Enantioselective total synthesis of avrainvillamide and the stephacidins

Article abstract of DOI:10.1021/ja061660s

In this article, full details regarding our total synthesis of avrainvillamide and the stephacidins are presented. After an introduction and summary of prior synthetic studies in this family of structurally complex anticancer natural products, the evoluti

Full text of DOI:10.1021/ja061660s

Published on Web 06/14/2006
Enantioselective Total Synthesis of Avrainvillamide and the
Stephacidins
Phil S. Baran,* Benjamin D. Hafensteiner, Narendra B. Ambhaikar,
Carlos A. Guerrero, and John D. Gallagher
Contribution from the Department of Chemistry, The Scripps Research Institute,
10550 North Torrey Pines Road, La Jolla, California 92037
Received March 9, 2006; E-mail: pbaran@scripps.edu
Abstract: In this article, full details regarding our total synthesis of avrainvillamide and the stephacidins
are presented. After an introduction and summary of prior synthetic studies in this family of structurally
complex anticancer natural products, the evolution of a final synthetic approach is described. Thus, a
thorough description of three separate model studies is provided for construction of the characteristic bicyclo-
[2.2.2]diazaoctane ring system common to these alkaloids. The first and second approaches sought to
build the core using formal Diels-Alder and vinyl radical pathways, respectively. Although these strategies
failed in their primary objective, they fostered the development of a new and mechanistically intriguing
method for the synthesis of indolic enamides such as those found in numerous bioactive natural products.
The scope and generality of this simple method for the direct dehydrogenation of tryptophan derivatives is
described. Finally, details of a third and successful route to the core of these alkaloids are described which
features oxidative C-C bond formation. Specifically, the first heterocoupling of two different types of carbonyl
species (ester and amide) is accomplished in good yield, on a preparative scale, and with complete
stereocontrol. The information gained in these model studies enabled an enantioselective total synthesis
of stephacidin A. The absolute configuration of these alkaloids was firmly established in collaboration with
Professor William Fenical. A full account of our successful efforts to convert stephacidin A into stephacidin
B via avrainvillamide is presented. Finally, the first analogues of these natural products have been prepared
and evaluated for anticancer activity.
by a Pfizer group in Japan and named CJ-17,665.6 The signature
bicyclo[2.2.2]diazaoctane ring system common to these alkaloids
has inspired numerous synthetic approaches.
Birch,7 Sammes,8 and Williams9 have proposed that this ring
system is derived biosynthetically from a Diels-Alder reaction.
This hypothesis has influenced several creative synthetic strate-
Introduction
In 2002, scientists from Bristol Myers Squibb added a new
entry to the list of structurally intriguing and bioactive indole
alkaloids with the discovery of stephacidins A and B (1 and 2
in Figure 1, respectively).1 Isolated from a fungal species found
in an Indian clay sample, they show obvious structural similari-
ties to the brevianamides,2 paraherquamides,3 and asperpara-
lines.4 In 2000, the Fenical group at the Scripps Institution of
Oceanography disclosed the structure of avrainvillamide (3) in
a patent.5 This compound was independently described in 2001
(5) Fenical, W.; Jensen, P. R.; Cheng, X. C. Avrainvillamide, a Cytotoxic
Marine Natural Product, and Derivatives thereof. U.S. Patent 6,066,635,
2000.
(6) Sugie, Y.; Hirai, H.; Inagaki, T.; Ishiguro, M.; Kim, Y.-S.; Kojima, Y.;
Sakakibara, T.; Sakemi, S.; Sugiura, A.; Suzuki, Y.; Brennan, L.; Duignan,
J.; Huang, L. H.; Sutcliffe, J.; Kojima, N. J. Antibiot. 2001, 54, 911-916.
(7) (a) Birch, A. J.; Wright, J. J. Tetrahedron 1970, 26, 2329-2344. (b) Baldas
J.; Birch, A. J.; Russell, R. A. J. Chem. Soc., Perkin Trans. 1 1974, 50-
52.
(1) (a) Qian-Cutrone, J.; Huang, S.; Shu, Y.-Z.; Vyas, D.; Fairchild, C.;
Menendez, A.; Krampitz, K.; Dalterio, R.; Klohr, S. E.; Gao, Q. J. Am.
Chem. Soc. 2002, 124, 14556-14557. (b) Qian-Cutrone, J.; Krampitz, K.
D.; Shu, Y.-Z.; Chang, L.-P.; Lowe, S. E. Stephacidin Antitumor Antibiotics.
U.S. Patent 6,291,461, 2001.
(8) Porter, A. E. A.; Sammes, P. G. J. Chem. Soc., Chem. Commun. 1970,
1103.
(2) (a) Birch, A, J.; Wright, J. J. J. Chem. Soc., Chem. Commun. 1969, 644-
645. (b) Birch, A. J.; Russell, R. A. Tetrahedron 1972, 28, 2999-3008.
(c) Coetzer, J. Acta Crystallogr. 1972, B30, 2254-2256. (d) Wilson, B.
J.;Yang, D. T. C.; Harris, T. M. Appl. Microbiol. 1973, 76, 633-635.
(3) (a) Yamazaki, M.; Okuyama, E.; Kobayashi, M.; Inoue, H. Tetrahedron
Lett. 1981, 22, 135-136. (b) Blanchflower, S. E.; Banks, R. M.; Everett,
J. R.; Reading, C. J. Antibiot. 1993, 46, 1355-1363. (c) Ondenyka, J. G.;
Goegelman, R. T.; Schaeffer, J. M.; Keleman, L.; Zitano, L. J. Antibiot.
1990, 43, 1375-1379. (d) Liesch, J. M.; Wichmann, C. F. J. Antibiot. 1990,
43, 1380-1386. (e) Banks, R. M.; Blanchflower, S. E.; Everett, J. R.;
Manger, B. R.; Reading, C. J. Antibiot. 1997, 50, 840-846. (f) Blanch-
flower, S. E.; Banks, R. M.; Everett, J. R.; Manger, B. R.; Reading, C. J.
Antibiot. 1993, 46, 1355-1363.
(9) (a) Stocking, E. M.; Sanz-Cervera, J. F.; Williams, R. M. Angew. Chem.,
Int. Ed. 2001, 40, 1296-1298. (b) Williams, R. M.; Glinka, T.; Kwast, E.;
Coffman, H.; Stille, J. K. J. Am. Chem. Soc. 1990, 112, 808-821. (c) Sanz-
Cervera, J. F.; Glinka, T.; Williams, R. M. Tetrahedron 1993, 49, 8471-
8482. (d) Domingo, L. R.; Sanz-Cervera, J. F.; Williams, R. M.; Picher,
M. T.; Marco, J. A. J. Org. Chem. 1997, 62, 1662-1667. (e) Williams, R.
M.; Sanz-Cervera, J. F.; Sanceno´n, F.; Marco, J. A.; Halligan, K. M. Biorg.
Med. Chem. 1998, 6, 1233-1241. (f) Sanz-Cervera, J. F.; Williams, R.
M.; Marco, J. A.; Lo´pez-Sa´nchez, J. M.; Gonza´lez, F.; Martinez, M. E.;
Sanceno´n, F. Tetrahedron 2000, 56, 6345-6358. (g) Stocking, E. M.; Sanz-
Cervera, J. F.; Williams, R. M. J. Am. Chem. Soc. 2000, 122, 1675-1683.
(h) Adams, L. A.; Gray, C. R.; Williams, R. M. Tetrahedron Lett. 2004,
45, 4489-4493. For very informative reviews, see: (i) Williams, R. M.;
Stocking, E. M.; Sanz-Cervera, J. F. Top. Curr. Chem. 2000, 209, 97-
173. (j) Williams, R. M.; Cox, R. J. Acc. Chem. Res. 2003, 36, 127-139.
(k) Stocking, E. M.; Williams, R. M. Angew. Chem., Int. Ed. 2003, 42,
3078-3115.
(4) (a) Hayashi, H.; Nishimoto, Y.; Nozaki, H. Tetrahedron Lett. 1997, 38,
5655-5658. (b) Hayashi, H.; Nishimoto, Y.; Akiyama, K.; Nozaki, H.
Biosci. Biotechnol. Biochem. 2000, 64, 111-115. (c) Steyn, P. S.
Tetrahedron 1973, 29, 107-120.
9
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Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Figure 1. The stephacidins and their presumed biogenetic origins.
Our interest in these natural products initially stemmed from
their considerable potential to inspire the development of new
chemistry; they later became an ideal stage to showcase the
power of oxidative enolate coupling in synthesis (Vide infra).
In this article we trace the evolution of our synthetic strategy,
a journey which culminated in the total syntheses of stephacidins
A and B and avrainvillamide.13 Aside from successfully applying
a strategy employing the first stereocontrolled oxidative enolate
heterocoupling to forge the bicyclo[2.2.2]diazaoctane nucleus,
several challenges were addressed including: (1) the design of
a scalable route to the benzopyran-containing subunit; (2) precise
selection and choreography of functional group masking devices;
(3) chemo- and position-selective oxidative conversion of
stephacidin A (1) into avrainvillamide (3); (4) the dimerization
of avrainvillamide to stephacidin B (2); and (5) determination
of the absolute configuration of this class of alkaloids. The
difficulty of this final issue was compounded by the fact that
the optical rotation of the stephacidins was not recorded.1
Results and Discussion
First Generation Retrosynthetic Analysis and Develop-
ment of a New Method for Tryptophan Dehydrogenation.
The obvious potential complications associated with a laboratory
conversion of stephacidin A into stephacidin B were temporarily
put aside, and the former was targeted for synthesis as the
precursor to the latter. Our first generation retrosynthetic analy-
sis, depicted in Figure 3, hinged on the use of a formal Diels-
Alder reaction to construct both the bicyclic skeleton and preny-
lated indole subunits in one step.14 Unlike the achiral intermedi-
Figure 2. Approaches to construct the bicyclo[2.2.2]diazaoctane nucleus.
gies for its construction, as shown in Figure 2. Thus, the groups
of Williams9 and Liebscher10 independently developed Diels-
Alder strategies with varying levels of success; the Williams
group also developed a novel stereoselective intramolecular SN2′
alkylation approach for the construction of this bicyclic core.9,11
In Myers’s recent total synthesis of stephacidin B, an elegant
acyl radical approach was utilized.12
(13) (a) Baran, P. S.; Guerrero, C. A.; Ambhaikar, N. A.; Hafensteiner, B. D.
Angew. Chem., Int. Ed. 2005, 44, 606-609. (b) Baran, P. S.; Guerrero, C.
A.; Hafensteiner, B. D.; Ambhaikar, N. A. Angew. Chem. Int. Ed. 2005,
44, 3892-3895.
(14) For examples of Diels-Alder reactions of 3-vinyl indoles: (a) Noland,
W. E.; Xia, G.-M.; Gee, K. R.; Konkel, M. J.; Wahlstrom, M. J.; Condoluci,
J. J.; Rieger, D. L. Tetrahedron 1996, 52, 4555-4572. (b) Gonzalez, E.;
Pindur, U.; Schollmeyer, D. J. Chem. Soc., Perkin Trans. 1 1996, 1767-
1771. (c) Le Strat, F.; Maddaluno, J. Org. Lett. 2002, 4, 2791-2793. (d)
Grieco, P. A.; Kaufman, M. D. J. Org. Chem. 1999, 64, 7586-7593. (e)
(10) Jin, S.; Wessig, P.; Liebscher, J. J. Org. Chem. 2001, 66, 3984-3997.
(11) For studies toward stephacidin A from the Williams group see: (a) Adams,
L. A.; Gray, C. R.; Williams, R. M. Tetrahedron Lett. 2004, 45, 4489-
4493. (b) Grubbs, A. W.; Artman, G. D., III; Williams, R. M. Tetrahedron
Lett. 2005, 46, 9013-9016.
(12) (a) Myers, A. G.; Herzon, S. B. J. Am. Chem. Soc. 2003, 125, 12080-
12081. (b) Herzon, S. B.; Myers, A. G. J. Am. Chem. Soc. 2005, 127, 5342-
5344.
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A R T I C L E S
Baran et al.
Figure 3. First generation retrosynthetic analysis for stephacidin A.
Scheme 1. Synthesis of Indolic Enamides from Tryptophans via a
ates in the proposed biosynthesis, this cyclization event benefited
from the presence of a quarternary center that could be used to
govern the absolute stereochemical outcome of the reaction. This
approach, although concise, was not without its potential pitfalls
and drawbacks. Although molecular models showed that the
diene/dienophile pretransition state assembly was nearly perfect,
there was no reason to assume a priori that the dienophile would
ever find itself in proximity to the diene. (This was based on
the assumption that the enamide olefin could be isomerized to
the E geometry (not shown) in the course of the cycloaddition.
Alternatively, the directly accessible Z geometry (see 5) would
be acceptable if the proposed cycloaddition reaction took place
in a stepwise rather than concerted manner.) Additionally, two
quaternary stereocenters would be generated in the course of
the cycloaddition. This consideration did not heavily influence
our planning, however, because it had been proposed by Birch,7
Sammes,8 and Williams9 that an equally complex event takes
place in the biosynthesis of the brevianamides (Figure 2). The
requisite substrate diene 5 could conceivably be accessed from
saturated diketopiperazine 6 Via a dehydrogenation event.
Peptide coupling of the proline fragment 7 with indole 8
followed by diketopiperazine formation should furnish 6.
The success of the “Diels-Alder” approach was contingent
on the availability of a mild protocol for the dehydrogenation
of tryptophans (e.g., 6 f 5). Known methods allow access to
these types of compounds; however, they typically involve
multistep syntheses.15 For instance, a published method for the
synthesis of simple tryptophan derivative 10 (Scheme 1) utilizes
a Wittig-type reaction with indole-3-carboxaldehyde. The
enzymatic dehydrogenation of certain tryptophans has been
reported by Hammadi using L-tryptophan-2′,3′-oxidase from
Chromobacterium Violaceum (ATCC 12472).16 Using DDQ as
the oxidant, certain indolic enamides have been shown to form
Cascade Ene/Group Transfer/Tautomerization Processa
Pindur, U.; Otto, C.; Molinier, M.; Massa, W. HelV. Chem. Acta 1991, 74,
727-738. (f) Pindur, U.; Kim, M. H.; Rogge, M.; Massa, W.; Molinier,
M. J. Org. Chem. 1992, 57, 910-915. (g) Fukazawa, T.; Shimoji, Y.;
Hashimoto, T. Tetrahedron: Asymmetry 1996, 7, 1649-1658. (h) Beccalli,
E. M.; Marchesini, A.; Pilati, T. Tetrahedron 1996, 52, 3029-3036. (i)
Wolter, M.; Borm, C.; Merten, E.; Wartchow, R.; Winterfeldt, E. Eur. J.
Org. Chem. 2001, 4051-4060.
a CAN ) ceric ammonium nitrate, DDQ ) 2,3-dichloro-5,6-dicyano-
1,4-benzoquinone.
in good yields from tryptophans possessing strongly electron-
withdrawing substituents such as a trifluoroacetyl group.17 Such
an approach, however, has not been employed in the dehydro-
genation of diketopiperazines such as 6. The model tryptophan
(15) (a) Krasovsky, A. L.; Druzhinin, S. V.; Nenajdenko, V. G.; Balenkova, E.
S. Tetrahedron Lett. 2004, 45, 1129-1132. (b) Hermkens, P. H. H.; Van
Maarseveen, J. H.; Berens, H. W.; Smits, J. M. M.; Kruse, C. G.; Scheeren,
H. W. J. Org. Chem. 1990, 55, 2200-2206. (c) Babievskii, K. K.;
Davidovich, Y. A.; Bakhmutov, V. I.; Rogozhin, S. V. Bull. Acad. Sci.
USSR DiV. Chem. Sci. 1988, 3, 649-653. (d) Harrington, P. J.; Hegedus,
L. S. J. Org. Chem. 1984, 49, 2657-2662.
(16) (a) Hammadi, A.; Me´nez, A.; Genet, R. Tetrahedron Lett. 1996, 37, 3309-
3312. (b) Hammadi, A.; Lam, J.; Gondry, M.; Me´nez, A.; Genet, R.
Tetrahedron 2000, 56, 4473-4477.
(17) (a) Yoshioka, T.; Mohri, K.; Oikawa, Y.; Yonemitsu, O. J. Chem. Res.
Miniprint. 1981, 2252-2281. (b) Oikawa, Y.; Yonemitsu, O. J. Org. Chem.
1977, 42, 1213-1216.
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Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Table 1. Direct Dehydrogenation of Tryptophan Derivatives
9 was screened with a variety of oxidants such as DDQ, CAN,
IBX, and Pd/C/O2 to elicit conversion to the enamide 10.
Although DDQ in THF17 led to the disappearance of the starting
material and the formation of polar byproducts, 10 could not
be detected. Faced with these failures, a new method was invent-
ed which exploited the unique reactivity of these systems. In
principle, the enophilic π bond of indole could be exploited such
that it could form an adduct susceptible to elimination (Cope-
like) followed by tautomerization to the desired R,â-unsaturated
system. This strategy would fill the gap in current methodology
and avoid the use of harsh oxidizing agents. Since indoles readily
react with singlet oxygen18 and triazolinediones19 it was reasoned
that nitroso compounds20,21 would react with similar facility.
Unfortunately, simply stirring nitrosobenzene and the tryptophan
9 in a wide variety of solvents at 20 °C or at elevated temper-
atures led to a full recovery of the starting material.
Inspired by Yamamoto’s work on Lewis and Brønsted acid
catalyzed reactions of enolates and enamines with nitroso com-
pounds,21 we decided to perform the above reaction in the pres-
ence of such acids. Indeed, when ZrCl4 (1 equiv) was added to a
solution of nitrosobenzene (2.5 equiv) and 9 in CH2Cl2, dehydro-
genated product 10 was isolated in 82% yield as a single alkene
isomer. The structure and Z-olefin geometry was confirmed by
X-ray crystallography as shown in Scheme 1. Mechanistically,
one of two pathways may be operative as depicted in Scheme
1 (paths A and B). These mechanisms differ by their initial
step: electrophilic attack by the indole on the nitrogen or oxygen
of nitrosobenzene. In path A, formation of aziridine N-oxide A
followed by fragmentation and Cope-like elimination of phe-
nylhydroxylamine from N-oxide B furnishes methylene indo-
lenine C which undergoes immediate tautomerization to 10. In
path B, Lewis or Brønsted activation of nitrosobenzene at nitro-
gen elicits conversion to O-linked intermediate D, which might
immediately fragment to C and phenylhydroxylamine. In light
of Yamamoto’s pioneering studies in this area, path B seems
to be more likely as O-linked adducts are generally formed with
nitroso compounds in the presence of acidic additives.
Several acids and solvents were screened using the phthal-
imide-protected tryptophan 11 (see Supporting Information for
a tabular comparison of acids tested). The reaction appears to
have reasonable generality using ZrCl4 as shown in Table 1.
Most importantly, this reaction is ideally suited to the task of
dehydrogenating unprotected diketopiperazines (entries 6 and
(18) (a) Nakagawa, M.; Watanabe, H.; Kodato, S.; Okajima, H.; Hino, T.;
Flippen, J. L.; Witkop, B. Proc. Natl. Acad. Sci. U.S.A. 1977, 74, 4730-
4733. (b) Weil, L.; Gordon, W. G.; Buchert, A. R. Arch. Biochem. Biophys.
1951, 33, 90-109. (c) Fontana, A.; Tonlolo, C. Fortschr. Chem. Org.
Naturst. 1976, 33, 309-449. (d) Nakagawa, M.; Kato, S.; Kataoka, S.;
Hino, T. J. Am. Chem. Soc. 1979, 101, 3136-3137. (e) Nakagawa, M.;
Kaneko, T.; Yoshikawa, K.; Hino, T. J. Am. Chem. Soc. 1974, 96, 624-
625. (f) Saito, I.; Matsuura, T.; Nakagawa, M.; Hino, T. Acc. Chem. Res.
1977, 10, 346-352.
(19) (a) Baran, P. S.; Guerrero, C. A.; Corey, E. J. Org. Lett. 2003, 5, 1999-
2001. (b) Baran, P. S.; Guerrero, C. A.; Corey, E. J. J. Am. Chem. Soc.
2003, 125, 5628-5629.
(20) (a) Banks, R. E.; Barlow, M. G.; Haszeldine, R. N. J. Chem. Soc. 1965,
4714-4718. For reviews, see: (b) Adam, W.; Krebs, O. Chem. ReV. 2003,
103, 4131-4146. (c) Johannsen, M.; Jørgensen, K. A. Chem. Rev. 1998,
98, 1689-1708. (d) Leach, A. G.; Houk, K. N. Chem. Commun. 2002,
1243-1255. (e) Leach, A. G.; Houk, K. N. Org. Biomol. Chem. 2003, 1,
1389-1403.
(21) (a) Yamamoto, Y.; Yamamoto, H. J. Am. Chem. Soc. 2004, 126, 4128-
4129. (b) Yamamoto, Y.; Momiyama, N.; Yamamoto, H. J. Am. Chem.
Soc. 2004, 126, 5962-5963. (c) Momiyama, N.; Yamamoto, H. J. Am.
Chem. Soc. 2005, 127, 1080-1081. (d) Momiyama, N.; Torii, H.; Saito,
S.; Yamamoto, H. Proc. Nat. Acad. Sci. U.S.A. 2004, 101, 5374-5378.
(e) Momiyama, N.; Yamamoto, H. Angew. Chem., Int. Ed. 2002, 41, 2986-
2988. (f) Yamamoto, H.; Momiyama, N. J. Chem. Soc., Chem. Commun.
2005, 3514-3525.
a Entries 1, 3, and 4 were run in toluene. b Isolated yield. c Yields based
on recovered starting material.
7) and will be discussed in further detail below. It should be
noted that other substituted nitroso compounds (nitrosotoluene
and 4-nitrosodiphenylamine) did not promote the reaction.
Significantly, this reaction may provide facile access to natural
products22 bearing unsaturated tryptophan or tryptamine subunits
from their readily available saturated analogues.
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A R T I C L E S
Baran et al.
Scheme 2. Synthesis of Reverse Prenylated Diketopiperazine 20a
a Reagents and conditions: (a) LHMDS (1.1 equiv), THF, -78 °C, 35
min; then 24 (1.1 equiv), -78 f 20 °C, 2 h, 83%; (b) p-TsOH (1.0 equiv),
toluene, reflux, 2 h, 84%; (c) Boc-L-Trp-OH (1.0 equiv), BOPCl (1.1 equiv),
i-Pr2EtN (3.0 equiv), CH2Cl2, 20 °C, 12 h, 48%; (d) 190 °C, neat, 45 min,
54%; (e) PhNO (2.5 equiv), ZrCl4 (1.0 equiv), CH2Cl2, 20 °C, 2.5 h, 92%.
LHMDS ) lithium bis(trimethylsilyl)amide, THF ) tetrahydrofuran,
p-TsOH ) p-toulenesulfonic acid monohydrate, BOPCl ) bis(2-oxo-3-
oxazolidinyl)phosphinic chloride.
Figure 4. Second and third generation approaches toward the construction
of the stephacidin A core.
ZrCl4) and protic acids; and also heating the substrate neat at
high temperatures (300 °C). Because of the ambiguous electronic
nature of 20, rhodium(I)-catalyzed cycloisomerization was also
attempted.27 In addition, photochemical conditions were ex-
plored in the hopes of eliciting a formal Diels-Alder reaction
Via radical intermediates. Substrate 20 was surprisingly robust
and emerged unscathed in these failed attempts. The bis-
acetylated derivative of 20 was also prepared and found to be
similarly unreactive. In retrospect, several factors may have
contributed to the failure of this cycloaddition: (1) the diene-
dienophile set may have been electronically mismatched; (2)
the unreactive s-trans configuration of the diene (s-cis is
depicted) might be highly favored to avoid steric clashing of
the diketopiperazine N-H and indole C2-H; and (3) the loss
of aromaticity and conjugation that would be required of the
immediate Diels-Alder product prior to tautomerization.
Second Generation Retrosynthesis. The difficulties encoun-
tered in our first approach prompted a slight retreat to a radical-
based strategy depicted in Figure 4 (path A). It was reasoned
that the model compound 28 could be accessed Via a vinyl radi-
cal cyclization28 directly from 22 (entry 7, Table 1) or in a step-
wise fashion Via the isolated intermediate 29. The synthesis of
vinyl precursor 22 was carried out in a manner analogous to that
used for the Diels-Alder precursor 20 as shown in Scheme 3.
Thus, N-Boc-L-proline methyl ester (23) was alkylated with
the known allylic bromide 3329 giving 34 in 83% yield (Scheme
3). This racemic proline derivative was deprotected using
p-TsOH25 (85%) (giving 35) and coupled with N-Boc-L-
With a simple and reliable method for dehydrogenation in
hand, the model diene 20 (entry 6, Table 1) was constructed as
shown in Scheme 2. Thus, N-Boc-L-proline methyl ester (23)
was alkylated23 with prenyl bromide (24) to afford racemic 25
in 83% yield. Subsequent N-Boc-deprotection with p-TsOH24
afforded 26 (84%) which was coupled with N-Boc-L-tryptophan
(1.1 equiv of BOPCl,25 3.0 equiv of i-Pr2EtN, 12 h, 20 °C) to
provide peptide 27 in 48% yield (unoptimized). Thermolysis
of 27 at 190 °C for 45 min resulted in N-Boc-deprotection26
and cyclization to diketopiperazine 19 in 54% yield (inconse-
quential mixture of two diastereomers). The key dehydrogena-
tion proceeded uneventfully and efficiently (92% isolated yield)
to afford Diels-Alder precursor 20.
Several conditions were explored to promote the key cy-
cloaddition such as refluxing in toluene, xylenes, or other high
boiling solvents; the use of various Lewis acids (AlCl3, TiCl4,
(22) (a) Shun, S.; Kakeya, H.; Osada, H.; Porco, J. A., Jr. Tetrahedron 2003,
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man, J.; Sjo¨gren, M.; Bohlin, L. Tetrahedron 2004, 60, 961-965.
(23) (a) Williams, R. M.; Cao, J. Tetrahedron Lett. 1996, 37, 5441-5444. (b)
Chan, C.-O.; Cooksey, C. J.; Crich, D. J. Chem. Soc., Perkin Trans. 1
1992, 777-780.
(27) Jolly, R. S.; Luedtke, G.; Sheehan, D.; Livinghouse, T. J. Am. Chem. Soc.
1990, 112, 4965-4966.
(28) (a) Stork, G.; Baine, N. H. J. Am. Chem. Soc. 1982, 104, 2321-2323. (b)
Stork, G.; Baine, N. H. Tetrahedron Lett. 1985, 26, 5927-5930. (c) Chen,
J.; Marx, J. N. Tetrahedron Lett. 1997, 38, 1889-1892.
(29) Wijnberg, J. B. P. A.; Wiering, P. G.; Steinberg, H. Synthesis 1981, 11,
901-903.
(24) Brinkman, H. R.; Landi, J. J., Jr.; Paterson, J. B., Jr.; Stone, P. J. Synth.
Commun. 1991, 21, 459-465. A modified procedure was used. See
Experimental Section for details.
(25) Cabre, J.; Palomo, A. L. Synthesis 1984, 5, 413-417. A modified procedure
was used. See Experimental Section for details.
(26) Rawal, V. H.; Jones, R. J.; Cava, M. P. J. Org. Chem. 1987, 52, 19-28.
9
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Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Table 2. Optimization Studies for Intramolecular Oxidative Coupling of Compound 44c
Oxidant
[O] Potential
(eV)
Yielda
(%)
Entry
(concentration)
1
2
3
4
5
6
7
8
I2 (0.05 M THF)
FeCl3 (0.2 M DMF)
Fe(Cp)2PF6 (0.2 M DMF)
Fe(acac)3 (0.2 M THF)
CuCl2 (0.2 M DMF)
Cu(OTF)2 (0.2 M DMF)
Cu(4-cyclohexylbutyrate)2 (0.1 M 1:1 THF:DMF)
Cu(2-ethylhexanoate)2 (0.1 M THF)
-0.39
0
31
26
65
39
35
19
29
b
-0.28
-1.14
b
b
b
-1.65
a Isolated yield. b No meaningful data could be obtained. c Measured using cyclic voltammetry; potentials relative to ferrocene-ferrocenium standard.
Scheme 3. Model Studies toward the Construction of Stephacidin
a relatively unexplored area of C-C bond formation and
chemical reactivity.
A Core via Radical Cyclizationa
Development of a Successful Strategy. In an ongoing project
in our laboratory involving the total synthesis of the fischerin-
dole,30 hapalindole,30 and welwitindolinone31 alkaloids it was
shown that enolates of carbonyl compounds undergo oxidative
coupling with indoles and pyrroles32 in the presence of metal
oxidants such as Cu(II). This method directly merges sp2 and
sp3 carbons, enabling the one-step preparation of a variety of
R-linked heterocycles without using protecting groups or
halogen atoms. Although the seminal work on oxidative
dimerization of enolates was performend more than thirty years
ago,33 these represented the first synthetically useful hetero-
couplings of different types of enolates (“enamines” with
ketones, esters, and amides). Oxidative enolate couplings had
received very little attention in the chemical literature and had
rarely been applied in the total synthesis of complex natural
products.34 Given our experience with these reactions, we
wondered whether ester and amide enolates could be coupled
together selectively in an intramolecular sense.
Although little is known regarding the pretransition state
assembly during such an oxidative coupling event, we reasoned
that the carbonyl oxygens (see 32 in Figure 4) of the two
possible enolates would be oriented in a head-to-head fashion
a Reagents and conditions: (a) LHMDS (1.1 equiv), THF, -78 °C, 35
upon binding to a single, multivalent metal counterion. The two
min; then 3329 (1.1 equiv), -78 °C, 2 h, 83%; (b) p-TsOH (1.0 equiv),
enolates should be forced together temporarily not only due to
nonbonded interactions with the proline ring, but also by the
Thorpe-Ingold effect imposed by the all-carbon quaternary
center at C-4 (see 32, Figure 4). In this arrangement, the sp2-
hybridized R-carbons of the enolized carbonyl species would
be essentially stacked on top of each other and forced to react
based on proximity. Models also indicated that the desired
stereochemical outcome would arise independent of the ester
toluene, reflux, 2 h, 85%; (c) Boc-L-Trp-OH (1.0 equiv), BOPCl (1.1 equiv),
i-Pr2EtN (3.0 equiv), CH2Cl2, 20 °C, 12 h, 54%; (d) (i) p-TsOH (1.0 equiv),
toluene, reflux, 2 h; (ii) toluene, reflux, 4 h, 46%; (e) PhNO (2.5 equiv),
then ZrCl4, CH2Cl2, 20 °C, 2 h, 79%. AIBN ) azobis(isobutyronitrile).
tryptophan furnishing 36 (54%, unoptimized) which was then
transformed to diketopiperazine 21 (46% yield) by p-TsOH-
catalyzed Boc removal25 and ring closure upon heating. Dehy-
drogenation proceeded smoothly as before using ZrCl4/PhNO
to afford vinyl bromide 22 in 79% yield. Unfortunately, we were
unable to effect the conversion of 22 to even the monocyclized
indole 29 under a variety of conditions. Ultimately, this strategy
was not intensely pursued for two reasons. First, even if bicycle
29 could be generated, its efficient conversion to hexacycle 28
was uncertain since there was no reason to believe either face
of the olefin in 29 would be more readily protonated or attacked
by the indole. Second, an enticing strategy employing oxidative
coupling was on the horizon and had the potential of revealing
(30) Baran, P. S.; Richter, J. M. J. Am. Chem. Soc. 2004, 126, 7450-7451.
(31) Baran, P. S.; Richter, J. M. J. Am. Chem. Soc. 2005, 127, 15394-15396.
(32) Baran, P. S.; Richter, J. M.; Lin, D. W. Angew. Chem., Int. Ed. 2005, 44,
609-612.
(33) (a) Maryanoff, C. A.; Maryanoff, B. E.; Tang, R.; Mislow, K. J. Am. Chem.
Soc. 1973, 95, 5839-5840. (b) Ito, Y.; Konoike, T.; Saegusa, T. J. Am.
Chem. Soc. 1975, 97, 2912-2914. (c) Ito, Y.; Konoike, T.; Harada, T.;
Saegusa, T. J. Am. Chem. Soc. 1977, 99, 1487-1493.
(34) (a) Paquette, L. A.; Bzowej, E. I.; Branan, B. M.; Stanton, K. J. J. Org.
Chem. 1995, 60, 7277-7283. (b) Cohen, T.; McNamara, K.; Kuzemko,
M. A.; Ramig, K.; Landi, J. J., Jr.; Dong, Y. Tetrahedron 1993, 49, 7931-
7942. (c) Terpin, A.; Polborn, K.; Steglich, W. Tetrahedron 1995, 51,
9941-9946.
9
J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006 8683

A R T I C L E S
Baran et al.
Scheme 4. Synthesis of Model Ester 44a
Scheme 5. Synthesis of Olefin 46 via Intramolecular Oxidative
Couplinga
a Reagents and conditions: (a) LDA (2.2 equiv), -78 °C, 5 min; then
Fe(acac)3 (2.2 equiv), THF, -78 f 20 °C, 65%; (b) (i) MeMgBr (5.0 equiv),
toluene, 0 °C, 2 h; (ii) Burgess reagent (3.0 equiv), benzene, 50 °C, 20
min, 41% overall, 39% recovered methyl ketone. LDA ) lithium diiso-
propylamide, Fe(acac)3 ) iron (III) acetylacetonate.
38 (96%) and deprotection of the Cbz group37 (H2, Pd/C)
furnished amine 39 in quantitative yield. The coupling of amine
39 with the bis-protected tryptophan 4038 proceeded optimally
using HATU39 to furnish amide 41 in 79% yield. Alternative
coupling agents such as EDC and BOPCl provided only modest
yields of 41. Cbz deprotection (H2, Pd/C) liberated the free
amine which cyclized upon heating in toluene to give dike-
topiperazine 42. In preparation for the key oxidative coupling
step, the free N-H of the diketopiperazine was protected with
a PMB group (NaH, PMBCl)40 to afford 43 in 72% isolated
yield. The use of a PMB group would facilitate structure
confirmation by comparison of our intermediates to those
synthesized by Williams and co-workers (Vide infra). The
oxidative coupling precursor ester 44 was prepared from TBS-
protected alcohol 43 by a standard deprotection/oxidation41
sequence depicted in Scheme 4 which required a single
purification and proceeded in 72% overall yield. To our delight,
ester-amide 44 readily underwent intramolecular oxidative
heterocoupling (LDA, -78 °C, 5 min then Fe(acac)3, -78 f
20 °C) to furnish 45 as a single diastereomer in 65% yield
(Scheme 5).
At the time, spectroscopic data indicated that the desired
oxidation had taken place; however, the stereochemistry at C-6
(stephacidin A numbering, see Scheme 5) could not be
decisively determined. Fortunately, in 1990 Williams9b and co-
workers completed a synthesis of brevianamide B (47) in which
both epimers (at C-6, stephacidin A numbering) of pentacycle
46 were synthesized. Correlating our data with those of the
Williams group allowed for an unambiguous stereochemical
determination. This comparison was made after compound 45
was converted to olefin 46 by tandem methyl Grignard addition42
(5 equiv, MeMgBr, toluene, 20 °C) and dehydration with the
a Reagents and conditions: (a) LHMDS (1.0 equiv), THF, -78 °C, 35
min; then allyl bromide (1.1 equiv), -78 °C, 2 h, 85%; (b) 9-BBN (2.0
equiv), THF, 20 °C, 3 h; then 3 M aq. NaOH/35% aq. H2O2 (1:1), 1 h,
92%; (c) TBSCl (1.1 equiv), imidazole (1.2 equiv), CH2Cl2, 20 °C, 30 min,
96%; (d) H2, 10% Pd/C (20% w/w), MeOH, 20 °C, 3 h, 100%; (e)
compound 40 (1.0 equiv), HATU (1.1 equiv), i-Pr2EtN (3.0 equiv), DMF,
20 °C, 12 h, 79%; (f) (i) H2, 10% Pd/C (20% w/w), EtOAc/MeOH 1:1, 20
°C, 5 h; (ii) toluene, reflux, 4 h, 74%; (g) NaH (1.0 equiv), DMF, 0 °C, 30
min; then PMBCl (1.2 equiv), 0 °C, 2 h, 72%; (h) (i) TBAF (3.0 equiv),
THF, 2 h; (ii) DMP (1.1 equiv), CH2Cl2, 20 °C, 1.5 h; (iii) NaClO2 (2.8
equiv), NaH2PO4‚H2O (3.0 equiv), 2-methyl-2-butene (20.0 equiv), THF/
H2O, 20 °C, 30 min; (iv) CH2N2, MeOH, 0 °C, 72%. 9-BBN )
9-borabicyclo[3.3.1]nonane; TBSCl ) tert-butyldimethylsilyl chloride;
HATU ) O-(7-azabenzotriazol-1-yl)-N,N,N′,N′-tetramethyluronium hexaflu-
orophosphate, DMF ) N,N-dimethylformamide, PMBCl ) p-methoxyben-
zyl chloride, TBAF ) tetrabutylammonium fluoride, DMP ) Dess-Martin
periodinane.
enolate geometry (however, one would expect the Z enolate to
be formed under kinetic enolization in the absence of HMPA,
Vide infra). Further encouragement came from observing that
in Williams’s studies,9b the stereochemical outcome of an SN2′
cyclization could be determined by proper cation selection and
suitable tuning of the reaction conditions to prevent or promote
tight ion pairing. Significantly, the quaternary stereocenter of
the proline subunit would control the facial selectivity of the
new bond being formed (relative to the diketopiperazine). From
the standpoint of synthetic design, this implied that a racemic
tryptophan could be utilized and only the quaternary proline
stereocenter needed to be controlled for an eventual enantiose-
lective synthesis. To evaluate these considerations, a synthesis
of model compound 44 was undertaken (Scheme 4).
Thus, N-Cbz-L-proline methyl ester (37) was treated with
LHMDS and allyl bromide to provide the racemic alkylated
product in 85% yield (see Scheme 4). Hydroboration of the
pendant olefin with 9-BBN35 gave the alcohol 38 in 92% yield
following oxidative workup. Subsequent TBS protection36 of
(37) Hartung, W. H.; Simonoff, R. Org. React. 1953, VII, 263-326.
(38) Abato, P.; Yuen, C. M.; Cubanski, J. Y.; Seto, C. T. J. Org. Chem. 2002,
67, 1184-1191.
(39) (a) Carpino, L. A. J. Am. Chem. Soc. 1993, 115, 4397-4398. (b) Carpino,
L. A.; El-Faham, A. J. Org. Chem. 1994, 59, 695-698. (c) Carpino, L.
A.; El-Faham, A.; Albericio, F. Tetrahedron Lett. 1994, 35, 2279-2282.
(40) Yamura, M.; Suzuki, T.; Hashimoto, H.; Yoshimura, J.; Shin, C. Chem.
Lett. 1984, 13, 1547-1548.
(35) (a) Scouten, C. G.; Brown, H. C. J. Org. Chem. 1973, 38, 4092-4094. (b)
Brown, H. C.; Knights, E. F.; Scouten, C. G. J. Am. Chem. Soc. 1974, 96,
7765-7770.
(41) Ireland, R. E.; Meissner, R. S.; Rizzacasa, M. A. J. Am. Chem. Soc. 1993,
115, 7166-7172.
(36) Corey, E. J.; Venkateswarlu, A. J. Am. Chem. Soc. 1972, 94, 6190-6191.
9
8684 J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006

Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Figure 5. (a) Chelated and nonchelated pretransition state assemblies. (b)
Possible bond forming scenarios.
Burgess reagent43 (3 equiv, 50 °C) in 41% overall yield
(unoptimized). Olefin 46 possessed the requisite stephacidin A
C-6 stereochemistry, in contrast to that found in brevianamide
B (47).
LDA (2.2 equiv) proved to be the base of choice to bring
about bis-enolization at -78 °C. Several oxidants with varying
oxidation potentials were investigated. The use of copper salts
to oxidize the enolates showed encouraging results (Table 2)
with significant product formation (isolated yields ranging from
19 to 39%, see Supporting Information for optimization studies).
Perhaps due to its unique oxidation potential, Fe(acac)3 furnished
the highest yield. Notably, the use of more than 2.2 equiv of
Fe(acac)3 led to a decreased yield. The optimal enolization time
was observed to be 5 min. When enolization times exceeded 5
min, undesired side products often formed, presumably arising
from competing Dieckmann cyclization and R-oxidation.
To the best of our knowledge, this transformation represents
the first example of an intramolecular oxidative heterocoupling
of two different types of carbonyl enolates. The good yield of
this reaction, which accomplishes the construction of adjacent
quaternary and tertiary stereocenters with complete stereocontrol,
might be due in part to exquisite selectivity of the iron oxidant
used. It is possible that this oxidant succeeds by selectively or
more rapidly oxidizing one of the enolate species (the amide
or ester). The resulting radical and remaining enolate can then
combine to give a ketyl-type radical anion that can be oxidized
further to the final product. Lower yields may result from
nonselective oxidation since either the amide or ester radical
may not react as desired for steric or electronic reasons. The
observed diastereoselectivity may be the result of a chelated
transition state (see Figure 5) or simply the innate bias of the
substrate. The former explanation is favored since the Williams
group has been able to control the stereochemistry of alkylations
in similar systems based on the presence or absence of metal
Figure 6. The stephacidins and related alkaloids listed in order of increasing
oxidation state.
chelation (tight ion pairing).9b It should be noted that both C-6
isomers of 46 have similar overall steric energies based on MM3
computations. The success of this C-C bond forming event is
impressive considering the wealth of possible side reactions that
can occur such as oxidation of either enolate to an R-hydroxy
species, dehydrogenation, dimerization, and inter- and intramo-
lecular Dieckmann condensations. Since the mechanism and
factors that govern oxidative enolate coupling are still poorly
understood, further work in this field is underway.
At this point in our studies, we had established the viability
of this strategy in forming the bicyclo[2.2.2]diazaoctane core
in a stereoselective manner. Our next task was to apply our
method to a system suitably functionalized to allow for eventual
elaboration into stephacidin A. As shown in Figure 6 (natural
series depicted), we planned to mimic the postulated biosynthesis
of these alkaloids in the final conversion of stephacidin A to
higher oxidized forms such as avrainvillamide. Although there
was very little precedent for climbing such an oxidative “ladder,”
this was viewed as an opportunity for invention. Friedel-Crafts
(42) (a) Bergmann, E.; Bondi, A. J. Am. Chem. Soc. 1936, 58, 1814. (b) Baran,
J. S.; Laos, I.; Langford, D. D.; Miller, J. E.; Jett, C.; Taite, B.; Rohrbacher,
E. J. Med. Chem. 1985, 28, 597-601.
(43) Burgess, E. M.; Penton, H. R., Jr.; Taylor, E. A. J. Org. Chem. 1973, 38,
26-31.
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A R T I C L E S
Baran et al.
Figure 7. Retrosynthetic analysis of stephacidin A.
Scheme 6. One-Step Synthesis of a Protected
Table 3. Development of an Efficient One-step Tryptophan
Synthesis
6-(tosyloxy)tryptophana
Productb
(% yield)
a Reagents and conditions: Pd(OAc)2 (0.05 equiv), DABCO (3.0 equiv),
TBAI (1.0 equiv), DMF, 105 °C, 4 h, 75%. DABCO ) 1,4-diaza-
bicyclo[2.2.2]octane, TBAI ) tetra-n-butylammonium iodide.
Entry
Conditionsa
1
Pd(OAc)2 (5 mol %), DABCO
(3 equiv), 105 °C , 3 h
Pd(OAc)2 (5 mol %), DABCO (3 equiv),
TBAI (1 equiv), 105 °C, 4 h
Pd(OAc)2 (5 mol %), DABCO (3 equiv),
TBAI (1 equiv), 105 °C, 2.5 h
Pd(OAc)2 (5 mol %), DABCO (3 equiv),
TBAI (1 equiv), MgSO4 (5 equiv),
105 °C, 5 h
Pd(PPh3)4 (10 mol %), DABCO (3 equiv),
TBAI (1 equiv), 105 °C, 10 h
35
75
18
39
2
3
4
cyclization of olefin 50 (see Figure 7), a tactic used successfully
by Williams,9b should furnish stephacidin A. The requisite olefin
50 should be accessible from ester 51 by a sequence of oxidative
C-C bond formation followed by functional group manipula-
tions. Ester 51 could in turn be constructed from two simple
building blocks: benzopyran tryptophan 52 and proline-derived
ester 53.
Development of a Scalable Route to Substituted Tryp-
tophans. One of the first aims of this project was to develop
an efficient and practical method for the synthesis of 6-hydrox-
ytryptophan. Our motivation transcended the urgent need
presented by the synthesis at hand, since dozens of indole
alkaloids bear this oxidation pattern. With brevity and practical-
ity in mind, we opted for a one-step pathway outlined in Scheme
6. Inspiration for this approach came from a publication by
Reider and co-workers44 at Merck for the palladium-catalyzed
Heck-type synthesis of indoles from o-haloanilines and alde-
hydes. A glutamate-derived aldehyde (such as 54) appeared to
be an ideal surrogate for the tryptophan side chain,45,46 while a
suitably protected aniline such as 55 could serve as a precursor
to the benzoypyran. In the event, reduced pyroglutamate 54 and
5
6
7
8
34
32
Pd(OAc)2 (5 mol %), Et3N (3 equiv),
TBAI (1 equiv), 105 °C, 1.5 h
Pd2dba3‚CHCl3 (10 mol %), DABCO (3 equiv),
TBAI (1 equiv), 105 °C, 1.5 h
a All reactions were conducted in 0.3 M DMF. b Isolated yield.
o-iodoaniline 55 could be efficiently coupled on a gram scale
furnishing 56 in 75% yield under conditions arrived at through
extensive optimization (see Table 3 for selected experiments).
The generality of this method is exemplified in Table 4.
This reaction was initially conducted using the originally
reported conditions,44 furnishing 56 in modest (10-35%) yields.
Investigation into the side products showed that a vast amount
of material, both iodoaniline and intermediate enamine, was
being stripped of iodine. With 6-hydroxytryptophan access being
crucial to our forward progress, the problem of deiodination
had to be overcome. In an attempt to elucidate the point of the
reaction when deiodination was occurring, the iodoaniline and
the preformed enamine were subjected to standard reaction
conditions. It was interesting to find that the enamine gave
deiodination products much more readily than did the iodoa-
(44) Chen, C.; Lieberman, D. R.; Larsen, R. D.; Verhoeven, T. R.; Reider, P.
J. J. Org. Chem. 1997, 62, 2676-2677.
(45) (a) Masumi, F.; Takeuchi, H.; Kondo, S.; Suzuki, K.; Yamada, S. Chem.
Pharm. Bull. 1982, 30, 3831-3833. (b) Dieter, R. K.; Sharma, R. R. J.
Org. Chem. 1996, 61, 4180-4184.
(46) While this manuscript was in preparation, a similar method for preparing
A-ring substituted tryptophans was reported: Jia, Y.; Zhu, J. Synlett 2005,
2469-2472.
9
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Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Table 4. Synthesis of Substituted Tryptophans
Scheme 7. Mechanistic Summary of the One-Step Tryptophan
Synthesis
Scheme 8. Synthesis of Benzopyran 52 from
6-(tosyloxy)tryptophan 56a
a Reagents and conditions: (a) Boc2O (1.0 equiv), DMAP (0.01 equiv),
CH2Cl2/MeCN (1:1), 20 °C, 10 min, 95%; (b) Mg0 (10.0 equiv), MeOH, 0
f 25 °C, 2.5 h; (c) CuCl (0.001 equiv), 65 (3 equiv), DBU (3.0 equiv),
CH2Cl2/MeCN (1:1), 0 °C, 24 h, 75% over two steps; (d) o-DCB, 190 °C,
10 min, 95%; (e) Boc2O (1.0 equiv), DMAP (0.01 equiv), CH2Cl2/MeCN
(1:1), 20 °C, 30 min, 77%; (f) LiOH (15.0 equiv), THF/H2O (1:1), 0 °C, 3
h, 100%. Boc2O ) di-tert-butyl dicarbonate, DMAP ) 4-(dimethylami-
no)pyridine, DBU ) 1,8-diazabicyclo[5.4.0]undec-7-ene, o-DCB ) ortho-
dichlorobenzene.
a Isolated yield
niline itself. The addition of tetraalkylammonium salts as
described by Jeffery,47 however, improved the yield, with tetra-
n-butylammonium iodide (TBAI) showing the most promise by
limiting the amount of deiodination. The addition of drying
44
agents such as 4 Å MS or MgSO4 did not improve the
efficiency of the reaction. The screening of other additives,
bases, and palladium sources indicated that the standard
conditions with the addition of TBAI were ideally suited for
the task at hand (see Table 3, entry 2).
Total Synthesis of Stephacidin A. The next task, namely
forging the benzopyran heterocycle onto tryptophan 56, was
accomplished as outlined in Scheme 8. Thus, Boc-protection
(Boc2O, DMAP)48 and Mg-mediated deprotection of the tosyl
group49 permitted installation of the requisite benzopyran car-
bons by treating the crude phenol with the vinylidene carbene
generated in situ from propargylic carbonate 65, DBU, and
catalytic CuCl50 giving 66 in 71% overall yield from 56. Allenyl
Claisen rearrangement then took place smoothly and in 95%
yield by heating in o-dichlorobenzene at 190 °C for 10 min
A likely mechanistic scenario for this process of indole
synthesis is shown in Scheme 7. Base-catalyzed condensation
of aniline 55 with the latent aldehyde of 54 followed by
tautomerization to the E-enamine and oxidative addition to the
iodine-aryl bond furnishes metalated intermediate 63. Migratory
insertion of the enamine olefin into the Pd-aryl bond generates
σ-complex 64, which can then undergo â-hydride elimination
involving Hb. The benzylic proton Ha in 64 is not involved in
this process since it is not syn to the metal. Tautomerization
then leads to 56.
(48) Franze´n, H. M.; Någren, K.; Grehn, L.; Långstro¨m, B.; Ragnarsson, U. J.
Chem. Soc., Perkin Trans. 1 1988, 497-502.
(49) Sridhar, M.; Kumar, B. A.; Narender, R. Tetrahedron Lett. 1998, 39, 2847-
2850.
(50) Tisdale, E. J.; Vong, B. G.; Li, H.; Kim, S. H.; Chowdhury, C.; Theodorakis
E. A. Tetrahedron 2003, 59, 6873-6887.
(47) Jeffery, T. Tetrahedron 1996, 52, 10113-10130.
9
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A R T I C L E S
Baran et al.
Scheme 9. Synthesis of Substituted Enantiomerically Pure
Prolines 69 and 70a
giving 74/75. Since Williams had reported difficulties in the
removal of PMB protecting groups from DKPs,9b a MOM group
was chosen and installed under standard conditions. Upon arrival
at ester 75, either directly Via the route from proline 53 or
through the more circuitous sequence from TBS-protected
proline 71, the stage was set for the key oxidative enolate
coupling. In accordance with model studies, ester 75 reacted
smoothly (LDA, followed by Fe(acac)3) to furnish 76 as a single
diastereomer in 61% isolated yield (along with 8% recovered
75), even on a gram scale. After some optimization (Vide infra),
the MOM group could be reliably dismantled using B-bro-
mocatecholborane54 in 63% yield. The resulting free DKP was
treated with an excess of MeMgBr (6 equiv) to furnish an
intermediate tertiary alcohol that dehydrated upon treatment with
the Burgess reagent43 furnishing 50 in 88% overall yield.
A seemingly trivial Friedel-Crafts alkylation was planned
to forge the seventh and final ring of 1, based on extensive
literature precedent.55 Olefin 50 was unstable to all Brønsted
acids, presumably due to the electron rich benzopyran ring (see
Supporting Information for Friedel-Crafts alkylation optimiza-
tion studies). Fortuitously, when 50 was simply heated in
sulfolane at 200 °C for 1 h, we were able to isolate stephacidin
A in variable yields (ca. 28-45%) along with recovered Boc-
deprotected 50. It is likely that this thermolytic sequence
involves initial Boc-removal26 followed by a formal ene reaction
to generate a spirocyclic intermediate56 such as 77 followed by
a 1,2-shift56 to afford 1. It is also possible that trace quantities
a Reagents and conditions: (a) 9-BBN (2.0 equiv), THF, 20 °C, 3 h;
then 3 M aq. NaOH/35% aq. H2O2 (1:1), 1 h, 92%; (b) TBSCl (1.1 equiv),
imid. (1.2 equiv), CH2Cl2, 20 °C, 30 min, 96%; (c) (i) PhI(OAc)2 (2.2 equiv),
TEMPO (0.2 equiv), MeCN/H2O (3:1), 20 °C, 5 h; (ii) CH2N2, EtOAc, 20
°C, 86% overall. TEMPO ) 2,2,6,6-tetramethyl-1-piperdinyloxy free radical.
furnishing 67. The Boc group was then re-appended, and the
ester was saponified (LiOH) resulting in acid 52 in 77% yield
from 67.
While a viable and scalable route to the benzopyran-
containing sector was developed, a route to the proline portion
took shape as shown in Scheme 9. This sector of stephacidin A
was prepared in enantiomerically pure form using Seebach’s
methodology for the asymmetric alkylation of amino acids.51
The hydrochloride salt of (R)-allylproline (generated from
L-proline) was esterified and protected as its benzyl carbamate
delivering 68 (see Supporting Information for details). Hy-
droboration and oxidative workup gave the primary alcohol, a
compound prepared previously in racemic form by Lectka.52
At this point, the first and second generation syntheses diverged.
Initially, it was feared that conversion of the primary alcohol
to its carboxylic ester would be incompatible with the depro-
tected amine required for amide bond formation. This concern
mandated that the alcohol be protected as its TBS ether (69)
and be unmasked after peptide coupling and amide function-
alization/protection. Our concerns, however, proved to be
unfounded since the alcohol could be converted to diester
analogue 70, deprotected, and coupled without complication
(Vide infra).
Scheme 10 details the union of the tryptophan and proline
fragments and their eventual conversion to ent-stephacidin A.
While reductive removal of the Cbz group of 69 and peptide
coupling with 52 proceeded uneventfully, amine diester 53
required swift peptide coupling with 52 immediately after
liberation of the amine to avoid formation of the corresponding
γ-lactam.52 In coupling 71 to 52 in our first strategy, we elected
to use BOPCl as the coupling agent allowing us to obtain 72 in
62% overall yield from 70. However, as observed in more
advanced model studies (Vide infra), HATU was found to be
the coupling reagent of choice and furnished amide 73 in 81%
overall yield from 53.
of acid are responsible for this transformation, although base-
washed glassware gave a similar result. With the exception of
optical rotation, which was not known at the time, synthetic 1
was found to be identical in all respects to a natural sample
kindly provided by Professor Fenical (Vide infra) and with data
reported by the Bristol-Myers Squibb group.1
Determination of Absolute Configuration. As alluded to
in the Introduction, optical rotation data were not reported for
the stephacidins.1 Although comparison of stephacidin A to other
related natural products such as the paraherquamides, asper-
paralines, and others9j might have allowed an educated guess
to be made with regards to its absolute configuration, we elected
to use natural L-proline until this uncertainty was resolved. As
of 2004 (or earlier), Bristol-Myers Squibb had exhausted their
supply of stephacidin A.57 Thus, the only means by which the
absolute configuration of stephacidin A could have been
determined would have been through reisolation or reduction
of avrainvillamide (or one of the aspergamides) to the level of
an indole followed by optical rotation measurement. This latter
option was of course dependent on the assumption that these
To form the diketopiperazine (DKP) ring system of 74/75,
chemoselective deprotection of the Cbz group of 72/73 was
achieved using modified conditions of those reported by
Ohfune53 followed by dissolution and heating in a mixed solvent
system of methanol (to hydrolyze the intermediate silyl car-
bamate) and DMF to elicit ring closure with loss of methanol
(54) Boeckman, R. K., Jr.; Potenza, J. C. Tetrahedron Lett. 1985, 26, 1407-
1410.
(55) See ref 9. Also see: (a) Mirand, C; Massiot, G.; Levy, J. J. Org. Chem.
1982, 47, 4169-4170. (b) Borschberg, H. J. HelV. Chem. Acta 1984, 67,
1878-1882. (c) Darbre, T.; Nussbaumer, C.; Borschberg, H. J. HelV. Chem.
Acta 1984, 67, 1040-1052. (c) Stoermer, D.; Heathcock, C. H. J. Org.
Chem. 1993, 58, 564-568.
(51) Seebach, D.; Boes, M.; Naef, R.; Schweizer, W. B. J. Am. Chem. Soc.
1983, 105, 5390-5398.
(52) Cox, C.; Lectka, T. J. Am. Chem. Soc. 1998, 120, 10660-10668.
(53) Sakaitani, M.; Ohfune, Y. J. Org. Chem. 1990, 55, 870-876.
(56) Jackson, A. H.; Smith, P. Tetrahedron 1968, 24, 2227-2239.
(57) Qian-Cutrone, J. Personal communication.
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Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Scheme 10. Total Synthesis of ent-Stephacidin A from Proline 69 or 70 and Benzopyran Acid 52a
a Reagents and conditions: (a) 10% Pd/C (20% w/w), H2, toluene, 20 °C, 2 h, 100% from 69; or 10% Pd/C (20% w/w), toluene, 20 °C, 2 h, 97% from
70; (b) 71, (1.5 equiv), BOPCl (1.1 equiv), i-Pr2EtN (1.1 equiv), CH2Cl2, 0 f 20 °C, 10 h, 62% from 71; or 53 (1.0 equiv), HATU (1.1 equiv), i-Pr2EtN
(3.0 equiv), DMF, 20 °C, 12 h, 81% from 53; (c) Pd2dba3‚CHCl3 (0.2 equiv), Et3SiH (40.0 equiv), Et3N (2.0 equiv), CH2Cl2, 20 °C, 4 h; then MeOH, reflux,
30 min; then toluene, reflux, 2 h, 53% overall from 72; or Pd2dba3‚CHCl3 (0.2 equiv), Et3SiH (40.0 equiv), Et3N (2.0 equiv), CH2Cl2, 20 °C, 2.5 h; then
MeOH, DMF (2:1), reflux, 2.5 h, 85% from 73; (d) NaH (1.2 equiv), DMF, 0 °C, 30 min; then MOMCl (1.1 equiv), 0 °C, 1 h, 65% from 74; or NaHMDS
(1.0 equiv), THF, -78 °C, 30 min; then MOMCl (1.3 equiv), -78 f 20 °C, THF, 1.5 h, 63% from 75; (e) (i) TBAF (3.0 equiv), THF, 20 °C, 1 h; (ii) DMP
(1.5 equiv), wet CH2Cl2, 20 °C, 2 h; (iii) NaClO2 (2.8 equiv), NaH2PO4‚H2O (3.0 equiv), 2-methyl-2-butene (20.0 equiv), THF, 20 °C, 20 min; (iv) CH2N2,
MeOH, 20 °C, 69% overall; (f) LDA (2.2 equiv), THF, -78 °C, 5 min; then Fe(acac)3 (2.2 equiv), -78 °C, 5 min; then -78 f 20 °C, 20 min, 61% (plus
8% recovered 75); (g) B-bromocatecholborane (1.5 equiv), CH2Cl2, 0 °C, 10 min, 63%; (h) (i) MeMgBr (6.0 equiv), toluene, 20 °C, 10 min; (ii) Burgess
reagent (2.0 equiv), benzene, 50 °C, 30 min, 88% yield over two steps; (iii) 200 °C, sulfolane, 1 h, 28-45%. Pd2dba3‚CHCl3 ) tris(dibenzylideneacetone)
dipalladium(0) chloroform complex, MOMCl ) chloromethyl methyl ether, NaHMDS ) sodium bis(trimethylsilyl)amide.
Total Synthesis of Avrainvillamide and Stephacidin B. As
shown in Figure 6, stephacidin A is at the lowest oxidation state
of this family of alkaloids. Initial analysis suggested that
oxidation of 1 to either of the aspergamides9j (48 or 49, Figure
6) would be a logical pathway to avrainvillamide (3). Analysis
of a sample of aspergamide A (48), kindly provided to us by
Professor Zeeck, supported this reasoning since upon arrival it
had dehydrated in quantitative yield to give avrainvillamide
(based on 1H NMR comparison; see Scheme 11). Aspergamide
B (48) was initially targeted since, in principle, a chemoselective
oxidation of the imine species would lead to 3. This appeared
to be an ideal opportunity to test the nitrosobenzene-mediated
dehydrogenation of tryptophans (Vide supra). Unfortunately,
despite repeated attempts, this new oxidation method failed
along with more conventional oxidants such as DDQ, p-
Figure 8. Comparison of synthetic and natural stephacidin A.
chloranil, IBX, CAN, and Pd/C/O2. In an effort to access
1
aspergamide A, 1 was exposed to O2 (generated in situ by
natural products were biogenetically related. Fortunately, Pro-
bubbling dry O2(g) in methanol while irradiating with a sunlamp
fessor Fenical had previously isolated avrainvillamide5 and still
in the presence of catalytic methylene blue) which cleanly
had active cultures of the organism, Aspergillus sp. CNC358,
delivered C-3 hydroxyindolenine 78 in 80% yield after reductive
that produced the natural product. Fermentation of the organism
workup with Me2S.58 Since there are numerous methods for
and analysis of the broth by LCMS suggested the presence not
only of the expected avrainvillamide but also of stephacidin A.
the oxidation of imines to nitrones,59 the conversion of 78 into
48 was explored. After considerable experimentation, this
Stephacidin B could not be detected. Purification of the extracts
approach was abandoned due to the lack of reactivity of the
and 1H NMR analysis confirmed these assignments. Comparison
imine nitrogen and the propensity of the benzopyran olefin to
of the CD data (see Figure 8) of natural (+)-stephacidin A to
those of our synthetic sample revealed that unnatural (-)-
stephacidin A had been synthesized previously in our labora-
tories. We therefore rectified our synthesis by using D-proline
from this point onward.
(58) See reference 18a.
(59) (a) Christensen, D.; Jørgensen, K. A. J. Org. Chem. 1989, 54, 126-131.
(b) Boyd, D. R.; Coulter, P. B.; McGuckin, M. R.; Sharma, N. D.; Jennings,
W. B.; Wilson, V. E. J. Chem. Soc., Perkin Trans. 1 1990, 301-306. (c)
Busque´, F.; de March, P.; Figueredo, M.; Font, J.; Gallagher, T.; Mila´n, S.
Tetrahedron: Asymmetry 2002, 13, 437-445.
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A R T I C L E S
Baran et al.
Scheme 11. Attempted Conversion of Stephacidin A (1) into Related Natural Products by Direct Oxidationa
a Reagents and conditions: (a) likely occurred gradually during storage/shipping, 100%; (b) sunlamp, cat. methylene blue, 3O2, MeOH, -28 °C, 30 min;
DMS (100 equiv), -28 f 20 °C, 10 min, 80%. DMS ) dimethyl sulfide.
be oxidized. This set of failures prompted the conception of a
new plan that would install the N-oxide functionality at an earlier
stage. Specifically, we considered preparing 79, the N-hydroxy
analogue of 1, since its tautomeric nitrone form bears a striking
resemblance to avrainvillamide.
dehydrogenation) and tautomerization gives the desired 1-hy-
droxyindole. Model compounds lacking the benzopyran het-
erocycle with and without N-protecting groups on the DKP (87
and 80, respectively) were evaluated under Somei conditions
(Scheme 13). Gribble reduction (NaBH3CN) led cleanly to the
indoline which was carried on in crude form following aqueous
workup. Surprisingly, indolines 88 and 89 exhibited markedly
different reactivities upon exposure to Na2WO4‚2H2O and H2O2.
Whereas the unprotected indoline was oxidized readily to the
1-hydroxyindole (91, fully characterized), PMB-protected in-
doline 88 was oxidized directly to R,â-unsaturated nitrone 92.
As expected, hydroxyindole 91 was amendable to further
oxidation to 93 under mild conditions (p-chloranil) in high yield
(88%). The observed difference in reactivity between the
indolines is difficult to explain but may be due to a subtle
electronic effect.
Armed with useful data from the model series, the total
synthesis of the natural products was pursued as shown in
Scheme 14. Thus, reduction of the indole in stephacidn A (1)
proceeded in quantitative yield to produce indoline 94 which
could be carried on in crude form in the ensuing oxidation.
When 94 was submitted to the Somei conditions (Na2WO4‚
2H2O), traces of 3 were sometimes observed but the overall
result was usually complete decomposition. After a vast
screening of different oxidation conditions it was discovered
that SeO2/H2O262 was capable of converting 94 into 3, albeit in
modest yield due to the instability of the product under the
reaction conditions. Subsequently it was found that avrainvil-
lamide (3) spontaneously dimerized (as did model 93, Vide infra)
using PTLC or by simply dissolving in DMSO and drying in
Vacuo. Myers also observed a similar phenomenon and found
that Et3N enables complete conversion of avrainvillamide (3)
to 2.12 Spectral data for synthetic 2 and 3 were found to be in
agreement with those of natural samples (synthetic (-)-2: [R]D
) -33 (c 0.1, CH3CN); natural (-)-2: [R]D ) -21.1 (c 0.19,
Keenly aware of Somei’s pioneering 1-hydroxyindole hy-
pothesis (Vide infra) and accompanying methodology for the
preparation of these elusive species,60 we set forth on a model
study. Using the reagents and general techniques of the
stephacidin synthesis, the model compound 80 was prepared
as shown in Scheme 12. Optimization of the deprotection of
the MOM protecting group (see Supporting Information) was
initially carried out on intermediate 84 and applied to the
stephacidin A synthesis once satisfactory conditions were found
(Vide supra). X-ray analysis of crystalline 85 thus secured its
relative configurational assignment and further bolstered the
assignments of related systems (Vide supra). Due to the absence
of the benzopyran ring system, olefin 86 cleanly underwent
Friedel-Crafts alkylation to stephacidin model 80 in 68%
isolated yield when treated with p-TsOH in toluene at reflux.
Hexacycle 80 possessed spectral properties that paralleled those
of stephacidin A. Furthermore, its physical properties were also
similar to those of 1; both are white powders and have poor
solubility in a number of common organic solvents. Thermal
cyclization of 86 to 80 also effected cyclization, but in lower
conversion (ca. 30%) than the acid-catalyzed process.
Somei’s hypothesis concerning 1-hydroxyindoles states that
these highly reactive species are widespread in Nature during
the biosynthesis of alkaloids. Since there is no direct method
for the conversion of an indole to its 1-hydroxy counterpart (in
the laboratory), the indole is first reduced to the indoline60,61
and then treated with catalytic Na2WO4‚2H2O and excess H2O2.
Presumably, this reagent combination initially hydroxylates the
indoline nitrogen after which further oxidation (presumably
(60) Somei, M. AdV. Heterocycl. Chem. 2002, 82, 101-155.
(61) Gribble, G. W.; Lord, P. D.; Skotnicki, K.; Dietz, S. E.; Eaton, J. T.;
Johnson, J. J. Am. Chem. Soc. 1974, 96, 7812-7814.
(62) Murahashi, S.-I.; Shiota, T. Tetrahedron Lett. 1987, 28, 2383-2386.
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Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Scheme 12. Synthesis of a Stephacidin A Model Systema
Scheme 13. Initial Oxidation Studies Performed on Simplified
Stephacidin A Modelsa
a Reagents and conditions: (a) NaBH3CN (10.0 equiv), AcOH, 20 °C,
12 h; (b) Na2WO4‚2H2O (0.2 equiv), aqueous 35% H2O2 (50 equiv), MeOH,
H2O, 20 °C, 6 h, 16% over two steps for 92; Na2WO4‚2H2O (2.0 equiv),
aqueous 35% H2O2 (50 equiv), MeOH, H2O, 20 °C, 50 min, 54% over
steps for 91; (c) p-chloranil (2.0 equiv), THF, reflux, 30 min, 88%.
forward 83 as single diastereomers (resulting from the use of
racemic 53) for the oxidative coupling event to aid in purfication
followed by combining the purified, oxidatively coupled
products for further processing. Presumably, homodimerization
occurs exclusively since stereochemical bias may retard the
dimerization in the mismatched case.
Biological Evaluation of Avrainvillamide and Simplified
Mimics. Biological assays of simplified analogues using the
human colon HCT-116 cell line delivered exciting results.
Although the benzopyran ring appears to be essential for the
anti-cancer activity stephacidin A (compare entries 1 and 5),
the higher oxidized analogs appear to restore activity. Interest-
ingly, in the assay which was used, analogs 91, 95, and 93
appear to be more active than 1. Of particular interest is that
model compound (()-93 is approximately half as effective as
(+)-avrainvillamide itself, a difference that may be ascribed to
the fact that, in preparing 93 for our studies, racemic 53 was
used (Vide supra).
a Reagents and conditions; (a) proline 53 (1.0 equiv), HATU (1.1 equiv),
i-Pr2EtN (3.0 equiv), DMF, 20 °C, 12 h, 74%; (b) 10% Pd/C (20% w/w),
H2, toluene, 20 °C, 18 h; toluene, reflux, 2 h, 87%; (c) NaHMDS (1.05
equiv), MOMCl (1.2 equiv), THF, -78 °C f 20 °C, 2.5 h, 86-92%; (d)
LDA (2.2 equiv), Fe(acac)3 (2.2 equiv), -78 f 20 °C, 53%; (e)
B-bromocatecholborane (2.0 equiv), CH2Cl2, 0 °C, 10 min, 68% after 1
recycle; (f) MeMgBr (10.0 equiv), toluene, 20 °C, 10 min; then Burgess
reagent (6.0 equiv), benzene, 50 °C, 30 min, 59% overall; (g) p-TsOH (1
equiv), toluene, reflux, 10 h, 68%.
CDCl3); synthetic (+)-3: [R]D ) +11 (c 0.1, CHCl3); natural
(+)-3: [R]D ) +10.7 (c 0.17, CHCl3)).
Avrainvillamide mimic 93 requires only nine steps for its
preparation from N-Cbz-N′-Boc-tryptophan (81), is readily
amenable to preparation on a multigram scale, and is qualita-
tively more shelf stable than avrainvillamide itself. As such it
represents a potentially useful candidate for in ViVo studies.
Based on the data in Table 5, we propose that the sole source
of bioactivity in these natural products (in anticancer screens)
stems from the electrophilic R,â-unsaturated nitrone moiety of
avrainvillamide. Indeed, Myers has shown this functionality to
be readily susceptible to nucleophilic attack (even methanol adds
in a conjugate fashion).12 It is possible that N-hydroxyindole
91 is autoxidized to 93 slowly during testing and that the
stephacidin B mimic (95) reverts to 93 to some extent, as seen
in the natural series.
With regard to the mechanism of dimerization of 3 or 93, it
seems most likely that the event takes place Via a double
Michael addition as shown in Scheme 15 in contrast to the
cationic mechanism proposed by the isolation chemists.1 Thus,
attack of the secondary amide (25′) of one molecule of 3 onto
electrophilic C-21 of the second molecule of 3 would generate
a nucleophilic 1-hydroxyindole species. This 1-hydroxyindole,
nucleophilic at C-20 (stephacidin A numbering), would be
forced to react with the proximate Michael acceptor at C-21′
of the second molecule. The fact that the electrophilic carbons
are sterically congested neopentyl sites may not be as significant
in a conformationally locked system such as this one.
The model avrainvillamide 93 also dimerized readily as
shown in Scheme 16. It is interesting to note that the dimer-
ization of 93 occurred starting with an uneven mixture of
enantiomers. These were generated as a result of carrying
Extensive literature precedent would suggest that avrainvil-
lamide functions by some sort of selective protein alkylation
9
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A R T I C L E S
Baran et al.
Scheme 15. Proposed Mechanism for the Dimerization of
Scheme 14. Total synthesis of stephacidin B (1) starting from
stephacidin A (1) via avrainvillamide (3)a
Avrainvillamide (3) to Stephacidin B (2)
Scheme 16. Dimerization of Avrainvillamide Model 93 to
Stephacidin B Analogue 95a
a Reagents and conditions: (a) NaBH3CN (40 equiv), AcOH, 20 °C, 24
h, >95%; (b) SeO2 (0.25 equiv), 35% aqueous H2O2, (50 equiv),
1,4-dioxane, 20 °C, 40 h, 27% with 50% recovered 94; (c) Procedure A:
preparative TLC (SiO2, EtOAc); Procedure B: Et3N (excess), MeCN, 20
°C, 1 h; Procedure C: DMSO, then solvent removal in Vacuo, approximately
2:1 mixture of 3 to 2, purified by preparative TLC.
event, but further mechanistic studies are needed to confirm
this suspicion.63
Conclusion
In this full account we have traced the evolution of our
synthetic strategy to the bicyclic core of stephacidin alkaloids
from a failed intramolecular Diels-Alder approach to a
conceptually unrelated and successful strategy based on oxida-
tive C-C bond formation. While the former approach did not
proceed as designed, it did lead to the development of a new
and mechanistically intriguing method for the direct dehydro-
genation of tryptophans to enamides. The operational simplicity
and stereoselectivity observed in this transformation is notable.
The highlight of these model studies, however, is the interesting
intramolecular oxidative coupling of 44 during the successful
synthesis of 45. This reaction reliably proceeds in good yield
a Reagents and conditions: (a) Et3N (100 equiv), MeCN, 20 °C, 1 h,
69%.
on a preparative scale with complete stereocontrol and represents
the first union of different types of carbonyl groups in an enolate
coupling. It is hypothesized that the high chemoselectivity in
this reaction is controlled by the unique oxidation potential of
(63) Drahl, C.; Cravatt, B. F.; Sorensen, E. J. Angew. Chem., Int. Ed. 2005, 44,
5788-5809.
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Total Synthesis of Avrainvillamide and the Stephacidins
A R T I C L E S
Table 5. Preliminary Biological Activity for Simplified Analoguesa
diazaoctane core. Further functional manipulations led to a
penultimate olefinic intermediate whose reluctance to undergo
Friedel-Crafts alkylation led to the discovery of a unique,
reagent-free method for the annulation of 3-substituted indoles.
Before the more highly oxidized members of the stephacidin
class of alkaloids were considered for synthesis, the absolute
configuration of stephacidin A, the parent member, was
elucidated by reisolation of the natural product. Once the identity
of the natural series was confirmed, synthetic studies continued.
Our inability to directly oxidize stephacidin A to either of the
aspergamides forced a reevaluation of our strategy that prompted
us to employ Somei’s method for generating 1-hydroxyindoles.
In some specific cases, indolines underwent unprecedented
three-stage oxidation to R,â-unsaturated nitrones directly, an
observation which could prove useful in other settings. Applying
this method led to avrainvillamide, stephacidin B, and models
thereof, making structure-activity studies possible for the first
time in the course of these studies. The first analogues of these
natural products have also been prepared and evaluated for
biological activity. Further studies are underway to find more
potent analogues and to probe the mechanism of action by which
these complex alkaloids operate.
Acknowledgment. This work is dedicated to Professor
William Fenical on the occasion of his 65th birthday. We are
grateful to the Fenical lab (specifically John B. MacMillan, Paul
Jensen, and Matt Woolery) for reculturing the avrainvillamide-
producing organism (Aspergillus sp. CNC358) and providing
us with the crude extracts and to Dr. J. Qian-Cutrone of Bristol-
Myers Squibb for copies of spectra, discussions, and an authentic
sample of 2. We would like to thank Michael DeMartino and
Professor M. G. Finn for providing the cyclic voltammetry data.
We thank Dr. D. H. Huang and Dr. L. Pasternack for NMR
spectroscopic assistance, and Dr. G. Siuzdak and Dr. Raj Chadha
for mass spectrometric and X-ray crystallographic assistance,
respectively. Biotage is acknowledged for a generous donation
of microwave process vials used in this study. Financial support
for this work was provided by The Scripps Research Institute,
Amgen, Astra Zeneca, Bristol-Myers Squibb, DuPont, Eli Lilly,
GlaxoSmithKline, Roche, the Searle Scholarship Fund, the Sloan
Foundation, and the NIH/NIGMS (GM071498 and a predoctoral
fellowship to C.A.G.).
a Activity ) IC50 against human colon HCT-116 cell line. b Values are
an average of three runs performed in the Fenical laboratory. c Researchers
at Bristol-Myers Squibb reported a value of 0.91 µg/mL.1a
the Fe-based oxidant and that complete stereoselectivity is
achieved by a metal chelated transition state.
Using this intramolecular oxidative enolate heterocoupling
as a powerfully simplifying transform, we have succeeded in
synthesizing avrainvillamide and the stephacidins. The issue of
synthesizing a benzopyran-substituted tryptophan was addressed
by extending the scope of a known indole synthesis. To this
subunit was appended a modified and enantiomerically pure
proline derivative that served as the source of chirality for the
synthesis. With all the carbon atoms of stephacidin A in place,
oxidative enolate coupling proceeded in good yield and excellent
diastereoselectivity furnishing the hallmark bicyclo[2.2.2]-
Supporting Information Available: Full characterization,
including copies of 1H and 13C NMR spectra, and experimental
procedures for selected compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
JA061660S
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J. AM. CHEM. SOC. VOL. 128, NO. 26, 2006 8693