Synthesis of (±)-anatoxin-α and analogues

Article abstract of DOI:10.1016/S0040-4020(99)00909-6

A new and highly efficient synthesis of the potent nicotinic acetylcholine receptor agonist, anatoxin-α and its analogues is described, which uses a β-lactam ring opening-transannular cyclisation sequence to set up the bridged bicyclic framework of the natural product. The synthesis involves a cycloaddition of chlorosulfonyl isocyanate with cyclooctadiene followed by Boc protection of the resulting β-lactam. Reaction of the β- lactam with a variety of nucleophiles, followed by selenium-mediated cyclisation and oxidation gave the skeleton of anatoxin-α bearing various sidechains. The approach offers a flexible entry to useful quantities of anatoxin-α and its analogues.

Full text of DOI:10.1016/S0040-4020(99)00909-6

TETRAHEDRON
Pergamon
Tetrahedron 56 (2000) 309–315
Synthesis of (^)-Anatoxin-a and Analogues
*
Philip J. Parsons, Nicholas P. Camp, Neil Edwards and L. Ravi Sumoreeah
The Chemical Laboratories, School of Chemistry, Physics & Environmental Science, Arundel Building, University of Sussex, Falmer,
Brighton BN1 9QJ, UK
Received 28 January 1999; revised 22 September 1999; accepted 7 October 1999
Abstract—A new and highly efficient synthesis of the potent nicotinic acetylcholine receptor agonist, anatoxin-a and its analogues is
described, which uses a b-lactam ring opening–transannular cyclisation sequence to set up the bridged bicyclic framework of the natural
product. The synthesis involves a cycloaddition of chlorosulfonyl isocyanate with cyclooctadiene followed by Boc protection of the resulting
b-lactam. Reaction of the b-lactam with a variety of nucleophiles, followed by selenium-mediated cyclisation and oxidation gave the
skeleton of anatoxin-a bearing various sidechains. The approach offers a flexible entry to useful quantities of anatoxin-a and its analogues.
᭧ 1999 Published by Elsevier Science Ltd. All rights reserved.
Introduction
and his co-workers have also made major contributions to
this area.9,10
Since its isolation and characterisation in the 1970s,1,2
anatoxin-a (1) has stimulated the scientific community
worldwide. Whilst synthetic chemists devised methods for
the construction of the novel 9-azabicyclo[4.2.1]nonane
ring system, medicinal chemists were intrigued by its
powerful biological activity. A recent review highlights
the various syntheses of this molecule, in both optically
pure and racemic form, achieved over the last 20 years.3
As acetylcholine deficiency is implicated in disease
states such as Alzheimer’s it is hoped that analogues of
anatoxin-a, possessing lower levels of toxicity, may be
used as acetylcholine replacements in the treatment of
brain disorders. With this aim in mind, we now wish to
report our improved synthesis of (^)-anatoxin-a, which
can be applied to the synthesis of anatoxin analogues by
introducing sidechains other than methyl at an early stage.
Results and Discussion
Our retrosynthetic analysis of anatoxin-a (1) recognised the
connection between the bridging nitrogen atom and the
methyl ketone carbonyl. A further disconnection between
the bridging nitrogen and C-6 would result in b-lactam (2),
which could be easily accessed from 1,5-cyclooctadiene
(Scheme 1).
Ingestion of pond water, containing this low molecular
weight alkaloid has proven fatal to wildlife, resulting in
many deaths.4 It has been deduced that anatoxin-a mimics
the neurotransmitter acetylcholine and therefore acts as a
potent agonist for the nicotinic acetylcholine receptor
nAChR.5 Due to its resistance to degradation by the enzyme
acetylcholine esterase, anatoxin-a remains available to
overstimulate muscle tissue, resulting in respiratory paraly-
sis and ultimately death. However, despite its poisonous
nature, anatoxin-a has become a useful pharmacological
probe, providing information with regard to acetylcholine-
mediated neurotransmission. The Rapoport group has been
the leading light in the field providing many analogues to
study the binding requirements of the nAChR.6–8 Gallagher
Our key idea involved a tandem methyllithium induced
b-lactam ring opening reaction, with concomitant intra-
molecular trapping of the resultant nitrogen anion onto the
Keywords: (^)-anatoxin-a; b-lactam; transannular cyclisation.
* Corresponding author. Tel.: ϩ44-1273-678-861; fax: ϩ44-1273-677-196;
e-mail: p.j.parsons@sussex.ac.uk
Scheme 1.
0040–4020/00/$ - see front matter ᭧ 1999 Published by Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(99)00909-6

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P. J. Parsons et al. / Tetrahedron 56 (2000) 309–315
should further increase the electrophilic nature of the
b-lactam carbonyl. To our delight, treatment of the Boc-
protected b-lactam with methylmagnesium bromide in
THF at Ϫ40ЊC for 2 h provided the methyl ketone (11a)
in 75% yield as a white solid.16 Only small quantities of
over-addition products were observed with the remaining
20% unreacted starting material. With the methyl ketone
(11a) in hand, we looked at several methods for the urethane
alkene cyclisation. As described earlier, we attempted to
activate the alkene with PdCl2, Pd(OAc)2 and RhCl3.
However, under various conditions, the desired cyclisation
of the urethane nitrogen onto the alkene failed; instead we
observed isomerisation of the alkene. Organoselenium-
induced cyclisations of olefinic urethanes are well
documented17–19 and offered us a route to alkene (13) via
oxidation and elimination of the intermediate selenide.
Treatment of the methyl ketone (11a) in acetonitrile with
1 equiv. of phenylselenenyl chloride gave a spot-to-spot
conversion by TLC to the selenide (12a) within 30 min.20
The crude selenide in THF was treated with aqueous
H2O2 in the presence of NaHCO3. The resulting selenoxide
was left to stir overnight in the presence of triethylamine,
to afford the alkene (13a) in 55% yield after purification.
Purification of the selenide (12a) by flash chromatography
prior to oxidation gave an improved 71% yield of the
alkene.
Scheme 2.
alkene, in the presence of a suitable electrophile (Eϩ)
(Scheme 2).
Potentially we could introduce the methyl ketone at the
correct oxidation level and create the desired ring system
in one synthetic operation. Additionally, if the tandem reac-
tion could be achieved in the presence of Pd(II), double
bond isomerisation towards the conjugated system of
anatoxin-a could be achieved. Although our early efforts
using Pd(II) alkene activation were unsuccessful, a recent
publication demonstrated a palladium-mediated trans-
annular cyclisation.11 The b-lactam epoxide (5) was
synthesised, and we successfully achieved the desired
tandem reaction to produce the bicyclic alcohol (6) in a
moderate 40% yield (Scheme 3). Attempts to dehydrate
the alcohol (6) failed, although conversion into the corre-
sponding iodide was achieved in good yield. To our great
disappointment, elimination of the iodide also proved fruit-
less, with alkene (8) remaining elusive to us. The iodide (7)
was reduced using tributyltin hydride and the natural
product was completed12,13 using the chemistry developed
by Rapoport.14
Using the method of Grieco21 we attempted to isomerise the
alkene (13a) using Rh(III) catalysis, to provide Boc-
protected anatoxin (16a). In accordance with the report of
Rapoport22 we observed a mixture of three alkenes with
increased reaction times and the addition of various bases
failing to drive the reaction to completion. To overcome
this, the alkene was hydrogenated in excellent yield to
provide Boc-dihydroanatoxin (14a). Introduction of the
conjugated double bond was achieved using the method-
ology developed by the Rapoport group.14 This general
route was also applicable to the synthesis of analogues of
anatoxin-a, although minor modifications were required. In
the case of the n-butyl analogue, b-lactam (10) could
only be ring-opened efficiently by treatment with n-butyl-
magnesium chloride, since the corresponding bromide led to
over-addition products. The completion of the synthesis was
carried out as described for anatoxin itself. The phenyl
analogue presented no problems. In the event, the route
was carried out smoothly and served to highlight the
We now wish to report an improved synthesis of (^)-
anatoxin-a, which allows access to analogues in useful
quantities and which also facilitated the synthesis of the
postulated key intermediate, alkene (8), inaccessible via
Scheme 3. Due to the moderate yield of the tandem
b-lactam/epoxide opening and the problems encountered
with dehydration and elimination, we decided upon a step-
wise approach (Scheme 4).
Treatment of b-lactam (9)13 in acetonitrile with (Boc)2O,
and DMAP15 afforded the Boc-protected b-lactam (10) in
70% yield. We reasoned that the urethane protecting group
Scheme 3.

P. J. Parsons et al. / Tetrahedron 56 (2000) 309–315
311
Scheme 4.
generality of this approach for the synthesis of C-10
modified anatoxin analogues.
chromatography was carried out using Merck Kieselgel 60
silica gel (Merck art. no. 9385, 230–400 mesh, 0.04–
0.063 nm). Melting points (electrothermal apparatus, open
capillary) are uncorrected. NMR spectra were recorded in
CDCl3, unless otherwise stated, at 300 MHz (1H) on a
Bruker Advance AC-300 instrument and at 75.5 MHz
(13C) on the same instrument, and chemical shifts are
recorded in parts per million (d) downfield from Me4Si
(1H) or relative to CDCl3 (central line of triplet at
77.0 ppm) (13C). 1H NMR data are tabulated in order: multi-
plicity (s, singlet; d, doublet; t, triplet; m, multiplet; br,
broad), coupling constant(s) in Hertz (Hz), number of
protons. In cases where DEPT experiments were undertaken
with 13C NMR acquisitions, the carbon multiplicities are
listed as (0) quaternary; (1) methine; (2) methylene; (3)
Experimental
Reactions were conducted in flame-dried or oven-dried
(175ЊC, overnight) glassware, under a dry nitrogen
atmosphere except when noted otherwise. Solvents and
reagents were freshly distilled as follows: tetrahydrofuran
(THF) was distilled from sodium/benzophenone; dichloro-
methane, acetonitrile, and triethylamine were distilled from
CaH2. Analytical thin layer chromatography (TLC) was
performed on Merck glass-backed thin layer chromato-
graphy plates pre-coated with a 0.25 mm layer of 60 F254
silica gel containing a fluorescent indicator. Visualisation
was achieved by ultraviolet light (254 nm), iodine or by
staining with alkaline potassium permanganate solution,
phosphomolybdic acid solution (5% in ethanol) or acidic
ceric ammonium nitrate, followed by heating. Evaporation
under reduced pressure was achieved using a Bu¨chi rotary
evaporator, using water aspirator pressure. Flash column
1
methyl. Both H and 13C NMR spectra of some of the
Boc-protected intermediates are complicated by the
presence of carbamate rotamers.
3-(tert-Butoxycarbonyl)-3-azabicyclo[6.2.0]dec-7-ene-2-
one (10). A 500 mL two-necked round-bottomed flask,
equipped with a nitrogen inlet adaptor, was charged with

312
P. J. Parsons et al. / Tetrahedron 56 (2000) 309–315
b-lactam 9 (13.1 g, 87 mmol), di-tert-butyl dicarbonate
(37.8 g, 174 mmol), and DMAP (1.07 g, 8.7 mmol) in
acetonitrile (200 mL) at 0ЊC. The reaction mixture was
allowed to warm to room temperature and left to stir over-
night. The solvent was removed under reduced pressure and
the residue dissolved in dichloromethane (200 mL) and
washed with NaHSO3 ꢀ2×50 mL†; saturated NaHCO3
ꢀ1×50 mL†; and brine ꢀ1×50 mL†: The organic layer was
dried over MgSO4, filtered, and concentrated to afford a
yellow oil. Purification using flash column chromatography
on silica gel (elution with 3:7 ether–petroleum ether)
provided 10 as a white solid (15.2 g, 70%): mp 82–84ЊC
(ether–petroleum ether); TLC (2:3 ether–petroleum ether)
Rf 0.42; IR (CH2Cl2) 1800, 1717 cmϪ1; 1H NMR d 1.50 (s,
9H, Boc), 1.85–2.00 (m, 1H, alkyl), 2.02–2.18 (m, 4H,
alkyl), 2.35–2.57 (m, 3H, alkyl), 3.27–3.38 (m, 1H,
CHCyO), 4.06–4.15 (m, 1H, CHNBoc), 5.64–5.79 (m,
2H, olefinic); 13C NMR d 24.41 (2), 25.56 (2), 26.13 (2),
29.77 (2), 30.02 (3), 54.71 (1), 58.21 (1), 84.85 (0), 132.46
(1), 132.72 (1), 150.08 (0), 170.28 (0); CIMS m/z (relative
intensity) 269 (MHϩϩNH3, 29), 252 (MHϩ, 20), 213
(MHϩϪ(H3C)2CCH2ϩNH3, 65), 196 (MHϩϪ(H3C)2CCH2,
100); HRMS, m/z calcd for C10H13NO3 (MϩϪ(H3C)2CCH2)
195.0895, found 195.0897; Anal. Calcd for C14H21NO3: C,
66.91; H, 8.42; N, 5.57. Found: C, 66.94; H, 8.46; N, 5.54.
127–130ЊC (ether–petroleum ether); TLC (2:3 ether–petro-
leum ether) Rf 0.52; IR (CHCl3) 2932, 1698 cmϪ1; 1H NMR
d 1.32 (s, 9H, Boc), 1.62–1.77 (m, 2H, alkyl), 1.88–2.28
(m, 4H, alkyl), 2.33–2.63 (m, 2H, alkyl), 3.87 (br s, 1H,
CHCyO), 4.12–4.24 (m, 1H, CHNBoc), 4.54–4.69 (m, 1H,
NH), 5.58–5.79 (m, 2H, olefinic), 7.33–7.53 (m, 3H,
aromatic), 7.80 (d, Jˆ7.9 Hz, 2H, aromatic) ppm; 13C
NMR d 23.90 (2), 26.70 (2), 28.74 (3), 33.46 (2), 47.45
(1), 50.41 (0), 79.73 (0), 126.28 (2), 128.70 (1), 129.07
(1), 130.13 (1), 130.40 (1), 133.27 (1), 147.97 (0), 155.45
(0) ppm; FABMS m/z (relative intensity) 352 (MϩNaϩ, 6),
330 (MHϩ, 20), 274 (MHϩϪ(H3C)2CCH2, 38), 230
(MHϩϪBoc, 82); HRMS m/z calcd for C16H19NO3
(MϩϪ(H3C)2CCH2) 273.1365, found 273.1365.
cis-5-Valeryl-6-([tert-butoxycarbonyl]amino)cyclooctene
(11c). Prepared by the general method described above, via
addition of n-butylmagnesium chloride to b-lactam (10) and
stirring at Ϫ40ЊC for 1 h. Purification using flash column
chromatography on silica gel (elution with 2:3 ether–petro-
leum ether) afforded the title compound as a colourless oil
(3.23 g, 97%): TLC (2:3 ether–petroleum ether) Rf 0.53; IR
1
(film) 2933, 1699 cmϪ1; H NMR d 0.65–0.73 (m, 3H,
butyl), 1.01–1.16 (m, 2H, butyl), 1.20 (s, 9H, Boc), 1.29–
1.40 (m, 2H, alkyl), 1.50–1.66 (m, 2H, alkyl), 1.76–2.08
(m, 4H, alkyl), 2.10–2.39 (m, 4H, alkylϩbutyl), 2.60–2.70
(m, 1H, CHCyO), 3.90–4.03 (m, 1H, CHNBoc), 4.57–4.69
(m, 1H, NH), 5.40–5.60 (m, 2H, olefinic); 13C NMR d 13.74
(2), 22.17 (2), 22.45 (2), 25.17 (2), 28.87 (3), 33.08 (2),
40.97 (1), 52.81 (1), 79.18 (0), 129.12 (1), 130.06 (1),
154.91 (0), 212.77 (0) ppm; EIMS m/z (relative intensity)
309 (Mϩ, 2), 253 (MϩϪ(H3C)2CCH2, 65), 209 (MϩϪBoc,
50); HRMS, m/z calcd for C18H31NO3 (Mϩ) 309.2304, found
309.2312.
cis-5-Acetyl-6-([tert-butoxycarbonyl]amino)cyclooctene
(11a). This procedure constitutes a modification to the
method described by Nozoe and co-workers.16 A 500 mL
two-necked round-bottomed flask, equipped with a nitrogen
inlet adaptor, was charged with Boc protected b-lactam 10
(11.73 g, 47 mmol), in THF (200 mL) and cooled to Ϫ40ЊC.
Methylmagnesium bromide (3 M in ether, 17 mL, 51 mmol)
was added dropwise to the solution and the reaction mixture
was left to stir at Ϫ40ЊC for 2 h. After this time, the reaction
was quenched with saturated NH4Cl solution (20 mL) and
poured into ether (200 mL).The organic layer was washed
with H2O ꢀ1×50 mL†; brine ꢀ1×50 mL†; dried over MgSO4,
filtered, and concentrated to afford a colourless oil. Purifi-
cation using flash column chromatography on silica gel
(elution with 2:3 ether–petroleum ether) provided 11a as
a white solid (9.36 g, 75%): mp 83–85ЊC (ether–petroleum
ether); TLC (2:3 ether–petroleum ether) Rf 0.34; IR
2-Acetyl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
non-4-ene (13a). A 500 mL two-necked round-bottomed
flask, equipped with a nitrogen inlet adaptor, was charged
with methyl ketone 11 (7.8 g, 29 mmol), in acetonitrile
(250 mL). Phenylselenenyl chloride (5.60 g, 29 mmol)
was added portionwise to the solution and the reaction
mixture was left to stir at room temperature for 30 min.
After this time, the solvent was removed and the residue
taken up in ether (200 mL). The organic layer was washed
with saturated NaHCO3 ꢀ2×50 mL†; dried over MgSO4,
filtered, and concentrated to afford the selenide 12a as a
dark yellow oil (11.83 g, 96%).
1
(CH2Cl2) 3442, 1708, 1704 cmϪ1; H NMR d 1.43 (s, 9H,
Boc), 1.75–1.87 (m, 2H, alkyl), 1.90–2.22 (m, 4H, alkyl),
2.19 (s, 3H, methyl), 2.25–2.48 (m, 2H, alkyl), 2.75–2.86
(m, 1H, CHCyO), 4.16–4.28 (m, 1H, CHNBoc), 4.81–4.92
(br, 1H, NH), 5.59–5.80 (m, 2H, olefinic); 13C NMR d
24.26 (2), 25.44 (2), 26.40 (2), 28.76 (3), 29.20 (3), 33.75
(2), 50.25 (1), 54.24 (1), 79.83 (0), 129.71 (1), 130.89 (1),
155.55 (0), 211.0 (0); FAB m/z (relative intensity) 290
(MϩNaϩ, 10), 268 (MHϩ, 42), 212 (MHϩϪ(H3C)2CCH2,
85), 168 (MHϩϪBoc, 100); HRMS, m/z calcd for
C15H25NO3 (Mϩ) 267.1834, found 267.1814; Anal. Calcd
for C15H25NO3: C, 67.38; H, 9.42; N, 5.24. Found: C,
67.31; H, 9.58; N, 5.22.
Oxidative elimination: To a solution of the crude selenide
12a (11.83 g, 28 mmol) in THF (200 mL) was added
NaHCO3 (4.7 g, 56 mmol) and 27% aqueous H2O2
(35.3 g, 280 mmol) dropwise at 0ЊC. After stirring at room
temperature for 1 h, triethylamine (8.6 mL, 62 mmol) was
added and the reaction mixture left to stir overnight. After
this time, the reaction mixture was poured into ether
(200 mL), washed with saturated NaHCO3 ꢀ2×50 mL†;
dried over MgSO4, filtered, and concentrated to yield a
colourless oil. Purification using flash column chromato-
graphy on silica gel (elution with 2:3 ether–petroleum
ether) provided 13a as a white crystalline solid (4.1 g,
55%): mp 88–90ЊC (ether–petroleum ether); TLC (2:3
ether–petroleum ether) Rf 0.27; IR (CH2Cl2) 1712,
cis-5-Benzoyl-6-([tert-butoxycarbonyl]amino)cyclo-
octene (11b). Prepared according to the general procedure
via addition of phenylmagnesium bromide to b-lactam (10).
Purification was carried out by column chromatography on
silica gel (elution with 3:7 ether–petroleum ether) affording
the title compound as a white crystalline solid (96%): mp
1
1691 cmϪ1; H NMR d 1.45 (s, 9H, Boc), 1.60–2.30 (m,

P. J. Parsons et al. / Tetrahedron 56 (2000) 309–315
313
4H, alkyl), 2.24 and 2.30 (s, 3H, methyl), 2.41–2.58 (m, 2H,
alkyl), 2.62–2.74 (m, 1H, alkyl), 4.48–4.59 (m, 1H, bridge-
head), 4.60–4.78 (m, 1H, bridgehead), 5.56–5.70 (m, 2H,
olefinic); 13C NMR (two rotamers 4:10) d 24.18, 24.96 (2),
27.98, 28.20 (3), 28.26, 28.32 (3), 29.52, 30.44 (2), 32.07,
33.86 (2), 54.77, 55.34 (1), 57.33, 57.67 (1), 61.49, 62.50
(1), 79.40, 80.23 (0), 125.39, 126.11 (1), 132.24, 133.46 (1),
153.28, 153.84 (0), 207.63, 208.8 (0); EIMS m/z (relative
intensity) 265 (Mϩ, 13), 209 (MϩϪ(H3C)2CCH2, 32), 192
(MϩϪ(H3C)2CCHϪH2O, 35), 165 (MϩϪBoc, 48); HRMS,
m/z calcd for C15H23NO3 (Mϩ) 265.1670, found 265.1678;
Anal. Calcd for C15H23NO3: C, 67.90; H, 8.74; N, 5.28.
Found: C, 67.81; H, 8.80; N, 5.13.
dried. Filtration and evaporation provided a pale oil which
was purified by column chromatography (elution with 3:2
ether–petroleum ether) to yield 14a as a colourless oil
(4.6 g, 95%): TLC (ether–hexane) Rf 0.23; IR (neat) 1710,
1
1687 cmϪ1; H NMR d (two rotamers 9:10) 1.43 and 1.45
(s, 9H, Boc), 1.47–2.30 (m, 10H, alkyl), 2.21 and 2.25 (s,
3H, methyl), 2.49–2.53 (m, 1H, alkyl), 4.15 and 4.32 (m,
1H, bridgehead), 4.45 and 4.55 (m, 1H, bridgehead); 13C
NMR d 23.33, 23.53 (2), 27.80, 28.16 (2), 29.07, 29.24,
29.69 (3), 29.79, 29.93 (1), 34.37, 35.14, 35.43, 36.98 (2),
57.14, 57.63 (1), 62.40, 62.92 (1), 80.61, 81.51 (0), 154.62,
155.10 (0), 210.07, 211.31 (0); EIMS m/z (relative intensity)
267 (Mϩ, 13), 194 (MϩϪ(H3C)2CCH2ϪH2O, 24), 167
(MϩϪBoc, 60), 149 (13), 124 (54), 96 (55), 82 (61), 57
(100); HRMS, m/z calcd for C15H25NO3 (Mϩ) 267.1834,
found 267.1829.
2-Benzoyl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
non-4-ene (13b). Prepared according to the general pro-
cedure. Purification was carried out using flash column
chromatography (elution with 2:3 ether–petroleum ether)
providing the title compound as a white crystalline solid
(3.38 g, 42%): mp 113–116ЊC (ether–petroleum ether);
TLC (2:3 ether–petroleum ether) Rf 0.44; IR (CHCl3)
2-Benzoyl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
nonane (14b). Prepared according to the general procedure.
Purification was carried out by flash column chromato-
graphy eluting with 2:3 ether–petroleum ether, to afford
the title compound as a white solid (4.66 g, 95%): mp
133–135ЊC (ether–petroleum ether); TLC (2:3 ether–petro-
leum ether) Rf 0.44; IR (CHCl3) 2932, 1684 cmϪ1; 1H NMR
d 1.26, 1.36 (s, 9H, Boc), 1.41–2.09 (m, 10H, alkyl), 2.38–
2.51 (m, 1H, alkyl), 3.15–3.20 (m, 1H, alkyl), 4.07–4.45
(m, 2H, bridgeheads), 7.22–7.46 (m, 3H, aromatic), 7.69–
7.80 (m, 2H, aromatic) ppm; 13C NMR d 22.45 (2), 26.66,
26.70, 26.83 (2), 28.64, 28.88 (3), 33.16 (2), 34.55 (2), 35.91
(2), 56.38, 56.61, 56.81 (1), 80.13 (0), 128.91, 129.02,
129.05 (1), 133.05 (1), 136.43 (0), 153.63 (0), 200.40 (0)
ppm; EIMS m/z (relative intensity) 329 (Mϩ, 17), 273
(MϩϪ(H3C)2CCH2, 14), 256 (MϩϪ(H3C)2CCHϪH2O,
33), 229 (MϩϪBoc, 92); HRMS m/z calcd for C20H27NO3
(Mϩ) 329.1991, found 329.1200.
1
2973, 1695 cmϪ1; H NMR d 1.37, 1.43 (s, 9H, Boc),
1.64–1.91 (m, 2H, alkyl), 2.06–2.23 (m, 2H, alkyl), 2.57–
2.80 (m, 2H, alkyl), 3.60, 3.63 (s, 1H, alkyl), 4.31–4.82 (m,
2H, bridgeheads), 5.52–5.80 (m, 2H, olefinic), 7.38–7.63
(m, 3H, aromatic), 7.87 (d, Jˆ7.4 Hz, 2H, aromatic) ppm;
13C NMR d 24.30 (2), 28.48, 28.73 (3), 29.92 (2), 34.70 (2),
55.75 (1), 58.05 (1), 58.69 (1), 80.34 (0), 126.37 (1), 128.76,
128.88, 128.99, 129.07 (1), 131.99, 133.17 (1), 136.34
(0), 153.67 (0), 199.66 (0) ppm; EIMS m/z (relative
intensity) 327 (Mϩ, 37), 271 (MϩϪ(H3C)2CCH2, 24), 254
(MϩϪ(H3C)2CCHϪH2O, 16), 227 (MϩϪBoc, 64); HRMS
calcd for C16H17NO3 (MϩϪ(H3C)2CCH2) 271.1208, found
271.1212.
2-Valeryl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
non-4-ene (13c). Prepared according to the general pro-
cedure. Purification was carried out using flash column
chromatography (elution with 3:1 petroleum ether–ether)
providing the title compound as a colourless oil (366 mg,
36%): TLC (3:1 petroleum ether–ether) Rf 0.16; IR (film)
2-Valeryl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]no-
nane (14c). Prepared according to the general procedure.
Purification was carried out by flash column chromato-
graphy eluting with 3:1 petroleum ether–ether, to afford
the title compound as a colourless oil (266 mg, 77%):
TLC (1:3 ether–petroleum ether) Rf 0.16; IR (film) 2932,
1692 cmϪ1; 1H NMR d 0.78–0.90 (m, 3H, butyl), 1.11–1.29
(m, 3H, butylϩalkyl), 1.33 and 1.38 (s, 9H, Boc), 1.40–1.65
(m, 5H, butylϩalkyl), 1.66–1.88 (m, 4H, alkyl), 1.88–2.09
(m, 1H, alkyl), 2.17–2.58 (m, 4H, butylϩalkyl), 4.09–4.52
(m, 2H, bridgeheads) ppm; 13C NMR d 14.36 (3), 22.52,
22.85 (2), 26.57, 26.62, 26.68, 26.85 (2), 28.00 (2), 28.64
(3), 28.85 (1), 32.87 (2), 33.90, 34.46 (2), 36.46 (2), 39.90,
40.84 (2), 56.33, 56.54 (1), 60.85, 61.43 (1), 79.42, 80.25
(0), 153.62 (0), 211.03, 212.10 (0) ppm; EIMS m/z (relative
intensity) 309 (Mϩ, 18), 253 (MϩϪ(H3C)2CCH2, 9), 236
(MϩϪ(H3C)2CCHϪH2O, 22), 209 (MϩϪBoc, 57); HRMS
m/z calcd for C18H31NO3 (Mϩ) 309.2304, found 309.2305.
1
2961, 1696 cmϪ1; H NMR d 0.78–0.90 (m, 3H, butyl),
1.13–1.31 (m, 2H, butyl), 1.39 (s, 9H, Boc), 1.44–1.70
(m, 4H, butylϩalkyl), 1.72–1.91 (m, 1H, alkyl), 1.99–
2.12 (m, 1H, alkyl), 2.30–2.65 (m, 5H, butylϩalkyl),
4.35–4.68 (m, 2H, bridgeheads), 5.48–5.62 (m, 2H, olefi-
nic); 13C NMR d 14.34 (3), 22.81 (2), 24.67, 25.38 (2),
26.41 (2), 28.64, 28.74 (3), 29.93, 30.94 (2), 32.55, 34.58
(2), 40.05, 40.59 (2), 55.15, 55.74 (1), 57.55, 58.04 (1),
61.19, 62.58 (1), 79.72, 80.50 (0), 125.94, 126.82 (1),
132.79, 134.00 (1), 153.77 (0), 210.08 (0); EIMS m/z
307 (Mϩ, 14), 251 (MϩϪ(H3C)2CCH2, 57), 243
(MϩϪ(H3C)2CCHϪH2O, 37), 207 (MϩϪBoc, 81); HRMS
m/z calcd for C18H29NO3 (Mϩ) 307.2147, found 307.2138.
2-Acetyl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
nonane (14a). A suspension of the alkene 13a (4.8 g,
18 mmol) and 20% Pd/C (960 mg) in methanol (120 mL)
was hydrogenated (balloon) for 3 h. The catalyst was
filtered off and thoroughly washed with methanol
(50 mL), and the combined filtrates were evaporated. The
residue was diluted with ether (150 mL), washed with
saturated NaHCO3 ꢀ1×30 mL†; brine ꢀ1×30 mL†; and
9-(tert-Butoxycarbonyl)-2-((Z)-1-(dimethyl-tert-butyl-
siloxy)ethylidene)-9-azabicyclo[4.2.1]nonane (15a). This
was prepared exactly according to the method of
Rapoport,14 using 14a (610 mg, 2.3 mmol) to yield 15a as
1
a colourless oil (820 mg, 94%): H NMR d (two rotamers)
0.04, 0.09, 0.10 and 0.13 (s, 6H, TBS), 0.79 and 0.80 (s, 9H,
TBS), 1.1–2.1 (m, 9H, alkyl), 1.28 and 1.31 (s, 9H, Boc),
1.67 (s, 3H, methyl), 2.15 (dd, Jˆ14.7, 7.0 Hz, 1H, alkyl),

314
P. J. Parsons et al. / Tetrahedron 56 (2000) 309–315
4.11 and 4.23 (m, 1H, bridgehead), 4.72 and 4.76 (d,
Jˆ8.4 Hz, 1H, bridgehead).
(s, 9H, Boc), 1.56–1.88 (m, 3H, alkyl), 2.01–2.21 (m, 2H,
alkyl), 2.30–2.51 (m, 3H, alkyl), 4.24–4.30, 4.39–4.47 (m,
1H, bridgehead), 4.96–5.10 (m, 1H, bridgehead), 6.24–6.40
(m, 1H, olefinic), 7.27–7.65 (m, 5H, aromatic) ppm; 13C
NMR d 24.77 (2), 28.76 (3), 30.67 (2), 32.40 (2), 55.15
(1), 56.53 (1), 80.00 (0), 128.50 (1), 129.96 (1), 132.30
(1), 138.13 (0), 144.59 (1), 150.35 (0), 153.45 (0), 197.68
(0) ppm; EIMS m/z (relative intensity) 327 (Mϩ, 10), 271
(MϩϪ(H3C)2CCH2, 18), 254 (MϩϪ(H3C)2CCHϪH2O, 9),
227 (MϩϪBoc, 26); HRMS m/z calcd C20H25NO3 (Mϩ)
327.1834, found 327.1835.
9-(tert-Butoxycarbonyl)-2-((Z)-1-(dimethyl-tert-butyl-
siloxy)benzylidene)-9-azabicyclo[4.2.1]nonane
(15b).
This was prepared exactly according to the method of
Rapoport,14 using 14b (485 mg, 1.47 mmol) to yield 15b
as a colourless oil (94%): TLC (1:9 ether–petroleum
ether) Rf 0.29; IR (CHCl3) 2928, 1695, 1653, 1252,
1
1106 cmϪ1; H NMR d Ϫ0.19 (s, 3H, TBS), 0.00 (s, 2H,
TBS), 0.05 (s, 1H, TBS), 0.95–1.09 (m, 9H, TBS), 1.32–
1.82 (m, 13H, alkylϩBoc), 1.82–2.28 (m, 4H, alkyl), 2.32–
2.58 (m, 2H, alkyl), 4.40–4.59 (m, 1H, bridgehead), 5.13–
5.24 (m, 1H, bridgehead), 7.32–7.50 (m, 5H, aromatic)
ppm; 13C NMR d 18.50 (0), 24.48, 24.65 (2), 26.09,
26.17, 26.23 (3), 26.50, 26.65 (2), 28.54 (3), 28.93 (2),
29.03, 29.08, 29.18 (3), 32.79, 33.45, 33.74, 34.15 (2),
56.81, 56.97 (1), 78.92, 79.23 (0), 125.84, 126.23 (0),
127.89, 128.00, 128.16, 128.27 (1), 130.03, 130.16,
130.28 (1), 139.14, 139.27 (0), 143.79, 144.30 (0), 154.06
(0) ppm; EIMS m/z (relative intensity) 443 (Mϩ, 30), 386
(MϩϪtert-Bu, 18), 370 (MϩϪ(H3C)2CCHϪH2O, 14), 342
(MϩϪBoc, 32); HRMS m/z calcd for C26H41NO3Si (Mϩ)
443.2856, found 443.2840.
Deprotection was effected using the method reported by
Gallagher et al.23 The N-Boc derivative (16b) was dissolved
in 1,4-dioxane, followed by the addition of 2 M hydrochlo-
ric acid. The reaction mixture was warmed to 50ЊC for 2 h,
and then concentrated in vacuo to give an orange solid
1
(100%): IR (CH2Cl2) 3401, 2924, 1642, 1596 cmϪ1; H
NMR (500 MHz, DMSO-d6) d 1.77–1.85 (m, 1H, alkyl),
1.88–2.07 (m, 3H, alkyl), 2.09–2.19 (m, 1H, alkyl), 2.34–
2.53 (m, 2H, alkyl), 2.55–2.65 (m, 1H, alkyl), 4.17–4.27
(m, 1H, bridgehead), 4.74–4.84 (m, 1H, bridgehead), 6.70–
6.77 (m, 1H, olefinic), 7.46–7.73 (m, 5H, aromatic), 9.05–
9.20 (m, 1H, NH), 9.69 (br s, 1H, NH) ppm; 13C NMR
(125.7 MHz, DMSO-d6) d 23.35 (2), 26.86 (2), 27.21 (2),
30.23 (2), 53.67 (1), 58.19 (1), 128.43 (1), 129.52 (1),
132.35 (1), 137.02 (0), 142.26 (0), 149.81 (1), 195.74 (0)
ppm; EIMS m/z (relative intensity) 227 (Mϩ, 100), 198 (50),
184 (24), 170 (17), 122 (43); HRMS m/z calcd for C15H17NO
(Mϩ) 227.1310, found 227.1308.
9-(tert-Butoxycarbonyl)-2-((Z)-1-(dimethyl-tert-butyl-
siloxy)pentylidene)-9-azabicyclo[4.2.1]nonane (15c).
This was prepared exactly according to the method of
Rapoport,14 using 14c (266 mg, 0.9 mmol) to yield 15c as
a colourless oil (65%): TLC (3:2 petroleum ether–ether) Rf
0.60; IR (film) 2929, 1699, 1657, 1258, 1109 cmϪ1
;
1H-NMR d Ϫ0.50 to 0.09 (m, 6H, TBS), 0.70–0.88 (m,
12H, TBSϩbutyl), 1.06–1.22 (m, 6H, butylϩalkyl), 1.27
and 1.30 (s, 9H, Boc), 1.35–1.70 (m, 5H, alkyl), 1.70–
2.25 (m, 5H, butylϩalkyl), 4.04–4.27 (m, 1H, bridgehead),
4.67–4.77 (m, 1H, bridgehead) ppm; 13C NMR d 14.55 (3),
18.60 (0), 22.66 (2), 23.08 (2), 24.60 (2), 26.27 (3), 28.46
(2), 28.96 (3), 30.72 (2), 32.01 (2), 32.62 (2), 34.26 (2),
56.55 (1), 56.79 (1), 79.06 (0), 123.85 (0), 144.41 (0),
153.96 (0) ppm; EIMS m/z (relative intensity) 423 (Mϩ,
14), 350 (MϩϪ(H3C)2CCHϪH2O, 10), 322 (MϩϪBoc,
25); HRMS calcd for C24H45NO3Si (Mϩ) 423.3169, found
423.3164.
2-Valeryl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
non-2-ene (16c). This was prepared exactly according to the
method of Rapoport,14 using 15c (210 mg, 0.5 mmol) to
yield 16c as a colourless oil (73%): TLC (3:2 Petrol/Et2O)
Rf 0.41; IR (CHCl3) 2960, 2932, 1694, 1632 cmϪ1; 1H NMR
d 0.91 (t, Jˆ7.0 Hz, 3H, butyl), 1.13–1.40 (m, 13H,
Bocϩbutylϩalkyl), 1.90–2.14 (m, 2H, alkyl), 2.29–2.41
(m, 2H, alkyl), 2.45–2.60 (m, 2H, butyl), 4.10–4.38 (m,
1H), 4.94–5.10 (m, 1H), 6.60–6.77 (m, 1H) ppm; 13C
NMR d 14.35 (3), 22.93 (2), 24.56 (2), 27.58 (2), 28.85
(3), 30.12 (2), 30.87 (2), 32.02 (2), 37.35 (2), 53.64 (1),
56.00 (1), 79.69 (0), 141.26 (1), 150.27 (0), 153.55
(0), 200.73 (0) ppm; EIMS m/z (relative intensity)
307 (Mϩ, 21), 251 (MϩϪ(CH3)2CCH2, 34), 234
(MϩϪ(CH3)2CCHϪH2O, 36), 207 (MϩϪBoc, 69); HRMS
m/z calcd for C18H29NO3 (Mϩ) 307.2147, found 307.2143.
2-Acetyl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
non-2-ene (16a). This was prepared exactly according to the
method of Rapoport,14 using 15a (820 mg, 2.2 mmol) to
1
yield 16a as a colourless oil (460 mg, 81%): H NMR d
(two rotamers) 1.38 and 1.45 (s, 9H), 1.6–1.7 (m, 3H),
2.1–2.5 (m, 5H), 2.30 (s, 3H), 4.30 and 4.42 (m, 1H),
5.14 and 5.19 (m, 1H), 6.84 (t, Jˆ5.9 Hz, 1H). Boc-anatoxin
was prepared from an authentic sample of anatoxin and gave
Deprotection was carried out using a modification to the
method reported by Gallagher et al.23 The N-Boc derivative
(16c) was dissolved in 1,4-dioxane, followed by the addition
of 2 M hydrochloric acid, and the reaction mixture was
stirred at room temperature overnight. The solvent was
removed in vacuo to afford a yellow oil (100%): IR
1
identical TLC and H NMR spectrum. Boc deprotection of
(16a) was carried out using TFA to provide anatoxin, which
1
gave identical TLC and H NMR characteristics to the
(CHCl3) 3407, 2959, 1662 cmϪ1 1H NMR (500 MHz,
;
authentic sample.
DMSO-d6) d 0.86 (t, Jˆ7.3 Hz, 3H, butyl), 1.22–1.32 (m,
2H, butyl), 1.47 (pent, Jˆ7.3 Hz, 2H, butyl), 1.80 (br s, 3H,
alkyl), 1.96 (br t, Jˆ10.1 Hz, 1H, alkyl), 2.06–2.09 (m, 1H,
alkyl), 2.25–2.30 (m, 2H, alkyl), 2.58–2.63 (m, 1H, alkyl),
2.65–2.69 (m, 2H, butyl), 4.13 (br s, 1H, bridgehead), 4.84
(br s, 1H, bridgehead), 7.33–7.34 (m, 1H, olefinic), 8.71 (br
s, 1H, NH), 9.68 (br s, 1H, NH) ppm; 13C NMR (125.7 MHz,
DMSO-d6) 13.67 (3), 21.63 (2), 22.92 (2), 26.23 (2), 26.71
2-Benzoyl-9-(tert-butoxycarbonyl)-9-azabicyclo[4.2.1]-
non-2-ene (16b). This was prepared exactly according to
the method of Rapoport,14 using 15b (450 mg, 1.02 mmol)
to yield 16b as a white solid (76%): mp 142–144ЊC (ether–
petroleum ether); TLC (2:3 ether–petroleum ether) Rf 0.35;
IR (CHCl3) 2925, 1686, 1644 cmϪ1; H NMR d 1.27, 1.41
1

P. J. Parsons et al. / Tetrahedron 56 (2000) 309–315
315
9. Magnus, N. A.; Ducry, L.; Rolland, V.; Wonnacott, S.;
Gallagher, T. J. Chem. Soc., Perkin Trans. 1 1997, 2313.
10. Wright, E.; Gallagher, T.; Sharples, C. G. V.; Wonnacott, S.
Bioorg. Med. Chem. Lett. 1997, 7, 2867.
11. Oh, C. Y.; Kim, K. S.; Ham, W. H. Tetrahedron Lett. 1998, 39,
2133.
(2), 27.14 (2), 29.69 (2), 35.71 (2), 51.38 (1), 57.69 (1),
142.50 (0), 146.46 (1), 198. 76 (1) ppm; EIMS m/z (relative
intensity) 242 (6, (MϪH)ϩHCl), 206 (69, (MϪH)ϩ), 178
(17), 137 (16), 122 (25); HRMS m/z calcd for C13H20NO
((MϪH)ϩ) 206.1545, found 206.1530.
12. Parsons, P. J.; Camp, N. P.; Underwood, J. M.; Harvey, D. M.
J. Chem. Soc., Chem. Commun. 1995, 1461.
13. Parsons, P. J.; Camp, N. P.; Underwood, J. M.; Harvey, D. M.
Tetrahedron 1996, 52, 11637.
14. Sardina, F. J.; Howard, M. H.; Koskinen, A. M. P.; Rapoport,
H. J. Org. Chem. 1989, 54, 4654.
Acknowledgements
We thank Professor R.C. Cookson for his continued interest
in this work and Tocris-Cookson Ltd for generous financial
support. We thank Richard Parlitz for technical assistance
and Dr Tony Avent for obtaining NMR spectra at 500 MHz.
15. Baldwin, J. E.; Adlington, R. M.; Gollins, D. W.; Schofield,
C. J. Tetrahedron 1990, 46, 4733.
16. Ohta, T.; Hosoi, A.; Kimura, T.; Nozoe, S. Chem. Lett. 1987,
2091.
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