Total synthesis of tryprostatin B: Generation of a nucleophilic prenylating species from a prenylstannane

Full text of DOI:10.1021/ja962954o

J. Am. Chem. Soc. 1996, 118, 12463-12464
Total Synthesis of Tryprostatin B: Generation of a
12463
Nucleophilic Prenylating Species from a
Prenylstannane
Kristopher M. Depew,1a Samuel J. Danishefsky,*,1a,b
Neal Rosen,1c and Laura Sepp-Lorenzino1c
Department of Chemistry, Columbia UniVersity
HaVemeyer Hall, New York, New York 10027
Laboratories for Bioorganic Chemistry
and Molecular Oncogenesis
The Sloan-Kettering Institute for Cancer Research
1275 York AVenue, New York, New York 10027
ReceiVed August 22, 1996
Recently, Osada et al. reported on the isolation, structure
proof, and biological activity of tryprostatins A (1) and B (2).2
These compounds, isolated from a particular strain (BM 939)
of Aspergillus fumigatus, were of interest to us on the basis of
several considerations. First, tryprostatin B, as well as its less
potent congener (A, 1), were claimed to be cell cycle progression
inhibitors of tsFT210 at the G2/M phase barrier. Given our
previous research3 directed to other indole-containing structures
involved in cell cycle modulation, access to these metabolites
(particularly the more potent B compound, 2) was much desired.
Our laboratory had already been concerned with synthesizing
naturally occurring indolic-isoprene constructs. In earlier work,4
we had developed a method for the introduction of a “reverse
prenyl” group at the 3-position of a pyrroloindole (see structure
3). In our recent synthesis of gypsetin,5 we had also described
the reaction of prenylborane (4) with an unstable 3-chloroin-
dolenine (5), unsubstituted at C2 or at N, to introduce a reverse
prenyl group onto the 2-position of a tryptophan-derived indole.
Of course, the elegant method of Gribble allows for the
metallation of C2 of an indole when the indolic nitrogen is
suitably protected.6 Thus, in theory, a prenyl group could be
introduced by alkylation of a 2-metallo derivative. However,
we were skeptical that such methods could be applied to an
L-tryptophan derivative with assured maintenance of its enan-
tiomeric homogeneity. In contrast, our chloroindolenine strategy
had been successfully conducted in the context of a tryptophan
system without compromising its optical purity (see Figure 1,
5 f 6). Therefore, we hoped to apply a conceptually related
formalism for the tryprostatins, requiring access to a reverse
prenylboron reagent, generalized as 7. Such an entity might
serve as a nucleophilic prenylating agent (via allylic transposi-
tion) to generate 8. Our initial attempts along these lines
involved reactions of tri-n-butylprenylstannane with 9-BBN-
Br or 9-BBN-OTf (9-BBN ) 9-borabicyclo[3.3.1]nonyl, OTf
) triflate), which we hoped would generate in situ a reagent of
type 7 prior to coupling with 5. Unfortunately, these kinds of
protocols were unsuccessful. Apparently, rearrangement of a
presumed species (7) to the prenyl system (4), occurs competi-
Figure 1.
tively with its coupling to the chloroindolenine leading to
mixtures of the previously encountered 6 along with the desired
8 (Vide infra) in low yield.
We wondered about the possibility of generating a usable
version of 7, keeping in mind the key contributions of Keck,7
Yamamoto,8 Denmark,9 Wardell,10 and Thomas11 which, read
in the aggregate, established the possibility of nucleophilic
allylation of Lewis acids by means of allylic and crotyltin
reagents. In line with our recently developed procedure,5
N-phthaloyl-L-tryptophan methyl ester (9) on treatment with tert-
butyl hypochlorite cleanly generated 5 at 0 °C.12 This CH2Cl2
solution was cooled to -78 °C and treated with stannane 10
followed by rapid addition of 2 equiv of BCl3. Upon workup,
an 83% yield of the desired 8 was obtained. Under these
conditions only ca. 2-3% of compound 6 could be detected.
Perhaps, reaction of 10 with BCl3 generates, transiently, 11
wherein reaction with chloroindolenine 5 would lead to the
“ate”-like structure 12. Intramolecular delivery of the prenyl
function (Scheme 1, arrows) would culminate in the formation
of 8.12
Following the same protocol (Table 1), indoles 13 and 1413
were prenylated to afford 15 and 16, respectively. Thus, a
simple method to introduce a prenyl function at the 2-position
of a 3-substituted indole is now available. We also note that
the nucleophilic prenylation of ketones14 (17, 18, and 19 leading
to 20, 21 and 22, respectively) by a related procedure has been
accomplished.
Even as the full scope of this method for nucleophilic
prenylation awaits definition, we focused on completion of the
(1) (a) Columbia University. (b) Laboratory for Bioorganic Chemistry,
The Sloan-Kettering Institute for Cancer Research. (c) Laboratory for
Molecular Oncogenesis, The Sloan-Kettering Institute for Cancer Research.
(2) (a) Cui, C.-B.; Kakeya, H.; Okada, G.; Onose, R.; Ubukata, M.;
Takahashi, I.; Isono, K.; Osasa, H. J. Antibiot. 1995, 48, 1382. (b) Cui,
C.-B.; Kakeya, H.; Okada, G.; Onose, R.; Osada, H. J. Antibiot. 1996, 49,
527. (c) Cui, C.-B.; Kakeya, H.; Osada, H. J. Antibiot. 1996, 49, 534.
(3) (a) Link, J. T.; Raghavan, S.; Danishefsky, S. J. J. Am. Chem. Soc.
1995, 117, 552. (b) Link, J. T.; Raghavan, S.; Gallant, M.; Danishefsky, S.
J. J. Am. Chem. Soc. 1996, 118, 2825.
(7) (a) Keck, G. E.; Abbot, D. E.; Boden, E. P.; Enholm, E. J.
Tetrahedron Lett. 1984, 25, 3927. (b) Keck, G. E.; Andrus, M. B.; Castellino,
S. J. Am. Chem. Soc. 1989, 111, 8136.
(8) Yamamoto, Y.; Maeda, N.; Maruyama, K. J. Chem. Soc., Chem.
Commun. 1983, 742.
(9) Denmark, S. E.; Wilson, T.; Willson, T. M. J. Am. Chem. Soc. 1988,
110, 984.
(10) Harston, P.; Wardell, J. L.; Marton, D.; Tagliavini, G.; Smith, P. J.
Inorg. Chim. Acta 1989, 162, 245.
(11) McNeill, A. H.; Thomas, E. J. Synthesis 1994, 322.
(12) Kutney, J. P.; Beck, J.; Bylsma, F.; Cook, J.; Cretney, W. J.; Fuji,
K.; Imhof, R.; Treasurywala, A. M. HelV. Chim. Acta 1975, 58, 1690.
(13) For example, see: Bennasar, M. L.; Zulaica, E.; Jimenez, J. M.;
Bosch, J. J. Org. Chem. 1993, 58, 7756.
(14) We note recent examples of prenylation via an apparent prenyl-
barium species: (a) Yanagisawa, A.; Ogasawara, K.; Yasue, K.; Yamamoto,
H. J. Chem. Soc., Chem. Commun. 1996, 367. (b) Yanagisawa, A.; Habaue,
S.; Yasue, K.; Yamamoto, H. J. Am. Chem. Soc. 1994, 116, 6130.
(4) Marsden, S. P.; Depew, K. M.; Danishefsky, S. J. J. Am. Chem. Soc.
1994, 116, 11143.
(5) (a) Schkeryantz, J. M.; Woo, J. C. G.; Danishefsky, S. J. J. Am. Chem.
Soc. 1995, 117, 7025. (b) See also: Parsons, R. L.; Berk, J. D.; Kuehne,
M. E. J. Org. Chem. 1993, 58, 7482.
(6) (a) Saulnier, M. G.; Gribble, G. W. J. Org. Chem. 1982, 47, 757. (b)
For another preparation of 2-substituted indoles, see: Fukuyama, T.; Chen,
X.; Peng, G. J. Am. Chem. Soc. 1994, 116, 3127.
S0002-7863(96)02954-X CCC: $12.00 © 1996 American Chemical Society

12464 J. Am. Chem. Soc., Vol. 118, No. 49, 1996
Communications to the Editor
Scheme 1a
2) with N-Boc-L-proline acid fluoride15 to afford 24. Depro-
tection16 led to 25 and then, by diketopiperazine formation, to
2. Tryprostatin B (2) (67% from 24; 46% from N-phthaloyl-
tryptophan methyl ester) exhibited a high-field 1H NMR
spectrum identical to that provided by Osada.2c Its optical
rotation was substantially the same ([R]27.5 ) -74.6° (c )
D
0.64, CHCl3) vs lit. [R]27 ) -71.1° (c ) 0.63, CHCl3)) as
D
that reported.
With tryprostatin B available to us through total synthesis,
we were able to initiate investigations as to its biological
properties. Given the earlier reports2 that tryprostatin B inhibits
progression of tsFT210 cells through the G2/M phase, we studied
the effectiveness of this drug in inhibiting the proliferation of
logarithmically growing MCF-7 and MDA MB-468 human
breast17 cancer cells and Colo-205 human colon carcinoma cells.
We found, in analogy with Cui et al.,2 that in logarithmically
growing cultures17 tryprostatin B inhibits cell proliferation only
at high concentrations (50 µg/mL). The growth of breast cell
lines was inhibited but, curiously, not the colon cancer cell line.
Furthermore, flow cytometric analysis of tryprostatin B-treated
cells showed no cell cycle dependent arrest since the distribution
of these cells in the G1, S, and G2/M phases was the same as
DMSO-treated cells (control). Given the earlier report of
complete inhibition of cell cycle progression at 12.5 µg/mL,
we then analyzed the effect of 2 on cell cycle progression in
cells arrested in G2/M by nocodazole18 treatment. We found
that, even at 50 µg/mL, 2 did not affect progression into G1.
One clue to some of these discrepencies is that a DMSO solution
of tryprostatin B, upon standing in air, undergoes slow conver-
sion to a mixture of products. Solutions in which detectable
byproducts have been produced are considerably more cytotoxic
(ca. 50-fold) than those containing apparently homogeneous
tryprostatin. Thus, while we have confirmed the activity starting
with homogeneous tryprostatin, we cannot rule out the pos-
sibility that 2 may also be, in effect, a prodrug for a much more
active entity.
a Reagents and conditions: (a) tert-butyl hypochlorite (1.2 equiv,
0.5 M in CCl4), Et3N (1 equiv), CH2Cl2, 0 °C; then at -78 °C, 10 (4
equiv) and BCl3 (2 equiv, 1.0 M in CH2Cl2, rapid addition) 83%. (b)
Hydrazine hydrate (3.5 equiv), 3:1 MeOH/CH2Cl2 (0.1 M), 24 h, 82%.
Table 1. Prenylation of Indoles and Ketones
substrate
product
yield (%)
Further investigations directed to the chemical transformations
of tryprostatin B are in progress. We will examine the biological
activity of these transformation products in the hope of
identifying the particularly potent principle suggested by the
data above. Also, the scope and generality of this new method
for nucleophilic prenylation, which was the key to the highly
concise total synthesis, are under study.
a The chloroindolenine was formed first at -78 °C as discussed in
the text. b BCl3 (1.1 equiv, 1.0 M in CH2Cl2 from Aldrich) was added
rapidly to a -78 °C CH2Cl2 solution (0.1 M) of the ketone (1.0 equiv)
and the stannane (2.0 equiv) under argon.
Acknowledgment. This research was supported by the National
Institutes of Health (Grant HL-25848). K.M.D. gratefully acknowledges
a Lilly predoctoral fellowship. We gratefully acknowledge Vinka
Parmakovich and Barbara Sporer of the Columbia Mass Spectral
Facility and, separately, Dr. Osada of the Institute of Physical and
Chemical Research (RIKEN) for copies of NMR spectral data of 1
and 2.
Scheme 2a
Supporting Information Available: Experimental procedures for
8, 15, 16 and 20-22 and graphs explaining the effect of fully synthetic
tryprostatin B on the (a) anchorage dependent growth and (b) cell cycle
distribution of logarithmically growing cells in the presence or absence
of 2 (6 pages). See any current masthead page for ordering and Internet
access instructions.
JA962954O
a Reagents and conditions: (a) N-Boc-L-Pro-F (1.5 equiv), CH2Cl2,
NaHCO3, H2O, 94%. (b) TMSI (1.2 equiv), MeCN, 0 °C. (c) NH3/
MeOH, 20 h, 67% from 24.
(15) Carpino, L. A.; Mansour, E.-S. M. E.; Sadat-Aalaee, D. J. Org.
Chem. 1991, 56, 2611.
(16) Lott, R. S.; Chauhan, W. S.; Stammer, C. H. J. Chem. Soc., Chem.
Commun. 1979, 495.
(17) Sepp-Lorenzino, L.; Ma, Z.; Rands, E.; Kohl, N. E.; Gibbs, J. B.;
Oliff, A.; Rosen, N. Cancer Res. 1995, 55, 5302.
(18) Zieve, G. W.; Turnbull, D.; Mullins, J. M.; McIntosh, J. R. Exp.
Cell Res. 1980, 126, 397.
total synthesis of tryprostatin B starting with 8. This goal was
smoothly accomplished by cleavage of the N-phthaloyl group,
followed by coupling of the resultant amino ester 23 (Scheme