Structural revision and total synthesis of azaspiracid-1, part 1: Intelligence gathering and tentative proposal

Article abstract of DOI:10.1002/anie.200460695

Phantom molecules from Nature: Degradation studies of azaspiracid-1 (see originally proposed, incorrect structure), the notorious marine neurotoxin isolated from contaminated mussels, revealed that the double bond of ring A was located at C7-C8 and not at

Full text of DOI:10.1002/anie.200460695

Communications
Natural Products Synthesis
Structural Revision and Total Synthesis of
Azaspiracid-1, Part 1: Intelligence Gathering and
Tentative Proposal**
K. C. Nicolaou,* Stepan Vyskocil, Theocharis V. Koftis,
Yoichi M. A. Yamada, Taotao Ling, David Y.-K. Chen,
Wenjun Tang, Goran Petrovic, Michael O. Frederick,
Yiwei Li, and Masayuki Satake*
In memory of D. John Faulkner
Structure 1-i (Figure 1, absolute stereochemistry and relative
stereochemistry between ABCDE and FGHIdomains
unknown) was proposed in 1998, on the basis of NMR
spectroscopic analysis, for a poisonous substance called
azaspiracid-1, which was isolated from the mussel family
Mytilus edulis.[1] A recent total synthesis[2,3] of 1-i and its
[4]
FGHIepimer proved that this assignment was incorrect.
Herein we report synthetic and degradation studies that led to
the revised structure 1-c or ABCD-epi-1-c as the most likely
depiction of the molecular architecture of this biotoxin
(Figure 1). The only doubt that remained was the absolute
configuration of the ABCD domain of the molecule, and this
question was resolved by total synthesis, as is described in the
following paper in this issue.[5]
Upon realizing that the originally proposed structure 1-i
of azaspiracid-1 was in error,[2,3] our plan for determining its
true structure called for the degradation of the natural
material into smaller fragments, whose chemical synthesis
may narrow down the location of the structural discrepancy
between the synthesized (originally proposed) and actual
Figure 1. Originally proposed structure 1-i (relative stereochemistry
between ABCDE (C1–C27)and FGHI (C28–C40)domains and absolute
stereochemistry unknown)and newly proposed structure 1-c or ABCD-
epi-1-c of azaspiracid-1.
structures. To this end, azaspiracid-1 was derivatized by
exposure to TMSCHN2 in MeOH, and the methyl ester
obtained was treated with NaIO4 in aqueous MeOH at
ꢀ
ambient temperature, resulting in the cleavage of its C20 C21
bond and in the formation of the corresponding aldehyde 3
(C1–C20 fragment) and lactone 4 (C21–C40 fragment) in
quantitative yield (Scheme 1). Compound 3 was further
elaborated by treatment with NaBH4 to afford, supposedly,
hydroxy methyl ester 5 (ꢁ 90% yield), followed by hydro-
genation to give the fully saturated compound 6 (ꢁ 90%
yield). On the other hand, cleavage of the exocyclic olefinic
bond within 4 with O3–Me2S to the corresponding ketone,
followed by saponification of the E-ring lactone with aqueous
NaOH in MeOH and esterification of the resulting carboxylic
acid with TMSCHN2 gave methyl ester 7 (ꢁ 90% yield).
Finally, treatment of compound 7 with NaIO4 furnished
amino acid 8 in high yield (ꢁ 90%). These degradatively
derived intermediates (i.e. 4–6 and 8, Scheme 1) were to serve
several purposes. First, spectroscopic comparisons with syn-
thetic materials would immediately pinpoint any errors in
each domain of the molecule. Second, comparison of amino
lactone 4 with the two synthetic FGHIepimers of the same
structure (i.e. 4) would clarify the relative stereochemistry
between the ABCDE and FGHIdomains of azaspiracid-1.
Third, optical rotation comparisons of the degradative and
synthetic samples may reveal the absolute configuration of
each fragment and, consequently, of the entire structure.
One drawback of the adopted degradation scheme was
the loss of stereochemical information regarding the C20 and
[*] Prof. Dr. K. C. Nicolaou, Dr. S. Vyskocil, Dr. T. V. Koftis,
Dr. Y. M. A. Yamada, Dr. T. Ling, Dr. D. Y.-K. Chen, Dr. W. Tang,
Dr. G. Petrovic, M. O. Frederick, Dr. Y. Li
Department of Chemistry and
The Skaggs Institute for Chemical Biology
The Scripps Research Institute
10550 North Torrey Pines Road, La Jolla, CA 92037 (USA)
Fax: (+1)858-784-2469
and
Department of Chemistry and Biochemistry
University of California, San Diego
9500 Gilman Drive, La Jolla, CA 92093 (USA)
E-mail: kcn@scripps.edu
Prof. Dr. M. Satake
Graduate School of Agriculture
Tohoku University
Tsutsumi-dori Amamiya, Aoba-ku, Sendai 981-8555 (Japan)
Fax: (+81)22-717-8817
E-mail: satake@biochem.tohoku.ac.jp
[**] We thank Dr. D. H. Huang and Dr. G. Siuzdak for NMR spectro-
scopic and mass spectrometric assistance, respectively. Financial
support for this work was provided by grants from the National
Institutes of Health (USA)and the Skaggs Institute for Chemical
Biology, and fellowships from the Ernst Schering AG Foundation (to
S.V.), the Japanese Society for the Advancement of Science (to
Y.M.A.Y.), the National Science Foundation (to M.O.F.), and Eli Lilly
(to Y.L.).
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Figure 2. Structures of C20-epi-1-i and C20-FGHI-epi-1-i (syn-
thesized and proven incorrect).
dithiane 9[2] was selectively monoacetylated (at the
primary position) with Ac2O in the presence of 2,4,6-
collidine (85% yield), and the resulting hydroxy
dithiane intermediate was deprotected by exposure to
[7]
PhI(OCOCF3)2 to afford the corresponding lactol,
whose oxidation with NIS–TBAI[8] completed the
synthesis of the required allylic acetate lactone 10.
The latter compound, 10, underwent Stille coupling
with stannanes 11 and ent-11 according to a previously
described procedure[3] to afford compounds 12 and 13,
respectively. These substrates were then smoothly
transformed into the targeted amino lactones 4 and
FGHI-epi-4, respectively, via intermediate compounds
14 (for 4) and 15 (for FGHI-epi-4) according to our
previous sequence[3] and as summarized in Scheme 2:
1) HF·py-induced selective TES removal; 2) NIS-ini-
tiated ring closure; 3) nBu3SnH–Et3B-facilitated
reductive deiodination; and 4) TAS-F-mediated desilylation.
The only deviation from the previously employed sequence[3]
was the use of TAS-F[9] instead of TBAF, as the former
reagent afforded superior results in the final deprotection
Scheme 1. Chemical degradation and derivatization of azaspiracid-1 (1-i: origi-
nally proposed structure)to C1–C20 alcohol 6, C21–C40 lactone 4, and C26–
C40 carboxylic acid 8. Reagents and conditions: a)TMSCHN 2, MeOH, 258C,
1 h; b)NaIO 4, MeOH/H2O (4:1), 258C, 1 h, ꢁ100% over two steps;
c)NaBH 4, MeOH, 258C, ꢁ90%; d)Pd/C (5%), H 2, MeOH, 258C, ꢁ90%;
e)O 3, Me2S, MeOH, ꢀ788C; f)NaOH (0.1 n), MeOH/H2O (4:1), 258C, 2 h;
g)TMSCHN 2, MeOH, 258C, 15 min, ꢁ90% over three steps; h)NaIO 4,
MeOH/H2O (4:1), 258C, 30 min, ꢁ90%. TMS=trimethylsilyl.
C21 hydroxy-bearing centers. However, if we assume that the
C21 hemiketal functionality would rest in its thermodynami-
cally most stable conformation, the only stereocenter in
question would be that at C20. As 1-i had already been
eliminated as the true structure, the C20-epi-azaspiracid-1
(C20-epi-1-i) structure and its FGHIepimer (C20- epi-FGHI-
epi-1-i) as logical targets moved to the front as possible
alternatives (see Figure 2). Their total syntheses along the
same lines as those described for 1-i,[2,3] however, also proved
them to be incorrect.[6]
Having established that the azaspiracid-1 structural prob-
lem did not reside exclusively at the C20 stereocenter, the
focus of our investigation shifted next to the synthesis of the
two diastereomeric EFGHIamino lactones 4 and FGHI-epi-4
(Scheme 2). Thus, the previously synthesized dihydroxy
1
step. Most gratifyingly, the H NMR spectral data of one of
these synthetic materials, namely FGHI-epi-4, matched those
of the sample derived from natural azaspiracid-1 (Scheme 1),
whereas its diastereomeric amino lactone 4 exhibited slightly
different and diagnostically distinct signals to those of the
naturally derived substance. Based on these results, we
concluded that the relative stereochemical arrangement for
the EFGHIdomain of azaspiracid-1 is that depicted by
structure FGHI-epi-4 (absolute stereochemistry unknown).[10]
The next goal was to establish the absolute stereochem-
istry of azaspiracid-1, and as we had confirmed the structural
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comparison purposes. This objective was met expeditiously,
starting with the previously prepared intermediate 16,[2] as
shown in Scheme 3. Thus, a two-step oxidative protocol
involving dihydroxylation (NMO–OsO4 cat.) of the terminal
olefin of 16, followed by NaIO4-induced cleavage of the
resulting 1,2-diol, provided aldehyde 17 in 80% overall yield.
Oxidation of this aldehyde (17) with NaClO2, followed by
TAS-F-mediated desilylation resulted in the formation of the
desired amino acid 8 in 35% yield over the two steps. As
expected, the NMR spectroscopic data of synthetic 8 matched
those of the degradatively obtained material. Furthermore,
the two samples exhibited comparable rotations, but with
opposite signs (synthetic: [a]2D5 = + 49.6 (c = 0.4, MeOH);
natural: [a]2D5 = ꢀ59.0 (c = 0.016, MeOH)). Furthermore, the
1H NMR spectrum of the (R)-PGME (phenyl glycine methyl
ester) derivative 18 (see Scheme 3 for preparation of the
PGME derivatives) of synthetic carboxylic acid 8 was
identical to the (S)-PGME derivative of the naturally derived
acid 8 (and (S)-PGME derivative 19 derived from synthetic 8
was identical to the (R)-PGME derivative of the naturally
derived 8), thereby establishing the absolute stereochemistry
of the FGHIdomain of azaspiracid-1 as the antipode of
compound 8 that we had prepared as shown in Scheme 3. By
virtue of the chemistry depicted in Scheme 2, the absolute
stereochemistry of the entire EFGHIdomain of azaspiracid-1
could now be shown as that in Scheme 3 (structure FGHI-epi-
4).
With the establishment of both the relative and absolute
stereochemistries of the EFGHIdomain of azaspiracid-1, our
attention was then turned to the remaining segment, namely
the ABCD (C1–C20) framework of the natural product, in
which we had, by now, suspected the structural discrepancy to
be embedded. As outlined in Scheme 4, a synthetic sample of
21, a substance corresponding to the degradatively derived
compound from the natural product (structure 5, Scheme 1)
was prepared from the previously synthesized intermediate
20[2] by desilylation (TBAF, 88% yield). The spectroscopic
data for these two samples (5 and 21) differed significantly,
thereby confirming our suspicions for the location of the
structural discrepancy (see Table 1). Furthermore, the main
differences between the two sets of spectra were associated
with protons located on ring A, namely 6-H, 7-H, 8-H, and 9-
H (see numbering on structure 21, Scheme 4). Despite these
clear differences, the final structural deconvolution of this
region of azaspiracid-1 remained elusive, primarily as a result
of the very similar 2D NMR spectral data for the naturally
derived and synthetic materials whose comparison implied
only subtle connectivity and spatial differences. Finally, it was
a discovery by C. Hopmann and the late Professor D. John
Faulkner that shed light on this issue. Indeed, the structural
elucidation of lissoketal (22, see Scheme 4),[11] a marine
natural product isolated by these workers in 1997, provided
the first clue to the azaspiracid-1 puzzle. This secondary
metabolite 22 bears a close resemblance to the proposed
structure of azaspiracid-1 (1-i) except for the fact that the
endocyclic double bond in ring A resides between C7 and C8,
rather than between C8 and C9. Intriguingly, close examina-
Scheme 2. Chemical synthesis of C21–C40 lactone 4 and its FGHI epimers,
FGHI-epi-4 and determination of the relative stereochemistry within the EFGHI
domain. Reagents and conditions: a)Ac 2O (5.0 equiv), 2,4,6-collidine
(10.0 equiv), CH2Cl2, 258C, 16 h, 85%; b)PhI(OCOCF 3)2 (1.5 equiv), MeCN/
pH 7 buffer (4:1), 08C, 10 min, 81%; c) NIS (10.0 equiv), TBAI (2.0 equiv),
CH2Cl2, 258C, 40 min, 74%; d) 10 (3.0 equiv), [Pd2dba3] (0.9 equiv), AsPh3
(0.9 equiv), LiCl (18 equiv), EtNiPr2 (12 equiv); then 11 or ent-11 (0.03m in THF,
syringe pump addition), NMP, 458C, 4 h; e)HF·py (excess), THF/py (1:1), 0
!
258C, 2 h; f)NIS (10.0 equiv), NaHCO (30 equiv), THF, 08C, 16 h, 26% for 14
3
and 38% for 15 over three steps; g)Et 3B (1.0m in hexanes, 0.2 equiv), nBu3SnH/
toluene (1:2), 08C, 5 min; h)TAS-F (5.0 equiv), DMF, 0 8C, 20 h, 70% for 4 and
74% for FGHI-epi-4 over two steps. NIS=N-iodosuccinimide, NMP=N-methyl-
pyrrolidinone, TBAI=tetra-n-butylammonium iodide, dba=dibenzylideneace-
tone, TAS-F=tris(dimethylamino)sulfur (trimethylsilyl)difluoride.
integrity of the FGHIdomain 8 of the molecule and had
secured its optical rotation from degradation studies (see
Scheme 1), we decided to synthesize the carboxylic acid 8 for
1
tion of the H NMR chemical shift values (see Table 1) and
the 2D NMR correlation pattern reported for lissoketal (22)
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structural arrangement. Furthermore, relo-
cating the endocyclic double bond from
ꢀ
ꢀ
C8 C9 to C7 C8, places the 6-H in a
doubly allylic environment, thereby provid-
ing a possible explanation for the signifi-
cant downfield shift for this proton exhib-
ited in the 1H NMR spectrum of the
naturally derived intermediate
5 (d =
4.79 ppm) relative to that observed for the
synthetic material 21 (d = 4.51 ppm).[12]
With the NMR spectroscopic parameters
associated with rings B, C, and D for the
naturally derived (5) and synthetic (21)
samples in good agreement, the alternative
structure 23 (see Scheme 4) was proposed
for the hydroxy methyl ester derived from
the natural product through intermediate 3
(see Scheme 1). To test this hypothesis, a
chemical synthesis of the newly proposed
structure 23 was required.
The synthesis of the newly proposed
structure 23 was initiated from the previ-
ously described compound 24[2] and pro-
ceeded as outlined in Scheme 5. Thus, the
free hydroxy group in 24 was protected as a
TES ether (95% yield), the pivaloate ester
was cleaved with DIBAL-H (96% yield),
and the liberated primary alcohol was
oxidized under Swern conditions to afford
aldehyde 25 (95% yield). Addition of vinyl
Grignard reagent to 25 (76% yield) fol-
lowed by acetylation (85% yield) yielded
allylic acetate 26 as a mixture of
diastereoisomers (ꢁ 2:1). Ireland–
Claisen rearrangement[13] of 26 fol-
lowed by methylation of the resulting
carboxylic acid with diazomethane
gave methyl ester 27 in 52% overall
yield. Selective removal of the TES
group (HF·py, 85% yield) from 27,
Swern oxidation of the resulting alco-
hol (90% yield) followed by treat-
ment with KHMDS–TMSCl led to
TMS enol ether 28, whose oxidation
to the corresponding enone (55%
Scheme 3. Synthesis of FGHI amino acid 8 and determination of the absolute stereochem-
istry of EFGHI domain of azaspiracid-1. Reagents and conditions: a)OsO 4 (0.1 equiv),
NMO (3.0 equiv), acetone/H2O (3:1), 258C, 3 h, 92%; b)NaIO 4 (3.0 equiv), MeOH/
pH 7 buffer (2.5:1), 258C, 1 h, 87%; c)NaClO 2 (10.0 equiv), NaH2PO4 (10.0 equiv), 2-
methyl-2-butene (excess), tBuOH/H2O (4:1), 258C, 30 min, 93%; d)TAS-F (5.0 equiv),
DMF, 08C, 16 h, 38%; e)( R)-PGME or (S)-PGME (5.0 equiv), EDC (5.0 equiv), HOBt
(5.0 equiv), NaHCO3 (10 equiv), DMF, 258C, 16 h, 75% for 18 and 63% for 19. NMO=N-
methylmorpholine N-oxide, PGME=phenyl glycine methyl ester, EDC=1-(3-dimethylamino-
propyl)-3-ethylcarbodiimide, HOBt=1-hydroxybenzotriazole.
overall
yield),
reduction
with
NaBH4, and acetylation (71% overall
yield) furnished allylic acetate 29.
Finally, exposure of 29 to catalytic
amounts of [Pd2dba3]·CHCl3 in the
presence of nBu3P and NaBH4,[14]
followed by treatment with TBAF
led to the targeted intermediate 23
Scheme 4. Chemical synthesis of C1–C20 alcohol 21 for comparison with naturally derived mate-
rial, structure of lissoketal (22), and second proposed structure, 23, for the ABCD domain of aza-
spiracid-1. Reagents and conditions: a)TBAF (2.0 equiv), THF, 25 8C, 2 h, 88%. TBAF=tetra-
n-butyl ammonium fluoride.
showed remarkable similarities to those observed for natural
azaspiracid-1.[1] In particular, a weak HMBC correlation
between C10 and 7-H as well as a weak COSY correlation
between 6-H and 9-H observed for the natural azaspiracid-1
are in line with observations reported by Hopmann and
Faulkner[11] and, therefore, point to the lissoketal ring A
(42% overall yield) through intermediate 30 (which was
chromatographically separated from its D8,9 isomer that
emerged as the minor product during the reduction step
(ꢁ 2:1)).
Much to our disappointment, however, the spectral data
of synthetic 23 did not match those of the degradatively
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Table 1: Key 1H NMR chemical shifts of compounds 5, and 21–23.
ring A was not the only problem with the originally proposed
structure (1-i) of azaspiracid-1. A more profound structural
change, perhaps having to do with the skeletal stereochem-
istry, was needed to accommodate the structure of the natural
product. Indeed, the fully hydrogenated synthetic product 31
(see Scheme 6) obtained from synthetic 21 (10% Pd/C, H2,
95% yield) proved to be different from the corresponding
hydrogenated fragment 6 obtained from 5, which was derived
by degradation and elaboration of the natural product as
already described above (see Scheme 1).
At this juncture, and having pinned down the location of
the double bond within ring A, we were still faced with 128
possible structures for the ABCD domain of azaspiracid-1
arising from its seven stereogenic centers. However, our
intelligence gathering at this stage extended to an additional
piece of information regarding the thermodynamic stability of
the degradatively derived ABCD fragments (e.g. 5) and of the
natural product itself. Their ABC double spiroketal bridge
H
5[a] (natural)
21
22
5[b]
23
d(1H)[ppm]
6-H
7-H
4.79
2.48
2.03
5.74
5.62
4.50
2.06
2.04
6.07
5.79
4.34
5.80
4.79
5.62
4.81
5.61
8-H
9-H
5.80
2.57
1.91
–
–
–
5.74
2.48
2.03
3.78
4.21
4.34
0.92
5.77
2.51
2.18
3.84
4.15
4.51
0.95
16-H
17-H
19-H
41-H
3.78
4.21
4.34
0.92
3.94
4.27
4.57
0.98
–
[a] Original incorrect assignments. [b] Revised (correct)assignments for
the D7,8 isomer at 5 (note that structure was still incorrect).
derived 5 (see Table 1), sending us, one more time, back to the
drawing board. Apparently, the location of the double bond in
Scheme 5. Chemical synthesis of the second proposed structure 23 for the ABCD domain. Reagents and conditions: a)TESCl (1.5 equiv), imida-
zole (3.0 equiv), 4-DMAP (0.1 equiv), CH2Cl2, 08C, 12 h, 95%; b)DIBAL (2.0 equiv), CH 2Cl2, ꢀ788C, 1 h, 96%; c)(COCl) 2 (5.0 equiv), DMSO
=
(11 equiv), CH2Cl2, ꢀ788C, 30 min, ꢀ608C, 1 h; then Et3N (22 equiv), ꢀ78!ꢀ258C, 95%; d)CH 2 CHMgBr (1.6 equiv), Et2O, 0!258C, 1 h,
76%; e)Ac 2O (5.0 equiv), py (10.0 equiv), 4-DMAP (0.1 equiv), CH2Cl2, 08C, 3 h, 85%; f)LDA (1.5 equiv), THF, ꢀ788C, 10 min, TBSCl (1.5 equiv),
HMPA (1.5 equiv), ꢀ78!258C, 24 h; g)CH 2N2 (excess), Et2O, 258C, 30 min, 52% over two steps; h)HF·py (10.0 equiv), py/THF (1:1), 0 8C, 2 h,
85%; i)(COCl) 2 (10.0 equiv), DMSO (22 equiv), CH2Cl2, ꢀ788C, 30 min, ꢀ608C, 1 h; then Et3N (44 equiv), ꢀ78!ꢀ258C, 90%; j)KHMDS
(1.5 equiv), THF, ꢀ788C, 1 h, TMSCl (1.6 equiv), ꢀ788C, 30 min; k)Pd(OAc) 2 (5.0 equiv), DMSO, 72 h, 55% over two steps; l) NaBH4
(3.0 equiv), CeCl3·7H2O (1.0 equiv), MeOH, ꢀ508C, 1 h; m)Ac 2O (20 equiv), py (40 equiv), 4-DMAP (0.1 equiv), CH2Cl2, 0!258C, 12 h, 71%
over two steps; n)[Pd 2dba3]·CHCl3 (0.2 equiv), nBu3P (0.4 equiv), NaBH4 (10.0 equiv), dioxane/H2O (9:1), 0!258C, 12 h; o)TBAF (3.0 equiv),
THF, 0!258C, 3 h, D7,8/D8,9 (2:1), 42% over two steps. TES=triethylsilyl, DMSO=dimethyl sulfoxide, TBS=tert-butyldimethylsilyl, LDA=lithium
diisopropylamide, KHMDS=potassium bis(trimethylsilyl)amide.
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1450, 1378, 1320, 1235, 1155, 1091, 1049, 1020, 978,
1
855, 800, 667 cmꢀ1; H NMR (500 MHz, CDCl3):
d = 6.07 (ddd, J = 9.9, 5.1, 2.2 Hz, 1H), 5.80–5.74
(m, 2H), 5.62 (dd, J = 15.7, 6.1 Hz, 1H), 4.60–4.55
(m, 1H), 4.52–4.48 (m, 1H), 4.28–4.26 (m, 1H),
3.94–3.93 (m, 1H), 3.83–3.81 (m, 1H), 3.75 (s,
3H), 3.58–3.53 (m, 1H), 2.49–2.42 (m, 4H), 2.31–
2.25 (m, 1H), 2.21–1.97 (m, 9H), 1.89–1.87 (m,
1H), 1.58–1.51 (m, 1H), 0.98 ppm (d, J = 6.6 Hz,
3H); 13C NMR (150 MHz, CDCl3): d = 173.4,
131.1, 130.1, 129.1, 128.5, 111.4, 104.2, 78.9, 76.3,
75.9, 68.6, 64.8, 51.5, 35.7, 33.9, 33.6, 31.0, 29.9,
Scheme 6. Preparation of fully saturated synthetic ABCD fragment 31. Reagents and condi-
tions: a)Pd/C (10%), H 2, CH3CO2Et, 258C, 1 h, 95%.
was noted to be stable under acidic conditions, whereas our
29.7, 27.6, 23.4, 15.6 ppm; HRMS (MALDI): calcd for C22H32O7Na+
[M+Na+]: 431.2040; found: 431.2060.
corresponding synthetic materials so far revealed their fleet-
ing nature under similar conditions. This intriguing observa-
tion, subtle as it was, proved to be crucial in shaping our next
hypothesis for solving the mystery of azaspiracid-1, as we will
describe in the following communication in this issue.[5]
Received: May 17, 2004 [Z460695]
Published Online: June 25, 2004
Keywords: degradation studies · natural products · neurotoxins ·
.
structure elucidation · total synthesis
Experimental Section
4: Rf = 0.30 (silica gel, EtOAc/MeOH 9:1); [a]2D5 = + 36.4 (MeOH, c =
0.14); IR (film): n˜max = 3416, 2954, 2919, 1727, 1430, 1261, 1096, 1044,
[1] M. Satake, K. Ofuji, H. Naoki, K. J. James, A. Fruey, T.
McMahon, J. Silke, T. Yasumoto, J. Am. Chem. Soc. 1998, 120,
9967 – 9968.
[2] K. C. Nicolaou, Y. Li, N. Uesaka, T. V. Koftis, S. Vyskocil, T.
Ling, M. Govindasamy, W. Qian, F. Bernal, D. Y.-K. Chen,
Angew. Chem. 2003, 115, 3777 – 3781; Angew. Chem. Int. Ed.
2003, 42, 3643 – 3648.
[3] K. C. Nicolaou, D. Y.-K. Chen, Y. Li, W. Qian, T. Ling, S.
Vyskocil, T. V. Koftis, M. Govindasamy, N. Uesaka, Angew.
Chem. 2003, 115, 3781 – 3783; Angew. Chem. Int. Ed. 2003, 42,
3649 – 3653.
[4] For synthetic studies directed toward azaspiracid-1, see: a) R. G.
Carter, D. J. Weldon, Org. Lett. 2000, 2, 3913 – 3916; b) R. G.
Carter, D. E. Graves, Tetrahedron Lett. 2001, 42, 6035 – 6039;
c) R. G. Carter, T. C. Bourland, D. E. Graves, Org. Lett. 2002, 4,
2177 – 2179; d) C. J. Forsyth, J. Hao, J. Aiguade, Angew. Chem.
2001, 113, 3775 – 3779; Angew. Chem. Int. Ed. 2001, 40, 3663 –
3667; e) A. B. Dounay, C. J. Forsyth, Org. Lett. 2001, 3, 975 – 978;
f) J. Hao, J. Aiguade, C. J. Forsyth, Tetrahedron Lett. 2001, 42,
817 – 820; g) J. Hao, J. Aiguade, C. J. Forsyth, Tetrahedron Lett.
2001, 42, 821 – 824; h) R. G. Carter, T. C. Bourland, D. E.
Graves, Org. Lett. 2002, 4, 2177 – 2179; i) R. G. Carter, D. E.
Graves, M. A. Gronemeyer, G. S. Tschumper, Org. Lett. 2002, 4,
2181 – 2184; j) M. Sasaki, Y. Iwamuro, J. Nemoto, M. Oikawa,
Tetrahedron Lett. 2003, 44, 6199 – 6201; k) Y. Ishikawa, S.
Nishiyama, Tetrahedron Lett. 2004, 45, 351 – 354; l) Y. Ishikawa,
S. Nishiyama, Heterocycles 2004, 63, 539 – 565; m) Y. Ishikawa, S.
Nishiyama, Heterocycles 2004, 63, 885 – 893.
1
797, 673 cmꢀ1; H NMR (600 MHz, CD3OD/0.5% CD3COOD): d =
5.35 (s, 1H), 5.31 (s, 1H), 4.52 (d, J = 10.1 Hz, 1H), 4.26 (br s, 1H),
3.81 (d, J = 1.8 Hz, 1H), 2.85–2.70 (m, 2H), 2.64–2.56 (m, 1H), 2.49
(dd, J = 14.5, 4.8 Hz, 1H), 2.44–2.38 (m, 1H), 2.40 (d, J = 14.5 Hz,
1H), 2.32 (d, J = 14.5 Hz, 1H), 2.30–2.22 (m, 2H), 2.10–1.95 (m, 3H),
1.85–1.71 (m, 2H), 1.60 (d, J = 12.7 Hz, 1H), 1.56–1.49 (m, 1H), 1.46–
1.39 (m, 2H), 1.27–1.23 (m, 2H), 1.23 (d, J = 7.0 Hz, 3H), 1.17–1.04
(m, 1H), 0.95 (d, J = 6.6 Hz, 3H), 0.93 (d, J = 6.1 Hz, 3H), 0.91 (d, J =
7.0 Hz, 3H), 0.89 ppm (d, J = 6.6 Hz, 3H); 13C NMR (125 MHz,
CD3OD): d = 177.2, 143.1, 120.4, 97.9, 95.9, 92.0, 79.6, 75.8, 72.6, 45.5,
43.4, 43.1, 42.4, 40.0, 38.0, 37.6, 37.0, 35.4, 33.1, 31.8, 26.4, 23.8, 19.8,
18.0, 17.5, 16.2 ppm; HRMS (MALDI): calcd for C26H41NO5H+
[M+H+]: 448.3057; found: 448.3051.
FGHI-epi-4: Rf = 0.29 (silica gel, EtOAc/MeOH 9:1); [a]D25
=
ꢀ18.0 (MeOH, c = 0.30); IR (film): n˜max = 3374, 2962, 2921, 2854,
1729, 1452, 1411, 1261, 1090, 1021, 865, 800, 701, 666 cmꢀ1; H NMR
1
(600 MHz, CD3OD/0.5% CD3COOD): d = 5.40 (s, 1H), 5.37 (s, 1H),
4.44 (d, J = 9.6 Hz, 1H), 4.37 (br s, 1H), 4.03 (d, J = 2.6 Hz, 1H), 3.25
(d, J = 12.3 Hz, 1H), 3.05 (t, J = 12.3 Hz, 1H), 2.72 (dd, J = 14.9,
4.4 Hz, 1H), 2.66–2.60 (m, 1H), 2.48 (d, J = 14.5 Hz, 1H), 2.40–2.30
(m, 1H), 2.32 (d, J = 14.5 Hz, 1H), 2.30–2.20 (m, 1H), 2.15–2.08 (m,
2H), 2.04–2.00 (m, 1H), 1.83 (dd, J = 14.0, 5.7 Hz, 1H), 1.73 (d, J =
14.0 Hz, 1H), 1.58–1.51 (m, 1H), 1.46–1.39 (m, 2H), 1.35–1.25 (m,
3H), 1.25 (d, J = 7.0 Hz, 3H), 1.18–1.09 (m, 2H), 1.00 (d, J = 6.6 Hz,
3H), 0.98 (d, J = 6.6 Hz, 3H), 0.97 (d, J = 6.2 Hz, 3H), 0.95 ppm (d,
J = 6.1 Hz, 3H); 13C NMR (150 MHz, CD3OD): d = 177.1, 142.5,
121.0, 98.1, 95.9, 92.4, 79.5, 75.8, 72.7, 45.7, 43.3, 43.2, 41.8, 40.0, 37.9,
37.7, 37.4, 35.6, 32.6, 31.8, 26.3, 23.9, 19.8, 17.9, 17.4, 16.2 ppm; HRMS
(MALDI): calcd for C26H41NO5H+ [M+H+]: 448.3057; found:
448.3057.
[5] See following Communication in this issue: K. C. Nicolaou, T. V.
Koftis, S. Vyskocil, G. Petrovic, T. Ling, Y. M. A. Yamada, W.
Tang, M. O. Frederick, Angew. Chem. 2004, 116, 4418; Angew.
Chem. Int. Ed. 2004, 43, 4318.
8: Rf = 0.19 (silica gel, CHCl3/MeOH/H2O 20:3:1); [a]2D5 = + 49.6
(MeOH, c = 0.4); IR (film): n˜max = 3359, 2922, 2948, 1700, 1575, 1455,
1393, 1257, 1116, 1064, 1043, 1012, 793, 621 cmꢀ1; 1H NMR (600 MHz,
C5D5N): d = 4.71 (br s, 1H), 4.46 (br s, 1H), 3.59 (br s, 1H), 2.95 (s,
2H), 2.92 (t, J = 12.3 Hz, 1H), 2.83–2.75 (m, 1H), 2.34–2.27 (m, 1H),
2.20–2.13 (m, 2H), 2.10–2.04 (m, 1H), 1.95–1.88 (m, 1H), 1.80–1.71
(m, 2H), 1.57–1.50 (m, 1H), 1.49–1.42 (m, 2H), 1.31–1.24 (m, 1H),
0.91 (d, J = 5.7 Hz, 3H), 0.78 (d, J = 4.8 Hz, 3H), 0.72 ppm (d, J =
5.3 Hz, 3H); 13C NMR (150 MHz, CD3OD): d = 177.4, 97.2, 95.7, 80.2,
75.3, 72.2, 52.0, 48.2, 43.1, 39.6, 37.7, 35.4, 31.2, 30.8, 26.5, 23.9, 19.6,
16.1 ppm; HRMS (MALDI): calcd for C18H29NO5H+ [M+H+]:
340.2118; found: 340.2122.
[6] As in the cases of synthetic 1-i and FGHI-epi-1-i,[3] both C20-epi-
1-i and C20-epi-FGHI-epi-1-i were found to exist as mixtures of
inseparable compounds, and neither of the samples matched by
TLC, HPLC, or 1H NMR with the natural azaspiracid-1.
Complete details will be discussed in the full account of this
work.
[7] G. Stork, K. Zhao, Tetrahedron Lett. 1989, 30, 287 – 2902.
[8] S. Hanessian, D. H. Wong, M. Therien, Synthesis 1981, 394 – 396.
[9] W. R. Roush, D. S. Coffey, D. J. Madar, J. Am. Chem. Soc. 1997,
119, 11331 – 11332.
[10] Both 4 and FGHI-epi-4 were found to be labile upon prolonged
standing in CD3OD or CD3OD/0.5% CD3COOD at ambient
temperature.
21: Rf = 0.22 (silica gel, EtOAc/hexanes 1:2); [a]2D5 = + 45.0
(CHCl3, c = 1.1); IR (film): n˜max = 3439, 2924, 2360, 2320, 1739,
Angew. Chem. Int. Ed. 2004, 43, 4312 –4318
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4317

Communications
[11] C. Hopmann, D. J. Faulkner, Tetrahedron Lett. 1997, 38, 169 –
170.
[12] For selected compounds with a doubly allylic oxymethine
hydrogen atom attached to a 3-dihydropyran ring, see: a) P. A.
Wender, S. G. Hedge, R. D. Hubbard, J. Am. Chem. Soc. 2002,
124, 4956 – 4957; b) C. Iwamoto, K. Minoura, T. Oka, T. Ohta, S.
Hagishita, A. Numata, Tetrahedron 1999, 55, 14353 – 14368;
c) L. V. Dunkerton, A. J. Serino, J. Org. Chem. 1992, 57, 2814 –
2816.
[13] R. E. Ireland, R. H. Mueller, J. Am. Chem. Soc. 1972, 94, 5897 –
5898.
[14] H. Nakamura, M. Ono, Y. Shida, H. Akita, Tetrahedron:
Asymmetry 2002, 13, 705 – 713.
4318
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 4312 –4318