Biomimetic studies towards the C28-C40 polycyclic domain of the azaspiracids

Article abstract of DOI:10.1016/S0040-4039(00)02156-0

An acyclic intermediate representing a putative biomimetic precursor of the C28-C40 domain of the novel marine toxin azaspiracid was constructed convergently from C28-C34 and C35-C40 fragments. In studying the assembly of the C28-C34 dioxabicyclo[3.3.1]nonane system via an intramolecular hetero-conjugate addition upon a C34-C36 enone, a stereoselective C-Michael addition intervened to provide a highly substituted cyclohexane.

Full text of DOI:10.1016/S0040-4039(00)02156-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 817–820
Biomimetic studies towards the C28–C40 polycyclic domain of
the azaspiracids
Josep Aiguade, Junliang Hao and Craig J. Forsyth*
Department of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA
Received 5 October 2000; revised 13 November 2000; accepted 14 November 2000
Abstract—An acyclic intermediate representing a putative biomimetic precursor of the C28–C40 domain of the novel marine toxin
azaspiracid was constructed convergently from C28–C34 and C35–C40 fragments. In studying the assembly of the C28–C34
dioxabicyclo[3.3.1]nonane system via an intramolecular hetero-conjugate addition upon a C34–C36 enone, a stereoselective
C-Michael addition intervened to provide a highly substituted cyclohexane. © 2001 Elsevier Science Ltd. All rights reserved.
Azaspiracid (1) is a marine natural product that was
isolated from the blue mussel Mytilus edulis as the
causative agent of human poisonings.1 The symptoms
resembled those of diarrhetic shellfish poisoning2 (DSP)
but the concentrations of the major DSP toxins such as
okadaic acid3 and dinophysistoxins4 were very low in
the contaminated mussels. Extensive bioassays per-
formed on 1 have demonstrated that the effects of the
toxin when administered to mice are distinct from those
caused by other marine toxins.5 Hence, azaspiracid
poisoning (AZP) defines a new type of neurotoxicity
associated with this environmental contaminant. Mass
spectrometric and extensive NMR spectroscopic analy-
sis revealed that azaspiracid is an v-amino acid with a
40-carbon backbone bearing an unprecedented array of
polycyclic, spiro-fused ring systems that terminate in a
spiroaminal.1
synthetic targets. Reported here are preliminary studies
aimed at a possible biomimetic preparation of the
unique C28–C40 domain of the azaspiracids that
resulted in a facile intramolecular C-Michael addition.
Assembly of the F–I ring system of 1–3 was envisioned
to occur via a polycyclization cascade (Scheme 1). An
a,b-unsaturated iminium species ii derived from the
hypothetical acyclic amino–ketone i may trigger the
formation of the F–G rings. Specifically, addition of an
axial hemiketal oxygen upon the conjugated iminium
iii, a tautomer of ii, would close the F–G ring system
(iv). Final closure to the spiroaminal v by addition of
the C33 alcohol upon the C36 iminium center would
complete the sequence towards the polycyclic system v.
To test this approach, an acyclic keto–enone 4 (Scheme
2) was chosen as a surrogate for the postulated iminium
species iii to initially study the viability of the hemi-
ketal/hetero-Michael addition sequence. A convergent
synthesis of 4 was developed from intermediates repre-
senting the C28–C34 (5) and C35–C40 (6) portions
of 1–3. Thereafter, closure of the 2,9-dioxabicyclo-
[3.3.1]nonane system via an intramolecular conjugate
addition of a cyclic C28 hemiketal upon a C34–C36
Michael acceptor could be examined.
Recently, two analogs of azaspiracid, azaspiracid-2 (2)
and azaspiracid-3 (3), have been isolated from similar
sources.6 However, neither the absolute configurations
nor the relative stereochemistries between their C1–C25
and C28–C40 domains of the azaspiracids have been
reported. The intriguing structures, stereochemical
ambiguities, distinct mechanism of action, and current
scarcity combine to make the azaspiracids important
Me
40
R1
HN
I
O
H
O
R1 R2
Me
Me Me
R2
H
Me
27
H
H
H
Me
O
A
O
O
C
B
O
1 Azaspiracid
2 Azaspiracid 2:
3 Azaspiracid 3:
:
H
HO
G
O
E
1
H
D
O
20
H
O
H
H
OH
H
F
Me
OH
Me
* Corresponding author. E-mail: forsyth@chem.umn.edu
0040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)02156-0

818
J. Aiguade et al. / Tetrahedron Letters 42 (2001) 817–820
40
HN
I
40
N
H
O
N
H
O
H
O
G
O
O
27
28
H
O
H
H
H
HO
R
H
H
28 33
F
R
R
O
O
32
v
iv
40
H2N
O
36
N
H
N
H
O
O
O H
HO
28 33
HO
28
H
H
HO
28
H
R
R
26
R
HO
32
O
HO 32
32
i
ii
iii
Scheme 1. Biosynthetic hypothesis for the C28–C40 domain of the azaspiracids.
The synthesis of aldehyde 5 began with oxazolidinone 7
(Scheme 3).7 The stereochemistry of 5 was arbitrarily
chosen to have the (30R,32S,33R)-absolute configura-
tion.8 Methylation afforded 8 as essentially a single
diastereomer.9 Reductive removal of the oxazolidinone
with lithium borohydride10 followed by treatment of the
resulting alcohol with NBS and Ph3P gave bromide 9.
Addition of the lithium anion derived from 9 to (R)-
glyceraldehyde acetonide provided alcohol 10 in moder-
ate yield as the major diastereomer of a 4:1 mixture.11
Compound 10 was assigned the (32S)-configuration by
Mosher ester analysis. Attempts to improve the yield by
changing temperature or reaction time, or using addi-
tives (TMEDA, HMPA) were unfruitful. Treatment of
10 with TBAF followed by acidic hydrolysis of the
acetonide group afforded a tetraol, which was con-
verted to the cyclic acetal 11. Protection of both
hydroxyl groups as TBS ethers followed by regioselec-
tive opening of the anisylidene with DIBAL gave pri-
mary alcohol 12.12 The regioselectivity of the acetal
opening was high, but the yield was inconsistent, due
mainly to cleavage of the silyl ethers.13 Finally, oxida-
tion of 12 with TEMPO14 afforded aldehyde 5 in high
yield.
The synthesis of the complementary ketophosphonate 6
began with the known racemic lactone 13 (Scheme 4).15
Ring opening with (R)-phenethylamine provided a 1:1
mixture of amide diastereomers that were chromato-
graphically separated after conversion of the primary
hydroxyl to the TBS ether 14.16 The desired
diastereomer (37R,39S)-14 was reduced to the corre-
PMB
O
O
O
28
H
40
OH
34
32
O
OTBS
Boc
O
N
40
OTBS
Boc
TBS
O
5
N
Ar
32
28
O
28
H
H
+
Ar
OTBS
O
O
P
40
O
4
Boc
32
N
(OMe)2
35
vi
Ph
6
Scheme 2. Retrosynthesis of enone 4.
O
O
O
O
O
O
1) LiBH4, THF
NaHMDS, MeI
THF, -78 oC
93%
CHO
OTBDPS
OTBDPS
OTBDPS
Br
N
O
tBuLi, Et2O
46%
O
N
2) NBS, PPh3
88%, 2 steps
Bn
OH
Bn
8
9
7
1) TBSOTf,
2,6-lutidine
OTBS
O
OH
TEMPO
oxidation
94%
O
1) TBAF
OTBS
OTBDPS
34
HO
5
28
2) TsOH, MeOH
O
O
2) DIBAL
83%, 2 steps
PMBO
OH
3) PMPCH(OMe)2,
TsOH
PMP
10
11
12
76%, 3 steps
Scheme 3. Synthesis of intermediate 5.

J. Aiguade et al. / Tetrahedron Letters 42 (2001) 817–820
819
O
O
, Et2O
NH2
1)
Ph
1) LiAlH4
O
Ph
N
H
OTBS
2) TBAF
2) TBSCl, Et3N, DMAP
CH2Cl2
3) Boc2O, NaOH
13
14
86%, 3 steps
90 %, 2 steps
O
O
P
40
40
36
1) (COCl)2, DMSO, Et3N
Ph
N
(OMe)2
Ph
N
OH
35
2) n-BuLi / CH3P(O)(OMe)2
3) Dess-Martin periodinane
Boc
Boc
15
6
77%, 3 steps
Scheme 4. Synthesis of the C35–C40 domain (6).
PMBO
O
40
28
1) HF.Pyr
i-Pr2NEt, LiCl,
Boc
N
+
5
6
OTBS
CH3CN, 5h, rt
92%
2) TEMPO oxidation
85%, 2 steps
Ar
OTBS
16
1)
Br
CrCl2 (NiCl2 1%)
THF, 15 min, 86%
PMBO
O
Boc
CHO
4
N
2) TPAP, NMO, 75%
3) DDQ, 90%
Ar
OTBS
17
Scheme 5. Synthesis of intermediate 4.
O
O
Na
O
O
Boc
N
Boc
N
NaH, THF
oC, 4 h
37
NaH, THF
Boc
O
N
OH
TBS
TBS
O
Ar
0
oC, 4 h
Ar
0
O
O
H
Ar
H
OTBS
28
32
O
HO
4
77%
( JHb-Ha = 11 Hz)
O
Ar
N
OTBS
O
O
Hb
Hb
Boc
OH
N
Boc
Boc
N
OH
TBS
O
O
Ar
Ar
OTBS
O
H
Ha
Ha
( JHa-Hb = 11 Hz)
O
O
18
18
Scheme 6. Facile intramolecular C-Michael addition.
ing portion of the azaspiracids. Oxidation of the newly
formed carbinol and subsequent cleavage of the PMB
ether with DDQ provided the acyclic intermediate 4.
sponding amine. Subsequent desilylation and carba-
mate formation gave 15. Swern oxidation of the alcohol
provided the C36 aldehyde. Addition of the lithium
anion of methyl dimethylphosphonate followed by oxi-
dation with Dess–Martin periodinane17 completed the
synthesis of ketophosphonate 6.
With the initial goal of studying the assembly of the
polycyclic domain of 1–3 in a stepwise fashion, keto–
enone 4 was subjected to a variety of different basic
conditions.20 Simple treatment of 4 with NaH in THF
at 0°C provided a mixture of cyclization products in
77% yield (Scheme 6). The major products were
intramolecular C-Michael adducts 18 epimeric at C37
(3:1) instead of the anticipated 2,9-dioxabicy-
clo[3.3.1]nonane system of vi (Scheme 2). The stereo-
chemistry of the two newly formed stereogenic centers
on cyclohexane 18 was assigned on the basis of the
observed large coupling constant (JHa,Hb=11 Hz)
between the newly generated methine protons.
Aldehyde 5 and ketophosphonate 6 were advanced
towards keto–enone 4 by coupling under Masamune–
Roush conditions18 to give (E)-16 (Scheme 5). At this
stage, an appropriate b,g-unsaturated ketone needed to
be installed at C28. Hence, the primary alcohol was
selectively liberated with HF·pyridine and oxidized with
TEMPO to afford aldehyde 17. A CrCl2/NiCl2 medi-
ated reaction19 of methallyl bromide with 17 provided
the corresponding homoallylic alcohol. Methallyl bro-
mide was chosen as a simple substitute for the adjoin-

820
J. Aiguade et al. / Tetrahedron Letters 42 (2001) 817–820
The generation of 18 can be explained by an intra-
molecular C-Michael addition reaction occurring in
preference to O-Michael addition (cf. iii“iv, Scheme 1).
The observed carbocyclization may be promoted by a
conformational predisposition of the tethered nucle-
ophile–electrophile pair to undergo rapid bond forma-
tion via a chair-like transition state. Furthermore, the
sodium cation of the enolate may be dually coordinated
with the enolate and enone oxygens to enforce the
reactive conformation and facilitate carbocyclization.
Once enolized, the C28 ketone is inert to intramolecular
attack by the C32 oxygen, thus preventing hemiketal
formation that would be associated with hetero-
Michael addition. Similar carbocyclization of 4 was
observed upon treatment with KOtBu.
2. Yasumoto, T.; Murata, M.; Oshima, Y.; Sano, M.;
Matsumoto, G. K.; Clardy, J. Tetrahedron 1985, 41,
1019.
3. Tachibana, K.; Scheuer, P. J.; Tsukitani, Y.; Kikuchi,
H.; Van Engen, D.; Clardy, J.; Gopichand, Y.;
Schmitz, F. J. J. Am. Chem. Soc. 1981, 103, 2469.
4. Zhao, J.; Lembeye, G.; Cenci, G.; Wall, B.; Yasumoto,
T. Dev. Mar. Biol. 1993, 3, 587.
5. Ito, E.; Satake, M.; Ofuji, K.; Kurita, N.; McMahon,
T.; James, K. J.; Yasumoto, T. Toxicon 2000, 38, 917.
6. Ofuji, K.; Satake, M.; McMahon, T.; Silke, J.; James,
K. J.; Naoki, H.; Oshima, Y.; Yasumoto, T. Nat. Tox-
ins 1999, 7, 99.
7. Jacobi, P. A.; Murphree, S.; Rupprecht, F.; Zheng, W.
J. Org. Chem. 1996, 61, 2413.
8. Carbon numbering corresponds to that of 1.
9. Evans, D. A.; Ennis, M. D.; Mathre, D. J. J. Am.
Chem. Soc. 1982, 104, 1737.
10. Penning, T. P.; Djuric, S. W.; Haack, R. A.; Kalish, V.
J.; Miyashiro, J. M.; Rowell, B. W.; Yu, S. S. Synth.
Commun. 1990, 20, 307.
11. Bailey, W. F.; Punzalan, E. R. J. Org. Chem. 1990, 55,
5404.
12. Takano, S.; Akiyama, M.; Sato, S.; Ogasawara, K.
Chem. Lett. 1983, 1593.
13. Corey, E. J.; Jones, G. B. J. Org. Chem. 1992, 57,
1028.
14. For a review on oxidations with TEMPO, see: De
Nooy, A. E. J.; Besemer, A. C.; Von Bekkum, H. Syn-
thesis 1996, 1153.
A putative acyclic precursor of the C28–C40 domain of
the azaspiracids was constructed from C28–C34 (5) and
C35–C40 (6) fragments. However, initial attempts to
induce formation of the 2,9-dioxabicyclo[3.3.1]nonane
system representing the F–G rings under basic condi-
tions resulted instead in a facile C-Michael addition.
The propensity of an acyclic keto–enone, with carbonyl
and Michael acceptor separated by 6-carbons, to
undergo stereoselective carbocyclization provides an
efficient and perhaps general method to form a highly
substituted cyclohexane. Alternative methods to induce
the postulated biomimetic assembly of azaspiracids’
unique F–I ring system are under study.
15. Collum, D. B.; McDonald, J. H.; Still, W. C. J. Am.
Chem. Soc. 1980, 102, 2120.
Acknowledgements
16. Miki, S.; Sato, Y.; Tabuchi, H.; Oikawa, H.; Ichihara,
A.; Sakama, S. J. Chem. Soc., Perkin Trans. 1 1990,
1228.
17. Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991,
113, 7277.
18. Blanchette, M. A.; Choy, W.; Davis, J. T.; Essenfeld,
A. P.; Masamune, S.; Rouch, W. R.; Sakai, T. Tetra-
hedron Lett. 1984, 25, 2183.
19. For a review on chromium mediated reactions, see:
Fu¨rstner, A. Chem. Rev. 1999, 99, 991.
This publication was made possible by grant number
ES10615 from the National Institute of Environmental
Health Sciences (NIEHS), NIH, and generous unre-
stricted grant support from Bristol-Myers Squibb. Its
contents are solely the responsibility of the authors and
do not necessarily represent the official views of the
NIEHS, NIH.
20. (a) Toshima, H.; Furumoto, Y.; Inamura, S.; Ichihara,
A. Tetrahedron Lett. 1996, 37, 5707; (b) Toshima, H.;
Aramaki, H.; Furumoto, Y.; Inamura, S.; Ichihara, A.
Tetrahedron 1998, 54, 5531; (c) Toshima, H.; Aramaki,
H.; Ichihara, A. Tetrahedron Lett. 1999, 40, 3587.
References
1. Satake, M.; Ofuji, K.; Naoki, H.; James, K. J.; Furey,
A.; McMahon, T.; Silke, J.; Yasumoto, T. J. Am.
Chem. Soc. 1998, 120, 9967.
.