Total synthesis of (+)-scyphostatin, a potent and specific inhibitor of neutral sphingomyelinase

Article abstract of DOI:10.1002/anie.200454192

The five crucial steps in the first total synthesis of (+)-scyphostatin from D-arabinose involve (see picture): a) stereoselective aldol coupling to form a quaternary stereocenter, b) ring-closing metathesis (RCM) to construct the cyclohexene ring, c) Neg

Full text of DOI:10.1002/anie.200454192

Angewandte
Chemie
Natural Products Synthesis
Total Synthesis of (+)-Scyphostatin, a Potent and
Specific Inhibitor of Neutral Sphingomyelinase**
Munenori Inoue, Wakako Yokota,
Modachur G. Murugesh, Takashi Izuhara, and
Tadashi Katoh*
Scyphostatin (1, Scheme 1), isolated from a culture broth of
Dasyscyphus mollissimus SANK-13892 by Ogita and co-
workers in 1997,[1] is a powerful and specific inhibitor of
neutral sphingomyelinase (N-SMase). It has been reported
that 1 inhibits N-SMase and acidic SMase (A-SMase) with
IC50 values of 1.0 mm and 49.3 mm, respectively.[2] Although a
large number of N-SMase inhibitors have been reported to
date,[3] 1 is still the most potent inhibitor of the enzyme.[1,2] N-
SMase inhibitors regulate the level of ceramide, the hydrol-
ysis product of sphingomyelin, in a wide variety of mamma-
lian cells. Therefore, 1 is expected to be a promising new agent
for the treatment of ceramide-mediated pathogenic states
such as AIDS,[4] inflammation, and immunological and
neurological disorders.[2] Structurally, 1 features a novel,
highly oxygenated cyclohexene ring and an aminopropanol
side chain linked to a C20 unsaturated fatty acid moiety. The
significant biological properties coupled with the unique
structural features make 1 an exceptionally intriguing and
timely target for total synthesis. Although several synthetic
approaches, including ours,[5] have appeared recently,[6] no
total synthesis has been reported to date. We have already
prepared the fully functionalized cyclohexene segment with
the N,O-protected aminopropanol side chain and the correct
stereogenic centers;[5b] however, all attempts to remove the
N,O-protecting group (the cyclic carbonate moiety) from the
cyclohexene segment met with failure.[5c] Herein we disclose
an alternative and more reliable synthetic strategy that led to
the completion of the total synthesis of 1.
Scheme 1. Retrosynthetic plan for scyphostatin (1). TBS=tert-butyldimethyl-
silyl, PMB=4-methoxybenzyl, Boc=tert-butoxycarbonyl, Ts=p-toluenesulfonyl,
TBDPS=tert-butyldiphenylsilyl, Bn=benzyl.
The retrosynthetic plan for scyphostatin (1) is outlined in
Scheme 1. Our preliminary model studies[5a,b] suggested that
the labile epoxycyclohexenone moiety should be elaborated
at a late stage of the synthesis. We envisaged that the
cyclohexene segment 2 would be produced through ring-
closing metathesis (RCM) of diene 4. The RCM substrate 4
would be prepared through aldol coupling of ester 5 with the
Garner aldehyde (6),[7] which would establish the requisite
quaternary stereocenter C4. During the course of our studies,
Hoye and Tennakoon reported that the fatty acid side chain
(the ethyl ester variant of 3) can be synthesized from tosylate
7 through Horner–Wadsworth–Emmons (HWE) olefina-
tion.[8] Therefore, we planned to employ 7 as a key inter-
mediate in our synthesis and explored an alternative
approach to 7, which features Negishi coupling of vinyl
iodide 8 and alkyl iodide 9.[23]
[*] Dr. M. Inoue, W. Yokota, Dr. M. G. Murugesh, Prof. Dr. T. Katoh+
Sagami Chemical Research Center
2743-1 Hayakawa, Ayase, Kanagawa 252-1193 (Japan)
E-mail: katoh@tohoku-pharm.ac.jp
Dr. T. Izuhara
The synthesis of the cyclohexene segment 2 started from
the known compound 10[9] (Scheme 2), readily prepared from
d-arabinose by O-benzylglycosylation and acetonide forma-
tion. PMB protection of 10 followed by debenzylation and
Wittig methylenation provided alcohol 11, which was sub-
sequently converted into methyl ester 5 through a three-step
sequence involving twofold oxidation and methyl esterifica-
tion. The crucial aldol coupling of 5 with 6 was effected by
treatment of 5 with NaN(SiMe3)2 (1.3 equiv) followed by
addition of 6 (1.1 equiv), which led to the desired product 12
as a single diastereomer in 69% yield.[10] The stereochemistry
at C4 in 12 was deduced by NOE studies[11] of the deoxy-
genated compound 13, which was obtained by applying the
procedure reported by Barton and McCombie.[12] The
Department of Electronic Chemistry
Tokyo Institute of Technology
Nagatsuta, Yokohama 226-8502 (Japan)
[+] Address correspondence to this author at
Tohoku Pharmaceutical University
4-4-1 Komatsushima, Aoba-ku, Sendai 981-8558 (Japan)
Fax: (+81)22-275-2013
[**] We are grateful to Dr. H. Kogen, Dr. T. Ogita, and Dr. T. Akiyama
(Sankyo Co., Ltd.) for fruitful discussions and a generous gift of
natural (+)-scyphostatin (1). We also thank Dr. N. Sugimoto and Dr.
N. Kawahara (National Institute of Health Sciences) for measure-
ments of HRMS and assistance with NMR spectroscopy experi-
ments. M.G.M thanks JST for an STA fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 4207 –4209
DOI: 10.1002/anie.200454192
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4207

Communications
Scheme 2. Synthesis of the cyclohexene segment 2. a) PMBCl, NaH,
DMSO, room temperature, 70%; b) H2, Raney Ni, EtOH, room temper-
ature, 86%; c) Ph3P+CH3Brꢀ, tBuOK, benzene, reflux, 86%; d) DMSO,
(COCl)2, CH2Cl2, ꢀ788C; iPr2NEt, ꢀ78!08C, 95%; e) NaClO2,
NaH2PO4, DMSO/H2O, room temperature; f) CH2N2, Et2O/MeOH,
08C, 78% (two steps); g) NaN(SiMe3)2, THF, ꢀ788C; Garner aldehyde
(6), ꢀ788C, 69%; h) NaN(SiMe3)2, THF, 08C; CS2; MeI, 08C!RT;
i) nBu3SnH, AIBN, toluene, reflux, 53% (two steps); j) DIBAL, CH2Cl2,
ꢀ1008C, 88%; k) vinylmagnesium bromide, THF, 08C, 93%;
Scheme 3. Synthesis of the fatty acid segment 3. a) PPh3, CBr4, CH2Cl2, 08C;
b) nBuLi, THF, ꢀ788C; MeI, 94%, (two steps); c) Cp2ZrHCl, benzene, 408C;
I2, RT, 63%; d) O3, CH2Cl2/MeOH, ꢀ788C; NaBH4, room temperature , 84%;
e) MsCl, Et3N, CH2Cl2, room temperature; f) NaI, acetone, reflux, 95% (two
steps); g) tBuLi, ZnCl2, Et2O, ꢀ78!08C; 8, [Pd(PPh3)4], THF, 81%; h) lith-
ium 4,4’-di-tert-butylbiphenylide, THF, ꢀ788C, 90%; i) TsCl, Et3N, DMAP,
CH2Cl2, RT; j) MeLi, CuI, Et2O, ꢀ408C, 97% (two steps); k) TBAF, THF,
room temperature, 91%; l) (nPr)4NRuO4, NMO, CH2Cl2, room temperature;
=
ꢀ
=
ꢀ
m) (MeO)2P(O)CH2CH CH CH CH CO2Me, LDA, THF, ꢀ78!ꢀ308C,
46% (two steps); n) KOH (2m), MeOH/THF, room temperature; o) (COCl)2,
DMF, CH2Cl2, room temperature. Ms=methanesulfonyl, DMAP=4-dimethyl-
aminopyridine, TBAF=tetrabutylammonium fluoride, NMO=4-methylmor-
pholine N-oxide, LDA=lithium diisopropylamide, DMF=N,N-dimethylforma-
mide.
=
l) (Cy3P)2RuCl2( CHPh) (10 mol%), CH2Cl2, reflux, 96%; m) TBSOTf,
2,6-lutidine, CH2Cl2, room temperature, 93%; n) PPTS, EtOH, 608C,
57%. DMSO=dimethyl sulfoxide, AIBN=2,2’-azobisisobutyronitrile,
DIBAL=diisobutylaluminum hydride, TBS=tert-butyldimethylsilyl,
Tf =trifluoromethanesulfonyl, Cy=cyclohexyl, PPTS=pyridinium p-tol-
uenesulfonate.
remarkable stereoselectivity at C4 observed in this coupling
reaction seems to be induced by the adjacent C5 stereocenter.
Reduction of 13 with DIBAL followed by vinylation of the
resulting aldehyde furnished diene 4. The critical RCM
reaction of 4 in the presence of the Grubbs catalyst[13]
proceeded smoothly and cleanly to give cyclohexenol 14 in
96% yield. Finally, 14 was converted into 2 by O-silylation
and selective acetonide cleavage.
The synthesis of the fatty acid segment 3 was next
investigated (Scheme 3). Vinyl iodide 8 was prepared from
the known aldehyde 15[14] via alkyne 16 by a Corey–Fuchs
reaction[15] and subsequent hydrozirconation/iodination.[16]
Alkyl iodide 9, the coupling partner of 8, was derived from
the known olefin 17[17] by ozonolysis with reductive workup
followed by mesylation and iodination. The key Negishi
coupling of 9 and 8 was carried out successfully by employing
the modified conditions of Smith et al.[18] to give the desired
coupling product (81% yield), which was converted into the
The projected synthesis was completed as shown in
Scheme 4. The Boc and PMB groups of the cyclohexene
segment 2 were simultaneously removed with TMSOTf[21] to
produce the corresponding amino diol, which was immedi-
ately treated with the fatty acid segment 3 to give, after
treatment of aqueous AcOH, the desired amide 20 in 73%.
Enone 21 was elaborated from 20 by selective acetylation,
mesylation, desilylation, and Dess–Martin oxidation. In
our previous work,[5a,b] the requisite epoxide moiety was
successfully installed by hydrolysis of the acetonide
with aqueous CF3CO2H, followed by treatment with base.
However, upon applying these conditions to 21, partial
isomerization of the C12’-C13’ double bond in the side
chain was observed during the acid hydrolysis. Eventually,
we overcame this problem by using aqueous CCl3CO2H
instead of CF3CO2H; subsequent treatment with NaOH
afforded the desired epoxide 22 without appreciable isomer-
ization of the double bond. The final step was to remove the
Hoye intermediate
7
by debenzylation and tosylation.
acetyl group of 22; deprotection was best brought about by
exposure of 22 to lipase PS in phosphate buffer,[22] furnishing
(+)-scyphostatin (1) in 60% yield, which was identical to
natural (+)-1 in all respects (1H and 13C NMR, HRMS, and
[a]D).[1,2]
According to the Hoye protocol,[8] 7 was transformed into
aldehyde 18 in a three-step sequence. Aldehyde 18 was then
subjected to HWE olefination[19] to yield methyl ester 19.
Spectral data (1H and 13C NMR, MS) of 19 were in good
agreement with those reported.[20] Finally, the desired acid
chloride 3 was derived from 19 in two steps.
In conclusion, we completed the first total synthesis of
(+)-scyphostatin (1) in an efficient and flexible way. Impor-
4208
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 4207 –4209

Angewandte
Chemie
T. Akiyama, J. Koizumi, S. Shibuya, Y. Tsuji, S. Soeda, H.
Shimeno, Bioorg. Med. Chem. Lett. 2003, 13, 229; l) M. Taguchi,
K. Sugimoto, K. Goda, T. Akama, K. Yamamoto, T. Suzuki, Y.
Tomishima, M. Nishiguchi, K. Arai, K. Takahashi, T. Kobori,
Bioorg. Med. Chem. Lett. 2003, 13, 1963; m) M. Taguchi, K.
Goda, K. Sugimoto, T. Akama, K. Yamamoto, T. Suzuki, Y.
Tomishima, M. Nishiguchi, K. Arai, K. Takahashi, T. Kobori,
Bioorg. Med. Chem. Lett. 2003, 13, 3681.
[4] S. Chatterjee, Arterioscler. Thromb. Vasc. Biol. 1998, 18, 1523.
[5] a) T. Izuhara, T. Katoh, Tetrahedron Lett. 2000, 41, 7651; b) T.
Izuhara, T. Katoh, Org. Lett. 2001, 3, 1653; c) T. Izuhara, W.
Yokota, M. Inoue, T. Katoh, Heterocycles 2002, 56, 553.
[6] a) M. K. Gurjar, S. Hotha, Heterocycles, 2000, 53, 1885; b) K. A.
Runcie, R. J. K. Taylor, Org. Lett. 2001, 3, 3237; c) H. Fujioka, N.
Kotoku, Y. Sawama, Y. Nagatomi, Y. Kita, Tetrahedron Lett.
2002, 43, 4825; d) R. Takagi, W. Miyanaga, Y. Tamura, K.
Ohkata, Chem. Commun. 2002, 2096; e) L. M. Murray, P.
OꢀBrien, R. J. K. Taylor, Org. Lett. 2003, 5, 1943; f) M. Eipert,
C. M. Mössmer, M. E. Maier, Tetrahedron 2003, 59, 7949.
[7] a) P. Garner, J. M. Park, J. Org. Chem. 1987, 52, 2361; b) A.
Dondoni, D. Perrone, Synthesis 1997, 527; c) A. Dondoni, D.
Perrone, Org. Synth. 1997, 77, 64.
[8] T. R. Hoye, M. A. Tennakoon, Org. Lett. 2000, 2, 1481.
[9] C. E. Ballou, J. Am. Chem. Soc. 1957, 79, 165.
[10] The stereochemistry at C3 in the coupling product 12 was
tentatively assigned based on the usual Felkin–Anh model.
[11] An NOE interaction between 3-H and 5-H was observed.
[12] D. H. R. Barton, S. W. McCombie, J. Chem. Soc. Perkin Trans. 1
1975, 1574.
[13] a) P. Schwab, M. B. France, J. W. Ziller, R. H. Grubbs, Angew.
Chem. 1995, 107, 2179; Angew. Chem. Int. Ed. Engl. 1995, 34,
2039; for a recent review, see: b) T. M. Trnka, R. H. Grubbs, Acc.
Chem. Res. 2001, 34, 18.
Scheme 4. Total synthesis of scyphostatin (1). a) TMSOTf, 2,6-lutidine,
CH2Cl2, room temperature; MeOH; b) 3, Et3N, CH2Cl2, room tempera-
ture; AcOH (aq.), 73% (two steps); c) Ac2O, pyridine, DMAP, CH2Cl2,
room temperature, 72%; d) MsCl, Et3N, CH2Cl2, room temperature,
93%; e) TBAF, THF, room temperature; f) Dess–Martin periodinane,
CH2Cl2, room temperature, 98% (two steps); g) CCl3CO2H, CH2Cl2/
H2O, reflux; NaOH (2m), room temperature, 45%; h) lipase PS, pH 7
phosphate buffer/acetone, room temperature, 60%. TMS=trimethyl-
silyl.
[14] T. F. Walsh, R. B. Toupence, F. Ujjainwalla, J. R. Young, M. T.
Goulet, Tetrahedron 2001, 57, 5233.
[15] E. J. Corey, P. L. Fuchs, Tetrahedron Lett. 1972, 3769.
[16] D. W. Hart, T. F. Blackburn, J. Schwartz, J. Am. Chem. Soc. 1975,
97, 679.
tantly, the synthesis has the potential for producing scyphos-
tatin analogues with a wide variety of fatty acid side chains.
[17] W. R. Roush, A. D. Palkowitz, K. Ando, J. Am. Chem. Soc. 1990,
112, 6348.
Received: March 5, 2004 [Z54192]
[18] A. B. Smith III, T. J. Beauchamp, M. J. LaMarche, M. D. Kauf-
man, Y. Qiu, H. Arimoto, D. R. Jones, K. Kobayashi, J. Am.
Chem. Soc. 2000, 122, 8654.
[19] M. Kinoshita, H. Takami, M. Taniguchi, T. Tamai, Bull. Chem.
Soc. Jpn. 1987, 60, 2151.
Keywords: aldol reaction · inhibitors · natural products ·
ring-closing metathesis · total synthesis
.
[20] S. Saito, N. Tanaka, K. Fujimoto, H. Kogen, Org. Lett. 2000, 2,
505.
[1] M. Tanaka, F. Nara, K. Suzuki-Konagai, T. Hosoya, T. Ogita, J.
Am. Chem. Soc. 1997, 119, 7871.
[21] M. Sakaitani, Y. Ohfune, J. Org. Chem. 1990, 55, 870.
[22] All attempts at deacetylation of 22 under a various conditions
(e.g. K2CO3, NaOMe, or KOH in MeOH; aqueous KOH in THF
or CH2Cl2; DBU or NH3 in THF) met with failure; presumably,
the epoxycyclohexenone moiety present in 22 and/or 1 is
sensitive to these basic conditions. Detailed results and dis-
cussions will be presented in a full account.
[23] Note added in proof: After submission of this manuscript, we
learnt of independent syntheses of the scyphostatin side chain in
which similar Negishi-type cross-coupling reactions were
employed: a) Z. Tan, E.-i. Negishi, Angew. Chem. 2004, 116,
2971; Angew. Chem. Int. Ed. 2004, 43, 2911; b) G. D. McAllister,
R. J. K. Taylor, Tetrahedron Lett. 2004, 45, 2551; we are grateful
to Professor Ei-ichi Negishi for kindly providing us with a
preprint of his paper prior to publication.
[2] a) F. Nara, M. Tanaka, T. Hosoya, K. Suzuki-Konagai, T. Ogita,
J. Antibiot. 1999, 52, 525; b) F. Nara, M. Tanaka, S. Masuda-
Inoue, Y. Yamasato, H. Doi-Yoshioka, K. Suzuki-Konagai, S.
Kumakura, T. Ogita, J. Antibiot. 1999, 52, 531.
[3] a) R. Uchida, H. Tomoda, Y. Dong, S. Omura, J. Antibiot. 1999,
52, 572; b) M. Tanaka, F. Nara, Y. Yamasato, S. Masuda-Inoue,
H. Doi-Yoshioka, S. Kumakura, R. Enokita, T. Ogita, J. Antibiot.
1999, 52, 670; c) M. Tanaka, F. Nara, Y. Yamasato, Y. Ono, T.
Ogita, J. Antibiot. 1999, 52, 827; d) C. Arenz, A. Giannis, Angew.
Chem. 2000, 112, 1498; Angew. Chem. Int. Ed. 2000, 39, 1440;
e) C. Arenz, A. Giannis, Eur. J. Org. Chem. 2001, 137; f) C.
Arenz, M. Thutewohl, O. Block, H.-J. Altenbach, H. Waldmann,
A. Giannis, ChemBioChem 2001, 2, 141; g) C. Arenz, M.
Gartner, V. Wascholowski, A. Giannis, Bioorg. Med. Chem.
2001, 9, 2901; h) T. Yokomatsu, H. Takechi, T. Akiyama, S.
Shibuya, T. Kominato, S. Soeda, H. Shimeno, Bioorg. Med.
Chem. Lett. 2001, 11, 1277; i) T. Hakogi, Y. Monden, M. Taichi, S.
Iwama, S. Fujii, K. Ikeda, S. Katsumura, J. Org. Chem. 2002, 67,
4839; j) C. C. Lindsey, C. Gómez-Díza, J. M. Villalba, T. R. R.
Pettus, Tetrahedron 2002, 58, 4559; k) T. Yokomatsu, T. Murano,
Angew. Chem. Int. Ed. 2004, 43, 4207 –4209
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4209