Total syntheses of (+)- and (-)-cacospongionolide B, cacospongionolide E, and related analogues. Preliminary study of structural features acquired for phospholipase A2 inhibition

Article abstract of DOI:10.1021/jo049285e

The total syntheses of the antiinflammatory marine sponge metabolites (+)-cacospongionolide B and E are described. The pivotal steps in the synthetic route include a three-step sequence that couples the two main regions of the natural product, as well as generates the side chain dihydropyran ring. The activity of the synthetic analogues against bee venom phospholipase A ₂ suggests that the cacospongionolides have enantiospecific interactions with the enzyme that may be independent of the γ- hydroxybutenolide moiety.

Full text of DOI:10.1021/jo049285e

Tota l Syn th eses of (+)- a n d (-)-Ca cosp on gion olid e B,
Ca cosp on gion olid e E, a n d Rela ted An a logu es. P r elim in a r y Stu d y
of Str u ctu r a l F ea tu r es Requ ir ed for P h osp h olip a se A2 In h ibition
Atwood K. Cheung, Ryan Murelli, and Marc L. Snapper*
Department of Chemistry, Merkert Chemistry Center, Boston College, 2609 Beacon Street,
Chestnut Hill, Massachusetts 02467
marc.snapper@bc.edu
Received April 28, 2004
The total syntheses of the antiinflammatory marine sponge metabolites (+)-cacospongionolide B
and E are described. The pivotal steps in the synthetic route include a three-step sequence that
couples the two main regions of the natural product, as well as generates the side chain dihydropyran
ring. The activity of the synthetic analogues against bee venom phospholipase A2 suggests that
the cacospongionolides have enantiospecific interactions with the enzyme that may be independent
of the γ-hydroxybutenolide moiety.
In tr od u ction
In this regard, the development of effective inhibitors of
sPLA2 could offer new opportunities for treating diseases
such as asthma, sepsis, and rheumatoid arthritis.6
The marine sponge metabolites, cacospongionolide B1
(1) and E2 (2), as well as related congeners have biological
properties that include antimicrobial, cytotoxic, and
antiinflammatory activities.3 The antiinflammation ac-
tivities, in particular, are thought to result from the
inhibition of secretory phospholipase A2 (sPLA2).4 This
activity was anticipated in light of the structural features
that the cacospongionolides share with manoalide (3),
another known inhibitor sPLA2.5
sPLA2 catalyzes the hydrolytic release of arachidonic
acid from phospholipids in the lipid bilayer. This unsat-
urated fatty acid leads to potent proinflammatory agents
such as prostaglandins, thromboxanes, and leukotrienes.
* Corresponding author. Phone: (617) 552-8096. Fax: (617) 552-
1442.
The biologically active region of manoalide is known
to include the γ-hydroxybutenolide moiety, a functionality
that can serve as a masked R,â-unsaturated aldehyde.
The γ-hydroxybutenolide is believed to form a covalent
interaction with a lysine residue at the PLA2-lipid
interface near the active site of the enzyme.7 Likewise,
the cacospongionolides may also involve this type of
interaction with sPLA2; however, no experimental evi-
dence has confirmed this hypothesis. Herein, we describe
the total syntheses of cacospongionolides B and E and
the initial study into the mechanism of their antiinflam-
matory activities.8
(1) (a) De Rosa, S.; Crispino, A.; De Giulio, A.; Iodice, C.; Amodeo,
P.; Tancredi, T. J . Nat. Prod. 1999, 62, 1316-1318. (b) De Rosa, S.;
Crispino, A.; De Giulio, A.; Iodice, C.; Benrezzouk, R.; Terencio, M. C.;
Ferra´ndiz, M. L.; Alcaraz, M. J .; Paya´, M. J . Nat. Prod. 1998, 61, 931-
935. (c) De Rosa, S.; Puliti, R.; Crispino, A.; De Giulio, A.; De Sena, C.;
Iodice, C.; Mattia, C. A. Tetrahedron 1995, 51, 10731-10736.
(2) De Rosa, S.; Crispino, A.; De Giulio, A.; Iodice, C.; Benrezzouk,
R.; Terencio, M. C.; Ferra´ndiz, M. L.; Alcaraz, M. J .; Paya´, M. J . Nat.
Prod. 1998, 61, 931-935.
(3) (a) De Rosa, S.; Crispino, A.; De Giulio, A.; Iodice, C.; Pronzato,
R.; Zavodnik, N. J . Nat. Prod. 1995, 58, 1776-1780. (b) Garcia Pastor,
P.; De Rosa, S.; De Giulio, A.; Paya´, M.; Alcaraz, M. J . Brit. J . Pharm.
1999, 126, 301-311. (c) De Rosa, M.; Giordano, S.; Scettri, A.; Sodano,
G.; Soriente, A.; Garc´ıa Pastor, P.; Alcaraz, M. J .; Paya´, M. J . Med.
Chem. 1998, 41, 1, 3232-3238.
(4) Cacospongionolide B is also reported to suppress expression of
inflammatory enzymes by inhibiting nuclear factor-κB activation.
Posadas, I.; De Rosa, S.; Terencio, M. C.; Paya´, M.; Alcaraz, M. J . Brit.
J . Pharm. 2003, 138, 1571-1579.
(5) (a) De Silva, E. D.; Scheuer, P. J . Tetrahedron Lett. 1980, 21,
1611-1614. (b) Glaser, K. B.; De Carvalho, S. M.; J acobs, R. S.; Kernan,
M. R.; Faulkner, D. J . Mol. Pharmocol. 1989, 36, 782-788. (c) de
Freitas, J . C.; Blankemeier, L. A.; J acobs, R. S. Experientia 1984, 40,
864-865. (d) J acobs, R. S.; Culver, P.; Langdon, R.; O’Brien, T.; White,
S. Tetrahedron 1985, 41, 981-984. (e) Mayer, A. M. S.; J acobs, R. S.
Biomedical Importance of Marine Organism; Fautin, D. G., Ed.;
California Academy of Sciences: San Francisco, 1988; pp 133-142. (f)
Mann, J . Nature 1992, 358, 540.
(6) (a) Six, D. A.; Dennis, E. A. Biochim. Biophys. Acta 2000, 1488,
1-19. (b) Tibes, U.; Friebe, W.-G. Exp. Opin. Invest. Drugs 1997, 6,
279-298. (c) Gelb, M. H.; J ain, M. K.; Berg, O. G. FASEB J . 1994, 8,
916-924. (d) Dennis, E. A. Drug. Dev. Res. 1987, 10, 205-220. (e)
Soriente, A.; De Rosa, M.; Scettri, A.; Sodano, G.; Terencio, M. C.; Paya´,
M.; Alcaraz, M. J . Curr. Med. Chem. 1999, 6, 415-431.
(7) (a) Potts, B. C. M.; Faulkner, D. J .; J acobs, R. S. J . Nat. Prod.
1992, 55, 1701-1717. (b) Potts, B. C. M.; Faulkner, D. J .; De Carvalho,
M. S.; J acobs, R. S. J . Am. Chem. Soc. 1992, 114, 5093-5100.
(8) For a preliminary report on the synthesis of cacospongionolide
B, see Cheung, A. K.; Snapper, M. L. J . Am. Chem. Soc. 2002, 124,
11584-11585.
10.1021/jo049285e CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/14/2004
5712
J . Org. Chem. 2004, 69, 5712-5719

Syntheses of (+)/ (-)-Cacospongionolide B and E and Analogues
preparation of the cacospongionolides, both fragments of
the molecule will need to be prepared with absolute
control over stereochemistry.
In itia l Ap p r oa ch to Ca cosp on gion olid e B. An
asymmetric preparation of the oxygenated side chain
began with the synthesis of the homoallylic alcohol 8.
This known compound was prepared utilizing Brown’s
asymmetric allylboration protocol.18 Treatment of 3-fur-
fural with allyl diisopinocampheylborane, followed by
oxidative workup provides the desired alcohol 8 in 73%
yield and 96% ee (eq 1). Other catalytic asymmetric
allylation procedures were considered; however, Brown’s
methodology proved convenient and effective.19
F IGURE 1. Retrosynthesis of cacospongionolide B.
Resu lts a n d Discu ssion
Etherification of alcohol 8 with propargyl bromide to
prepare enyne 7 was investigated next.20 Optimal yields
of the desired propargyl ether 7 with minimal contami-
nation by allenyl ether 9 was possible by using NaH in
benzene with 15-crown-5 to catalyze the alkylation.
To provide access to a wide range of cacospongionolide
structural variants, a convergent strategy was pursued.9
The coupling of various reaction partners could give rise
to new compounds with useful handles for attaching
fluorescent,10 radiolabel,11 biotin,12 or photoaffinity la-
bels.13
Retr osyn th etic An a lysis of Ca cosp on gion olid e B.
As illustrated in Figure 1, cacospongionolide can be
dissected into two sections; an aliphatic Decalin region
possessing four contiguous stereocenters and a γ-hy-
droxybutenolide-containing section with a remote ste-
reogenic center.14
Our plan was to join the two fragments at the C9-
C11 bond15 through a reductive alkylation of the type
pioneered by Stork and co-workers in the early 1960s.16
The requisite Decalin 6 is similar to the known Wieland-
Mischer ketone.17 We thought to create the dihydropyran
ring functionality found in 5 through an enyne ring-
closing metathesis. Enyne 5 is accessible from the known
alcohol 8. To accomplish a diastereo- and enantioselective
With compound 7 in hand, the intramolecular enyne
reaction was investigated.21 As observed by Mori and co-
workers,21e a dramatic improvement in the efficiency of
the metathesis was noted when the reaction was carried
out under an atmosphere of ethylene using ruthenium
(18) Racherla, U. S.; Liao, Y.; Brown, H. C. J . Org. Chem. 1992, 57,
6614-6617.
(19) (a) Denmark, S. E.; Coe, D. M.; Pratt, N. E.; Griedel, B. D. J .
Org. Chem. 1994, 59, 6161-6163. (b) Keck, G. E.; Geraci, L. S.
Tetrahedron Lett. 1993, 34, 7827-7828. (c) Nakamura, K.; Nakamura,
H.; Yamamoto, Y. J . Org. Chem. 1999, 64, 2614-2615. (d) Nakajima,
M.; Saito, M.; Shiro, M.; Hashimoto, S. J . Am. Chem. Soc. 1998, 120,
6419-6420. (e) Yamada, K. Tozawa, T.; Nishida, M.; Mukaiyama, T.
Bull. Chem. Soc. J pn. 1997, 70, 2301-2308. (f) Sugimoto, K.; Aoyagi,
S.; Kibayashi, C. J . Org. Chem. 1997, 62, 2322-2323. (g) Bedeschi,
P.; Casolari, S.; Costa, A. L.; Tagliavini, E.; Umani-Ronchi, A.
Tetrahedron Lett. 1995, 36, 7897-7900. (h) Yu, C.-M.; Choi, H.-S.;
J ung, W.-H.; Lee, S.-S. Tetrahedron Lett. 1996, 37, 7095-7098. (i) Held,
C.; Fro¨hlich, R.; Metz, P. Angew. Chem., Int. Ed. 2001, 40, 1058-1060.
(20) (a) Sharma, G. V. M.; Krishnudu, K. Tetrahedron Lett. 1995,
36, 2661-2664. (b) Mandal, P. K.; Maiti, G.; Roy, S. C. J . Org. Chem.
1998, 63, 2829-2834. (c) Rochet, P.; Vatele, J .-M.; Gore, J . Synthesis
1994, 8, 795-799. (d) Frissen, A. E.; Marcelis, A. T.; Buurman, D. G.;
Pollmann, C. A. M.; Van der Plas, H. C. Tetrahedron 1989, 45, 5611-
5620.
(9) For alternative approaches to cacospongionolide B, see De Rosa,
M.; Soriente, A.; Giudo, S.; Scettri, A. Tetrahedron 2000, 56, 2095-
2102 and references therein. For the total synthesis of cacospongiono-
lide F, see Darntew, D.; Forsyth, C. J . Org. Lett. 2003, 5, 991-994.
(10) (a) Casaubon, R. L.; Snapper, M. L. Bioorg. Med. Chem. Lett.
2001, 11, 133-136. (b) Radeke, H. S.; Digits, C. A.; Casaubon, R. L.;
Snapper, M. L. Chem. Biol. 1999, 6, 639-647. (c) Radeke, H. S.; Digits,
C. A.; Bruner, S. D.; Snapper, M. L. J . Org. Chem. 1997, 62, 2823-
2831.
(11) (a) Ulmer, J . B.; Palade, G. E. Proc. Natl. Acad. Sci. U.S.A.
1989, 86, 6992-6996. (b) Sampath, D.; Varki, A.; Freeze, H. H. J . Biol.
Chem. 1992, 267, 4440-4455.
(12) (a) J ones, K. Am. Biotechnol. Lab. 1990, 8, 26-30. (b) Fretz,
H.; Albers, M. W.; Galat, A.; Standaert, R. F.; Lane, W. S.; Burakoff,
S. J .; Bierer, B. E.; Schreiber, S. L. J . Am. Chem. Soc. 1991, 113, 1409-
1411.
(13) Radeke, H. S.; Snapper, M. L. Bioorg. Med. Chem. 1998, 6,
1227-1232.
(14) Soriente, A.; De Rosa, M.; Scettri, A.; Sodano, G.; Terencio, M.
C.; Paya´, M.; Alcaraz, M. J . Curr. Med. Chem. 1999, 6, 415-431.
(15) For a review on the generating quaternary carbon centers, see
Martin, S. F. Tetrahedron 1980, 36, 419-460.
(16) (a) Stork, G.; Rosen, P.; Goldman, N. L. J . Am. Chem. Soc. 1961,
83, 2965-2966. (b) Stork, G.; Rosen, P.; Goldman, N. L.; Coombs, R.
V.; Tsuji, J . J . Am. Chem. Soc. 1965, 87, 275-286.
(17) (a) Swaminathan, S.; Newman, M. S. Tetrahedron 1958, 2, 88-
99. (b) Eder, U.; Sauer, G.; Wiechert, R. Angew. Chem., Int. Ed. Engl.
1971, 10, 496-497.
(21) (a) Kim, S.-H.; Bowden, N.; Grubbs, R. H. J . Am. Chem. Soc.
1994, 116, 10801-10802. (b) Kinoshita, A.; Mori, M. Synlett 1994,
1020-1022. (c) Kinoshita, A.; Mori, M. J . Org. Chem. 1996, 61, 8356-
8357. (d) Kinoshita, A.; Sakakibara, N.; Mori, M. J . Am. Chem. Soc.
1997, 119, 12388-12389. (e) Mori, M.; Sakakibara, N.; Kinoshita, A.
J . Org. Chem. 1998, 63, 6082-6083. For recent reviews, see (f) Poulsen,
C. S.; Madsen, R. Synthesis 2003, 1-18. (g) Mori, M. Top. Organomet.
Chem. 1998, 1, 133-154. For other examples of enyne metatheses,
see (h) Katz, T. J .; Sivavec, T. M. J . Am. Chem. Soc. 1985, 107, 737-
738. (i) Trost, B. M.; Tanoury, G. J . J . Am. Chem. Soc. 1988, 110, 1636-
1638. (j) Watanuki, S.; Ochifuji, N.; Mori, M. Organometallics 1994,
13, 4129-4130. (k) Chatani, N.; Morimoto, T.; Muto, T.; Murai, S. J .
Am. Chem. Soc. 1994, 116, 6049-6050. (l) Aubert, C.; Buisine, O.;
Malacria, M. Chem. Rev. 2002, 102, 813-834.
J . Org. Chem, Vol. 69, No. 17, 2004 5713

Cheung et al.
alkylidene 10. Optimized conditions provided the desired
diene 11 in nearly quantitative yield.
Å molecular sieves at room temperature to generate
enone 15 in 83% yield.26
With access to the two regions of the molecule, the key
reductive coupling reaction was investigated. Metal/
ammonia reduction of enone 15 provided the desired
enolate that was examined as the nucleophilic partner
in an alkylation of our electrophilic side-chain precur-
sor.27 Unfortunately, conditions could not be identified
where the enolate 16, used either directly from the metal/
ammonia reduction or derived from the corresponding
trimethylsilyl enol ether,28 provided more than a minor
amount (i.e., 10%) of the desired alkylation product.29 The
major byproducts from these reactions were diene 11 and
the protonated enolate (eq 4). Since conditions could be
found for alkylating other electrophiles, such as iodopen-
tane (75% yield), the problem apparently resided with
facile elimination of iodide 5.
Further functionalization of diene 11 into the requisite
homoallylic iodide 5 was examined next. A hydrozircona-
tion/iodination sequence was envisioned as an attractive
single-step solution;22 however, this strategy proceeded
in unacceptably low yields in our hands. A two-step
solution to the electrophilic side chain proved more
successful. Hydroboration of the diene 11 with 9-BBN
generated homoallylic alcohol 12 in 87% yield.23 Alcohol
12 was then converted to iodide 5 in 83% yield with PPh3,
imidazole, I2, in Et2O/CH3CN (3:1) at 0 °C.24
With the precursor to cacospongionolide’s oxygenated
side chain in hand, the preparation of the Decalin portion
was investigated. An asymmetric Robinson annulation,
as optimized by Hagiwara and Uda, appeared particu-
larly well-suited for preparing this functionality.25 Com-
mercially available 2-methyl-1,3-cyclohexadienone was
treated with ethyl vinyl ketone, KOH in methanol at 65
°C to provide triketone 14 in 95% yield. Compound 14 in
the presence of D-phenylalanine and camphorsulfonic acid
in DMF with careful control over reaction temperature,
30-70 °C, 10 °C per day for 4 days, generated enone 6
in 79% yield and 93% ee. Recrystallization from Et2O/
hexanes afforded enantiomerically pure material (i.e.,
>99% ee). Effective differentiation of the carbonyls was
achieved with ethylene glycol as solvent, p-TsOH, and 4
To improve the results of the alkylation, other condi-
tions, including various alkylation partners and addi-
tives, were studied. The enolate counterion and solvent
additives were investigated to moderate the basicity of
the enolate and perhaps favor the alkylation over the
elimination. The addition of HMPA/Me2Zn to the lithium
enolate30 provided only a few percent increase in the yield
of desired product. Similarly, modification of the enolate
with copper cyanide was also not successful.31 Metal
enolates generated from Et3B,32 n-Bu2BOTf,33 Ti(On-
35
Bu)4,34 and TiCl4 yielded similar results. Basically, the
hindered nature of the enolate and the enhanced acidity
of the homoallyic iodide presented an insurmountable
problem at this point. Without the key reductive coupling
reaction, we opted to explore an alternate approach that
would generate the dihydropyran functionality after the
key coupling reaction.
(22) (a) White, J . D.; Hanselmann, R.; Wardrop, D. J . J . Am. Chem.
Soc. 1999, 121, 1106-1107. (b) Nicolaou, K. C.; Bertinato, P.; Piscapio,
A. D.; Chakraborty, T. K.; Minowa, N. J . J . Chem. Soc. Chem. Commun.
1993, 619-622. (c) Ragan, J . A.; Nakatsuka, M.; Smith, D. B.; Uehling,
D. E.; Schreiber, S. L. J . Org. Chem. 1989, 54, 4267-4268. (d) Negishi,
E.; Takahashi, T. Aldirchim. Acta 1985, 2 (18), 31-47. (e) Bertelo, C.
A.; Schwartz, J . J . Am. Chem. Soc. 1976, 98, 262-264. (f) Hart, D.
W.; Schwartz, J . J . Am. Chem. Soc. 1974, 96, 8115-8116. (g) Schwartz,
J .; Labinger, J . A. Angew. Chem., Int. Ed. Engl. 1976, 15, 333-339.
For a recent review, see (h) Wipf, P.; J ahn, H. Tetrahedron 1996, 52,
12853-12910. (i) Wipf, P.; Kendall, C. Chem.sEur. J . 2002, 8, 1778-
1784. (j) Wipf, P.; Takahashi, H.; Nian, Z. Pure Appl. Chem. 1998, 70,
1077-1082. (k) Wipf, P.; Xu, W.; Takahashi, H.; J ahn, H.; Coish, P.
D. G. Pure Appl. Chem. 1997, 69, 639-644.
(23) (a) Chambers, M. S.; Thomas, E. J . J . Chem. Soc., Perkin Trans.
1 1997, 4, 417-431. (b) Kranz, M.; Kessler, H. Tetrahedron Lett. 1996,
37, 5359-5362. (c) Amoo, V. E.; Bernardo, De S.; Weigele, M.
Tetrahedron Lett. 1988, 29, 2401-2404.
(24) (a) Theodorakis, E. A.; Drouet, K. E. J . Am. Chem. Soc. 1999,
121, 456-457. (b) Tse, B. J . Am. Chem. Soc. 1996, 118, 7094-7100.
(c) Li, J .; Lan, J .; Liu, Z.; Li, Y.; Li, Y. Tetrahedron: Asymmetry 1996,
7, 2851-2862.
(25) (a) Hagiwara, H.; Uda, H. J . Org. Chem. 1988, 53, 2308-2311.
(b) Hajos, Z. G.; Parrish, D. R. J . Org. Chem. 1974, 12, 1615-1621. (c)
Eder, U.; Sauer, G.; Wiechert, R. Angew. Chem., Int. Ed. Engl. 1971,
10, 496-497. (d) Danishefsky, S.; Cain, P. J . Am. Chem. Soc. 1976,
98, 4975-4983. (e) Hiroi, K.; Yamada, S. I. Chem. Pharm. Bull. 1975,
23, 1103-1109. (f) Brown, K. L.; Damm, L.; Dunitz, J . D.; Eschenmoser,
A.; Hobi, R.; Kratky, C. Helv. Chim. Acta 1978, 61, 3108-3135. (g)
Agami, C.; Meynier, F.; Puchot, C.; Guilhem, J .; Pascard, C. Tetrahe-
dron 1984, 40, 1031-1038. (h) Agami, C.; Puchot, C.; Sevestre, H.
Tetrahedron Lett. 1986, 27, 1501-1504. (i) Puchot, C.; Samuel, O.;
Dunach, E.; Zhao, S.; Agami, C.; Kagan, H. B. J . Am. Chem. Soc. 1986,
108, 2353-2357. (j) Agami, C.; Puchot, C. J . Mol. Catal. 1986, 38, 341-
343. (k) Agami, C. Bull. Soc. Chim. Fr. 1987, 3, 499-507. (l) Hoang,
L.; Bahnmanyar, S.; Houk, K. N.; List, B. J . Am. Chem. Soc. 2003,
125, 16-17.
Alter n a tive Retr osyn th esis of Ca cosp on gion olid e
B. The failure of the key fragment coupling reaction
(26) Ciceri, P.; Demnitz, F. W. J . Tetrahedron 1997, 38, 389-390.
(27) For related alkylations, see (a) Ruggeri, R. B.; Hansen, M. M.;
Heathcock, C. H. J . Am. Chem. Soc. 1988, 110, 8734-8736. (b) Larsen,
S. D.; Monti, S. A. J . Am. Chem. Soc. 1977, 99, 8015-8020. (c) Corey,
E. J .; Hartmann, R.; Vatakencherry, P. A. 1962, 84, 2611-2614.
(28) (a) House, H. O.; Czuba, L. J .; Gall, M.; Olmstead, H. D. J . Org.
Chem. 1969, 34, 2324-2336. (b) Stork, G.; Hudrlik, P. F. J . Am. Chem.
Soc. 1968, 90, 4462-4464. (c) Stork, G.; Hudrlik, P. F. J . Am. Chem.
Soc. 1968, 90, 4464-4465. (d) Snitman, D. L.; Tsai, M.-Y.; Watt, D.
S.; Edwards, C. L.; Stotter, P. L. J . Org. Chem. 1979, 44, 2838-2842.
(e) Smith, A. B., III; Mewshaw, R. J . Org. Chem. 1984, 49, 3685-3689.
(29) Smith and Mewshaw observed no alkylation of similar lithium
enolate with trans-1-iodo-3-pentene: Smith, A. B., III; Mewshaw, R.
J . Org. Chem. 1984, 49, 3685-3689.
(30) Morita, Y.; Suzuki, M.; Noyori, R. J . Org. Chem. 1989, 54,
1785-1787.
(31) Posner, G. H.; Lentz, C. M. J . Am. Chem. Soc. 1979, 101, 934-
946.
(32) Rathke, M. W.; Lindert, A. Synthetic Commun. 1978, 8, 9-14.
(33) Evans, D. A.; Nelson, J . V.; Vogel, E.; Taber, T. R. J . Am. Chem.
Soc. 1981, 103, 3099-3111.
(34) Yachi, K.; Shinokubo, H.; Oshima, K. J . Am. Chem. Soc. 1999,
121, 9465-9466.
(35) Mukaiyama, T.; Banno, K.; Narasaka, K. J . Am. Chem. Soc.
1974, 96 (6), 7503-7509.
5714 J . Org. Chem., Vol. 69, No. 17, 2004

Syntheses of (+)/ (-)-Cacospongionolide B and E and Analogues
SCHEME 3. Syn th esis of Vin yl Keton e 19a
a
Reagents: (a) NaH, THF, 0 °C-rt; BrCH2CO2C2H5, 0 °C (84%
at 76% conv.). (b) LiOH, MeOH/H2O (3:1), rt. (c) MeO(Me)NH•HCl,
DIC, DMAP, TEA, CH2Cl2, 0 °C-rt (86%, two steps). (d) H2Cd
CHMgBr, THF, -78 to 0 °C, (83%).
20 through an etherification with ethyl bromoacetate in
84% yield (based on 76% conversion). The saponification
of ester 20 and conversion to the Weinreb amide proceeds
in 86% yield for the two steps.37 Addition of vinyl
Grignard into the Weinreb amide occurred in 83% yield
and completes the preparation of vinyl ketone 19 in an
enantiomerically enriched fashion.
The key Michael addition of the Decalin enolate into
the vinyl ketone 19 was explored next.38 In THF, the
Michael addition afforded only 25% of the desired alkyla-
tion product and 5% of multiple alkylation products.
Switching to Et2O proved to be the key for optimizing
the addition and suppressing undesired proton exchange
and over alkylation products. As depicted in eq 5, lithium
ammonia reduction of enone 15 in the presence of
t-BuOH generates the trans-decalin lithium enolate that
can be trapped by vinyl ketone 19 to provide the alkyla-
tion product 21 in 70% yield as a single diastereostere-
omer.39 The high diastereoselectivity of this reaction
arises through the kinetic facial selectivity of the Michael
addition. Approach of the electrophile from one face of
the Decalin enolate is blocked partially by the axial
methyl group. The other face, however, is free of such
steric encumbrance.40
F IGURE 2. Revised retrosynthesis of cacospongionolide B.
SCHEME 1. P r ep a r a tion of Hom oa llylic Iod id e 5a
a
(a) 9-BBN, THF, rt; H2O2, NaOH (87%). (b) PPh3, imid., I2,
Et2O/CH3CN, 0 °C (83%).
SCHEME 2. Asym m etr ic Robin son An n u la tion a
a
(a) Ethyl vinyl ketone, MeOH, KOH, 65 °C (95%). (b) D-Phe,
(+)-CSA, DMF, 30-70 °C (79%). (c) Ethylene glycol, p-TsOH, 4 Å
MS, rt (83%).
forced us to reexamine the synthetic plan. To circumvent
the problematic alkylation, we thought to introduce the
side chain via a Michael addition.36 This strategy would
require an R,â-unsaturated carbonyl, such as vinyl ketone
19, as the electrophilic precursor to the oxygenated side
chain (Figure 2). In this approach, the natural product’s
dihydropyran ring would be formed at a later stage in
the synthesis through a selective ring-closing metathesis
between the two olefins on the side-chain.
Tota l Syn th esis of Ca cosp on gion olid e B. Prepara-
tion of the vinyl ketone intermediate 19 was the initial
focus of our investigation. As illustrated in Scheme 3,
accessing the necessary vinyl ketone 19 should be pos-
sible from the same homoallyl alcohol 8 used in the first
generation strategy. Alcohol 8 can be converted to ester
With the alkylation product in hand, the next two steps
in the plan were to generate the dihydropyran ring and
complete the core structure of the natural product
(Scheme 4). Bis-olefination of diketone 21 was accom-
plished in the presence of the Ph3PdCH2 in DMSO.
Purification of the crude material by basic alumina
chromatography provided a 60% yield of the desired
triene 22.
(37) (a) Piscopio, A. D.; Minowa, N.; Chakraborty, T. K.; Kiode, K.;
Bertinato, P.; Nicolaou, K. C. J . Chem. Commun. 1993, 617-618. (b)
Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 39, 3815-3818.
(38) For examples of related transformations, see (a) Stork, G.;
Ganem, B. J . Am. Chem. Soc. 1973, 95, 6152-6153. (b) Stork, G.;
Singh, J . J . Am. Chem. Soc. 1974, 96, 6181-6182. (c) Boeckman, R.
K., J r. J . Am. Chem. Soc. 1973, 95, 6867-6869. (d) Boeckman, R. K.,
J r. J . Am. Chem. Soc. 1974, 96, 6179-6181.
(36) For related Michael reactions, see (a) Stork, G.; Ganem, B. J .
Am. Chem. Soc. 1973, 95, 6152-6153. (b) Marshall, J . A.; Fanta, W. I.
J . Org. Chem. 1964, 29, 2501-2505. (c) Boeckman, R. K., J r. J . Am.
Chem. Soc. 1973, 95, 6867-6869. (d) Boeckman, R. K., J r. J . Am.
Chem. Soc. 1974, 96, 6179-6181. (e) Stork, G.; Singh, J . J . Am. Chem.
Soc. 1974, 96, 6181-6182.
(39) The minor diastereomer resulting from the enantiomer of 19
was not detected by 1H NMR (i.e., <5%).
(40) For
a related example, see Winkler, J . D.; Rouse, M. B.;
Greaney, M. F.; Harrison, S. J .; J eon, Y. T. J . Am. Chem. Soc. 2002,
124, 9726-9728.
J . Org. Chem, Vol. 69, No. 17, 2004 5715

Cheung et al.
SCHEME 4. In sta lla tion of Dih yd r op yr a n Moietya
SCHEME 6. F in a l Step s to Ca cosp on gion olid e Ba
a
(a) 1 N HCl/THF (1:2), rt (90%). (b) Ph3PdCH2, DMSO, 75 °C
(84%). (c) O2, rose bengal, i-Pr2NEt, 150 W tungsten lamp, CH2Cl2
-78 °C (69%).
a
(a) Ph3PdCH2, DMSO, 75 °C (60%). (b) Cy3P(iMes)(Cl)2Rud
CHPh, CH2Cl2, rt (91%).
of 23 showed significant loss of the trisubstituted pyranyl
olefin, in addition to the desired reduction of the exo-
olefin.
SCHEME 5. Red u ction of Deca lin Olefin
To improve the diastereoselectivity of the hydrogena-
tion step, other potentially more selective catalyst/ligand
systems were considered.43 Interestingly, a more selective
reduction was obtained with the chiral rhodium catalyst
developed by Takaya and co-workers.44 As depicted in
Scheme 5, the (R)-BINAP/Rh(I) catalyst (20 mol %) in
CH2Cl2 at 30 °C under H2 (3 atm) provided the desired
isomer 24 in 91% yield (based on 76% conversion)45 in a
3.3:1 mixture of diastereomers. The desired isomer was
separated by silica gel chromatography. The double
diastereoselectivity imparted by the chiral ligand was
slight with the (S)-BINAP ligand providing the same
reduction product in a 3:1 diastereomeric ratio. This
result suggests that the improved diastereoselectivity of
this catalyst/ligand system was influenced by the steric
bulk of the BINAP ligands and not necessarily the result
of a matched/mismatched transition state. X-ray crystal-
lography confirmed that all the stereocenters of caco-
spongionolide B were in place in reduced intermediate
24.
Triene 22 is now ready for the intramolecular RCM
reaction. We were confident that the RCM reaction would
occur selectively generating the stable six-membered ring
with a trisubstituted olefin over the less stable alternate
ring closures. As anticipated, exposure of triene 22 to 8
mol % of Grubbs’ Ru metathesis catalyst41 in CH2Cl2
provided the dihydropyran 23 in 91% yield and completed
the carbon skeleton of cacospongionolide B.
With all the stereocenters in place, completion of the
synthesis required only the conversion of the protected
ketone to the exo-olefin and installation of the γ-hydroxy-
butenolide. As depicted in Scheme 6, removal of the
acetal (24) under acidic conditions and subsequent Wittig
olefination of the ketone with excess methyl triphen-
ylphosphonium ylide in DMSO at 75 °C proceeded to the
desired product in 76% overall for the two steps. Photo-
oxidation of the furan moiety under basic conditions46
unmasked the desired γ-hydroxybutenolide functionality
in 69% yield,47 thus completing the first total synthesis
With the natural product’s skeleton in place, the next
challenge was to introduce appropriate functionality
throughout the molecule. A diastereoselective reduction
of the exocyclic olefin on the Decalin (C8) to a methyl
group was explored first. Selective reductions of similar
olefins of trans-decalin ring systems were known with
catalysts such as Pd/C, Rh/C, Ir-black, and Pt-black.42
These catalysts, however, proved nonselective under a
variety of conditions, reducing both isolated olefins as
well as the furan ring in some cases. Wilkinson’s catalyst
[(Ph3P)3RhCl], on the other hand, reduced the desired
disubstituted olefin selectively in high yield (90%), how-
ever, with only modest diastereoselectivity (1.7:1) favor-
ing the desired product 24 (Scheme 5). Crabtree’s iridium
catalyst [(cod)IrPCy3Pyr]PF6 is known to be more reactive
than Wilkinson’s catalyst, and its use in the reduction
(43) For recent reviews on catalytic asymmetric hydrogenation, see
(a) Noyori, R. Acta Chem. Scand. 1996, 50, 380-390. (b) Nugent, W.
A.; RajanBabu, T. V.; Burk, M. J . Science 1993, 259, 479-483. (c)
Ratovelomanana-Vidal, V.; Geneˆt, J .-P. J . Organomet. Chem. 1998,
567, 163-171. (d) Helmchen, G.; Pfaltz, A. Acc. Chem. Res. 2000, 33,
336-345. (e) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345-
350.
(44) Otha, T.; Ikegami, H.; Miyake, T.; Takaya, H. J . Organomet.
Chem. 1995, 502, 169-176.
(45) Higher conversions gave significant amounts of pyran-olefin
reduction that can be removed at a later stage by RP-HPLC.
(46) Kernan, M. R.; Faulkner, D. J . J . Org. Chem. 1988, 53, 2773-
2776.
(47) For other syntheses of γ-hydroxybutenolide rings, see (a)
Boukouvalas, J .; Lachance, N. Synlett 1998, 31-32. (b) Gerlach, K.;
Hoffmann, H. M. R.; Wartchow, R. J . Chem. Soc., Perkin Trans. 1 1998,
3867-3872. (c) Katsumura, S.; Hori, K.; Fujiwara, S.; Isoe, S. Tetra-
hedron Lett. 1985, 26, 4625-4628. (d) Larcheveque, M.; Legueut, C.;
Debal, A.; Lallemand, J . Y. Tetrahedron Lett. 1981, 22, 1595-1598.
(41) Scholl, M.; Ding, S.; Lee, C. W.; Grubbs, R. H. Org. Lett. 1999,
1, 953-956.
(42) (a) Bruner, S. D.; Radeke, H. S.; Tallarico, J . A.; Snapper, M.
L. J . Org. Chem. 1995, 60, 1114-1115. (b) Almstead, J .-I. K.; Demuth,
T. P., J r.; Ledoussal, B. Tetrahedron: Asymmetry 1998, 9, 3179-3183.
(c) Marko´, I. E.; Wiaux, M.; Warriner, S. M.; Giles, P. R.; Eustace, P.;
Dean, D.; Bailey, M. Tetrahedron Lett. 1999, 40, 5629-5632. (d) Ling,
T.; Xiang, A. X.; Theodorakis, E. A. Angew. Chem., Int. Ed. Engl. 1999,
38, 3089-3091. (e) Kende, A. S.; Rustenhoven, J . J .; Zimmermann, K.
Tetrahedron Lett. 2000, 41, 843-846.
5716 J . Org. Chem., Vol. 69, No. 17, 2004

Syntheses of (+)/ (-)-Cacospongionolide B and E and Analogues
SCHEME 7. Syn th esis of Ca cosp on gion olid e Ea
a
(a) RhCl3•H2O, EtOH/CHCl3, 70 °C (70%). (b) 1O2, rose bengal,
i-Pr2NEt, CH2Cl2, -78 °C (54%).
complished utilizing cacospongionolide B’s synthetic strat-
egy. As depicted in Scheme 8, enone 15 was coupled with
the (S)-antipode of vinyl ketone 19 through a reduction/
Michael addition sequence to yield diketone 30 in 60%
as the major diastereomer. Bis-olefination of the diketone
30 provided a triene in 52% yield. RCM completed the
ring system of the side chain to give 31 in 88% yield.
As described previously, the diastereoselective reduc-
tion of the Decalin exo-olefin was affected with Rh(I)/(R)-
BINAP, and H2 (3 atm) in CH2Cl2 at room temperature
proceeds in 65% yield (90% conversion) as a mixture of
diastereomers (3.3:1) again favoring the stereochemistry
of the natural product.50 Acetal deprotection with aqueous
acid followed by Wittig olefination of the resulting ketone
gave a furan in 47% yield for the two steps. Photooxida-
tion with in situ generated singlet oxygen under basic
conditions unmasked the γ-hydroxybutenolide of ana-
logue 27 in 42% yield.
of (+)-cacospongionolide B (1). In an analogous fashion,
the enantiomer of the natural product was also prepared
using the appropriate enantiomeric catalysts and re-
agents.
Syn th esis of Ca cosp on gion olid e E. Cacospongiono-
lide E (2) differs from cacospongionolide B (1) only in the
position of its olefin in the Decalin ring system. On the
basis of biological studies, this region may be crucial for
inhibiting PLA2 activity.48 In any case, we expected that
a metal-catalyzed isomerization of the exo-olefin of 1
could be used to yield the more potent isomer 2.
The synthesis of cacospongionolide E (2) was ac-
complished in two steps from the furanyl intermediate
25 (Scheme 7). The exo-cyclic olefin of 25 was isomerized
to the more stable internal position utilizing RhCl3•H2O
in a mixture of EtOH/CHCl3 (1:1). Heating the mixture
to 70 °C for 6 days provided the desired olefin isomer-
ization product 26 in 70% yield with no detectable pyran
olefin isomerization by 1H NMR. Adduct 26 was con-
verted to cacospongionolide E in 54% yield by the same
photooxidation procedure used for completing compound
1. The spectroscopic data for the synthetic cacospon-
gionolide E matched data reported for the natural
product.
The synthetic route to cacospongionolides B and E
allowed us to begin exploring the structural requirements
necessary for these compound’s antiinflammatory activ-
ity. In this regard, the synthesis was particularly well-
suited to prepare diastereomers of the natural products.
For example, compound 27 includes a stereochemical
inversion at the dihydropyran ring. The use of the
enantiomeric side chain in the Michael addition could
give rise to this isomer. To access the alternate config-
uration for the C8 methyl group on the Decalin ring in
compound 28, the distereoselectivity of the exo-olefin
reduction could be inverted. In addition, as a step toward
preparing functional cacospongionolide probe reagents,
linker groups could be added to the natural product.49
For example, the exo-olefin on cacospongionolide B could
be converted to the N-Boc-protected linker 29.
As discussed earlier, the diastereoselectivity for the
reduction of the exo-olefin of compound 23 is moderate
(1.7:1) with the use of Wilkinson’s catalyst [(Ph3P)3RhCl].
This lack of selectivity in the hydrogenation allowed us
to use the minor diastereomer to access the structural
variant 28 (Scheme 9). The reaction mixture containing
the minor component in the hydrogenation51 can be
SCHEME 8. Syn th esis of Ca cosp on gion olid e
Dia ster eom er a
Syn th esis of Ca cosp on gion olid e Dia ster eom er s.
Preparation of analogue 27 with the S-configuration at
the stereocenter on the dihydropyran ring was ac-
a
Reagents: (a) Li/NH3, t-BuOH, Et2O, -33 °C; (S)-19, Et2O,
(48) De Rosa, S.; Crispino, A.; De Giulio, A.; Iodice, C.; Benrezzouk,
R.; Terencio, M. C.; Ferra´ndiz, M. L.; Alcaraz, M. J .; Paya´, M. J . Nat.
Prod. 1998, 61, 931-935.
(49) (a) Casaubon, R. L.; Snapper, M. L. Chem. Biol. 1999, 6, 639-
647. (b) Radeke, H. S.; Snapper, M. L. Bioorg. Med. Chem. 1998, 6,
1227-1232.
-78 °C (60%). (b) Ph3PdCH2, DMSO, 75 °C (52%). (c) Cy3P(iMes)-
(Cl)2RudCHPh, CH2Cl2, rt (88%). (d) [(cod)RhCl]2, (R)-BINAP, H2
(3 atm), CH2Cl2, 30 °C (65%). (e) 1 N HCl/THF (1:2), rt. (f)
Ph3PdCH2, DMSO, 75 °C (47%, two steps). (g) O2, rose bengal,
i-Pr2NEt, 150 W tungsten lamp, CH2Cl2, -78 °C (42%).
J . Org. Chem, Vol. 69, No. 17, 2004 5717

Cheung et al.
SCHEME 9. Syn th esis of Ca cosp on gion olid e
Dia ster eom er a
due to the absence of the exo-olefin on the Decalin,
functionality that is likely to also react with singlet
oxygen.
a
(a) Ph3PdCH2, DMSO, 75 °C (79%). (b) O2, rose bengal,
i-Pr2NEt, 150 W tungsten lamp, CH2Cl2, -78 °C (62%).
SCHEME 10. Syn th esis of C4 Lin k er An a logu e 29a
a
(a) NaBH4, EtOH (80%). (b) EDCI, DMAP, BocHN(CH2)5CO2H,
CH2Cl2 (66%). (c) O2, rose bengal, i-Pr2NEt, 150 W tungsten lamp,
CH2Cl2 -78 °C (87%).
In h ibition of Bee Ven om sP LA2. With access to
cacospongionolide B (1), cacospongionolide E (2), and
structural variants, we could begin screening these
natural and nonnatural compounds against sPLA2. We
chose to first evaluate the potency of the inhibitors
utilizing an in vitro enzyme activity assay.53 The ease of
use and microtiter plate nature of the in vitro sPLA2
assay makes it particularly well-suited for rapid com-
pound screening.54
deprotected to ketone 33 and then isolated by RP-HPLC
to provide the desired diastereomer in approximately 23%
yield. The ketone 33 was then converted in two steps to
the corresponding diastereomeric analogue 28. Treat-
ment of ketone 33 with excess Ph3PCH2 then oxidation
provides the R-diastereomeric structural variant 28 in
62% yield.
Syn th esis of C4 Lin k er Va r ia n t. Analogue 29 was
pursued to determine the feasibility of linking reporter
groups at the C4 position on the Decalin ring. The
compound’s preparation was achieved in three steps from
the cacospongionolide B precursor 34 (Scheme 10). Di-
astereoselective reduction of the ketone was affected with
NaBH4 in EtOH to give a 7:1 mixture of diastereomeric
alcohols in 80% favoring compound 35.52 Isolation by
silica gel chromatography and coupling with N-Boc-6-
aminohexanoic acid in the presence of EDCI and DMAP
in CH2Cl2 provided the C4 esterified precursor 36 in 66%
yield. Unmasking of the γ-hydroxybutenolide moiety with
in situ generated singlet oxygen under basic conditions
gave access to the desired Decalin-substituted variant 29
in 87% yield. The higher yield for this oxidation may be
Manoalide (2), a known inhibitor of bee venom sPLA2,
(+)-cacospongionolide B (1) and its enantiomer (-)-1, and
their respective furan precursors, (+)-25 and (-)-25, were
compared. The inhibition of each compound was deter-
mined over a range of concentrations (400-1 µM), and
from those results, the IC50 values were calculated. The
results are summarized in Table 1. Several important
aspects about the interaction between the substrates and
their target enzyme were noted. The natural enantiomer
of cacospongionolide, (+)-1, is two times more potent as
an inhibitor of sPLA2 than the unnatural enantiomer (-)-
1, suggesting that the inhibition of bee venom sPLA2 is
enantiospecific. Also of note, the furan precursor with the
natural stereochemistry (+)-25 shows potency greater
than that of the γ-hydroxybutenolide containing (-)-
(50) At higher conversions, lower yields of the desired compound
are obtained due to reduction of the trisubstituted dihydropyran olefin.
(51) Contaminated with starting material and over-reduced materi-
als.
(52) (a) Dutcher, J . S.; Macmillan, J . G.; Heathcock, C. H. J . Org.
Chem. 1976, 41, 2663-2669. (b) Radeke, H. S.; Digits, C. A.; Bruner,
S. D.; Snapper, M. L. J . Org. Chem. 1997, 62, 2823-2831.
(53) Most sPLA2-inhibitory assays involve live animals or radiola-
beled phospholipids. For examples, see (a) Garcia Pastor, P.; De Rosa,
S.; De Giulio, A.; Paya´, M.; Alcaraz, M. J . Brit. J . Pharm. 1999, 126,
301-311. (b) Potts, B. C. M.; Faulkner, D. J .; J acobs, R. S. J . Nat.
Prod. 1992, 55, 1701-1717.
(54) sPLA2 Assay Kit from Cayman Chemical, Ann Arbor, MI
(Catalog 765001).
5718 J . Org. Chem., Vol. 69, No. 17, 2004

Syntheses of (+)/ (-)-Cacospongionolide B and E and Analogues
TABLE 1. sP LA2 In h ibition
Su m m a r y
inhibitor
IC50 (µM)
The first total syntheses of (+)- and (-)-cacospongiono-
lide B have been accomplished in a longest linear
sequence of 12 steps from commercially available starting
materials with an overall yield of 8%. The pivotal
transformations include a key three-step reaction se-
quence that couples the two fragments of the natural
product and generates the dihydropyran ring. In addition,
cacospongionolide E and other structural variants of the
natural product were prepared through this synthetic
strategy.
manoalide (3)
38 ((3)
49 ((9)
114 ((7)
72 ((4)
167 ((32)
27 ((7)
19 ((7)
29 ((13)
44 ((21)
(+)-cacospongionolide B (1)
(-)-cacospongionolide B (1)
(+)-furan (25)
(-)-furan (25)
Cacospongionolide E (2)
(S)-dihydropyran (27)
C8-methyl diastereomer (28)
C4-linker (29)
cacospongionolide B. This result suggests that more than
the pyranofuranone portion of the molecule interacts with
the enzyme and that the hydrophobic region may provide
some yet-to-be-determined role in inhibition.55
The synthetic cacospongionolides were utilized to
investigate the structural requirements necessary for
secretory phospholipase A2 inhibition. Eight compounds
including the natural product, its enantiomer, synthetic
precursors to the natural product, cacospongionolide E
(2), two diastereomeric variants of cacospongionolide B
(27 and 28), and a linker-containing analogue 29 have
been prepared and compared with manoalide. The activ-
ity of these analogues against bee venom sPLA2 suggests
that binding is enantiospecific and independent of the
γ-hydroxybutenolide moiety. This information provides
new directions for the preparation of more potent sPLA2
inhibitors.
Compound 27, which incorporates a stereochemical
inversion on the dihydropyran ring, is the most potent
inhibitor with an IC50 value of 19 µM. The result suggests
that the stereocenter on the dihydropyran ring is rel-
evant. The diastereomeric methyl group on the Decalin
ring of compound 28 does not adversely affect its binding
to the enzyme. The addition of the N-Boc-protected linker
to the C4 position of variant 29, however, does appear to
reduce its ability to inhibit bee venom sPLA2. Cacospon-
gionolide E (2) with an IC50 value of 27 µM has been
shown to be a more potent inhibitor than cacospongiono-
lide B (1) due to its internal olefin on the Decalin ring.56
A major modification to the C4 region, such as variant
29, however, was detrimental toward enzyme activity.
The assay results suggest that for use as a molecular
probe, conjugation of cacospongionolide analogues to
reporter functionality at the C8 methyl may provide a
more effective strategy.
Ack n ow led gm en t. We thank Dr. Salvatore De Rosa
for providing authentic samples of cacospongionolide B.
We also thank Mr. J arred Blank for assistance with
X-ray crystallographic studies and Schering-Plough for
X-ray facility support. The NIH (CA R01-66617) is
acknowledged for the financial support of this research.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures and data on new compounds (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(55) Glaser, K. B.; De Carvalho, S. M.; J acobs, R. S.; Kernan, M.
R.; Faulkner, D. J . Mol. Pharmocol. 1989, 36, 782-788 and ref 3c.
(56) De Rosa, S.; Crispino, A.; De Giulio, A.; Iodice, C.; Benrezzouk,
R.; Terencio, M. C.; Ferra´ndiz, M. L.; Alcaraz, M. J .; Paya´, M. J . Nat.
Prod. 1998, 61, 931-935.
J O049285E
J . Org. Chem, Vol. 69, No. 17, 2004 5719