A synthetic approach toward the proposed tetracyclic aziridinomitosene derived from FK317

Article abstract of DOI:10.1021/jo040211c

A synthesis of the FK317 derivative 25 is described using internal Michael addition. Tin-lithium exchange of the deuterated stannylaziridine 18 generated the key lithioaziridine intermediate, followed by cyclization and aromatization of the pyrrole ring t

Full text of DOI:10.1021/jo040211c

A Syn th etic Ap p r oa ch tow a r d th e P r op osed Tetr a cyclic
Azir id in om itosen e Der ived fr om F K317
Musong Kim and Edwin Vedejs*
Department of Chemistry, University of Michigan, 930 North University Avenue,
Ann Arbor, Michigan 48109
edved@umich.edu
Received J une 7, 2004
A synthesis of the FK317 derivative 25 is described using internal Michael addition. Tin-lithium
exchange of the deuterated stannylaziridine 18 generated the key lithioaziridine intermediate,
followed by cyclization and aromatization of the pyrrole ring to give 7. Ester reduction from 7 to
23 was effected via temporary aldehyde protection as the silylimidazole adduct 22, and conversion
to the carbamate 25 was carried out using FmocNCO and FMOC cleavage. Structure 25 is the
N-trityl-protected derivative of the proposed intermediate from bioactivation of FK317 that is
responsible for DNA cross-linking. Attempted nitrogen deprotection of 25 using MsOH/i-Pr3SiH
resulted in replacement of the C(10) carbamate by hydride. Deprotection of the more stable 21
gave the desired aziridine 26.
Mitomycin C (1; Figure 1) has long been used against
a variety of solid tumors despite significant side effects.1
The related aziridine-containing antitumor agents 2 and
3 were isolated more recently from Streptomyces san-
daensis, and the semisynthetic derivative FK973 (4) was
found to have potent antitumor activity.2,3 However,
attempts to develop FK973 were terminated due to
toxicity from vascular leak syndrome (VLS).4 In 1998,
FK317 (5)5 was shown to have improved antitumor
activity compared to FK973 or mitomycin C, but without
the VLS side effects. The mode of action of FK317 is
believed to involve a sequence of metabolic activation
steps, including deacylation, reductive N-O bond cleav-
age, and cyclization to give the mitosene-like intermedi-
ate 6 as the activated form responsible for DNA-DNA
cross-linking.6
Although FK317 has not yet lived up to its early
promise,7 it has attracted considerable interest due to the
structural similarity between the proposed intermediate
6 and the aziridinomitosene intermediates responsible
for the antitumor activity of mitomycins. We have
therefore initiated a synthetic effort to prepare 6 and to
learn whether structures containing the sensitive sub-
stitution pattern can be isolated. Here we report our first
attempts to synthesize the fully functionalized 6 using
an internal Michael addition approach based on lithiated
aziridines.8
Resu lts a n d Discu ssion
Our strategy involves the convergent synthesis of 8 via
the intermediates 9 and the known 108 (Scheme 1). In
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10.1021/jo040211c CCC: $27.50 © 2004 American Chemical Society
Published on Web 09/21/2004
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J . Org. Chem. 2004, 69, 7262-7265

Tetracyclic Aziridinomitosene Derived from FK317
F IGURE 1.
SCHEME 1
SCHEME 2
potassium carbonate in ethanol/THF/water (3:1:1) then
afforded the desired indole 9 (78% from 17).13
Indole 9 was deprotonated with NaHMDS and coupled
with the aziridine mesylate 10. To force the reaction of
the mesylate 10 to completion, it was necessary to use
indole 9 in excess. When 3 equiv of 9 was used, 8 was
obtained in 95% yield based on 10, and 64% of 9 was
recovered (97% material balance) and could be reused in
subsequent experiments.
earlier studies, we had demonstrated the key bond
formation between C(1) and C(9a) (mitomycin number-
ing) using a model substrate 11. Treatment of 11 with
methyllithium generated an aziridinyllithium intermedi-
ate 12, followed by cyclization via internal Michael
addition to give the tetracyclic enolate 13. Enolate
trapping with PhSeCl then gave the desired tetracyclic
indole 14 in good yield. The presence of C(9a) deuterium
at the stage of tin-lithium exchange was essential.
Without the kinetic isotope effect of a deuterium blocking
group, competing lithiation at C(9a) was the major
pathway and effectively prevented the internal Michael
addition step.
The more highly functionalized indole 159 (Scheme 2)
required to access aziridinomitosene-like structures re-
lated to 6 was prepared in four steps from pyrrole-2-
carboxaldehyde using a recently optimized procedure as
described elsewhere.10,11 The ester side chain of 15 was
reduced with LiAlH4 and subsequent silylation afforded
16 in 91% yield. Carboxylation at the C(9) position of
indole 16 was carried out by bromination, lithium
halogen exchange, and quenching of the resulting anion
with ethyl chloroformate.12 Selective N-desilylation with
In preparation for the key internal Michael cyclization,
the C(9a) position of indole 8 was deprotonated using
freshly prepared mesityllithium (MesLi),14 and the re-
sulting lithiated indole was quenched with EtOD to give
18 in excellent yield. In our model study where there
were no substituents at C(6) or C(8), PhLi had been used
as the base to prepare 11 with no complications.8
However, in the case of 8, significant addition of PhLi to
the ethyl ester was observed under the same conditions.
Although TMEDA15 suppressed the undesired addition
of PhLi to the ester, MesLi gave the best result (98% yield
1
with >95% D1 by H NMR). The deuterated indole 18
(13) (a) Repke, D. B.; J ahangir; Clark, R. D.; Nelson, J . T. Hetero-
cycles 1988, 27, 2289. (b) Repke, D. B.; J ahangir; Clark, R. D.;
MacLean, D. B. J . Chem. Soc., Chem. Commun. 1988, 439. (c) Sauer,
G.; Heindl, J .; Wachtel, H. Tetrahedron Lett. 1988, 29, 6425. (d) Repke,
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1989, 45, 2541. (e) Angle, S. R.; Henry, R. M. J . Org. Chem. 1998, 63,
7490. (f) Wynne, J . H.; Stalick, W. M. J . Org. Chem. 2002, 67, 5850.
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24, 627. (b) Pelter, A.; Singaram, B.; Wilson, J . W. Tetrahedron Lett.
1983, 24, 631. (c) Vedejs, E.; Chapman, R. W.; Lin, S.; Mueller, M.;
Powell, D. R. J . Am. Chem. Soc. 2000, 122, 3047.
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(10) Vedejs, E.; Kim, M. J . Org. Chem. 2004, 0000.
(11) A gift of indole 15 by Albany Molecular Research, Inc., greatly
facilitated this work.
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(15) Hope, H.; Power, P. P. J . Am. Chem. Soc. 1983, 105, 5320.
J . Org. Chem, Vol. 69, No. 21, 2004 7263

Kim and Vedejs
SCHEME 3
SCHEME 4
This resulted in the neutralization of alkoxides as well
as hydrolysis of the N,O acetal to give 23 (82% from 21).
The indole 23 was then treated with freshly prepared
Fmoc-NCO16c to give the Fmoc-carbamate 24, and cleav-
age of Fmoc with Et3N in a one-pot procedure afforded
the desired 25 (55% from 23).16e
The challenging removal of the N-trityl group could
now be explored in the highly sensitive environment.
Preliminary attempts to deprotect the most sensitive
substrate 25 were not promising, so deprotection of the
ester aldehyde 21 was studied in the expectation that
the electron-withdrawing substituents would help to
stabilize the product 26 (Scheme 4). Electrospray mass
spectroscopy (ES/MS) was used to monitor events in
small scale experiments based on the observation that
all of the tetracyclic structures prepared up to this point
had given distinct peaks for the M + Na+ ions, charac-
teristic of the expected structures. We anticipated the
same behavior for 26 because removal of the trityl group
was not expected to adversely affect the stability of the
aziridine. However, the recently optimized deprotection
conditions (MsOH/Et3SiH at 0 °C)18 gave no mass peak
for the desired aziridine 26, and identifiable components
could not be separated from the complex product mixture.
Attempts to control the deprotection of 21 with the Et3-
SiH/MsOH reagent at lower temperatures were not
successful, but a similar experiment using the bulkier
triisopropylsilane at -42 °C (10 min) gave the desired
aziridine 26 (41%) together with recovered 21 (16%). The
structure of 26 was supported by ES/MS and NMR data
and by comparisons with the simpler structure 27
prepared earlier in our laboratory.8
The reoptimized detritylation procedure (-42 °C) was
then applied to 25 with monitoring by ES/MS. However,
no clear evidence to support the formation of the de-
protected aziridine 6 was obtained, and an alternative
reductive pathway was encountered. The major product
(65%) still contained an N-trityl group, as well as the
characteristic 1H NMR signals for the bridgehead protons
of an intact aziridine and the adjacent CH2 protons in
the five-membered ring. Structure 29 was assigned,
based on the presence of a new methyl singlet at δ 2.52
ppm, together with the disappearance of the character-
istic AB pattern for the C(10) protons. In contrast to the
tetracyclic structures 21, 23, or 25, no M + Na+ ion was
observed in the ES/MS output for 29. Instead, a strong
was then reacted with MeLi followed by PhSeCl to give
tetracycle 7 in an overall yield of 79% via the same
sequence of tin-lithium exchange, internal Michael
addition, and aromatization steps as in the model study.8
The most direct strategy at this point would be to
attach the C(10) carbamate and then to deprotect and
modify the C(6) hydroxymethyl ether group of tetracycle
7. Ester 7 was reduced with LiAlH4 to give hydroxy indole
19 with relative ease (Scheme 3). However, all attempts
to install the carbamate moiety from 19 using prece-
dented methods16 failed due to the sensitivity of the
desired product 20. This was no surprise because 20
contains potential leaving groups at C(1) (aziridine C-N)
and C(10) (carbamate C-O) that are activated by dona-
tion from the electron-rich indole nitrogen. To suppress
leaving group reactivity, an alternative sequence was
followed from 7 via initial deprotection of the C(6) silyl
ether with TBAF followed by TPAP oxidation to the
aldehyde 21 (65% from 7). The electron-withdrawing
CHO group in 21 was expected to improve stability by a
delocalization effect due to the conjugated, vinylogous
formamide subunit, resulting in lower electron density
in the indole and a higher barrier for leaving group
departure. On the other hand, the presence of a C(6)
aldehyde would not be compatible with reduction of the
C(10)-ester. Fortunately, temporary protection of the
CHO group was possible using TMS-imidazole17 in re-
fluxing CH2Cl2 to give the N,O acetal 22. Without
purification of 22, the ester group was reduced with
LiAlH4, followed by workup using solid Na2SO4‚10H2O.
(16) (a) Kocovsky, P. Tetrahedron Lett. 1986, 27, 5521. (b) Dong,
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2000, 122, 5401. (d) Coleman, R. S.; Chen, W. Org. Lett. 2001, 3, 1141.
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Chem. Commun. 1973, 55. (b) Gasparini, J . P.; Grassend, R.; Maire,
J . C.; Elguero, J . J . Organomet. Chem. 1980, 188, 141. (c) Gasparini,
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7264 J . Org. Chem., Vol. 69, No. 21, 2004

Tetracyclic Aziridinomitosene Derived from FK317
peak corresponding to M + MeOH + Na+ was found,
suggesting that 29 undergoes solvolytic aziridine ring
opening under electrospray sampling conditions due to
increased electron density in the pyrrole ring of 29
compared to 26.
Analysis of the minor chromatography fractions by
NMR was uninformative, but ES/MS assay revealed the
characteristic mass of an iminium ion 28. The same ion
was observed in the electrospray mass spectrum of the
carbamate 25 as well as the precursor alcohol 23, so this
ion can be taken as evidence for the presence of a C(10)-
heteroatom that is capable of heterolysis, as well as for
the survival of the N-trityl group. Weak ions were also
detected in several fractions corresponding to the adduct
of 28 + Na + OMe or 28 + MeOH, but these peaks were
always small compared to that of 28.
controlled conditions suggests that 6 will also prove to
be an isolable substance. Prior synthetic efforts have
accessed tetracyclic aziridinomitosene derivatives stabi-
lized by the presence of the quinone ring,16b,c,19 but reports
describing the isolation of solvolytically sensitive ana-
logues of 6 or 25 have been rare.8c,20 Further studies
toward this goal are underway.
Ack n ow led gm en t. This work was supported by the
National Institutes of Health (CA17918). The authors
thank Dr. J .G. Reid and Albany Molecular Research Inc.
for providing a generous sample of 15.
Su p p or tin g In for m a tion Ava ila ble: Experimental de-
tails, characterization, and NMR spectra for new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
Detritylated products were detected in trace amounts
based on a weak ion corresponding to 30, but no products
were found having the characteristic mass of the depro-
tected carbamate 6. When detritylation was attempted
on the alcohol 23 in small scale test experiments, the
formation of the same over-reduction product 29 was
indicated by ES/MS. Evidently, the C(10) hydroxyl group
of 23 is also easily activated for heterolysis and reduction,
presumably via the acid-induced formation of the imini-
um ion 28. We conclude that the reductive detritylation
of 25 via N-protonation and cleavage to give the trityl
cation cannot compete with the undesired conversion into
the iminium ion 28.
From the above evidence, it is likely that the late stage
deprotection of 25 will not be feasible. On the other hand,
detritylation is possible earlier, prior to reduction of the
ester group, as demonstrated in the conversion from 21
to 26. Although the earlier deprotection would require
adjustments in the carbamoylation chemistry and the
timing of redox events, the stability of 25 under carefully
J O040211C
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