Brief total synthesis of the cell cycle inhibitor tryprostatin B and related preparation of its alanine analogue

Article abstract of DOI:10.1021/jo034703l

Tryprostatin B was synthesized in 32% overall yield from the readily available dipeptide anhydride cyclo-(L-Trp-L-Pro). Its tandem C-3 prenylation/cyclization gave the corresponding pentacyclic pyrroloindole systems bearing a prenyl group at the indole C-3 position. These compounds were then submitted to acid-catalyzed opening of the newly formed ring, with concomitant migration of the prenyl group to the indole C-2 position. The alanine analogue of tryprostatin B was also prepared using a similar sequence. The successful implementation of this strategy strengthens the case for a biosynthetic route for the tryprostatins along similar lines.

Full text of DOI:10.1021/jo034703l

Br ief Tota l Syn th esis of th e Cell Cycle In h ibitor Tr yp r osta tin B
a n d Rela ted P r ep a r a tion of Its Ala n in e An a logu e
Esmeralda Caballero, Carmen Avendan˜o, and J . Carlos Mene´ndez*
Departamento de Qu´ımica Orga´nica y Farmace´utica, Facultad de Farmacia, Universidad Complutense,
28040 Madrid, Spain
josecm@farm.ucm.es
Received May 23, 2003
Tryprostatin B was synthesized in 32% overall yield from the readily available dipeptide anhydride
cyclo-(L-Trp-L-Pro). Its tandem C-3 prenylation/cyclization gave the corresponding pentacyclic
pyrroloindole systems bearing a prenyl group at the indole C-3 position. These compounds were
then submitted to acid-catalyzed opening of the newly formed ring, with concomitant migration of
the prenyl group to the indole C-2 position. The alanine analogue of tryprostatin B was also prepared
using a similar sequence. The successful implementation of this strategy strengthens the case for
a biosynthetic route for the tryprostatins along similar lines.
In tr od u ction
The cell cycle is an attractive target for the develop-
ment of chemotherapeutic agents, with much current
interest being focused on the search for antimitotic
agents.1 Since cancer can be considered to arise from
uncontrolled cell proliferation with loss of regulation of
the cell cycle, new inhibitors of this cycle are good
candidates for cancer chemotherapy2 and can also be
useful probes for investigating the control of the cell
cycle.3
F IGURE 1. Structures of Tryprostatins A and B.
Tryprostatins A and B (1 and 2, Figure 1) are two
natural products isolated from a marine strain (BM939)
of Aspergillus fumigatus.4 They can be considered as
members of a small family of indole-isoprene alkaloids
that, among others, includes compounds such as echinu-
lin, from Aspergillus echinulatus,5 the neoequinulines,
from Aspergillus amstelodami,6 and deoxybrevianamide
E7 and related compounds,8 from Aspergillus ustus. The
tryprostatins9 and also some related compounds such as
the diketopiperazine alkaloid phenylahistin,10 the spiro-
tryprostatins,11 and the cyclotryprostatins12 are inhibitors
of the mammalian cell cycle. The tryprostatins prevent
cell cycle progression at the G2/M phase through a unique
mechanism, consisting of inhibition of the interaction
between one of the microtubule-associated proteins (MAP-
2) and the C-terminal end of tubuline.13 Recently, a
diastereomer of tryprostatin B has shown a more potent
cytotoxic activity than etoposide against three human
cancer cell lines.14a
The considerations outlined above prompted us to
study a synthetic route to the tryprostatins, in particular
tryprostatin B. All total15-17 and formal18 syntheses of
the tryprostatins published to date rely on the prepara-
* Corresponding author. Telephone: 34-91-3941840. Fax: 34-91-
3941822.
(1) Crews, C. M.; Mohan, R. Curr. Opin. Chem. Biol. 2000, 4, 47-
53.
(10) Kanoh, K.; Kohno, S.; Ashari, T.; Harada, T.; Katada, J .;
Muramatsu, M.; Kawashima, H.; Sekiya, H.; Uno, I. Bioorg. Med.
Chem. Lett. 1997, 7, 2847-2852.
(2) Osada, H. J pn. J . Cancer Chemother. 1997, 24, 1541-1546.
(3) Osada, H. J . Antibiotics 1998, 51, 973-982.
(4) (a) Cui, C.; Kakeya, H.; Okada, G.; Onose, R.; Ubukata, M.;
Takahashi, I.; Isono, K.; Osada, H. J . Antibiot. 1995, 48, 1382-1384.
(b) Cui, C.; Kakeya, H.; Okada, G.; Onose, R.; Osada, H. J . Antibiot.
1996, 49, 527-533. (c) Cui, C.; Kakeya, H.; Osada, H. J . Antibiot. 1996,
49, 534-540.
(11) Cui, C. B.; Kakeya; Osada, H. Tetrahedron 1996, 52, 12651-
12666.
(12) Cui, C. B.; Kakeya; Osada, H. Tetrahedron 1997, 53, 59-72.
(13) Usui, T.; Kondoh, M.; Cui, C.; Mayumi, T.; Osada, H. Biochem.
J . 1998, 333, 543-548.
(14) (a) Zhao, S.; Smith, K. S.; Deveau, A. M.; Dieckhaus, C. M.;
J ohnson, M. A.; Macdonald, T. L.; Cook, J . M. J . Med. Chem. 2002,
45, 1559-1562. For additional work on the biological activity of
tryprostatin analogues, see: (b) Sanz-Cervera, J . F.; Stocking, E. M.;
Usui, T.; Osada, H.; Williams, R. M. Bioorg. Med. Chem. 2000, 8, 2407-
2415. (c) Wang, H.; Usui, T.; Ganessan, A. J . Med. Chem. 2000, 43,
1577-1585.
(15) (a) Depew, K. M.; Danishefsky, S. J .; Rosen, N.; Sepp-Lorenzino,
L. J . Am. Chem. Soc. 1996, 118, 12463-12464. (b) Schkeryantz, J . M.;
Woo, J . C. G.; Siliphaivanh, P.; Depew, K. M.; Danishefsky, S. J . J .
Am. Chem. Soc. 1999, 121, 11964-11975.
(5) (a) Birch, A. J .; Blance, G. E.; David, S.; Smith, H. J . Chem.
Soc. 1961, 3128. (b) Takamatsu, N.; Inoue, S.; Kishi, Y. Tetrahedron
Lett. 1971, 4657-4660 and 4665-4668.
(6) (a) Dossena, A.; Marchelli, R.; Pochini, A. J . Chem. Soc., Chem.
Commun. 1974, 771-772. (b) Plate, R.; Nivard, R. J . F.; Ottenheijm,
H. C. J . Chem. Soc., Perkin Trans. 1 1987, 2473-2480.
(7) Steyn, P. S. Tetrahedron Lett. 1971, 3331-3334.
(8) Steyn, P. S. Tetrahedron 1973, 21, 107-120.
(9) (a) Kondoh, M.; Usui, T.; Mayumi, T., Osada, H. J . Antibiot.
1998, 51, 801-804. (b) Usui, T.; Kondoh, M.; Cui, C.-B.; Mayumi, T.;
Osada, H. Biochem. J . 1998, 333, 543-548.
10.1021/jo034703l CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/12/2003
6944
J . Org. Chem. 2003, 68, 6944-6951

Total Synthesis of Tryprostatin B
SCHEME 1. P r eviou s Tota l Syn th eses of Tr yp r osta tin B
SCHEME 2. Retr osyn th etic An a lysis of
Tr yp r osta tin B
tion of a suitable 2-prenylindole intermediate. Regarding
tryprostatin B itself, two total syntheses have been
published. The one due to Danishefsky15 is based on the
preparation of a 2-prenyltryptophan derivative by attack
of a nucleophilic species generated from tributylprenyl-
stannane and boron trichloride onto an unstable 3-chlor-
oindolenine, obtained from a suitably protected tryp-
tophan derivative (Scheme 1a). The synthesis developed
by Cook16c is also based on the preparation of a 2-pre-
nyltryptophan derivative, in this case by directed ortho-
lithiation of a N-BOC-indolymethyl derivative of the
Scho¨llkopf chiral auxiliary, followed by alkylation and
hydrolysis of the pyrazine moiety (Scheme 1b). This route
has been subsequently adapted to work on a solid phase
by attaching the prenylindole derivative to a resin
through a silyl linker.19 Also relevant are the alternative
syntheses of tryprostatin B developed by Cook, which are
also based on the multistep preparation of a suitable
2-prenyltryptophan, either by halogen-lithium exchange
of a 2-bromo-3-methylindole derivative16a,b followed by
alkylation and incorporation of the R-amino acid moiety
using Scho¨llkopf chemistry or by a palladium-mediated
heteroannulation reaction.16d
other amino acids as the starting material, this route can
be made amenable to the preparation of tryprostatin
analogues.
To prepare the required prenylated pentacyclic inter-
mediate from cyclo-(L-Trp-L-Pro), we initially considered
the possibility of using the procedure developed by Crich
for introducing a prenyl group at the 3a position of a
hexahydropyrrolo[2,3-b]indole system during his synthe-
sis of ent-debromoflustramine B,21 which relies on the
regioselective bromination of a benzylic position with
preference to a captodative one. However, we have
recently shown that cyclic tautomers of cyclo-(L-Trp-L-
Ala) derivatives behave differently than hexahydropyr-
rolo[2,3-b]indoles toward bromination, which does not
take place at the benzylic position as required.22 We
therefore turned our attention to the possibility of car-
rying out a tandem prenylation-cyclization of cyclo-(L-
Trp-L-Pro). Tandem C-3 protonation/cyclizations of tryp-
tophan and tryptamine derivatives23 and 2,5-pipera-
We describe here an alternative strategy for the syn-
thesis of tryprostatin B based on the preparation of a
fused pentacyclic compound from cyclo-(L-Trp-L-Pro),
followed by rearrangement of the prenyl chain20 with
concomitant ring opening and rearomatization of the in-
dole system (Scheme 2). By simply replacing L-Pro by
(16) (a) Gan, T.; Cook, J . M. Tetrahedron Lett. 1997, 38, 1301-
1304. (b) Gan, T.; Liu, R.; Yu, P.; Zhao, S.; Cook, J . M. J . Org. Chem.
1997, 62, 9298-9304. (c) Zhao, S.; Gan, T.; Yu, P.; Cook, J . M.
Tetrahedron Lett. 1998, 39, 7009-7012. (d) Ma, C.; Liu, X.; Li, X.;
Flippen-Anderson, J .; Yu, S.; Cook, J . M. J . Org. Chem. 2001, 66, 4525-
4542.
(17) Zhao, S.; Gan, T.; Cook, J . M. Tetrahedron Lett. 1998, 39, 7009-
7012.
(18) (a) Cardoso, A. S.; Lobo, A. M.; Prabhakar, S. Tetrahedron Lett.
2000, 41, 3611-3613. (b) Lobo, A. M.; Prabhakar, S. J . Heterocycl.
Chem. 2002, 39, 429-436.
(19) Wang, B.; Chen, L.; Kim, K. Tetrahedron Lett. 2001, 42, 1463-
1466.
(21) Bruncko, M.; Crich, D.; Samy, R. J . Org. Chem. 1994, 59, 5543-
5549.
(20) For precedent of prenyl migration from C-3 to C-2 triggered by
ring opening in related systems, see: (a) Nakagawa, N. M.; Matsuki,
K.; Hasumi, K.; Taniguchi, M.; Hino, T. Heterocycles 1982, 19, 156
(symposium abstract). (b) Hino, T.; Hasumi, K.; Yamaguchi, H.;
Taniguchi, M.; Nakagawa, M. Chem. Pharm. Bull. 1985, 33, 5202-
5206. (c) Plate, R.; Ottenheijm, H. C. J . Tetrahedron Lett. 1986, 27,
3755-3758.
(22) Caballero, E.; Avendan˜o, C.; Mene´ndez, J . C. Tetrahedron 1999,
55, 14185-14198.
(23) (a) For
a review of the chemistry of cyclic tautomers of
tryptophan and tryptamines, see: Hino, T.; Nakagawa, M. In The
Alkaloids; Brossi, A., Ed.; 1988; Vol. 34, pp 1-75. See also: (b) Crich,
D.; Natarajan, S. J . Org. Chem. 1995, 60, 6237-6241 and references
therein.
J . Org. Chem, Vol. 68, No. 18, 2003 6945

Caballero et al.
SCHEME 3. Ta n d em P r en yla tion -Cycliza tion of cyclo(L-Tr p -L-P r o)
zinediones containing a tryptophan moiety24 are well-
known in the literature. On the other hand, the related
reactions initiated by C-3 alkylation have been much less
employed and do not normally give yields above 20% in
the case of tryptophan and tryptamine derivatives.25 To
our knowledge, the cyclization in 15% yield of cyclo-(L-
Trp-L-Ala) in the presence of prenyl bromide in acetic
buffer by Casnati26 is the only tandem prenylation-
cyclization of a diketopiperazine containing a tryptophan
subunit described in the literature, although the stereo-
chemical outcome of the reaction was not studied.
Our work leading to tryprostatin B is summarized in
Scheme 3. Slow addition of prenyl bromide to a solution
of cyclo-(L-Trp-L-Pro) (3)27 and magnesium nitrate25b in
acetic buffer gave a mixture of diastereomers (4a , 5a )
arising from alkylation of the indole ring at C-3 and
subsequent cyclization by nucleophilic attack of the
neighboring piperazinedione nitrogen, together with their
N-prenylated derivatives 4b and 5b. The reaction also
gave 22% of an inseparable mixture of compound 4a and
tryprostatin B (2) and a trace (4%) of the diprenylated
pentacyclic derivative 6, arising from C3-prenylation/
cyclization of tryprostatin B. Use of prenyl iodide28 as the
electrophile led to increased yields of the N-prenyl
derivatives: 6% of a 4a + 2 mixture, 30% of 4b, 6% of
5a , and 29% of 5b. Since even C-3-substituted indoles
are normally alkylated at C-3 preferably to C-2, the
formation of tryprostatin B observed by us can in prin-
ciple be explained by acid-catalyzed rearrangement of 4a
or 5a under the reaction conditions, although direct al-
kylation at C-2 cannot be completely excluded29 and rear-
rangement of 5a can be discarded, as shown by later
results.
To trigger the desired rearrangement of the prenyl
chain of compound 4a , a number of acidic conditions were
assayed, involving various trifluoroacetic acid concentra-
tions and reaction times, leading always to mixtures of
compound 7 and tryprostatin B (2). Since we were unable
to prevent the addition of trifluoroacetic acid to the
double bond of 2, we decided to use a relatively concen-
trated acid solution and a long reaction time in order to
ensure the isolation of 7 as the only reaction product and
then transform it into the target compound by base-
catalyzed elimination. Thus, the mixture of 4a and 2 was
exposed to 1:10 trifluoroacetic acid-dichloromethane for
20 h, giving a quantitative yield of trifluoroacetate ester
7, which was characterized via its hydrolysis to 8. Finally,
exposure of compound 7 to triethylamine in methanol
gave tryprostatin B in 96% yield (Scheme 4). We expected
that compound 5a could also be transformed into trypros-
tatin by a similar rearrangement of its prenyl chain.
However, its treatment with several trifluoroacetic acid-
dichloromethane mixtures gave only starting material,
and forcing conditions (neat trifluoroacetic acid) led to
compound 9 from addition of trifluoroacetic acid to the
prenyl double bond, which, upon attempted silica gel
chromatography, reverted to the starting material. De-
spite this drawback, we found that 5a could also be
transformed into tryprostatin B in 50% yield by prolonged
treatment with ytterbium triflate in nitromethane, thus
raising the overall yield of the route to 32% (Scheme 4).
Ytterbium triflate is a well-known Lewis acid that
promotes the addition of nucleophiles to aldehydes30 and
has also found application in the deprotection of prenyl
ethers,31 but we are unaware of any literature precedent
on its use for promoting a rearrangement similar to the
5a f 2 transformation.
(24) See ref 23a, and also: (a) Sammes, P. G.; Weedon, A. C. J .
Chem. Soc., Perkin Trans. 1 1979, 3048-3052. (b) Caballero, E.;
Avendan˜o, C.; Mene´ndez, J . C. Tetrahedron: Asymmetry 1998, 9, 967-
981. (c) Taniguchi, M.; Yamamoto, I.; Nakagawa, M.; Hino, T. Chem.
Pharm. Bull. 1985, 33, 4783-4791.
(25) (a) Muthusubramanian, P.; Carle´, J . S.; Christophersen, C. Acta
Chem. Scand. B 1983, 37, 803-807. (b) Mitchell, M. O.; Le Quesne, P.
W. Tetrahedron Lett. 1990, 31, 2681-2684. (c) Mitchell, M. O.; Dorroh,
P. Tetrahedron Lett. 1991, 32, 7641-7642.
(26) Bocchi, V.; Casnati, G.; Marchelli, R. Tetrahedron 1978, 34,
929-932.
(27) Compound 3 has been prepared from N-Cbz-L-prolyl-L-tryp-
tophan methyl ester (ref 8). Our procedure employing the N-Boc
derivative is more convenient and can be found in Supporting Informa-
tion.
(28) Prenyl iodide used in this experiment was prepared in 71% yield
from prenyl alcohol and Olah’s chlorotrimethylsilane/sodium iodide
reagent. See: Olah, G. A.; Narang, S. B.; Gupta, B.; Malhotra, R. J .
Org. Chem. 1979, 44, 1247-1251.
(29) (a) J oule, J . A.; Mills, K.; Smith, G. F. Heterocyclic Chemistry,
3rd ed.; Chapman & Hall: New York, 1995; p 303. (b) Casnati, G.;
Dossena, A.; Pochini, A. Tetrahedron Lett. 1972, 5277-5280.
(30) Molander, G. A.; Harris, C. R. In Encyclopedia of Reagents for
Organic Synthesis; Paquette, L. A., Ed.; J ohn Wiley and Sons: New
York, 1995; Vol. 8, pp 5522-5523.
(31) Sharma, G. V. M.; Ilangovan, A.; Mahalingam, A. K. J . Org.
Chem. 1998, 63, 9103-9104.
6946 J . Org. Chem., Vol. 68, No. 18, 2003

Total Synthesis of Tryprostatin B
SCHEME 4. F in a l Sta ges of th e Syn th esis of
Tr yp r osta tin Ba
bonds and the C11a-H bond in 4a , which are ap-
proximately parallel to C13dO, can force the carbonyl
group slightly out of conjugation with N-12, thus facili-
tating its protonation and subsequent rearrangement of
the prenyl chain through indolenine I. This effect does
not exist in compound 5a , which is almost completely
planar at the amide bond, as shown by the C11a-N12-
C13-O13 dihedral angle, which is -0.95° for 5a and
-3.05° for 4a according to MM2 calculations, this differ-
ence being consistent with the one obtained using AM1
semiempirical calculations (ca. 3° lower for 5a ). Addition-
ally, the higher steric crowding of the N-12 atom in
compound 5a may be the cause of a rate difference in its
protonation with respect to compound 4a , which under-
goes the rearrangement (Scheme 5).
On the other hand, lanthanide(III) species are consid-
ered to be hard acids according to Pearson’s HSAB
classification and therefore show affinity toward hard
bases such as oxygen donor ligands. Because of this
oxophilicity and the high electron density of its C-13
carbonyl oxygen, compound 5a would be expected to react
with ytterbium triflate, giving the intermediate indole-
nine III, which would then undergo prenyl rearrange-
ment to give tryprostatin B (Scheme 5). Because of the
expected interference of the C5a-H bond, this reaction
would be expected to proceed slowly, which is in agree-
ment with the experimental observations.
a
Reagents and conditions: (i) 1:10 CF3CO2H, CH2Cl2, rt, 20 h;
(ii) NaHCO3, H2O-THF, rt, 37 h; (iii) MeOH, Et3N, rt, 60 h; (iv)
Yb(OTf)3, CH3NO2, reflux, 72 h; (v) CF3CO2H, rt, 2 h; (vi) SiO2
chromatography (AcOEt).
The preparation of tryprostatin B described above
paved the way for the synthesis of analogues by simply
replacing proline by other amino acids, although the
practical realization of this modification might not be
completely trivial since the presence of an additional NH
group in the starting piperazinedione created a potential
The different behavior of compounds 4a and 5a toward
acid-induced rearrangement can be ascribed to different
basicities of their N-12 atoms and to the higher steric
crowding of N-12 in compound 4a . Steric compression
between the C-13 carbonyl oxygen and one of the C1-H
SCHEME 5. P r op osed Exp la n a tion for th e Differ en t Beh a vior s of Com p ou n d s 4a a n d 5a
J . Org. Chem, Vol. 68, No. 18, 2003 6947

Caballero et al.
SCHEME 6. Syn th esis of th e Ala n in e An a log of Tr yp r osta tin B
F IGURE 2. NOE effects in compounds 11-13.
site for competitive alkylation in the first step of the
synthesis. Fortunately, this problem did not arise, and
we found that slow addition of prenyl bromide to a
solution of cyclo-(L-Trp-L-Ala) (10) gave a mixture of
diastereomers (11a , 12a ), similarly formed by alkylation
of the indole ring at C-3 and tandem cyclization by
nucleophilic attack of the neighboring piperazinedione
nitrogen. Their N-prenylated derivatives 11b and 12b,
respectively, as well as compound 13, prenylated at both
atoms of the indoline-pyrrolidine fusion bond, were also
obtained. Treatment of compound 11a with dilute tri-
fluoroacetic acid gave the desired tryprostatin analogue
14 as the major product (60% yield), together with 25%
of the trifluoroacetate 15, which could be transformed
into 14 in 81% yield by reaction with triethylamine in
methanol (Scheme 6).
The stereochemical assignments of the intermediate
tetracyclic derivatives in the alanine series (compounds
11-13) were based on the NOE experiments summarized
in Figure 2. We assume that epimerization of the Trp
stereocenter in 12a takes place after cyclization and is
prompted by the considerable steric crowding of the
nonisolated compound IV due to the interaction between
the C5a-H, C11-H, and C10b-prenyl substituents, which
is partly relieved by its transformation into 12. This steric
crowding does not exist in compounds 11, which therefore
6948 J . Org. Chem., Vol. 68, No. 18, 2003

Total Synthesis of Tryprostatin B
F IGURE 4. NOE effects in compound 5a and structure of
compound V, the nonisolated 5a-epimer of 5a .
V (5a-epi-5a ) would be forced to have its piperazinedione
ring in a chair conformation (Figure 4), which does not
happen in its alanine analogue.
As a final note, we believe that the tryprostatin B
synthesis described in this work has implications regard-
ing the yet unknown biosynthetic pathway of the trypro-
statins32 in that it provides an alternative to the acid-
catalyzed rearrangement of 1-allylindole derivatives,
which has been shown to give product mixtures contain-
ing variable amounts of 2-allylindoles in work aimed at
clarifying the biosynthetic pathways of the echinulins, a
related group of prenylindole alkaloids.33
F IGURE 3. Comparison between the structures of compounds
11 and 12 and their nonisolated epimer at C-11a (IV).
do not epimerize under acidic conditions (e.g., dilute CF3-
CO2H in the transformation of 11a into 14 and 15). Both
results, namely, the observed epimerization at the tryp-
tophan stereocenter of 12 and the absence of epimeriza-
tion in 11, are consistent with our previous findings on
the tandem protonation-cyclization reactions of cyclo-
(Trp-Ala)24b (Figure 3).
Exp er im en ta l Section
Ta n d em P r en yla t ion -Cycliza t ion of cyclo(L-Tr p -L-
P r o) (3). To a vigorously stirred solution of cyclo(L-Trp-L-Pro)
327 (350 mg, 1.24 mmol) and magnesium nitrate hexahydrate
(1.59 g, 5 equiv) in 50 mL of aqueous acetic acid-sodium
acetate buffer (pH 2.9, prepared from 8 g of sodium acetate,
20 mL of water, and 100 mL of acetic acid) was slowly added
at room-temperature prenyl bromide (0.85 mL, 6 equiv) over
20 h, via syringe pump, under an argon atmosphere. The
solution was neutralized with solid Na2CO3 and extracted with
CHCl3 (10 × 10 mL), filtering off any inorganic precipitate
formed during the extraction. The combined organic layers
were dried over anhydrous Na2SO4 and evaporated. The
residue was evaporated to dryness and chromatographed on
silica gel, eluting with 1:1 petroleum ether-ethyl acetate,
yielding (in order of elution) 31 mg (4%) of (5aS,6aS,11aS,-
13aS)-6a,11a-diprenyl-2,3,5,5a,6,6a,11,11a,13,13a-decahydro-
1H-pyrrolo[1′′,2′′-4′,5′]pyrazino[2′,1′-5,1]pyrrolo[2,3-b]indole-
5,13-dione (6), 67 mg (13%) of (5aS,6aR,11aR,13aS)-6a,11-
diprenyl-2,3,5,5a,6,6a,11,11a,13,13a-decahydro-1H-pyrrolo[1′′,2′′-
4′,5′]pyrazino[2′,1′-5,1]pyrrolo[2,3-b]indole-5,13-dione (5b), 96
mg (19%) of (5aS,6S,11aS,13aS)-6a,11-diprenyl-2,3,5,5a,6,6a,-
11,11a,13,13a-decahydro-1H-pyrrolo[1′′,2′′-4′,5′]pyrazino[2′,1′-
5,1]pyrrolo[2,3-b]indole-5,13-dione (4b), 93 mg (21%) of (5aS,-
6aR,11aR,13aS)-6a-prenyl-2,3,5,5a,6,6a,11,11a,13,13a-deca-
hydro-1H-pyrrolo[1′′,2′′-4′,5′]pyrazino[2′,1′-5,1]pyrrolo[2,3-
On the other hand, the pentacyclic derivatives related
to tryprostatin B (compounds 4-6) were not equally
amenable to NOE studies, and we had to resort to
1
comparisons of some key H NMR chemical shifts and
multiplicities to establish their stereochemistries. In the
alanine series, the H-11 signals appear as two dd at ca.
2.60 and 2.30 ppm for compounds with a trans relation-
ship between the substituents at positions 5a-10b and
the H-3 proton (compounds 11 and 13) but as multiplets
centered at ca. 2.30 ppm for their diastereomers where
this relationship is cis (compounds 12). Another signifi-
cant difference can be found in the chemical shift of the
bridgehead proton H-5a, which is influenced by prenyl-
ation at N-6 in the case of compounds 12 (5.29 ppm for
12a and 5.51 ppm for 12b), but not in the case of
compounds 11 and 13, where it is observed at 5.40 ppm
in all cases. Spectral data of compounds 4 and 5 fit very
well into these criteria, allowing assignment of their
stereochemistries. Epimerization at the tryptophan ster-
eocenter was considered to be dubious because, according
to the literature, this epimerization does not take place
in the protonation-cyclization of cyclo-(L-Trp-L-Pro).24c
Although the key NOE effects of the tryptophan H-5a
proton were ambiguous because the signals due to one
of the H-6 protons and the prenyl methylene are over-
lapped, the fact that 5a can be rearranged to tryprostatin
B confirms the proposed configuration at H-5a. The
difference between the alanine and proline series in
terms of epimerization of the tryptophan stereocenter can
be attributed to the fact that the nonisolated compound
(32) For a review of the biosynthesis of prenylated alkaloids derived
from tryptophan, see: Williams, R. M.; Stocking, E. M.; Sanz-Cervera,
J . F. Top. Curr. Chem. 2000, 209, 98-173.
(33) (a) Casnati, G.; Marchelli, R.; Pochini, A. J . Chem. Soc., Perkin
Trans. 1 1974, 754-757. (b) Inada, S.; Nagai, K.; Takayanagi, Y.;
Okazaki, M. Bull. Chem. Soc. J pn. 1976, 49, 833-834. (c) Sammes, P.
G.; Weedon, A. C. J . Chem. Soc., Perkin Trans. 1 1979, 3053-3059.
(d) Grundon, M. F.; Hamblin, M. R.; Harrison, D. M.; Logue, J . N. D.;
Maguire, M.; McGrath, J . A. J . Chem. Soc., Perkin Trans. 1 1980,
1294-1298. (e) See also ref 18.
J . Org. Chem, Vol. 68, No. 18, 2003 6949

Caballero et al.
b]indole-5,13-dione (5a ), and 123 mg (44%) of a 1:1 inseparable
mixture of compound 4a 34 and tryprostatin B (2).
on silica gel, eluting with ethyl acetate. The yield of trypro-
statin B (2) was 25 mg (50%).
Da ta for 4b. Pale yellow oil. [R]D ) + 1.60 (c ) 1.35,
Hyd r olysis of Com p ou n d 7. To a solution of 7 (8.5 mg,
0.024 mmol) in THF (1 mL) was added a solution of NaHCO3
(44.75 mg, 22 equiv) in water (1 mL). The solution was stirred
at room temperature under an argon atmosphere for 37 h and
extracted with CHCl3 (7 × 5 mL). The combined extracts were
dried over anhydrous Na2SO4 and evaporated. The residue was
chromatographed on silica gel, eluting with 4:1 AcOEt-MeOH,
yielding 4 mg (60%) of (3S,8aS)-3-(2′-(3′′-hydroxy-3′′,3′′-di-
methylpropyl)-3′-indolylmethyl)-1,2,3,4,6,7,8,8a-octahydropyr-
rolo[1,2-a]pyrazine-1,4-dione (8). Mp 90-92 °C. [R]D25 ) + 35.0
(c ) 0.08, CHCl3). IR (NaCl): 3349; 1663 cm-1. HRMS: calcd
25
CHCl3). Anal. Calcd for C26H33N3O2, M ) 419: C, 74.43; H,
7.93; N, 10.02. Found: C, 74.45; H, 7.87; N, 9.94. For spectral
data, see Supporting Information.
Da ta for 5a . Off-white solid. Mp 65-69 °C. [R]D25 ) -236.8
(c ) 0.72, CHCl3). Anal. Calcd for C21H25N3O2, M ) 351: C,
71.77; H, 7.17; N, 11.96. Found: C, 71.71; H, 6.97; N, 11.99.
For spectral data, see Supporting Information.
Da ta for 5b. Pale yellow oil. [R]D ) -197.8 (c ) 0.69,
25
CHCl3). For spectral data, see Supporting Information.
Da ta for 6. Off-white solid. Mp 63-65 °C. [R]D ) -210.1
25
for C21H27N3O3 (M+) 369.2052, found 369.2044; calcd for C14H18
-
(c ) 0.69, CHCl3). Anal. Calcd for C26H33N3O2, M ) 419: C,
74.46; H, 7.88; N, 10.02. Found: C, 74.79; H, 7.68; N, 9.81.
For spectral data, see Supporting Information.
NO (M+ - C7H9N2O2), 216.1388, found 216.1386; calcd for
C
14H16N (M+ - C7H11N2O3) 198.1283, found 198.1281.
Rea ction of Com p ou n d 5a w ith Tr iflu or oa cetic Acid .
Tr a n sfor m a tion of th e 4a + 2 Mixtu r e in to P u r e
Tr yp r osta tin B. To a solution of the equimolecular mixture
of 4a and 2 (27 mg, 0.077 mmol) in CH2Cl2 (5 mL) was added
CF3CO2H (0.5 mL). The solution was stirred at room temper-
ature for 20 h, under an argon atmosphere, and then it was
poured onto a cooled (5 °C), vigorously stirred mixture of 20%
aqueous Na2CO3 (7 mL) and CH2Cl2 (7 mL). After further
extraction with CH2Cl2 (7 × 5 mL), the combined organic layers
were dried over anhydrous Na2SO4 and evaporated, the residue
being identified by 1H NMR and 19F NMR as (3S,8aS)-3-[2′-
(3′′,3′′-dimethy-3′′-trifluoroacetoxy)propyl-3′-indolylmethyl]-
1,2,3,4,6,7,8,8a-octahydropyrrolo[1,2-a]pyrazine-1,4-dione (7).35
A solution of the crude compound 7 in methanol (2 mL) and
triethylamine (2 mL) was stirred at room temperature for 60
h under an argon atmosphere. The solvent was evaporated
under reduced pressure, and the residue was dissolved in CH2-
Cl2 and washed with a saturated aqueous solution of NH4Cl
(5 mL). The organic layer was dried over anhydrous Na2SO4
and evaporated, yielding 25 mg (96%) of tryprostatin B (2).
A solution of 5a (27 mg, 0.077 mmol) in neat trifluoroacetic
acid (1 mL) was stirred at room temperature for 2 h and poured
onto a cooled (0 °C), stirred mixture of 20% aqueous Na2CO3
(5 mL) and CH2Cl2 (5 mL). The aqueous phase was further
extracted with CH2Cl2 (7 × 5 mL), and the combined organic
extracts were dried over anhydrous Na2SO4 and evaporated
under reduced pressure, giving compound 936 (35 mg, 98%).
Attempted silica gel chromatography, eluting with ethyl
acetate, gave the starting material 5a .
Ta n d em P r en yla tion -Cycliza tion of cyclo(L-Tr p -L-Ala )
(10). To a vigorously stirred solution of cyclo(L-Trp-L-Ala) (10)37
(500 mg, 1.95 mmol) and magnesium nitrate hexahydrate (2.5
g, 5 equiv) in 50 mL of acetic acid-sodium acetate buffer (pH
) 2.9) was slowly added at room-temperature prenyl bromide
(1.5 mL, 6 equiv) over 12 h, via syringe pump, under an argon
atmosphere. The solution was neutralized with solid Na2CO3
and extracted with CHCl3 (10 × 10 mL), filtering off any
inorganic precipitate formed during the extraction. The com-
bined organic layers were dried over anhydrous Na2SO4 and
evaporated. The residue was evaporated to dryness and
chromatographed on silica gel, eluting with 1:1 petroleum
ether-ethyl acetate, yielding 132 mg (21%) of (3S,5aS,10bS,-
11aS)-3-methyl-10b-prenyl-1,3,4,5a,6,10b,11,11a-octahydro-
2H-pyrazino[2′,1′-5,1]pyrrolo[2,3-b]indole-1,4-dione (11a ), 129
mg (20%) of its (10bR,11aR)-diastereomer (12a ), 101 mg (13%)
of (3S,5aS,10bS,11aS)-3-methyl-6,10b-diprenyl-1,3,4,5a,6,10b,-
11,11a-octahydro-2H-pyrazino[2′,1′-5,1]pyrrolo[2,3-b]indole-
1,4-dione (11b), 29 mg (4%) of its (10bR,11aR)-diastereomer
(12b), and 78 mg (10%) of (3S,5aS,10bS,11aS)-3-methyl-5a,-
10b-diprenyl-1,3,4,5a,6,10b,11,11a-octahydro-2H-pyrazino[2′,1′-
5,1]pyrrolo[2,3-b]indole-1,4-dione (13).
Off-white solid. Mp 102-104 °C; lit.4a 102-105 °C. [R]D
)
25
-83.3 (c ) 0.03, CHCl3); lit4a [R]D25 ) - 71.1 (c ) 0.63, CHCl3).
IR (NaCl): 3284; 1665 cm-1 1H NMR (CDCl3, 250 MHz): δ
.
7.91 (br s, 1H); 7.48 (d, 1H, J ) 7.4 Hz); 7.32 (d, 1H, J ) 7.8
Hz); 7.12 (m, 2H); 5.63 (br s, 1H); 5.31 (m, 1H); 4.37 (m, 1H);
4.06 (m, 1H); 3.66 (m, 3H); 3.48 (app d, 1H, J ) 7.6 Hz); 2.95
(dd, 1H, J ) 15.0 and 11.5 Hz); 2.34 (m, 1H); 2.03 (m, 3H);
1.79 (s, 3H); 1.76 (s, 3H).13C NMR (CDCl3, 63 MHz): δ 169.5;
165.9; 136.6; 135.8; 135.5; 128.1; 121.9; 120.0; 119.9; 117.8;
110.9; 104.7; 59.4; 54.7; 45.5; 28.4; 25.9; 25.7; 25.2; 22.7; 18.3.
MS, m/z (%): 351 (13.2, M+); 282 (8.7, M+ - prenyl); 198 (100,
M+ - C7H9N2O2). HRMS: calcd for C21H25N3O2 (M+) 351.1947,
found 351.1949; calcd for C16H16N3O2 (M+ - prenyl) 282.1243,
found 282.1242; calcd for C14H16N (M+ - C7H9N2O2) 198.1283,
found 198.1279.
Tr a n sfor m a tion of 5a in to Tr yp r osta tin B. A solution
of compound 5a (50 mg, 0.14 mmol) in nitromethane (2 mL)
was treated with ytterbium triflate (9 mg, 0.014 mmol) and
refluxed in an oil bath at 115 °C for 72 h. The reaction mixture
was diluted with water (5 mL) and extracted with CHCl3 (5 ×
10 mL). The combined extracts were dried over anhydrous Na2-
SO4 and evaporated, and the residue was chromatographed
Da ta for 11a . White solid. Mp 187-188 °C. [R]D25 ) +122.3
(c ) 0.13, CHCl3). Anal. Calcd for C19H23N3O2, M ) 325: C,
70.15; H, 7.08; N, 12.92. Found: C, 69.93; H, 6.88; N, 13.09.
For spectral data, see Supporting Information.
Da ta for 11b. Off-white solid. Mp 60-61 °C. [R]D25 ) -40.3
(c ) 0.66, CHCl3). Anal. Calcd for C24H31N3O2, M ) 393: C,
73.28; H, 7.89; N, 10.69. Found: C, 73.00; H, 7.97; N, 10.42.
For spectral data, see Supporting Information.
Da ta for 12a . Off-white solid. Mp 70-72 °C. [R]D25 ) -353.0
(c ) 0.20, CHCl3). Anal. Calcd for C19H23N3O2, M ) 325: C,
70.15; H, 7.08; N, 12.92. Found: C, 69.91; H, 6.93; N, 12.87.
For spectral data, see Supporting Information.
(34) 1H NMR (CDCl3, 250 MHz): δ 7.07 (m, 2H); 6.75 (td, 1H, J )
7.4 and 0.8 Hz); 6.58 (d, 1H, J ) 7.7 Hz); 5.42 (s, 1H); 5.10 (t, 1H, J )
7.9 Hz); 4.36 (t, 1H, J ) 8.6 Hz); 4.14 (m, 1H); 3.48 (m, 2H); 2.50 (m,
4H); 2.32 (m, 1H); 2.22-1.87 (m, 3H); 1.69 (s, 3H); 1.62 (s, 3H). 13C
NMR (CDCl3, 63 MHz): δ 168.1; 166.0; 147.5; 135.5; 132.8; 128.4;
123.0; 119.3; 118.9; 109.4; 80.9; 60.7; 59.8; 56.2; 45.2; 38.7; 35.4; 27.8;
26.1; 23.3; 18.1.
Da ta for 12b. Pale yellow oil. [R]D ) - 220.8 (c ) 0.06,
25
CHCl3). Anal. Calcd. for C24H31N3O2, M ) 393: C, 73.28; H,
7.89; N, 10.69. Found: C, 73.44; H, 7.53; N, 10.86. For spectral
data, see Supporting Information.
(35) 1H NMR (CDCl3, 250 MHz): δ 8.05 (br s, 1H); 7.48 (d, 1H, J )
7.6 Hz); 7.33 (d, 1H, J ) 7.3 Hz); 7.12 (m, 2H); 5.63 (br s, 1H); 4, 38
(m, 1H); 4.08 (m, 1H); 3.64 (m, 3H); 2.95 (m, 1H); 2.82 (m, 2H); 2.35
(m, 1H); 2.20 (m, 2H); 2.03 (m, 3H); 1.63 (s, 3H); 1.62 (s, 3H) ppm. 19F
NMR (CDCl3): δ -76.00 (s).
Da ta for 13. Off-white solid. Mp 65-64 °C. [R]D25 ) -170.9
(c ) 0.43, CHCl3). Anal. Calcd for C24H31N3O2, M ) 393: C,
73.28; H, 7.89; N, 10.69. Found: C, 73.58; H, 7.60; N, 10.50.
(36) 1H NMR (CDCl3, 250 MHz): δ 7.10 (m, 2H); 6.82 (t, 1H, J )
7.4 Hz); 6.63 (d, 1H, J ) 7.7 Hz); 5.25 (s, 1H); 4.09 (m, 2H); 3.55 (m,
2H); 2.72 (dd, 1H, J ) 13.8 and 8.1 Hz); 2.32 (m, 3H); 2.24-1.54 (m,
8H); 1.47 (s, 3H); 1.25 (s, 3H). 19F NMR (CDCl3): δ -76.1 (s).
(37) Compound 10 has been isolated from natural sources. See, for
instance: Hamasaki, T.; Nagayama, K.; Hatsuda, Y. Agric. Biol. Chem.
1976, 40, 2487. For its preparation, see ref 24b.
6950 J . Org. Chem., Vol. 68, No. 18, 2003

Total Synthesis of Tryprostatin B
Rea ction of Com p ou n d 11a w ith Tr iflu or oa cetic Acid .
To a solution of compound 11a (50 mg, 0.154 mmol) in CH2-
Cl2 (1.5 mL) was added trifluoroacetic acid (1 mL). The solution
was stirred under an argon atmosphere for 4.5 h and poured
onto a vigorously stirred biphasic system formed by 20%
aqueous Na2CO3 (7 mL) and CH2Cl2 (5 mL), kept in a bath at
4 °C. The aqueous layer was extracted with CH2Cl2 (5 × 5 mL),
and the combined organic layers were dried over Na2SO4 and
evaporated. The residue was chromatographed on silica gel,
eluting with ethyl acetate, giving 30 mg (60%) of (3S,6S)-3-
(2′-prenyl-3′-indolylmethyl)-6-methyl-2,5-piperazinedione 14,
the alanine analogue of tryprostatin B, as a white solid, and
17 mg (25%) of (3S,6S)-3-[2′-(3′′,3′′-dimethyl-3′′-trifluoroac-
etoxy)propyl-3′-indolylmethyl]-6-methyl-2,5-piperazinedione (15),
as a pale yellow solid.
Tr a n sfor m a tion of Com p ou n d 15 in to 14. To a solution
of compound 15 (10 mg, 0.03 mmol) in methanol (2 mL) was
added triethylamine (2 mL). The solution was stirred at room
temperature, under an argon atmosphere, for 18 h. The solvent
was evaporated under reduced pressure, and the residue was
dissolved in CH2Cl2 (10 mL) and washed with a saturated
aqueous solution of NH4Cl (5 mL). The organic layer was dried
over Na2SO4 and evaporated, yielding 6 mg (81%) of compound
14.
Ack n ow led gm en t. We thank Dr. M. T. Ramos for
helpful discussions. Financial support of this research
from CICYT (Grant SAF-2000-0130) is also gratefully
acknowledged.
Da ta for 14. White solid. Mp 240-242 °C. [R]D ) -21.1
25
(c ) 0.09, CHCl3). HRMS: calcd for C19H23N3O2 (M+) 325,1790,
found 325.1789; calcd for C14H16N (M+ - C5H7N2O2) 198.1283,
found 198.1278. For spectral data, see Supporting Information.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails of the synthesis of starting materials, spectral data, and
spectra of representative compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
Da ta for 15. Pale yellow solid. Mp 273-275 °C. [R]D
)
25
-7.4 (c ) 0.27, EtOH). Anal. Calcd for C29H24F3N3O4, M )
439: C, 57.40; H, 5.47; N, 9.57. Found: C, 57.37; H, 5.70; N,
9.59. For spectral data, see Supplementary Information.
J O034703L
J . Org. Chem, Vol. 68, No. 18, 2003 6951