cyanate 5 which, under the
reaction conditions, underwent a rapid [3,3]-sigmatropic re-
arrangement to furnish the intermediate isocyanate 6 as the sole
diastereoisomer. Our first choice of N-protecting group was Boc
but, initially, the isocyanate 6 proved to be completely unreactive
towards t-butanol or its metal alkoxides. However, using the
newly developed Bruckner protocol,30 with molybdenum (VI)
dichloride dioxide as a catalyst, the isocyanate 6 reacted
smoothly with t-butanol to give the Boc derivative (+)-7. This
approach allowed the conversion of urethane 4 to Boc-derivative
7 as a one pot operation, without isolation of the sensitive iso-
cyanate 6, in an overall yield of 76%.
Suzuki cross coupling31 of alkenyl bromide (+)-7 with com-
mercially available 6-chloropyridin-3-ylboronic acid proceeded
smoothly and gave compound (+)-8. Fluoride-induced removal
of the O-silyl protecting group gave allylic alcohol (+)-9. The
next step involved catalytic stereoselective reduction of the hin-
dered, trisubstituted alkene with retention of the chloropyridine
group. It was envisaged that the N and O substituents would
direct addition of hydrogen to the alkene face anti to these
groups.32 In a previous synthesis of racemic epibatidine it was
reported that the 6-chloropyridine was prone to hydrogenolysis32
but Kibayashi et al.13 and Baker et al.6 have successfully
reduced an alkene bond in the presence of a 6-chloropyridyl
group. In our hands when hydrogenation of alkene (+)-9, using
platinum oxide as catalyst in ethanol as solvent, was allowed to
proceed to full conversion of alkene (+)-9, only intractable
material was produced. However, when the hydrogenation reac-
tion was stopped after ca. 70% completion, the desired reduction
product (−)-10 was obtained as a single diastereoisomer;
its properties were identical to those previously reported by
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for preparing epibatidine analogues for further biological
evaluation.
3
Experimental section
Scheme 2 Reagents and conditions: (i) 10% PdCl2(dppf), PhMe :
EtOH : H2O 1 : 1 : 1, Na2CO3, 6 h, 100 °C (82%).
cis-Dihydrodiol (+)-1, was obtained from the biotransformation
of bromobenzene, using the dioxygenase enzymes present in the
whole cell cultures of P. putida UV4 and the reduction to (−)-2
is as detailed elsewhere.33,26 All reagents were used as purchased
from Aldrich and 6-chloropyridin-3-ylboronic acid was donated
by Frontier Scientific. 1H-NMR and 13C-NMR spectra were
recorded on Bruker Avance DPX-300 and AV-400 instruments
and coupling constants are reported in Hz. Mass spectra were
run at 70 eV, on an AE1-MS90 Mass Spectrometer updated by
VG Autospec, using a heated inlet system. Accurate molecular
weights were determined, by the peak matching method, with
perfluorokerosene as standard. For optical rotation ([α]D)
measurements (ca. 20 °C, 10−1 degree cm2 g−1) a Perkin-Elmer
214 polarimeter was used. Flash chromatography and preparative
layer chromatography (PLC) were performed on Merck Kieselgel
type 60 (250–400 mesh) and PF254/366 respectively. Merck Kie-
selgel type 60F254 analytical plates were used for TLC.
Evans.12 Subsequently, using the Evans protocol, compound
(−)-10 was converted in a two step sequence to bromide (−)-12
with inversion of configuration. On removal of the N-protecting
group, compound 13 cyclised to afford (−)-epibatidine, whose
spectral and chiroptical properties were identical to those pre-
viously reported.12,13
Including the initial biotransformation, this synthesis, overall,
employed five reactions which generated six new stereocentres to
achieve the desired stereochemistry at the final three chiral
centres in (−)-epibatidine. In each of these stereo defining reac-
tions the product formed was a single stereoisomer, as deter-
mined by NMR spectroscopy, making the synthesis completely
stereoselective.
A potential shortcoming of the chemoenzymatic approach is
that it often leads to only one of the two possible enantiomers.
As (−)-epibatidine has pseudo symmetry in its structure, the
shift of the chloropyridine substituent to the adjacent methylene
carbon will yield the opposite enantiomer i.e. (+)-epibatidine
(Fig. 1). In practice this could be achieved by shifting the
bromine substituent in intermediate (+)-7 to the adjacent alkene
carbon (Scheme 1).
During our attempts to carry out Suzuki cross coupling of
allylic alcohol (−)-14 with 6-chloropyridin-3-yl boronic acid, an
unusual high yielding reaction was observed which resulted in
the formation of ketone (−)-15 (Scheme 2). Formally, the
alcohol was oxidised to a ketone and the carbon bromine bond
was simultaneously reduced (disproportionation).
Further investigation revealed that ketone (−)-15 was formed
even in the absence of 6-chloropyridin-3-yl boronic acid but the
palladium catalyst and the base were both essential components
of the reaction. It is presumed that the reaction involves a palla-
dium-catalysed migration of the double bond to give the enol of
the ketone which rapidly tautomerises to form a β-bromoketone.
The ketone then loses hydrogen bromide under the basic reaction
conditions to yield enone (−)-15 which is the enantiomer of the
intermediate used by Trost in the first asymmetric synthesis of
epibatidine. In the absence of a deuterium labelling experiment,
the proposed disproportionation (migration/tautomerisation/
dehydrobromination) mechanism for the formation of enone
(−)-15 remains speculative. A bromine atom can be sub-
sequently introduced, regioselectively, into the α-position of
enone (−)-15 as described by Trost and Cook.8 This novel dis-
proportionation reaction thus constitutes a formal chemoenzy-
matic synthesis of (+)-epibatidine.
(−)- (1S,6S)-2-Bromo-6-(tert-butyldimethylsilyloxy)cyclohex-2-
enol 3
tert-Butyldimethylsilyl chloride (1.76 g, 0.014 mol) was added
to a solution of cis-tetrahydrodiol (−)-2 (2.26 g, 0.012 mol) in
dry pyridine (5 mL) under a nitrogen atmosphere. The reaction
mixture was allowed to stir overnight at room temperature.
Excess of pyridine was removed under reduced pressure, the
residue dissolved in EtOAc (50 mL) and the solution washed,
successively, with a sat. solution of NaHCO3 (15 mL), brine
(15 mL) and dried (Na2SO4). Removal of solvent gave TBS
derivative 3 as a light yellow oil (3.10 g, 86%), which was used
for the next stage without further purification. Rf 0.41 (10%
EtOAc in hexanes); [α]D −43.7 (c 1.1, CHCl3); HRMS (ES):
found [M + H]+ 307.0732, C12H24O2BrSi requires 307.0729;
1H-NMR (400 MHz, CDCl3): δ 0.11 (3H, s, SiMe2), 0.12 (3H,
s, SiMe2), 0.91 (9H, s, CMe3), 1.63 (1H, m, 5-H), 1.83 (1H, m,
5′-H), 2.04 (1H, m, 4-H), 2.22 (1H, m, 4′-H), 2.78 (1H, d, J 3.7
Hz, OH), 3.94 (1H, ddd, J 10.2, 4.1, 3.4, 6-H), 4.10 (1H, appar
t, J 3.6, 1-H), 6.20 (1H, dd, J 4.8, 3.3, 3-H); 13C-NMR (CDCl3,
100 MHz): δ −4.8, −4.5, 18.1, 25.3, 25.5, 25.8 (3C), 70.8, 72.3,
121.9, 132.4; LRMS (EI): m/z 277 (30), 215 (25), 149 (100),
104 (65).
(+)-tert-Butyl-(1R,4S)-2-bromo-4-(tert-butyldimethylsilyloxy)
cyclohex-2-enylcarbamate 7
Trichloroacetyl isocyanate (0.88 mL, 7.43 mmol) was added to
an ice cooled solution of silyl ether (−)-3 (1.18, 3.84 mmol) in
CH2Cl2 (25 mL). After stirring the mixture for 0.5 h at 0 °C the
solvent was evaporated, the residue dissolved in a mixture of
MeOH (20 mL) and Et2O (5 mL) and the solution stirred at
room temperature for 5 h with aq. K2CO3 solution (1 M,
20 mL). Most of the methanol was removed under reduced
pressure and the aqueous reaction mixture extracted with Et2O
In conclusion, we have demonstrated that bromobenzene cis-
dihydrodiol (+)-1 can be converted, with complete stereocontrol
of the three new chiral centres, to (−)-epibatidine. A newly dis-
covered palladium-catalysed disproportionation reaction allows
entry into the enantiomeric series. Since a wide range of aro-
matic boronic acids are commercially available this route is ideal
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(+)-tert-Butyl-(1R,4S)-2-(6-chloropyridin-3-yl)-4-
hydroxycyclohex-2-enylcarbamate 9
(3 × 30 mL). The combined organic extract was washed with
brine (60 mL), dried (MgSO4) and concentrated under reduced
pressure to give urethane 4 as an off-white solid (1.25 g, 90%),
which was used in the next step without further purification.
To a solution of urethane 4 (1.25 g, 3.45 mmol) in anhydrous
CH2Cl2 (20 mL) were added triphenylphoshine (2.71 g,
10.3 mmol) and Et3N (2.79 mL, 20.01 mmol). The mixture was
cooled to −10 °C and CBr4 (3.43 g, 10.4 mmol) solution in dry
CH2Cl2 (30 mL) was added to it drop-wise over 1 h. The reac-
tion mixture was kept stirring at −10 °C for a further 1.5 h,
freshly distilled tert-butanol (0.51 g, 6.90 mmol) and molyb-
denum (VI) dichloride dioxide (6.9 mg, 0.034 mmol) were then
added sequentially and it was allowed to warm to room tempera-
ture. After stirring the mixture for another 16 h, it was washed
with 10% aq. NaHCO3 (20 mL), saturated brine (20 mL), dried
(Na2SO4) and concentrated in vacuo. The resulting residue was
purified by flash chromatography (5% EtOAc in hexanes) to
yield compound (+)-7 as a clear oil (1.19 g, 76%, 2 steps). Rf
0.45 (10% EtOAc in hexanes); [α]D +18.3 (c 0.7, CHCl3);
HRMS (ES): found [M + H]+ 406.1413, C17H33BrNO3Si
To an ice cooled solution of carbamate (+)-8 (0.90 g,
2.05 mmol) in THF (8 mL) was added a THF solution of tetra-
butylammonium fluoride (1 M, 4.48 mL, 4.51 mmol). The reac-
tion mixture was stirred at room temperature for 1 h under
nitrogen. Saturated aq. NH4Cl (15 mL) was then added and the
mixture extracted with EtOAc (3 × 15 mL). The combined
organic extract was dried (Na2SO4), concentrated in vacuo and
the crude product purified by flash chromatography (20% EtOAc
in hexanes, then 50% EtOAc in hexanes) to give compound
(+)-9 as an off-white low melting solid (0.63 g, 95%); m.
p. 54–56 °C; [α]D +80.3 (c 0.6, CHCl3); Rf 0.22 (50% EtOAc in
hexanes); HRMS (ES): found [M
+
H]+ 325.1308,
C16H22ClN2O3 requires 325.1319; 1H-NMR (400 MHz, CDCl3):
δ 1.37 (9H, s, OCMe3), 1.63–1.71 (2H, m, 5-H, OH) 1.84–1.95
(1H, m, 6-H), 1.96–2.10 (2H, m, 5′-H, 6′-H), 4.37 (1H, m, 4-H),
4.63–4.70 (2H, m, 1-H, NHBoc), 6.23 (1H, d, J 3.0, 3-H), 7.28
(1H, d, J 8.4, Ar-5-H), 7.70 (1H, dd, J 8.4, 2.5, Ar-4-H), 8.44
(1H, d, J 2.5, Ar-2-H); 13C-NMR (100 MHz, CDCl3): δ 27.2,
28.0, 28.3 (3C), 45.6, 66.3, 80.0, 123.8, 133.0, 133.2, 136.2,
136.4, 147.6, 150.6, 155.2; LRMS (ES): m/z 649 (10), 325
(100), 269 (40), 217 (25).
1
requires 406.1408; H-NMR (400 MHz, CDCl3): δ 0.07 (3H, s,
SiMe2), 0.08 (3H, s, SiMe2), 0.89 (9H, s, SiCMe3), 1.46 (9H, s,
OCMe3), 1.49–1.63 (1H, m, 5-H), 1.79–1.97 (3H, m, 5′-H, 6-H,
6′-H), 4.17 (1H, dddd, J 8.9, 5.0, 3.1, 1.4, 4-H), 4.28 (1H, m, 1-
H), 4.78 (1H, d, J 8.7, NHBoc), 6.13 (1H, ddd, J 3.0, 0.9, 0.9,
3-H); 13C-NMR (CDCl3, 100 MHz): δ −4.6, −4.6, 18.2, 25.9
(3C), 28.0, 28.1, 28.5 (3C), 51.7, 68.4, 79.9, 125.3, 137.3,
155.4; LRMS (EI): m/z 292 (15), 248 (10), 118 (25), 74 (100).
(−)-tert-Butyl-(1R,2R,4S)-2-(6-chloropyridin-3-yl)-4-
hydroxycyclohexylcarbamate 10
To a solution of allylic alcohol (−)-9 (100 mg, 0.304 mmol), in
EtOH (7 mL) was added PtO2 (10 mg, 10% by wt) and the
mixture stirred for 5 h under 1 atm. of hydrogen at room temp-
erature. When the reaction had progressed to 70% completion, as
(+)-tert-Butyl-(1R,4S)-4-(tert-butyldimethylsilyloxy)-2-
(6-chloropyridin-3-yl)cyclohex-2-enylcarbamate 8
1
monitored by H-NMR spectroscopy, the catalyst was removed
by filtration and the filtrate concentrated in vacuo. The residue
obtained was purified by flash chromatography (60% EtOAc in
hexanes) to give compound (−)-10, a white solid (60 mg, 60%)
and unreacted starting material (24 mg, 24%); [α]D −41.5 (c 0.5,
CHCl3); Rf 0.15 (50% EtOAc in hexanes); HRMS (ES): found
[M + Na]+ 349.1290, C16H23ClN2O3Na requires 349.1295;
1H-NMR (400 MHz, CDCl3): δ 1.26 (9H, s, OCMe3), 1.37–1.39
(1H, m, 6-H), 1.65–1.81 (2H, m, 3-H, 5-H), 1.93–2.12 (3H, m,
3′-H, 5′-H, 6′-H), 2.91 (1H, d, J 13.5, 2-H), 3.79 (1H, m, 4-H),
4.03 (1H, br s, 1-H), 4.72 (1H, d, J 9.6, NHBoc), 7.24 (1H, d, J
8.2, Ar-5-H), 7.50 (1H, dd, J 8.2, 2.4, Ar-4-H), 8.22 (1H, d, J
2.0, Ar-2-H); 13C-NMR (100 MHz, CDCl3): δ 28.2 (3C), 29.6,
29.8, 34.4, 42.2, 48.5, 69.8, 79.6, 123.9, 136.2, 138.4, 148.7,
149.8, 155.0; LRMS (ES): m/z 1387 (3), 1061 (13), 739 (10),
675 (28), 556 (70), 413 (30), 349 (100), 293 (40), 239 (25). The
spectral characteristics of compound (−)-10 were identical to
those reported.12,13
A
mixture of 6-chloropyridin-3-ylboronic acid (0.46 g,
2.95 mmol), EtOH (10 mL) and aq. Na2CO3 (2M, 8 mL) was
added to a round bottom flask containing a solution of tert-butyl
(1R,4S)-carbamate (+)-7 (1.00 g, 2.46 mmol) in toluene
(10 mL). After flushing the contents of the flask with a stream of
nitrogen, PdCl2(dppf) (40 mg, 0.055 mmol) was added and the
mixture was refluxed with stirring for 5 h under nitrogen. The
reaction mixture was transferred into a separating funnel, the
organic layer was separated and the aq. layer extracted with
EtOAc (3 × 20 mL). The combined organic extract was washed
with brine (50 mL), dried (Na2SO4) and concentrated in vacuo
to give a dark brown oil. Purification of the oil by flash chrom-
atography (5–10% EtOAc in hexanes) gave compound (+)-8 as a
white crystalline solid (1.00 g, 93%); m.p. 106–107 °C; [α]D
+36.6 (c 1.2, CHCl3); Rf 0.3 (10% EtOAc in hexanes); HRMS
(ES): found [M + H]+ 439.2192, C22H36ClN2O3Si requires
1
439.2184; H-NMR (400 MHz, CDCl3): δ 0.12 (3H, s, SiMe2),
0.13 (3H, s, SiMe2), 0.93 (9H, s, SiCMe3), 1.37 (9H, s,
OCMe3), 1.56–1.68 (1H, m, 5-H), 1.79–2.02 (3H, m, 5′-H, 6-H,
6′-H), 4.32 (1H, ddd, J 8.4, 5.6, 2.8, 4-H), 4.67 (2H, m, 1-H,
NHBoc), 6.11 (1H, d, J 2.7, 3-H), 7.27 (1H, d, J 8.5, Ar-5-H),
7.69 (1H, dd, J 8.5, 2.6, Ar-4-H), 8.44 (1H, d, J 2.6, Ar-2-H);
13C-NMR (100 MHz, CDCl3): δ −4.6, −4.4, 18.3, 26.0 (3C),
27.5, 28.0, 28.4 (3C), 45.3, 67.3, 79.9, 123.8, 133.8, 134.7,
134.7, 136.4, 147.7, 150.5, 155.2; LRMS (ES): m/z 877 (30),
439 (100), 383 (10).
(−)-(1S,3R,4R)-4-(tert-Butoxycarbonylamino)-3-(6-
chloropyridin-3-yl)cyclohexyl methanesulphonate 11
An ice cooled solution of alcohol (−)-10 (50 mg, 0.15 mmol) in
dry CH2Cl2 (4 mL) containing Et3N (0.31 mL, 2.20 mmol) was
treated with methanesulphonyl chloride (0.15 mL, 1.97 mmol).
After stirring the reaction mixture for 20 minutes at 0 °C, it was
diluted with EtOAc (20 mL) and then washed successively with
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sat. NaHCO3 (2 × 10 mL), sat. NH4Cl (2 × 10 mL) and brine
(20 mL). The organic layer was dried (Na2SO4), concentrated
in vacuo, and the yellow oil obtained was purified by PLC (50%
EtOAc in hexanes) to furnish mesylate (−)-11 as a colourless oil
(56 mg, 91%); [α]D −56.7 (c 0.96, CH2Cl2); Rf 0.2 (50% EtOAc
in hexanes); HRMS (ES): found [M+H]+ 405.1257,
C17H26ClN2O5S requires 405.1251; 1H-NMR (400 MHz,
CDCl3): δ 1.25 (9H, s, OCMe3), 1.65–1.69 (1H, m, 6-H),
1.75–1.86 (1H, m, 5-H), 1.96–2.09 (2H, m, 2-H, 5′-H),
2.15–2.23 (1H, m, 6′-H), 2.24–2.34 (1H, m, 2′-H), 2.97 (1H, br
d, J 13.8, 3-H), 3.03 (3H, s, SO2Me), 4.06 (1H, br s, 4-H), 4.76
(1H, dddd, J 11.2, 11.2, 5.3, 5.3, 1-H), 7.26 (1H, m, Ar-5-H),
7.50 (1H, dd, J 8.3, 2.5, Ar-4-H), 8.22 (1H, br s, Ar-2-H);
13C-NMR (100 MHz, CDCl3): δ 14.3, 21.1, 22.7, 28.2 (3C),
31.8, 39.0, 42.2, 60.5, 79.2, 124.0, 135.1, 138.2, 148.6, 150.1,
171.3; LRMS (ES): m/z 405 (98), 239 (32), 217 (100). The spec-
tral characteristics of compound (−)-11 were identical to those
reported.12,13
ddd, J 14.1, 12.8, 3.1, 3-H), 3.35 (1H, m, 1-H), 3.44 (1H, dt, J
12.8, 3.1, 2-H), 4.87 (1H, t, J 3.1, 4-H), 7.29 (1H, d, J 8.2, Ar-5-
H), 7.53 (1H, dd, J 8.2, 2.6, Ar-4-H), 8.28 (1H, d, J 2.6, Ar-2-
H); 13C-NMR (CDCl3,100 MHz): δ 27.8, 28.6, 29.8, 32.0, 50.1,
54.3, 124.2, 137.5, 138.4, 149.5, 149.9.
(−)-Epibatidine
A solution of amine bromide 13 (16 mg, 0.055 mmol) in CHCl3
(3 mL) was heated in a sealed tube at 55 °C for 3 days. The reac-
tion mixture was diluted with CHCl3 (30 mL) washed with 10%
aqueous K2CO3 (15 mL) and the organic layer separated. The
aq. layer was extracted with CHCl3 (20 mL), the combined
organic extract dried (K2CO3) and concentrated to give a light
yellow oil. Purification by PLC (95 : 5 : 0.5, CH2Cl2 : MeOH :
NH4OH) furnished (−)-epibatidine as a white solid (11 mg,
95%); [α]D −6.1 (c 1.0, CHCl3); HRMS (LCMS): found [M +
H]+ 209.0840, C11H14ClN2 requires 209.0840; 1H-NMR
(400 MHz, CDCl3): δ 1.48–1.66 (5H, m, 3-H, 5-H, 5′-H, 6-H,
6′-H), 1.91 (1H, dd, J 12.2, 9.3, 3′-H), 2.27 (1H, br s, NH), 2.77
(1H, dd, J 9.3, 4.9, 2-H), 3.57 (1H, br d, J 1.9, 1-H), 3.80 (1H, t,
J 4.3, 4-H), 7.23 (1H, d, J 8.3, Ar-5-H), 7.76 (1H, dd, J 8.3, 2.5,
Ar-4-H), 8.27 (1H, d, J 2.5, Ar-2-H); 13C-NMR (100 MHz,
CDCl3): δ 30.2, 31.4, 40.4, 44.7, 56.6, 62.9, 124.1, 137.8,
141.1, 148.9, 149.1. LRMS (ES): m/z 209 (62), 192 (100), 166
(54), 126 (82). The spectral data of (−)-epibatidine was in good
agreement with literature values.12,13
(−)-tert-Butyl (1R,2R,4R)-4-bromo-2-(6-chloropyridin-3-yl)
cyclohexylcarbamate 12
Anhydrous LiBr (129 mg, 1.48 mmol) was added to a solution
of mesylate (−)-11 (30 mg, 0.07 mmol) in dry THF (5 mL) and
the mixture heated at 60 °C for 36 h. The reaction mixture was
cooled, diluted with EtOAc (40 mL), washed with brine (2 ×
30 mL), dried (Na2SO4) and concentrated to give a yellow oil.
Purification of the oil by PLC (20% EtOAc in hexanes) yielded
compound (−)-12 as a white crystalline solid (23 mg, 78%);
[α]D −26.2 (c 1, CHCl3); Rf 0.36 (20% EtOAc in hexanes);
HRMS (ES): found [M + Na]+ 411.0430, C16H22ClN2O2NaBr
requires 411.0451; 1H NMR (CDCl3): δ 1.26 (9H, br s,
OCMe3), 1.84 (1H, m, 6-H), 1.96 (1H, m, 5-H), 2.06 (1H, m, 5′-
H), 2.13–2.36 (3H, m, 6′-H, 3-H, 3′-H), 3.52 (1H, dt, J 12.2,
3.4, 2-H), 4.10 (1H, br s, 1-H), 4.62 (1H, br s, NHBoc), 4.81
(1H, m, 4-H), 7.27 (1H, d, J 2.3, Ar-5-H), 7.52 (1H, dd, J 8.5,
2.3, Ar-4-H), 8.22 (1H, br s, Ar-2-H); 13C-NMR (CDCl3,
100 MHz): δ 26.5, 28.1 (3C), 29.1, 33.3, 38.4, 49.3, 51.6, 79.9,
123.9, 136.1, 138.6, 148.7, 149.9, 155.0; LRMS (ES): m/z 507
(20), 467 (30), 413 (100), 239 (60). The spectral characteristics
of compound (−)-12 were identical to those reported.12,13
(+)-tert-Butyl (1R,4S)-2-bromo-4-hydroxycyclohex-2-
enylcarbamate 14
To an ice cooled solution of silylether (+)-7 (102 mg,
0.192 mmol) in THF–H2O (2 mL, 3 : 1) was added a THF sol-
ution of tetrabutylammonium fluoride (1 M, 1 mL, 1.0 mmol)
and the mixture stirred overnight at room temperature. The sol-
vents were removed under reduced pressure and the residual
light yellow oil purified by PLC (40% EtOAc in hexanes) to
give compound (+)-14 as a clear oil (46 mg, 82%); [α]D +36.6 (c
0.50, CHCl3); Rf 0.31 (40% EtOAc in hexanes); HRMS (ES):
found [M + Na]+ 314.0352, C11H18NO3BrNa requires 314.0368;
1H-NMR (400 MHz, CDCl3): δ 1.47 (9H, s, CMe3), 1.62 (1H,
m, 5-H), 1.97 (3H, m, 5′-H, 6-H, 6′-H), 4.22 (2H, m, 1-H, 4-H),
4.86 (1H, s, NHBoc), 6.29 (1H, d, J 2.69, 3-H); 13C-NMR
(100 MHz): δ 27.5, 28.0, 28.5 (3C), 52.0, 67.4, 80.1, 127.3,
135.9, 155.4; LRMS (EI): m/z 279 (5) 277 (5), 234 (10), 236
(10), 217 (30), 175 (10), 156 (100), 94 (50).
(1R,2R,4R)-4-Bromo-2-(6-chloropyridin-3-yl)cyclohexanamine
13
Trifluoroacetic acid (0.4 mL) was added to a solution of pro-
tected amine (−)-12 (23 mg, 0.06 mmol), in dry CH2Cl2 (3 mL)
and the mixture stirred at room temperature for 1.5 h. The reac-
tion mixture was diluted with CHCl3 (25 mL), washed with 10%
aq. K2CO3 (25 mL) and the organic layer separated. The aq.
layer was extracted with CHCl3 (15 mL), the combined organic
extract dried (K2CO3) and concentrated under reduced pressure
to give the crude amine bromide 13 as light yellow oil (16 mg,
94%). It showed spectral characteristics identical to those
reported12,13 and was used for the next step without further
purification. 1H-NMR (400 MHz, CDCl3): δ 1.67 (1H, dq, J
13.5, 3.4, 6-H), 1.91 (1H, m, 5-H), 1.98 (1H, m, 3-H), 2.26 (1H,
tt, J 13.4, 3.1, 6′-H), 2.35 (1H, tt, J 13.5, 3.3, 5′-H), 2.67 (1H,
(−)- (R)-tert-Butyl 4-oxocyclohex-2-enylcarbamate 15
A slow stream of argon gas was bubbled through a mixture of
allylic alcohol (+)-14 (32 mg, 0.11 mmol), toluene (1 mL) and
aq. Na2CO3 (2 M, 0.4 mL) to displace any oxygen present in the
solution. PdCl2(dppf) (9.4 mg, 0.013 mmol) was added to the
mixture under a flow of argon. The reaction mixture was then
heated under reflux with stirring for 6 h. The solvent was
removed under reduced pressure, the residue taken up in EtOAc
(5 mL), washed with brine (2 × 5 mL), dried (Na2SO4) and con-
centrated to give the crude product 15 as a yellow oil.
2778 | Org. Biomol. Chem., 2012, 10, 2774–2779
This journal is © The Royal Society of Chemistry 2012

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Purification by PLC (20% EtOAc in hexane) yielded the enone
(−)-15 as a white solid (19 mg, 82%); m.p. 117–118 °C, lit. m.p.
117–118; Rf 0.21 (20% EtOAc in hexane); [α]D +120.0, (c 0.50,
CH2Cl2); lit. (ent) [α]D −123.6 (c 1.14, CH2Cl2);8 HRMS (EI):
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155.4, 198.4. LRMS (EI): m/z 155 (48), 138 (20), 95 (50), 94
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Acknowledgements
We gratefully acknowledge financial support from DEL
(PABMI), The Queen’s University of Belfast and Dr Robert
Cundell, Frontier Scientific, for a generous gift of 6-chloropyri-
din-3-ylboronic acid. We would also like to thank Dr Colin Mc
Roberts and Mr Stewart Floyd for providing mass spectra.
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