Investigation of a unified strategy for the synthesis of anatoxin analogues: Scope and limitations

Article abstract of DOI:10.1055/s-0029-1217021

Syntheses of the potent neurotoxins and biochemical probes anatoxin-a and homoanatoxin and several analogues by a combined two-directional synthesis-tandem reaction strategy are presented. Key steps include an oxidative desymmetrisation and a tandem Micha

Full text of DOI:10.1055/s-0029-1217021

PAPER
3775
Investigation of a Unified Strategy for the Synthesis of Anatoxin Analogues:
Scope and Limitations
Synthesis of
A
t
natoxi
e
A
na
p
logues hen J. Roe,a David L. Hughes,a Pooja Aggarwal,b Robert A. Stockman*a,b
a
School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich, NR4 7TJ, UK
School of Chemisty, University of Nottingham, Nottingham, NG7 2RD, UK
E-mail: Robert.Stockman@Nottingham.ac.uk
b
Received 11 June 2009
O
P
O
Et2O(O)P
O
R
Abstract : Syntheses of the potent neurotoxins and biochemical
probes anatoxin-a and homoanatoxin and several analogues by a
combined two-directional synthesis–tandem reaction strategy are
presented. Key steps include an oxidative desymmetrisation and a
tandem Michael–intramolecular Mannich cyclisation.
H
N
EtO
EtO
O
–
PG
N+
R
R
R'O
O
PG
1
3
N
Key words: anatoxin, homoanatoxin, two-directional, tandem, cas-
cade, Michael, iminium, desymmetrisation
O
1 R = Me, anatoxin-a
2 R = Et, homoanatoxin
4 (R' = H, Me)
PG = protecting group
Anatoxin-a was initially isolated from the blue-green al-
gae Anabena flos aquae in the 1970’s,1 after being first
identified as the cause of incidents of fatal poisoning of
wild and domestic animals by cyanobacterial blooms in
North America and Europe,2 leading to it being initially
called VFDF (very fast death factor). Subsequently, ana-
toxin-a has been isolated from several other toxic strains
of freshwater cyanobacteria.3 Homoanatoxin (2) was first
reported as a synthetic analogue of anatoxin-a by Gallagh-
er in 1992,4 and was later identified as a natural product
from strains of Oscillatoria formosa5 and Rhabidopsis
mediterranea.6 Anatoxin-a (1) and derivatives have been
the subject of significant interest7 due to their potent and
selective activity as agonists at nicotinic acetylcholine re-
ceptors (nAChR), which has led to them being used as
tools to investigate muscle and neuronal nAChR.8 Due to
their modulatory role in the brain, nAChRs have attracted
attention as potential targets for therapeutic intervention
in a range of disease states including neurodegenerative
diseases, attention deficit disorder, pain, smoking cessa-
tion, schizophrenia and epilepsy.9 Recently, we reported a
concise and efficient total synthesis of anatoxin-a and
homoanatoxin10 using a combined two-directional synthe-
sis and tandem reaction strategy which allowed both nat-
ural products to be accessed from a common intermediate.
Herein, we report in full our investigations into an expan-
sion of this work for the synthesis of several anatoxin an-
alogues using the same common intermediate.
NHPG
OH
6
5
Figure 1 Retrosynthetic analysis
keto-phosphonate used. Iminium precursor 4 would be
available through two-directional synthesis.
The key transformation in our synthesis of anatoxin-a and
homoanatoxin was a iodotrimethylsilane (TMSI)-mediat-
ed tandem iminium formation–Bayliss–Hillman type cy-
clisation, developed from previously reported HCl-
mediated cyclisations by Speckamp,11 Somfai12 and
Tanner13 on similar substrates for the formation of the
anatoxin core structure (Scheme 1).
MeO
MeO
O
P
O
Ts
Ts
EtO
EtO
N
N
O
O
NaH, THF
4
7
TMSI,
CH2Cl2, –78 °C
TMSO
Ts
N+
I
Our retrosynthetic analysis of the anatoxin framework is
shown in Figure 1. We reasoned that a Mannich and a
Horner–Wadsworth Emmons disconnective approach
would allow the use of iminium synthon 3 as a precursor
to both anatoxin-a and homologues through the choice of
Ts
O
Ts
O
N
N
DBU
toluene
70% overall
I
SYNTHESIS 2009, No. 22, pp 3775–3784
Advanced online publication: 23.09.2009
x
x.
x
x
.
2
0
0
9
8
9
DOI: 10.1055/s-0029-1217021; Art ID: P08709SS
Scheme 1 Previously developed cyclisation to N-tosyl-protected
© Georg Thieme Verlag Stuttgart · New York
anatoxin-a

3776
S. J. Roe et al.
PAPER
Table 1 Effect of Nitrogen Protecting Group on the Cyclisation to the Anatoxin Core Structure
MeO
PG
O
N
PG
1. TMSI, CH2Cl2, –78 °C
2. DBU, toluene, r.t.
N
O
7a–e
9a–e
Entry
Substrate 7
PG
TMSI (equiv)
DBU (equiv)
Product 9
9a
Yield (%)a
1
2
3
4
5
6
7
8
7a
7a
7a
7bc
7bc
7c
SES
SES
SES
Ts
1.2
2.4
2.4
2.0
2.0
2.4
2.4
2.4
–
25b
47
66
70
50d
78
15e
–
1.4
2.8
2.8
2.8
2.8
2.8
2.8
9a
9a
9bc
9bc
9c
Ts
2-NaphS
Ms
7d
7e
9d
Boc
9e
a Isolated yields.
b 51% yield of a single iodo-diastereoisomer 8 was also isolated.
c See Roe and Stockman.10
d Carried out on a 10.5 g scale with respect to starting material 7b.
e This material is prone to facile polymerisation and the yield refers to product immediately isolated after column chromatography as a single
spot by TLC. Concentration of this compound from solution leads to polymerisation.
The studies described herein investigate the substrate from Table 2, the yields for cyclisation and elimination
range for this cyclisation. In our work directed towards the steps providing the corresponding anatoxins 9 decreased
synthesis of anatoxin-a,10 we had screened around twenty as the alkyl chain substituent increased in size. Also, at-
conditions for the cyclisation reaction, of which TMSI– tempts at cyclisation with any functionality other than a
dichloromethane followed by treatment with DBU was ketone [R = -(CO)alkyl] resulted in only decomposition
found to be optimal.14 Initially, we decided to investigate and no evidence of cyclisation (entries 6–10). In the pen-
the effects of different nitrogen protecting groups on the ultimate example (entry 9) we hoped that the imide would
yields of the cyclisation. The results of these studies are be sufficiently similar to a ketone to enable cyclisation
shown in Table 1.
and also potentially facilitate a kinetic resolution. Unfor-
tunately, since this imide substrate also failed to cyclise,
this approach was not explored further. It is unclear why
the cyclisation appears to be limited to ketone substrates.
Indeed a few similar types of cyclisation in the literature
have been shown to work on enal substrates but not on ke-
tone substrates.16 Thus, in this context, the current proto-
col does provide a complimentary protocol to these other
methodologies. All but the longest alkyl chain analogue
9e (entry 5) were crystallised so that their X-ray crystal
structures could be collected (Figure 2).
An investigation into the effects of the number of equiva-
lents of TMSI used suggested that more than one equiva-
lent was necessary in order to attain good yields of
product. Also, a separate dehydrohalogenation step was
necessary. 2-Trimethylsilylethanesulfonyl (SES), tosyl
(Ts) and 2-naphthalenesulfonyl groups (2-NaphS) were
all found to give good yields of the cyclised products, al-
though mesityl (Ms) and tert-butoxycarbonyl (Boc) pro-
tected products were found to degrade under the reaction
conditions. As N-tosyl anatoxin-a had successfully been
previously deprotected, we decided to conduct the ana- Having explored the scope of the cyclisation and with a
logue studies using this protecting group.
number of anatoxin analogues in hand, we looked into
their deprotection. As in our initial work on anatoxin, we
turned to a procedure developed by Bäckvall et al. who
In order to undertake analogue syntheses,15 and to probe
the scope of the cyclisation, a series of phosphonates 11
were reacted with the N-sulfonyliminium ion cyclisation
precursor 4, to give a series of alkenes 7. These were then
subjected to the optimised cyclisation conditions, to give
N-tosyl anatoxin-a analogues 9a–e (Table 2).
had
successfully
deprotected
N-tosyl-protected
ferruginine17 via the dioxolane-protected ketone with
magnesium in methanol under sonication.18 Thus, diox-
olane protection of the ketone functionality in the ana-
logues 9 followed by treatment with 30 equivalents of
magnesium powder in methanol under sonication for 40
minutes, followed by another 30 equivalents of magne-
sium and further sonication for 30 minutes, successfully
The yields for the sequential ozonolytic desymmetrisation
and olefination steps, providing the cyclisation precursors
7, were consistently very good. However, as can be seen
Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York

PAPER
Synthesis of Anatoxin-a and Analogues
3777
cleaved the tosyl group giving the amine/dioxolane. The
latter, fortuitously, swiftly deprotected on aqueous acidic
workup affording the anatoxin analogues as shown in
Table 3.
It was found that the determining factor that dictates the
efficiency of the overall deprotection protocol was the ini-
tial ketone protection step, giving dioxolanes 12. The best
yield obtained for this step was obtained when the parent
tosyl protected anatoxin-a was simply reacted under
Dean–Stark conditions with ethylene glycol on a 1 g scale
(Table 3, entry 1). However, since the classical Dean–
Stark conditions could not be applied to the other ana-
logues (entries 2–6) because of the small scale of the reac-
tions, these analogues were reacted with ethylene glycol
under catalytic Lewis acidic conditions with BF3·Et2O in
the presence of triethyl orthoformate as a water scavenger.
In conclusion, we have developed a unified strategy for
the synthesis of anatoxin analogues from a common inter-
mediate using a TMSI-mediated tandem cyclisation. The
scope and limitations of the cyclisation have been investi-
gated and a robust procedure for the deprotection of N-ar-
ylsulfonyl anatoxin analogues has been developed.
Figure 2 Tosyl anatoxin-a analogue crystal structures
Table 2 Synthesis of N-Tosyl-Protected Anatoxin Analoguesa
O
P
H
MeO
MeO
R
MeO
PG
N
PG
N
PG
c
a
11
R
N
O
N
b
R
PG
O
9
7
10
4
10a, 4a PG = SES
10b, 4b PG = Ts
10c, 4c PG = 2-NaphS
10d, 4d PG = Ms
10e, 4e PG = Boc
Entry
Phosphonate 11c
R
Yield (%)b
1
2
11ad
11b
11c
11d
11e
11f
11g
11h
11i
–
-(CO)Me
-(CO)Et
-(CO)n-Pr
-(CO)n-Bu
-(CO)n-Pent
-CN
7be
7f
76
65
81
78
73
64
64
66
76
59f
9be
9fe
9g
9h
9i
–
70
55
52
50
20
–
3
7g
7h
7i
4
5
6
7j
7
-CO2Et
7k
7l
–
–
8
-(CO)-3-pyridyl
p087_t2-1a.eps
CHO
–
–
9
7m
7n
–
–
10
–
–
a Reagents and conditions: (a) i. O3, CH2Cl2–MeOH (5:1), –78 °C; ii. TsOH, r.t., 1.5 h; iii. NaHCO3, r.t., 15 min; iv. Me2S, r.t., 16 h; (b) NaH,
phosphonate, THF, 0 °C → r.t., 18 h; (c) i. TMSI, CH2Cl2, –78 °C → r.t.; ii. DBU, r.t., toluene.
b Isolated yield after two steps.
c Phosphonates were prepared according to previous literature.19
d Commercially available diethyl (2-oxopropyl)phosphonate was used in this example.
e See Roe and Stockman.10,
f Compound 7n was synthesised by reduction of nitrile 7j with DIBAL-H in 50% yield.
Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York

3778
S. J. Roe et al.
PAPER
Table 3 Deprotection of Anatoxin Analoguesa
Cl–
N+
HO
OH
PG
H
H
O
PG
O
O
PhH, PPTS,
Dean–Stark
N
N
O
Mg powder (2 x 30 equiv)
R
R
R
MeOH, sonication, 1 h
then HCl
or
BF3⋅Et2O, HC(OEt)3,
Δ, benzene, 23 h
9
12
13
Entry
Substrate 9
PG
R
Yield (%)b
12c
13
13 (2 steps)
1
2
3
4
5
6
9bd
9fd
9g
9h
9i
Ts
Me
12a
12b
12c
12d
12e
12f
95e
71
13a
13b
13c
13d
13e
13a
86f
94
87
84
82
67
61
44
–
Ts
Et
Ts
n-Pr
n-Bu
n-Pent
Me
70
Ts
52
g
Ts
52
–
9c
2-NaphS
90h
87
78
a Reagents and conditions for stage one: Ethylene glycol, cat. BF3·Et2O, HC(OEt)3, benzene, reflux, 23 h.
b Isolated yields.
c Two rotomers are seen in the NMR spectra for these compounds at r.t. due to hindered rotation around the C–C bond connecting the dioxolane
ring to the bicycle.
d See Roe and Stockman.10
e Reagents and conditions for stage one: Ethylene glycol, cat. PPTS, benzene, Dean–Stark.
f A sample of this material was converted into its N-Boc derivative, the data for which matched those reported in the literature.4
g Reaction not performed due to insufficient amounts of material.
h Material taken directly into the next step after purification.
H2O refers to deionised water, and brine to sat. aq NaCl. The evap-
oration of solvents was carried out on a Buchi rotary evaporator at
1H NMR (300 or 400 MHz) and 13C NMR (75 or 101 MHz) spectra
were recorded on Varian Gemini 300 or Varian 400 Lambda spec-
trometers. All NMR spectra were obtained from solutions of CDCl3,
unless otherwise stated. Signals are quoted in ppm as d-shifts down-
field from TMS (d = 0.00 ppm) and coupling constants (J) are given
in Hertz. Infrared spectra were recorded on a Perkin–Elmer 1720X
FT-IR spectrophotometer as either thin films or Nujol mulls using
KBr or NaCl plates. Elemental analyses were preformed by Mr
Stephen Boyer at the London Metropolitan University with analyt-
ical data being quoted to the nearest 0.01%. Low resolution mass
spectra EI, ES, CI and HRMS were obtained via the EPSRC Nation-
al Mass Spectroscopy Service Centre at the University of Wales,
Swansea. TLC analysis was carried out on Fluka glass-backed silica
gel 60 F254 coated plates, or Merck aluminium-backed aluminium
oxide 60 F254 coated plates, and were visualised by one or a combi-
nation of the following methods: (a) viewing under UV at 254 nm;
(b) exposure to iodine vapour; (c) exposure to aq KMnO4 solution
containing: KMnO4 (3 g), K2CO3 (20 g), aq NaOH (2 M, 5 mL),
H2O (300 mL); (d) exposure to an 8% ethanolic phosphomolybdic
acid solution; (e) exposure to an ethanolic vanillin solution, contain-
ing: 4-hydroxy-3-methoxybenzaldehyde (6 g), EtOH (250 mL) and
aq H2SO4 (12 M, 2.5 mL). Column chromatography was performed
at r.t. using Merck silica gel 60 (0.063–0.200 mm) or BDH neutral
aluminium oxide. Solvent systems are given as volume ratios. Melt-
ing points are uncorrected and were recorded using a Stuart Scien-
tific SMP1 melting point apparatus.
reduced pressure. Temperatures quoted in the reaction conditions
refer to those of the cooling or heating bath, and all reactions were
carried out under anhydrous argon or nitrogen using flame-dried
glassware.
TMSI-Mediated Cyclisation of Hemi-aminal Ethers 7 to 9a–d
and 9f–i; General Procedure
To a solution of the enone 7 (1.0 equiv) in CH2Cl2 (100 vols) at
–78 °C, was added TMSI (2.4 equiv) dropwise. The reaction mix-
ture was covered to exclude light, then allowed to warm to r.t. slow-
ly in the cardice–acetone bath over 22 h. The reaction mixture was
quenched with H2O (20 vols) and extracted with CH2Cl2 (3 × 10
vols). The combined organic layers were dried over anhydrous
Na2SO4, filtered and concentrated in vacuo to give a brown oil
which was then taken up in toluene (100 vols), and DBU (2.8 equiv)
was added. The reaction mixture was stirred at r.t. for 2 h, then con-
centrated in vacuo, and purified by column chromatography over
silica gel, affording the pure N-protected anatoxin 9.
N-SES Anatoxin-a (9a); Method I
MeOH (3.9 mL) was saturated with HCl gas at –68 °C and a drying
tube was fitted to the flask. A solution of 7a (50.0 mg, 0.138 mmol)
in MeOH (0.2 mL) was then added, and the reaction mixture was
stirred at –61 °C for 1 h and then allowed to warm to r.t. slowly in
the cardice–acetone bath over an additional 10 h. The reaction was
poured into cold sat. aq NaHCO3 (80 mL) and extracted with
CH2Cl2 (3 × 80 mL). The combined organic layers were dried over
anhydrous Na2SO4, filtered, evaporated, and purified by column
chromatography over silica gel (EtOAc–hexane, 1:2) to afford a
mixture of stage one intermediate diastereoisomers (PG = SES)
along with a small amount of product 9a (51 mg). This mixture was
dissolved in toluene (5 mL) and DBU (29 mL, 0.194 mmol) was
Unless otherwise stated, all chemicals were obtained from commer-
cial sources and used as received. THF was freshly distilled from
sodium and benzophenone; CH2Cl2 was freshly distilled from
CaH2; anhydrous MeCN, benzene, DMF, DCE, EtOH, MeOH and
toluene were obtained commercially from Aldrich, in SureSeal™
grade packaging and stored over molecular sieves under argon. Oth-
er solvents were SLR-grade and used without further purification.
Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York

PAPER
Synthesis of Anatoxin-a and Analogues
3779
added. The reaction mixture was then stirred at reflux for 6.5 h,
cooled, evaporated in vacuo, and purified by column chromatogra-
phy over silica gel (EtOAc–hexane, 1:2) to afford 9a.
Yield: 68.8 mg (15%); pale-yellow oil; Rf = 0.31 (EtOAc–hexane,
3:1).
IR (thin film): 1659 (carbonyl), 1323 and 1145 (sulfonamide) cm–1.
Yield: 17.0 mg (37%); waxy white solid; mp 50–52 °C; Rf = 0.38
(EtOAc–hexane, 1:1).
IR (CDCl3): 1664 (ketone), 1331 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d = 6.92 (t, J = 6.0 Hz, 1 H), 5.21 (d,
J = 8.7 Hz, 1 H), 4.45 (m, 1 H), 2.83 (s, 3 H), 2.67–1.96 (m, 6 H),
2.28 (s, 3 H), 1.78–1.62 (m, 2 H).
1H NMR (400 MHz, CDCl3): d = 6.93 (dd, J = 7.0, 4.9 Hz, 1 H),
5.19 (d, J = 8.7 Hz, 1 H), 4.49 (dt, J = 7.4, 3.8 Hz, 1 H), 2.88 (t, J =
7.8 Hz, 2 H), 2.61 (m, 1 H), 2.45 (m, 1 H), 2.37–1.99 (m, 3 H), 2.29
(s, 3 H), 1.81–1.67 (m, 3 H), 0.96–0.90 (m, 2 H), 0.01 (s, 9 H).
13C NMR (101 MHz, CDCl3): d = 197.5, 148.4, 143.4, 58.5, 55.3,
48.6, 33.1, 32.5, 30.4, 25.3, 24.2, 10.4, –2.0.
13C NMR (101 MHz, CDCl3): d = 197.8, 147.7, 143.9, 58.8, 55.7,
39.7, 33.2, 32.5, 30.4, 25.6, 24.5.
+
MS (CI): m/z (%) = 261.1 (100) [M + NH4 ], 244.1 (22).
HRMS: m/z [M + H+] calcd for C11H18NO3S: 244.1002; found:
244.1000.
+
MS (CI): m/z (%) = 347.2 (100) [M + NH4 ], 330.2 (20).
1-(9¢-Tosyl-9¢-azabicyclo[4.2.1]non-2¢-en-2¢-yl)butan-1-one (9g)
Following the general procedure for TMSI-promoted cyclisation,
7g (0.90 g, 2.37 mmol) was reacted. Purification by column chro-
matography over silica gel (EtOAc–hexane, 1:2) gave 9g.
HRMS: m/z [M + NH4 ] calcd for C15H31N2O3S28Si: 347.1819;
+
found: 347.1817.
Anal. Calcd for C15H27NO3SSi: C, 54.67; H, 8.26; N, 4.25. Found:
C, 54.65; H, 8.27; N, 4.32.
Yield: 0.43 g (52%); clear needles; mp 109–110 °C; Rf = 0.32
(EtOAc–hexane, 1:2).
15 (PG = SES)
IR (CDCl3): 1663 (carbonyl), 1342 and 1161 (sulfonamide) cm–1.
Rf = 0.32 and 0.23 (EtOAc–hexane, 1:2).
1H NMR (400 MHz, CDCl3): d = 7.72 (d, J = 8.3 Hz, 2 H), 7.26 (d,
J = 8.3 Hz, 2 H), 6.86 (t, J = 5.9 Hz, 1 H), 5.19 (d, J = 8.7 Hz, 1 H),
4.43 (m, 1 H), 2.71–2.50 (m, 3 H), 2.40 (s, 3 H), 2.39–2.28 (m,
1 H), 2.21–2.09 (m, 1 H), 1.82–1.33 (m, 7 H), 0.91 (t, J = 7.4 Hz,
3 H).
1H NMR (400 MHz, CDCl3): d (Characteristic proton shifts for the
intermediate chlorides; Major diastereoisomer) = 4.52 (ddd, J =
12.0, 9.8, 2.4 Hz, 1 H), 4.46 (dd, J = 10.1, 2.5 Hz, 1 H), 4.34 (m,
1 H), 3.66 (m, 1 H).
13C NMR (101 MHz, CDCl3): d = 199.8, 147.1, 143.2, 141.6, 137.2,
129.7, 127.0, 58.9, 56.6, 38.9, 33.5, 32.0, 29.8, 24.3, 21.5, 18.1,
13.8.
N-SES Anatoxin-a (9a); Method II
Following the general procedure for TMSI-promoted cyclisation,
7a (500 mg, 1.38 mmol) was reacted. Purification by column chro-
matography over silica gel (EtOAc–hexane, 1:1) gave 9a (303 mg,
66%) with identical spectral data to those produced via method I.
Characteristic spectral data for the intermediate iodide (major dia-
stereoisomer):
+
MS (CI): m/z (%) = 365.3 (52) [M + NH4 ], 348.3 (39) [M + H+],
194.2 (100).
HRMS: m/z [M + H+] calcd for C19H26NO3S: 348.1628; found:
348.1628.
Rf = 0.55 (EtOAc–hexane, 1:1).
1-(9¢-Tosyl-9¢-azabicyclo[4.2.1]non-2¢-en-2¢-yl)pentan-1-one
(9h)
Following the general procedure for TMSI-promoted cyclisation,
7h (0.95 g, 2.41 mmol) was reacted. Purification by column chro-
matography over silica gel (EtOAc–hexane, 1:2) gave 9h.
1H NMR (400 MHz, CDCl3): d = 4.60 (ddd, J = 12.1, 10.1, 1.9 Hz,
1 H), 4.48 (dd, J = 10.1, 2.6 Hz, 1 H), 4.36 (m, 1 H), 3.23 (d, J =
10.1 Hz, 1 H).
13C NMR (101 MHz, CDCl3): d = 69.9, 59.6, 58.9, 26.1.
Yield: 445 mg (50%); clear needles; mp 108–109 °C; Rf = 0.37
(EtOAc–hexane, 1:1).
IR (thin film): 1661 (carbonyl), 1342 and 1161 (sulfonamide) cm–1.
N-(2-Naphthalenesulfonyl) Anatoxin-a (9c)
Following the general procedure for TMSI-promoted cyclisation,
7c (0.48 g, 1.24 mmol) was reacted. Purification by column chro-
matography over silica gel (EtOAc–hexane, 1:1) gave 9c.
1H NMR (400 MHz, CDCl3): d = 7.71 (d, J = 7.6 Hz, 2 H), 7.25 (d,
J = 7.6 Hz, 2 H), 6.84 (t, J = 5.9 Hz, 1 H), 5.18 (d, J = 8.5 Hz, 1 H),
4.43 (m, 1 H), 2.69–2.54 (m, 3 H), 2.39 (s, 3 H), 2.38–2.29 (m,
1 H), 2.20–2.10 (m, 1 H), 1.80–1.42 (m, 7 H), 1.30 (m, 2 H), 0.89
(t, J = 7.3 Hz, 3 H).
13C NMR (101 MHz, CDCl3): d = 199.9, 147.1, 143.1, 141.6, 137.2,
129.6, 127.0, 58.9, 56.6, 36.7, 33.5, 32.0, 29.9, 26.8, 24.3, 22.4,
21.5, 13.9.
Yield: 342 mg (78%); colourless needles; mp 184–186 °C; Rf = 0.31
(EtOAc–hexane, 1:1).
IR (thin film): 1662 (carbonyl), 1341 and 1160 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d = 8.40 (s, 1 H), 7.99–7.85 (m, 3 H),
7.81 (d, J = 8.6 Hz, 1 H), 7.65–7.56 (m, 2 H), 6.86 (t, J = 5.9 Hz,
1 H), 5.33 (d, J = 8.5 Hz, 1 H), 4.52 (m, 1 H), 2.65 (m, 1 H), 2.37
(m, 1 H), 2.24 (s, 3 H), 2.23–2.14 (m, 1 H), 1.81–1.42 (m, 5 H).
13C NMR (101 MHz, CDCl3): d = 197.4, 147.4, 143.3, 137.1, 134.6,
132.1, 129.3, 129.2, 128.6, 128.0, 127.8, 127.4, 122.3, 58.9, 56.2,
33.3, 31.9, 29.8, 25.3, 24.3.
+
MS (CI): m/z (%) = 379.3 (100) [M + NH4 ], 362.3 (64) [M + H+],
208.2 (81).
HRMS: m/z [M + H+] calcd for C20H28NO3S: 362.1784; found:
362.1784.
+
MS (CI): m/z (%) = 373.2 (31) [M + NH4 ], 166.0 (100).
HRMS: m/z [M + H+] calcd for C20H22NO3S: 356.1315; found:
356.1318.
1-(9¢-Tosyl-9¢-azabicyclo[4.2.1]non-2¢-en-2¢-yl)hexan-1-one (9i)
Following the general procedure for TMSI-promoted cyclisation, 7i
(0.64 g, 1.57 mmol) was reacted. Purification by column chroma-
tography over silica gel (EtOAc–hexane, 1:2) gave 9i.
N-Mesyl Anatoxin-a (9d)
Following the general procedure for TMSI-promoted cyclisation,
7d (0.51 g, 1.86 mmol) was reacted. Purification by column chro-
matography over silica gel (EtOAc–hexane, 1:1) gave 9d.
Yield: 118 mg (20%); pale-yellow oil; Rf = 0.40 (EtOAc–hexane,
1:1).
IR (thin film): 1664 (carbonyl), 1342 and 1158 (sulfonamide) cm–1.
Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York

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S. J. Roe et al.
PAPER
1H NMR (400 MHz, CDCl3): d = 7.69 (d, J = 8.2 Hz, 2 H), 7.24 (d,
J = 8.2 Hz, 2 H), 6.83 (t, J = 5.9 Hz, 1 H), 5.18 (d, J = 8.4 Hz, 1 H),
4.42 (m, 1 H), 2.67–2.57 (m, 1 H), 2.56 (t, J = 7.2 Hz, 2 H), 2.38 (s,
3 H), 2.38–2.28 (m, 1 H), 2.18–2.08 (m, 1 H), 1.79–1.40 (m, 7 H),
1.34–1.16 (m, 4 H), 0.86 (t, J = 7.2 Hz, 3 H).
13C NMR (101 MHz, CDCl3): d = 199.9, 147.0, 143.1, 141.6, 137.2,
129.6, 126.9, 58.8, 56.5, 36.9, 33.4, 32.0, 31.4, 29.8, 24.3, 24.2,
22.4, 21.5, 13.9.
Intermediate Aldehyde 4b
In an identical reaction, purification of the crude intermediate alde-
hyde 4b by column chromatography over silica gel (EtOAc–hex-
ane, 1:2) gave 4b as predominately a single diastereoisomer ≥90%.
Yield: 72.0 mg (62%); clear oil; Rf = 0.30 (EtOAc–hexane, 1:2).
IR (CDCl3): 1724 (aldehyde), 1335 and 1146 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d (major diastereoisomer) = 9.76 (m,
1 H), 5.02 (dd, J = 7.9, 4.6 Hz, 1 H), 3.81 (m, 1 H), 3.32 (s, 3 H),
2.94–2.71 (m, 2 H), 2.66–2.35 (m, 2 H), 2.19–1.41 (m, 6 H), 0.99
(m, 2 H), 0.02 (s, 9 H).
+
MS (CI): m/z (%) = 393.4 (100) [M + NH4 ], 376.3 (63) [M + H+],
222.2 (86).
+
HRMS: m/z [M + NH4 ] calcd for C21H33N2O3S: 393.2206; found:
13C NMR (101 MHz, CDCl3): d (major diastereoisomer) = 201.7,
92.2, 59.6, 54.9, 47.4, 39.8, 32.4, 29.4, 28.6, 9.9, –2.1.
393.2202.
Ozonolytic Desymmetrisation of Cycloheptenes and Subse-
quent Olefination; General Procedure
6-[5¢-Methoxy-N-(naphthalene-2¢¢-sulfonyl)pyrrolidin-2¢-
yl]hex-3-en-2-one (7c)
Ozone was bubbled through a solution of the cycloheptene deriva-
tive (1.0 equiv) in CH2Cl2–MeOH (5:1, 6 mL) at –78 °C until a
deep-blue colouration persisted. Next, the solution was degassed
with argon, and acidified with p-toluenesulfonic acid monohydrate
(0.084 equiv), and stirred at r.t. for 1.5 h, after which the reaction
was basified with anhydrous NaHCO3 (0.34 equiv). The suspension
was stirred for 15 min, then Me2S (2.0 equiv) was added and the re-
action was stirred for an additional 18 h. The reaction mixture was
concentrated in vacuo and the crude residue was taken up in CH2Cl2
(2 × 3 mL), then washed with H2O (2 mL). The aqueous layer was
extracted with CH2Cl2 (2 × 5 mL) and the combined organic layers
were dried over anhydrous Na2SO4, filtered and concentrated in
vacuo to give the crude intermediate aldehyde 4. To NaH (60% in
mineral oil, 1.05 equiv) under an atmosphere of argon, was added
n-pentane (3 mL). The slurry was mixed thoroughly, allowed to set-
tle and the supernatant removed and the residue dried in vacuo. THF
(2 mL) was added followed by the appropriate phosphonate (1.2
equiv) dropwise. Once hydrogen evolution had ceased, the reaction
solution was cooled to 0 °C and a solution of the crude aldehyde 4
in THF (0.5 mL) was added dropwise over a period of 5 min. The
reaction was stirred at r.t. for 16 h then quenched with H2O (1 mL)
and concentrated in vacuo. The residue was taken up in CH2Cl2 (3
mL) and washed with H2O (2 mL) and brine (2 mL), then dried over
anhydrous Na2SO4, filtered, and concentrated in vacuo to give the
crude product, which was purified by column chromatography over
silica gel.
Following the general procedure for ozonolysis/olefination, 10c
(0.705 g, 2.339 mmol) and diethyl (2-oxopropyl)phosphonate (11a;
0.54 mL, 2.81 mmol) were reacted. Purification by column chroma-
tography over silica gel (EtOAc–hexane, 1:1) gave 7c as a single
diastereoisomer.
Yield: 0.658 g (73%); waxy white solid; mp 72–74 °C; Rf = 0.36
(EtOAc–hexane, 1:1).
IR (CDCl3): 1672 (ketone), 1626 (alkene), 1346 and 1160 (sulfon-
amide) cm–1.
1H NMR (400 MHz, CDCl3): d = 8.31 (d, J = 1.7 Hz, 1 H), 7.99–
7.94 (m, 2 H), 7.91 (d, J = 8.0 Hz, 1 H), 7.74 (dd, J = 8.0, 1.7 Hz,
1 H), 7.68–7.60 (m, 2 H), 6.83 (dt, J = 16.0, 6.7 Hz, 1 H), 6.10 (dt,
J = 16.0, 1.4 Hz, 1 H), 5.17 (d, J = 5.0 Hz, 1 H), 3.57 (m, 1 H), 3.46
(s, 3 H), 2.40–2.08 (m, 2 H), 2.24 (s, 3 H), 1.87–1.64 (m, 5 H), 1.11
(m, 1 H).
13C NMR (101 MHz, CDCl3): d = 198.5, 147.4, 135.2, 134.7, 132.0,
131.5, 129.6, 129.2, 128.9, 128.7, 127.9, 127.7, 122.2, 92.8, 60.2,
55.0, 35.1, 32.1, 29.3, 28.4, 26.8.
+
MS (CI): m/z (%) = 405.2 (5) [M + NH4 ], 356.2 (18), 166.0 (100).
+
HRMS: m/z [M + NH4 ] calcd for C21H29N2O4S: 405.1843; found:
405.1843.
6-(5¢-Methoxy-N-mesyl-pyrrolidin-2¢-yl)hex-3-en-2-one (7d)
Following the general procedure for ozonolysis/olefination, 10d
(638 mg, 3.37 mmol) and diethyl (2-oxopropyl)phosphonate (11a;
0.78 mL, 4.04 mmol) were reacted. Purification by column chro-
matography over silica gel (EtOAc–hexane, 3:1) gave 7d as a single
diastereoisomer.
6-(5¢-Methoxy-N-SES-pyrrolidin-2¢-yl)hex-3-en-2-one (7a)
Following the general procedure for ozonolysis/olefination, 10a
(0.50 g, 1.82 mmol) and diethyl (2-oxopropyl)phosphonate (11a;
0.42 mL, 2.18 mmol) were reacted. Purification by column chroma-
tography over silica gel (EtOAc–hexane, 1:1) gave 7a as a single
diastereoisomer.
Yield: 87 mg (63%); pale-yellow oil; Rf = 0.32 (EtOAc–hexane,
3:1).
IR (CDCl3): 1672 (ketone), 1336 and 1154 (sulfonamide) cm–1.
Yield: 519 mg (79%); clear oil; Rf = 0.40 (EtOAc–hexane, 1:1).
1H NMR (400 MHz, CDCl3): d = 6.77 (dt, J = 16.0, 6.8 Hz, 1 H),
6.06 (dt, J = 16.0, 1.5 Hz, 1 H), 4.96 (d, J = 4.7 Hz, 1 H), 3.69 (m,
1 H), 3.32 (s, 3 H), 2.82 (s, 3 H), 2.34–1.46 (m, 8 H), 2.21 (s, 3 H).
13C NMR (101 MHz, CDCl3): d = 198.5, 147.1, 131.5, 92.1, 60.4,
54.8, 38.0, 35.0, 32.3, 29.3, 28.4, 26.8.
IR (CDCl3): 1674 (ketone), 1626 (alkene), 1335 and 1145 (sulfon-
amide) cm–1.
1H NMR (400 MHz, CDCl3): d = 6.78 (dt, J = 15.9, 6.7 Hz, 1 H),
6.07 (d, J = 15.9 Hz, 1 H), 5.01 (d, J = 4.3 Hz, 1 H), 3.76 (m, 1 H),
3.33 (s, 3 H), 2.96–2.71 (m, 2 H), 2.33–1.57 (m, 8 H), 2.22 (s, 3 H),
1.10–0.92 (m, 2 H), 0.03 (s, 9 H).
+
MS (CI): m/z (%) = 293.2 (73) [M + NH4 ], 261.1 (99), 244.1 (100).
13C NMR (101 MHz, CDCl3): d = 198.5, 147.2, 131.5, 92.0, 60.1,
54.9, 47.7, 35.2, 32.4, 29.4, 28.5, 26.8, 9.9, –2.0.
+
HRMS: m/z [M + NH4 ] calcd for C12H25N2O4S: 293.1530; found:
293.1529.
+
MS (CI): m/z (%) = 379.2 (48) [M + NH4 ], 330.2 (100).
+
HRMS: m/z [M + NH4 ] calcd for C16H35N2O4S28Si: 379.2081;
found: 379.2080.
6-(5¢-Methoxy-N-Boc-pyrrolidin-2-yl)hex-3-en-2-one (7e)
Following the general procedure for ozonolysis/olefination, 10e
(276 mg, 1.31 mmol) and diethyl (2-oxopropyl)phosphonate (11a)
were reacted. Purification by column chromatography over silica
gel (EtOAc–hexane, 1:2) gave 7e as an inseparable mixture of dia-
stereoisomers.
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PAPER
Synthesis of Anatoxin-a and Analogues
3781
Yield: 234 mg (60%); colourless oil; Rf = 0.50 (EtOAc–hexane,
2.11–1.98 (m, 1 H), 1.82–1.62 (m, 4 H), 1.62–1.49 (m, 2 H), 1.35–
1:1).
1.14 (m, 4 H), 1.11–0.98 (m, 1 H), 0.85 (t, J = 7.0 Hz, 3 H).
1H NMR (400 MHz, CDCl3): d (rotomers) = 6.73 (dt, J = 15.7, 7.5
Hz, 1 H), 5.99 (d, J = 15.7 Hz, 1 H), 5.17–5.00 (m, 1 H), 3.78–3.58
(m, 1 H), 3.21 (s, 3 H), 2.14 (s, 3 H), 2.10–1.41 (m, 8 H), 1.38 (s,
9 H).
13C NMR (101 MHz, CDCl3): d = 200.8, 146.1, 143.6, 135.4, 130.5,
129.7, 127.2, 92.7, 60.2, 54.8, 40.0, 35.1, 32.0, 31.4, 29.2, 28.4,
23.9, 22.4, 21.4, 13.9.
+
MS (CI): m/z (%) = 425.3 (20) [M + NH4 ], 376.3 (100).
+
HRMS: m/z [M + NH4 ] calcd for C22H37N2O4S: 425.2469; found:
425.2469.
7-(5¢-Methoxy-N-tosyl-pyrrolidin-2¢-yl)oct-5-en-4-one (7g)
Following the general procedure for ozonolysis/olefination, 10b
(1.00 g, 3.77 mmol) and dimethyl (2-oxopentyl)phosphonate (11c;
1.02 g, 4.90 mmol) were reacted. Purification by column chroma-
tography over silica gel (EtOAc–hexane, 1:2) gave 7g as a single
diastereoisomer.
5-(5¢-Methoxy-N-tosyl-pyrrolidin-2¢-yl)pent-2-ene-nitrile (7j)
Following the general procedure for ozonolysis/olefination, 10b
(1.00 g, 3.77 mmol) and diethyl cyanomethylphosphonate (0.77
mL, 4.90 mmol) were reacted. Purification by column chromatog-
raphy over silica gel (EtOAc–hexane, 1:2) gave 7j as a 1:1.3 mix-
ture of cis/trans isomers and a single diastereoisomer.
Yield: 1.16 g (81%); colourless oil; Rf = 0.33 (EtOAc–hexane, 1:2).
IR (thin film): 1670 (carbonyl), 1344 and 1159 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d = 7.60 (d, J = 8.2 Hz, 2 H), 7.24 (d,
J = 8.2 Hz, 2 H), 6.77 (dt, J = 15.9, 6.8 Hz, 1 H), 6.05 (d, J = 15.9
Hz, 1 H), 4.97 (d, J = 5.2 Hz, 1 H), 3.47–3.36 (m, 1 H), 3.35 (s,
3 H), 2.46 (t, J = 7.3 Hz, 2 H), 2.35 (s, 3 H), 2.26–2.10 (m, 2 H),
2.08–1.97 (m, 1 H), 1.80–1.50 (m, 6 H), 1.09–0.96 (m, 1 H), 0.87 (t,
J = 7.4 Hz, 3 H).
13C NMR (101 MHz, CDCl3): d = 200.6, 146.1, 143.6, 135.4, 130.5,
129.7, 127.1, 92.7, 60.1, 54.8, 41.8, 35.1, 32.0, 29.2, 28.3, 21.4,
17.6, 13.7.
Yield: 811 mg (64%); colourless oil; Rf = 0.47 (EtOAc–hexane,
1:2).
IR (thin film): 2221 (nitrile), 1343 and 1157 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d = 7.64 (d, J = 8.2 Hz, 2 H), 7.30 (d,
J = 8.2 Hz, 2 H), 6.73 (dt, J = 16.3, 6.8 Hz, 0.56 H), 6.52 (dt, J =
10.9, 7.6 Hz, 0.44 H), 5.37 (dt, J = 16.3, 1.5 Hz, 0.56 H), 5.33 (dt,
J = 10.9, 1.1 Hz, 0.44 H), 5.01 (d, J = 5.2 Hz, 1 H), 3.52–3.42 (m,
1 H), 3.40 (s, 3 H), 2.48–2.20 (m, 2 H), 2.41 (s, 3 H), 2.15–1.91 (m,
1 H), 1.88–1.56 (m, 4 H), 1.15–0.99 (m, 1 H).
13C NMR (101 MHz, CDCl3): d = 155.1, 154.3, 143.8, 143.7, 135.3,
129.8, 127.1, 117.4, 115.8, 100.0, 99.8, 92.8, 92.7, 60.1, 59.8, 54.9,
54.8, 35.2, 34.4, 32.0, 31.9, 29.2, 29.1, 28.0, 21.4.
+
MS (CI): m/z (%) = 397.3 (4) [M + NH4 ], 348.2 (24), 194.1 (100).
+
HRMS: m/z [M + NH4 ] calcd for C20H33N2O4S: 397.2156; found:
397.2156.
+
MS (CI): m/z (%) = 352.3 (12) [M + NH4 ], 303.2 (100), 149.1 (98).
7-(5¢-Methoxy-N-tosyl-pyrrolidin-2¢-yl)non-3-en-5-one (7h)
Following the general procedure for ozonolysis/olefination, 10b
(1.00 g, 3.77 mmol) and dimethyl (2-oxohexyl)phosphonate (11d;
1.02 g, 4.90 mmol) were reacted. Purification by column chroma-
tography over silica gel (EtOAc–hexane, 1:2) gave 7h as a single
diastereoisomer.
+
HRMS: m/z [M + NH4 ] C17H26N3O3S: 352.1689; found: 352.1694.
Ethyl 5-(5¢-Methoxy-N-tosyl-pyrrolidin-2¢-yl)pent-2-enoate
(7k)
Following the general procedure for ozonolysis/olefination, 10b
(1.00 g, 3.77 mmol) and triethyl phosphonoacetate (0.97 g, 4.90
mmol) were reacted. Purification by column chromatography over
silica gel (EtOAc–hexane, 1:2) gave 7k as a single diastereoisomer.
Yield: 1.16 g (78%); colourless oil; Rf = 0.35 (EtOAc–hexane, 1:2).
IR (thin film): 1668 (carbonyl), 1344 and 1159 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d = 7.61 (d, J = 8.3 Hz, 2 H), 7.25 (d,
J = 8.3 Hz, 2 H), 6.79 (dt, J = 15.9, 6.8 Hz, 1 H), 6.07 (d, J = 15.9
Hz, 1 H), 4.98 (d, J = 5.2 Hz, 1 H), 3.48–3.38 (m, 1 H), 3.37 (s,
3 H), 2.49 (t, J = 7.6 Hz, 2 H), 2.37 (s, 3 H), 2.30–2.11 (m, 2 H),
2.10–1.98 (m, 1 H), 1.79–1.59 (m, 4 H), 1.59–1.45 (m, 2 H), 1.35–
1.13 (m, 2 H), 1.11–0.96 (m, 1 H), 0.86 (t, J = 7.4 Hz, 3 H).
13C NMR (101 MHz, CDCl3): d = 200.7, 146.1, 143.6, 135.4, 130.5,
129.7, 127.1, 92.7, 60.2, 54.8, 39.7, 35.1, 32.0, 29.2, 28.3, 26.3,
22.3, 21.4, 13.8.
Yield: 913 mg (64%); colourless oil; Rf = 0.50 (EtOAc–hexane,
1:2).
IR (thin film): 1712 (carbonyl), 1340 and 1159 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d = 7.66 (d, J = 8.2 Hz, 2 H), 7.29 (d,
J = 8.2 Hz, 2 H), 6.96 (dt, J = 15.6, 6.8 Hz, 1 H), 5.83 (dt, J = 15.6,
1.4 Hz, 1 H), 5.03 (d, J = 5.1 Hz, 1 H), 4.18 (q, J = 7.1 Hz, 2 H),
3.52–3.42 (m, 1 H), 3.40 (s, 3 H), 2.41 (s, 3 H), 2.32–2.14 (m, 2 H),
2.13–1.99 (m, 1 H), 1.84–1.63 (m, 4 H), 1.28 (t, J = 7.1 Hz, 3 H),
1.15–1.02 (m, 1 H).
13C NMR (101 MHz, CDCl3): d = 166.5, 148.2, 143.6, 135.5, 129.8,
127.2, 121.6, 92.7, 60.2, 60.1, 54.8, 35.1, 32.1, 29.3, 28.2, 21.5,
14.3.
+
MS (CI): m/z (%) = 411.3 (9) [M + NH4 ], 362.2 (41), 208.2 (100).
+
HRMS: m/z [M + NH4 ] calcd for C21H35N2O4S: 411.2312; found:
411.2309.
+
MS (CI): m/z (%) = 399.3 (19) [M + NH4 ], 350.3 (81), 196.2 (100).
7-(5¢-Methoxy-N-tosyl-pyrrolidin-2¢-yl)dec-3-en-5-one (7i)
Following the general procedure for ozonolysis/olefination, 10b
(0.53 g, 1.99 mmol) and dimethyl (2-oxoheptyl)phosphonate (11e;
0.58 g, 2.59 mmol) were reacted. Purification by column chroma-
tography over silica gel (EtOAc–hexane, 1:2) gave 7i as a single
diastereoisomer.
+
HRMS: m/z [M + NH4 ] calcd for C19H31N2O5S: 399.1948; found:
399.1949.
5-(5¢-Methoxy-N-tosyl-pyrrolidin-2¢-yl)-1-pyridin-3¢¢-yl-pent-2-
en-1-one (7l)
Following the general procedure for ozonolysis/olefination, 10b
(1.00 g, 3.77 mmol) and dimethyl (2-oxo-2-pyridin-3-ylethyl)phos-
phonate (11h; 1.12 g, 4.90 mmol) were reacted. Purification by col-
umn chromatography over silica gel (EtOAc–hexane, 5:1) gave 7l
as a single diastereoisomer.
Yield: 0.59 g (73%); colourless oil; Rf = 0.37 (EtOAc–hexane, 1:2).
IR (thin film): 1669 (carbonyl), 1344 and 1158 (sulfonamide) cm–1.
1H NMR (400 MHz, CDCl3): d = 7.62 (d, J = 8.3 Hz, 2 H), 7.26 (d,
J = 8.3 Hz, 2 H), 6.80 (dt, J = 15.9, 6.7 Hz, 1 H), 6.08 (dt, J = 15.9,
1.4 Hz, 1 H), 5.00 (d, J = 5.1 Hz, 1 H), 3.50–3.40 (m, 1 H), 3.38 (s,
3 H), 2.50 (t, J = 7.5 Hz, 2 H), 2.38 (s, 3 H), 2.31–2.12 (m, 2 H),
Yield: 1.03 g (66%); colourless oil; Rf = 0.17 (EtOAc–hexane, 5:1).
IR (thin film): 1671 (carbonyl), 1342 and 1157 (sulfonamide) cm–1.
Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York

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S. J. Roe et al.
PAPER
1H NMR (400 MHz, CDCl3): d = 9.12 (d, J = 1.5 Hz, 1 H), 8.75 (dd,
J = 4.9, 1.5 Hz, 1 H), 8.20 (dt, J = 7.9, 1.5 Hz, 1 H), 7.64 (d, J = 8.3
Hz, 2 H), 7.41 (dd, J = 7.9, 4.9 Hz, 1 H), 7.27 (d, J = 8.3 Hz, 2 H),
7.10 (dt, J = 15.4, 6.7 Hz, 1 H), 6.90 (dt, J = 15.4, 1.3 Hz, 1 H), 5.02
(d, J = 5.2 Hz, 1 H), 3.53 (m, 1 H), 3.41 (s, 3 H), 2.47–2.30 (m, 2 H),
2.38 (s, 3 H), 2.13 (m, 1 H), 1.86–1.67 (m, 4 H), 1.16–1.03 (m,
1 H).
13C NMR (101 MHz, CDCl3): d = 189.3, 153.0, 150.3, 149.8, 143.7,
135.9, 135.4, 133.0, 129.8, 127.1, 125.7, 123.5, 92.8, 60.1, 54.9,
35.0, 32.0, 29.3, 28.7, 21.4.
Preparation of Dioxolanes 12b–f; General Procedure
To a solution of the N-protected anatoxin derivative 9 (1.0 equiv) in
benzene (5 mL), was added ethylene glycol (5.0 equiv), triethyl
orthoformate (1.2 equiv) and 1 drop of BF3·Et2O and the resulting
mixture was heated at reflux for 23 h. The reaction was washed with
sat. aq NaHCO3 (5 mL), H2O (5 mL), then brine (5 mL) and dried
over anhydrous Na2SO4. The solvent was removed in vacuo and the
residue was purified by column chromatography over silica gel.
2-(2¢-Propyl[1,3]dioxolan-2¢-yl)-9-tosyl-9-azabicyclo[4.2.1]non-
2-ene (12c)
Following the general procedure for dioxolane preparation, 9g (71.9
mg, 0.207 mmol) was reacted. Purification by column chromatog-
raphy over silica gel (EtOAc–hexane, 1:3) gave 12c.
MS (CI): m/z (%) = 415.3 (100) [M + H+], 229.2 (70).
HRMS: m/z [M + NH+] calcd for C22H27N2O4S: 415.1686; found:
415.1682.
Yield: 57.0 mg (70%); colourless oil; Rf = 0.55 (EtOAc–hexane,
1:1).
4(S)-Benzyl-3-[5¢¢-(5¢-methoxy-N-tosyl-pyrrolidin-2¢-yl)pent-
2¢¢-enoyl]oxazolidin-2-one (7m)
Following the general procedure for ozonolysis/olefination, 10b
(1.00 g, 3.77 mmol) and diethyl {2-[4(S)-benzyl-2-oxo-oxazolidin-
3-yl]-2-oxo-ethyl}phosphonate (11i; 1.74 g, 4.90 mmol) were re-
acted. Purification by column chromatography over silica gel
(EtOAc–hexane, 1:2) gave 7m as a single diastereoisomer.
1H NMR (400 MHz, CDCl3): d = 7.86 (d, J = 8.0 Hz, 2 H), 7.21 (d,
J = 8.0 Hz, 2 H), 5.73 (dd, J = 7.6, 3.7 Hz, 1 H), 4.53 (d, J = 8.6 Hz,
1 H), 4.38 (m, 1 H), 3.94–3.58 (m, 4 H), 2.35 (s, 3 H), 2.29–2.09
(m, 2 H), 1.89–1.16 (m, 10 H), 0.85 (t, J = 7.4 Hz, 3 H).
13C NMR (101 MHz, CDCl3): d = 146.2, 143.1, 137.1, 129.6, 127.1,
125.9, 111.1, 64.4, 64.1, 60.1, 57.6, 38.9, 33.1, 32.9, 28.4, 23.1,
21.5, 16.9, 14.1.
Yield: 1.47 g (76%); amorphous white foam; Rf = 0.22 (EtOAc–
hexane, 1:2).
MS (CI): m/z (%) = 392.3 (100) [M + H+], 238.2 (56).
HRMS: m/z [M + H+] calcd for C21H30NO4S: 392.1890; found:
IR (thin film): 1778 (carbamate), 1688 (amide), 1354 and 1162 (sul-
fonamide) cm–1.
392.1896.
1H NMR (400 MHz, CDCl3): d = 7.69 (d, J = 8.2 Hz, 2 H), 7.36–
7.17 (m, 9 H), 5.05 (d, J = 5.1 Hz, 1 H), 4.74 (m, 1 H), 4.24–4.14
(m, 2 H), 3.57–3.48 (m, 1 H), 3.42 (s, 3 H), 3.35–3.30 (m, 1 H),
2.80 (dd, J = 13.4, 9.5 Hz, 1 H), 2.43–2.31 (m, 5 H), 2.16–2.05 (m,
1 H), 1.88–1.67 (m, 4 H), 1.17–1.05 (m, 1 H).
13C NMR (101 MHz, CDCl3): d = 164.4, 153.0, 150.0, 143.2, 135.1,
129.4, 129.0, 128.4, 126.8, 126.7, 126.0, 120.3, 92.4, 65.7, 59.9,
54.7, 54.3, 37.2, 34.7, 31.6, 28.8, 28.3, 21.0.
2-(2¢-Butyl[1,3]dioxolan-2¢-yl)-9-tosyl-9-azabicyclo[4.2.1]non-
2-ene (12d)
Following the general procedure for dioxolane preparation, 9h (193
mg, 0.534 mmol) was reacted. Purification by column chromatog-
raphy over silica gel (EtOAc–hexane, 1:3) gave 12d.
Yield: 113 mg (52%); colourless oil; Rf = 0.60 (EtOAc–hexane,
1:1).
1H NMR (400 MHz, CDCl3): d = 7.68 (d, J = 8.2 Hz, 2 H), 7.20 (d,
J = 8.2 Hz, 2 H), 5.73 (dd, J = 7.6, 3.7 Hz, 1 H), 4.53 (d, J = 8.6 Hz,
1 H), 4.37 (m, 1 H), 3.94–3.59 (m, 4 H), 2.34 (s, 3 H), 2.30–2.09
(m, 2 H), 1.90–1.12 (m, 12 H), 0.83 (t, J = 7.0 Hz, 3 H).
MS (ES): m/z (%) = 535.3 (8) [M + Na+], 481.1 (22), 79.0 (100).
+
HRMS: m/z [M + NH4 ] calcd for C27H36N3O6S: 530.2319; found:
530.2314.
5-(5¢-Methoxy-N-tosyl-pyrrolidin-2¢-yl)pent-2-enal (7n)
To a solution of nitrile 7j (20 mg, 0.060 mmol) in anhydrous toluene
(3 mL), was added DIBAL-H (1.5 M in toluene, 0.10 mL, 0.15
mmol) dropwise at –78 °C. The reaction mixture was stirred for 1.5
h under argon and then quenched with EtOAc (5 mL) and sat. aq
NH4Cl (10 mL). The aqueous layer was extracted with CH2Cl2
(3 × 10 mL), and the combined organic layers were washed with
brine (10 mL), dried over anhydrous MgSO4, and concentrated in
vacuo. Purification by column chromatography over silica gel
(EtOAc–petroleum ether, 20 → 50%) gave aldehyde 7n as a single
diastereoisomer.
13C NMR (101 MHz, CDCl3): d = 146.2, 143.1, 137.1, 129.6, 127.1,
125.9, 111.1, 64.4, 64.1, 60.1, 57.6, 36.5, 33.0, 32.9, 28.4, 25.6,
23.1, 22.7, 21.5, 14.1.
MS (CI): m/z (%) = 406.3 (100) [M + H+], 252.2 (69).
HRMS: m/z [M + H+] clacd for C22H32NO4S: 406.2047; found:
406.2047.
2-(2¢-Pentyl[1,3]dioxolan-2¢-yl)-9-tosyl-9-azabicyclo[4.2.1]non-
2-ene (12e)
Following the general procedure for dioxolane preparation, 9i (61.2
mg, 0.163 mmol) was reacted. Purification by column chromatog-
raphy over silica gel (EtOAc–hexane, 1:3) gave 12e.
Yield: 10 mg (50%); clear oil; Rf = 0.60 (EtOAc–petroleum ether,
1:1).
IR (thin film): 1689 (aldehyde), 1446, 1344, 1212, 1161, 1093,
1076 cm–1.
Yield: 35.5 mg (52%); colourless oil; Rf = 0.63 (EtOAc–hexane,
1:1).
1H NMR (300 MHz, CDCl3): d = 9.52 (d, J = 7.9 Hz, 1 H), 7.67 (d,
J = 8.2 Hz, 2 H), 7.31 (d, J = 8.2 Hz, 2 H), 6.88 (dt, J = 15.6, 6.7 Hz,
1 H), 6.14 (ddt, J = 15.6, 7.9, 1.5 Hz, 1 H), 5.04 (d, J = 5.1 Hz, 1 H),
3.53 (m, 1 H), 3.43 (s, 3 H), 2.46–2.35 (m, 1 H), 2.43 (s, 3 H), 2.16–
2.03 (m, 1 H), 1.88–1.71 (m, 5 H), 1.19–1.07 (m, 1 H).
13C NMR (75 MHz, CDCl3): d = 194.0, 157.9, 143.8, 135.5, 133.2,
129.9, 127.3, 92.9, 60.1, 55.0, 34.8, 32.1, 29.3, 28.7, 21.5.
1H NMR (400 MHz, CDCl3): d = 7.90 (d, J = 7.9 Hz, 2 H), 7.20 (d,
J = 7.9 Hz, 2 H), 5.73 (dd, J = 7.6, 3.7 Hz, 1 H), 4.54 (d, J = 8.6 Hz,
1 H), 4.37 (m, 1 H), 3.93–3.60 (m, 4 H), 2.35 (s, 3 H), 2.29–2.09
(m, 2 H), 1.91–1.03 (m, 14 H), 0.81 (m, 3 H).
13C NMR (101 MHz, CDCl3): d = 146.3, 143.1, 137.2, 129.6, 127.2,
125.9, 111.1, 64.4, 64.2, 60.1, 57.6, 36.8, 33.1, 33.0, 31.9, 28.4,
23.2, 23.1, 22.7, 21.5, 14.1.
MS (ESI): m/z (%) = 360 (100) [M + Na+], 306 (25), 355 (4).
MS (CI): m/z (%) = 420.3 (100) [M + H+], 266.2 (78).
HRMS: m/z [M + Na+] calcd for C17H23NO4SNa: 360.1240; found:
HRMS: m/z [M + H+] calcd for C23H34NO4S: 420.2203; found:
360.1247.
420.2201.
Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York

PAPER
Synthesis of Anatoxin-a and Analogues
3783
2-(2¢-Methyl[1,3]dioxolan-2¢-yl)-9-(naphthalene-2¢¢-sulfonyl)-9-
azabicyclo[4.2.1]non-2-ene (12f)
HRMS: m/z [M + H+ – HCl] calcd for C13H22NO: 208.1696; found:
208.1693.
Following the general procedure for dioxolane preparation, 9c (80.0
mg, 0.225 mmol) was reacted. Purification by column chromatog-
raphy over silica gel (EtOAc–hexane, 1:2) gave 12f which was tak-
en directly into the preparation of 13a without further analysis.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.
Yield: 80.8 mg (90%); colourless oil; Rf = 0.52 (EtOAc–hexane,
1:1).
Acknowledgment
Funding for this work was provided by EPSRC (R.A.S., P.A., S.R.).
Mass spectra were recorded by the EPSRC Mass Spectroscopy Ser-
vice, Swansea.
Preparation of ( )-Anatoxin-a Hydrochlorides 13a–d; General
Procedure
To a solution of the dioxolane 12 (1.0 equiv) in MeOH (5 mL) was
added powdered magnesium (30 equiv) and the resulting suspen-
sion was sonicated at r.t. for 40 min. Additional powdered magne-
sium (30 equiv) was then added and sonicated for a further 30 min.
The reaction was quenched with 3 N aq HCl until all solids had dis-
solved, then made basic (pH >12) by addition of solid K2CO3 and
the aqueous phase was extracted with CHCl3 (3 × 10 mL). The com-
bined organic layers were dried over anhydrous Na2SO4, filtered,
and concentrated in vacuo. The crude product was dissolved in
CHCl3 (10 mL) and anhydrous HCl gas was bubbled through the so-
lution for 10 s. Concentration in vacuo followed by purification by
column chromatography over silica gel (MeOH–CHCl3, 1:9) gave
the product as its hydrochloride salt.
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( )-1-(9¢-Azabicyclo[4.2.1]non-2¢-en-2¢-yl)butan-1-one Hydro-
chloride (13c)
Following the general procedure for preparation of anatoxin hydro-
chlorides, 12c (27.0 mg, 69.0 mmol) was reacted and purified to give
13c.
Yield: 13.8 mg (87%); colourless glass; Rf = 0.09–0.25 (MeOH–
CHCl3, 1:9).
IR (neat): 1674 (ketone), 1587 (alkene) cm–1.
1H NMR (400 MHz, CDCl3): d = 10.12–9.95 (br s, 1 H), 9.43–9.28
(br s, 1 H), 7.18–7.09 (m, 1 H), 5.28–2.16 (m, 1 H), 4.38–4.28 (m,
1 H), 2.75–2.23 (m, 6 H), 1.98–1.71 (m, 4 H), 1.69–1.50 (m, 2 H),
0.90 (t, J = 6.7 Hz, 3 H).
13C NMR (101 MHz, CDCl3): d = 198.7, 144.2, 143.5, 58.3, 52.3,
38.8, 30.3, 27.7, 27.5, 23.5, 17.9, 13.8.
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MS (ES): m/z (%) = 194.0 (100) [M + H+ – HCl].
HRMS: m/z [M + H+ – HCl] calcd for C12H20NO: 194.1539; found:
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( )-1-(9¢-Azabicyclo[4.2.1]non-2¢-en-2¢-yl)pentan-1-one Hydro-
chloride (13d)
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Following the general procedure for preparation of anatoxin hydro-
chlorides, 12d (50.0 mg, 0.138 mmol) was reacted and purified to
give 13d.
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1997, 62, 6704.
Yield: 28.3 mg (84%); colourless oil; Rf = 0.10–0.30 (MeOH–
CHCl3, 1:9).
IR (neat): 1668 (ketone), 1587 (alkene) cm–1.
1H NMR (400 MHz, CDCl3): d = 10.13–9.96 (br s, 1 H), 9.42–9.25
(br s, 1 H), 7.18–7.09 (m, 1 H), 5.26–5.18 (m, 1 H), 4.37–4.29 (m,
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1.39–1.23 (m, 2 H), 0.90 (t, J = 7.2 Hz, 3 H).
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(14) A table of conditions screened is provided in the supporting
information.
Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York

3784
S. J. Roe et al.
PAPER
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Synthesis 2009, No. 22, 3775–3784 © Thieme Stuttgart · New York