Reactivity of aziridinomitosene derivatives related to FK317 in the presence of protic nucleophiles

Article abstract of DOI:10.1021/jo202286a

The syntheses and reactivity of N-TBDPS and N-trityl protected derivatives of an aziridinomitosene corresponding to FK317 are described. New reactivity patterns were observed for these highly sensitive and functionally dense heterocycles under mild nucleophilic conditions approaching the threshold for degradation. Thus, the silyl or trityl protected aziridinomitosene reacted with Cs₂CO₃/CD₃OD to give isomeric products where substitution occurred at C(10) and C(9a) (mitomycin numbering) providing a CD₃ ether and a CD₃ hemiaminal, respectively. These findings show that heterolysis at C(10) is faster than at aziridine C(1), in contrast to the behavior of typical aziridinomitosenes in the mitomycin series. The labile N-TBDPS hemiaminal and the more stable N-trityl hemiaminal resemble the mitomycin K substitution pattern. A reagent consisting of CsF in CF ₃CH₂OH/CH₃CN desilylated a simple N-TBDPS aziridine but caused nucleophilic cleavage at C(1) as well as C(10) without cleavage of the N-TBPDS group in the fully functionalized penultimate aziridinomitosene. The high reactivity of the C(10) carbamate with nucleophiles precludes the use of deprotection methodology that requires N-protonation for fully functionalized aziridinomitosenes in the FK317 series.

Full text of DOI:10.1021/jo202286a

Article
pubs.acs.org/joc
Reactivity of Aziridinomitosene Derivatives Related to FK317 in the
Presence of Protic Nucleophiles
Susan D. Wiedner† and Edwin Vedejs*
Department of Chemistry, University of Michigan, 930 North University Avenue, Ann Arbor, Michigan 48109, United States
S
* Supporting Information
ABSTRACT: The syntheses and reactivity of N-TBDPS and N-trityl protected derivatives of an aziridinomitosene
corresponding to FK317 are described. New reactivity patterns were observed for these highly sensitive and functionally
dense heterocycles under mild nucleophilic conditions approaching the threshold for degradation. Thus, the silyl or trityl
protected aziridinomitosene reacted with Cs2CO3/CD3OD to give isomeric products where substitution occurred at C(10) and
C(9a) (mitomycin numbering) providing a CD3 ether and a CD3 hemiaminal, respectively. These findings show that heterolysis
at C(10) is faster than at aziridine C(1), in contrast to the behavior of typical aziridinomitosenes in the mitomycin series. The
labile N-TBDPS hemiaminal and the more stable N-trityl hemiaminal resemble the mitomycin K substitution pattern. A reagent
consisting of CsF in CF3CH2OH/CH3CN desilylated a simple N-TBDPS aziridine but caused nucleophilic cleavage at C(1) as
well as C(10) without cleavage of the N-TBPDS group in the fully functionalized penultimate aziridinomitosene. The high
reactivity of the C(10) carbamate with nucleophiles precludes the use of deprotection methodology that requires N-protonation
for fully functionalized aziridinomitosenes in the FK317 series.
reductive detritylation conditions (i-Pr3SiH/MsOH)16 gave the
INTRODUCTION
The FR family of anticancer prodrugs 1−4 is structurally
■
N−H aziridine 7 in 42% yield. However, attempted
related to mitomycin C 6 (Figure 1)1−4 and has a similar
detritylation of the late stage intermediate 10, containing the
correct C(9) carbamate functionality, was not possible due to
requirement for reductive conversion into an active form that is
competing heterolysis and reductive trapping at C(10).15 The
greater sensitivity of 10 vs 7 or 8 reflects increased electron
density in the indole ring, an effect that is expected to promote
heterolysis at C(10) as well as C(1) in the absence of the
stabilizing C(9) ester group. We now report further exploration
of 7 and derivatives as part of our efforts to probe the chemistry
of labile aziridinomitosenes and to carry out late stage synthetic
transformations in the highly sensitive, fully functionalized
environment. These new studies help to better define the
competing pathways in reactions at the C(1), C(9a), and C(10)
positions with nucleophiles.
responsible for DNA cross-linking at the 5′CG 3′ sequence.5,6
Fukuyama proposed that the active form is the labile
aziridinomitosene 5 (R = Y = X = H) resulting from reductive
N−O bond cleavage to a benzazocinone core followed by
cyclization and loss of water.7 Although detection of the labile
azirdinomitosene has not been reported, the DNA lesion
expected from activation of FR66979 (2) and DNA alkylation
by the C(1) and C(10) positions of 5 has been characterized.8
Members of the FR family were reported to have better activity
and lower toxicity compared to the mitomycins,4,9,10 and the
semisynthetic 4 (FK317), a diacetate analogue of 1, was
identified as having a promising activity profile with decreased
incidence of vascular leak syndrome compared to 1.4b These
developments have stimulated extensive studies that establish
access to 1−3 by total synthesis,11,12 but relatively little is
known regarding the corresponding activated intermediates
related to the aziridinomitosenes 5. Given the exceptional
solvolytic reactivity of 5 in the mitomycin series, it remains to
be seen whether any FR- or FK-derived analogue of 5 (Y = X =
H) is stable enough to prepare by synthetic means.
RESULTS
■
Key substrates were prepared using the previously reported
convergent sequence via alkylation of a protected indole ester
1115 with the mesylate 13 derived from tributylstannylaziridine
alcohol 1214 to afford 14 (Scheme 1). Introduction of a
deuterium blocking group (15) to prevent competing indole
lithiation (16) followed by tin−lithium exchange resulted in
intramolecular 1,4-addition to form the enolate 17. Subsequent
selenenylation/oxidative elimination afforded tetracycle 18,
functional group manipulations provided 8, and reductive
Earlier reports from our laboratory described a synthetic
approach to structures 7 and 8, aziridinomitosene analogues
that are related to the hypothetical activated form 9
corresponding to FK317 (4) (Scheme 1).13−15 Selective
deprotection of 8 by cleavage of the N-trityl group using
Received: November 11, 2011
Published: December 29, 2011
© 2011 American Chemical Society
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Figure 1. Natural and semisynthetic precursors of aziridinomitosenes 5.
Scheme 1
aziridine methine signals at 4.04 and 3.60 ppm was observed
upon warming the sample to ca. 45 °C. The variable NMR
spectra for the N-H aziridinomitosene 7 reflect the presence of
nitrogen invertomers. The structure of 7 was confirmed by
treating the material with trityl chloride (1 equiv) and Hunig’s
̈
base (2.2 equiv; CH2Cl2, 0 °C to rt) to regenerate 8 (34%
recovered). The N−H aziridine 7 is more susceptible to
decomposition than 8 due to removal of the bulky trityl group,
but purified 7 can be stored at low temperature for extended
periods.
Early stage aziridine deprotection complicates handling of
products during and after the selective reduction at ester C(10).
This conversion is challenging under the best of circumstances
because the resulting primary alcohol at C(10) is highly
sensitive under acidic or neutral protic conditions in the
absence of the electron-withdrawing C(10) ester group that
helps to prevent aziridine heterolysis. Another complication is
the presence of the C(6) formyl group. Temporary protection
at C(6) is essential during ester reduction, but masking the
C(6) formyl also temporarily cancels its stabilizing electronic
effect and contributes to the solvolytic reactivity of the product.
Therefore, the aldehyde protecting group must be removed
using basic conditions immediately after reduction, but prior to
product isolation. In principle, this approach restores the
stabilizing effect of the formyl group and improves chances for
product survival during purification. The initial plan was to
mask the C(6) aldehyde using N-trimethylsilylimidazole 21
(Scheme 2), following a procedure that had been developed
detritylation16 gave the N−H aziridine 7 following the
published route with small adjustments in late stage procedures
(see Supporting Information). The technically challenging
conversion from 14 to 18 was carried out in acceptable 75%
yield, but the N-alkylation from the aziridine 13 to 14 proved
more troublesome and the high yields reported previously
could not be reproduced. Typical experiments produced 14 in
54% yield after 4 d at 60 °C with 3 equiv of 11 and 3 equiv of
NaHMDS in 2:1 THF: DMPU, along with 17% of the
undesired desilylated alcohol 19. Repeated attempts failed to
identify or suppress the nucleophile responsible for the
premature desilylation, so the options for the conversion
from 11 to 14 were re-evaluated. More consistent results were
obtained in the N-alkylation when mesylate 13 was replaced by
the iodide 20, available in excellent yield from 12 using a
Mitsunobu procedure.17,18 This gave 14 in 65% isolated yield
based on the iodide 20 and only traces of 19, and the reaction
time was considerably shorter (3 h) compared to the mesylate
reaction (4 d).
Scheme 2
Most samples of the N-H aziridinomitosene 7 prepared in
the current study were found to consist of two aziridine
invertomers according to NMR evidence,16 in contrast to the
single species observed for the material isolated previously.
Thus, two pairs of broad aziridine methine signals appeared
between 1H δ 3 and 4 ppm, and a coupling constant of 4.2 Hz
was extracted for one of the invertomers by homonuclear
decoupling and D2O exchange experiments. This coupling is
small for a cis aziridine, but it is consistent with the spectra of
other aziridinomitosenes.18−20 Coalescence into a single pair of
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from the N-trityl aziridine 8.15 However, numerous attempts to
perform the conversion from 7 to 22 failed to reach an
acceptable level of conversion according to NMR assay.
Furthermore, crude 22 proved to be surprisingly labile
compared to the N-trityl analogue 8. Together with the
substantial increase in polarity, the stability problem frustrated
all attempts to purify 22. Finally, the presence of decomposition
products as well as presumed aziridine invertomers in 22
resulted in complex spectra, and left the outcome of these
experiments open to question.
desilylation might be possible using nucleophilic “silaphiles”
without resorting to acidic conditions that destroy the sensitive
aziridinomitosene core.19,25−27 Accordingly, the t-butyldiphe-
nylsilyl (TBDPS) group was installed by treating 7 with 1 equiv
of freshly prepared TBDPS triflate (TBDPSOTf)28 and Hunig’s
̈
base to provide 30 (Scheme 3) after workup with a basic buffer
Scheme 3. TBDPS-Protected Aziridinomitosenes
As an alternative to 22, the base-labile pyrrole hemiaminal
strategy developed by Evans21 and Dixon22 was investigated for
the purpose of masking the formyl group. Not surprisingly,
installation of the pyrrole on tetracycle 7 also proved to be
difficult. Attempted addition of C4H4NLi followed by workup
with proton donors such as water or methanol always returned
mixtures containing mostly recovered 7 as well as the pyrrole
hemiaminal. Evidently, the basic medium resulted in equili-
brium between the desired 25 and the alkoxide 24, and
promoted cleavage of the adduct back to the starting material 7.
To avoid the equilibrium problem, as well as to minimize
potential risks of acid-induced aziridine cleavage, a nonaqueous
quenching procedure was developed using hexafluoroisopropyl
alcohol (HFIP)23 as the proton donor (1:2 of C4H4NLi/
HFIP). This procedure improved conversion and allowed the
isolation of 25, but recovery of the starting 7 was still
problematic. Finally, it was found that adding one equiv of NaH
to 7 prior to addition of excess C4H4NLi provided 25 in
improved 63% yield. The role of the NaH is not known, but
temporary deprotonation of the potentially troublesome
aziridine N−H is one possibility. Once purified, the pyrrole
hemiaminal 25 was reasonably stable if stored in the freezer.
The stage was now set for the challenging reduction of the
ester 25 with LiAlH4. Selective reduction requires that the
intermediate alkoxide 26 does not dissociate to the aldehyde in
the presence of the reducing agent. Furthermore, isolation of
the primary alcohol 27 with the pyrrole hemiaminal intact at
C(6) increases the risk of C(1)-N or C(10)-O heterolysis due
to the absence of the stabilizing π acceptor as already
mentioned. With these issues in mind, 25 was treated with
excess LiAlH4 at 0 °C13 and the reaction was quenched with
ethyl acetate containing trace dissolved water, thereby avoiding
the risky aqueous workup. The ethoxide generated during
reduction of the ethyl acetate facilitated cleavage of the pyrrole
hemiaminal 27 in one pot to regenerate the C(6) formyl group
(28). Filtration through Celite with MeOH provided a crude
product that was difficult to assay due to poor solubility in
organic solvents, but two interesting characteristics could be
solution (pH 10). A similar experiment using TIPSOTf/
Hunig’s base did not give acceptable conversion, and the N-
̈
TIPS aziridinomitosene was not sufficiently stable for
purification. In contrast, 30 survived Et3N-deactivated silica
gel chromatography, could be stored at −20 °C in frozen
benzene, and was sufficiently stable for the next stages.
Attention was turned to masking the aldehyde group of 30
by treatment with N-lithiopyrrolidine to give the pyrrole
hemiaminal 31 (Scheme 3). In addition to the risk of C(1) and
C(10) heterolysis, the conversion to 31 must also deal with the
potential problem of premature N-desilylation. Indeed, aqueous
workup led to mixtures of 7, 30 and 31. Fortunately, the
nonaqueous workup using hexafluoroisopropyl alcohol
(HFIP)/ether quenching allowed recovery of pyrrole hemi-
aminal 31 in >60% yield. Although 31 could be separated from
impurities on deactivated silica gel, solvent removal or storage
at −20 °C resulted in unmasking of the aldehyde and
decomposition. In particular, attempts to remove the last traces
of Et3N and residual water obtained by elution of the buffered
silica gel resulted in time-dependent decomposition. Similar
problems was encountered in all subsequent stages of this
investigation, and were never fully controlled, although limiting
the time scale for solvent removal allowed recording
informative NMR spectra of material containing residual
Et3N, water, and traces of elution solvent. In the best isolation
experiment, 31 was obtained along with 6% of aldehyde 30, but
it was necessary to perform subsequent operations on material
1
observed: a formyl proton at δ = 9.9 ppm in the H NMR
spectrum (CD3OD), and a broad IR stretch at 3450 cm−1. This
information suggests successful reduction at C(10), subsequent
unmasking at C(6) and formation of 28. However, the C(10)
methylene and aziridine protons were obscured by other signals
and could not be observed, and the product could not be
purified or further characterized due to its limited quantity, high
polarity, and insolubility. Therefore, installation of the C(10)
carbamate at the stage of 28 was not attempted and an
alternative strategy was evaluated.
Plans were adjusted to exploit the apparent correlation
between better stability and solubility observed for aziridino-
mitosenes containing the bulky, hydrophobic N-trityl group (9
or 29) compared to N−H aziridines (7 or 28) by replacing the
N-trityl group with a bulky N-silyl group.24 In principle,
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immediately after chromatography without allowing time for
removal of the last traces of elution solvents.
alternative method for carbamate installation using the milder
FmocNCO reagent was investigated.18,31 Initially, 33 was
treated with freshly prepared FmocNCO/Et3N to give a crude
product having characteristic C(10) methylene signals near 5
ppm, suggesting formation of 35. Attempts to remove the
Fmoc group from the presumed 35 using previously optimized
conditions (Et3N/CH3CN) resulted in significant decomposi-
tion.29 However, treatment of 33 with FmocNCO/Et3N
followed by excess TBAF on alumina provided the free
carbamate 36 as a white solid after filtration and solvent
removal.32,33 Although this selective cleavage was welcome, it
was rather sobering. We had imagined that the fluoride reagent
might also cleave the previously labile N-TBDPS group, an
event that would solve all of the deprotection problems in one
The reduction of 31 to 32 must contend with the risk of N-
desilylation, the ever present hazards of heterolysis at C(10)−O
and C(1)−N, the polarity of 32, and the challenge of avoiding
reduction at C(6). On the other hand, polarity should be lower
and stability should be improved at the aldehyde stage (33)
while the N-TBDPS group should facilitate recovery compared
to the N−H analogue 28. In the event, freshly chromato-
graphed 31 was treated with 5 mol equiv of LiAlH4 at 0 °C
followed by quenching as described starting from 25. In
preliminary experiments, two major products were detected by
1H NMR spectroscopy (ca. 1:1 ratio), one of which was stable
enough to isolate and was assigned as the aldehyde 33 based on
the characteristic signal at δ 9.95 ppm. Aldehyde 33 also
showed an ABX pattern (δ 4.95 and 4.76 ppm) coupled to a
doublet of doublets (δ 2.88 ppm), assigned as the C(10)
methylene protons and the hydroxyl proton, respectively
(Table 1, entry 4). The distinctive C(1) and C(2) aziridine
1
operation, but this was not to be. The H NMR spectrum
clearly showed the characteristic TBDPS signals as well as the
C(1) and C(2) methine signals of an intact, N-silylated
aziridine (Table 1, entry 7).
Treatment of 36 with anhydrous CsF or KHF2 in THF at rt
did not effect desilylation, while Et3N-HF and TAS-F resulted
in decomposition.34 This initially surprising resistance to
fluoride donors stimulated an experiment to confirm that the
N-TBDPS group is sensitive to protic conditions, as noted in
connection with attempts to prepare ester 30. Thus, 30 was
dissolved in CD3OD and monitored by 1H NMR spectroscopy
over 20 h. During this time, the signals of N−H aziridine 7
slowly appeared, along with a new TBDPS signal at δ 1.01 ppm,
as expected from attack of nucleophilic deuterated methanol at
the silicon to form TBDPSOCD3. In the hope that these
conditions would be mild enough, aziridinomitosene 36 was
a
1
Table 1. Characteristic H NMR Signals of 30−36
entry
1
tetracycle
C(1)
C(2)
C(3)
C(10)
30
3.62 d
3.38 dd
4.27 d
4.07 dd
4.17 d
3.95 dd
obsc
2
3
4
5
6
7
31
32
33
34
35
36
3.57 d
3.22 d
3.29 d
3.53 d
3.36 d
3.41 d
3.31 d
3.28 dd
3.6 dd
obsc
4.93 dd
4.73 dd
4.95 dd
4.76 dd
5.66 d
4.21 d
4.03 dd
obsc
1
dissolved in CD3OD and the sample was monitored by H
NMR spectroscopy. The expected TBDPSOCD3 signal at δ =
1.01 ppm appeared within 45 min, but new C(1) and C(2)
methine signals corresponding to the deprotected N−H
5.56 d
3.29 dd
3.36 dd
obsc
5.65 d
1
5.20 d
aziridinomitosene were never observed. Instead, the H NMR
4.23 d
5.47 ABq
spectrum showed complex new signals between 3.8 and 4.2
ppm, consistent with the chemical shift range for products of
aziridine ring cleavage.26b
4.02 dd
a
Chemical shifts in ppm.
Since the undesired aziridine C(1) heterolysis implies N-
protonation as the activating event, the methanolysis experi-
ment was repeated in the presence of a base in the hope that
nucleophilic attack at Si may become competitive (Scheme 4).
protons were also present, but the C(1) doublet moved upfield
from δ = 3.62 ppm (30) to δ = 3.29 ppm (33), thereby
confirming a change in oxidation state at C(10) (Table 1,
entries 1 and 4). Like the ester 31, aldehyde 33 survived rapid
chromatography, but decomposed during prolonged (>1d)
storage in frozen benzene. The other product from reduction
was too sensitive to isolate, but was assigned as 32 due to the
similarity of the upfield 1H NMR signals to those of 33 (Table
1, entry 3), plus signals consistent with the presence of the
pyrrole moiety. Fortunately, the undesired recovery of 32 could
be avoided by increasing the LAH quenching time with ethyl
acetate to ca. 1.5 h at rt, resulting in a 14:1 ratio of 33:32 in the
best case.
Scheme 4
The inherent risk of heterolysis at the stage of 33 severely
restricts the options for carbamoylation at C(10) (Scheme 3).
Following precedents,29,30 33 was treated with 1 equiv of
trichloroacetyl isocyanate between −78 °C and rt for 1 h to
give material having 1H NMR signals consistent with the
expected C(10) methylene AB pattern of 34 (δ ca. 5.6 ppm;
Table 1, entry 5). However, attempted cleavage of the
trichloroacetyl group from the presumed 34 with K2CO3 in
MeOH29 resulted in disappearance of the C(10) signal, but
produced no signals consistent with simple cleavage of the
trichloroacetyl group, nor with recovery of 33. Therefore, an
Thus, carbamate 36 and Cs2CO3 were dissolved in deuterated
methanol, and the reaction was monitored by 1H NMR
spectroscopy over a 10 h period. No evidence was found for the
desired silyl group cleavage. Instead, the NMR spectrum
indicated that tetracycle 36 had undergone surprising structural
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changes. Multiple new peaks appeared while the signals
corresponding to 36 decreased in intensity. Two new products
were observed after 17 h at rt in a 1:1 ratio, but after 6 d only
susceptibility of the carbamate 36 to nucleophilic attack at
C(9a) as well as C(10) severely limits the options for selective
removal of the TBDPS group in the sensitive environment.
In an attempt to better understand, and hopefully to
improve, the N-desilylation process using fluoride sources, a
model aziridine 44 was briefly investigated (eq 1). Treatment of
1
one of these products (37) was present. According to the H
NMR spectrum, the aziridine C(1) and C(2) signals as well as
the C(3) methylene signal of the more stable product had
moved upfield compared to carbamate 36 (compare Table 2,
a
Table 2. Characteristic 1H NMR Signals of Key Tetracycles
entry
tetracycle
C(1)
C(2)
C(3)
C(10)
1
37
2.66 d
2.43 dd
3.55 d
3.34 dd
obsc
6.28 d
5.54 d
4.79 ABq
6.32 s
44 with commercial TBAF/DMF gave 45 (56% after 3 h at
rt),36 presumably because residual water37 in TBAF acts as the
source of the N−H proton. In contrast, the nonhygroscopic,
crystalline tetrabutylammonium difluorotriphenylsilicate
(TBAT)38 did not desilylate 44 in deuterated DMF even
after 3 d (1H NMR assay). However, when the TBAT
experiment was repeated in the presence of 1 equiv of
trifluoroethanol as a stoichiometric proton source,23 desilyla-
tion to 45 did occur (75% conversion in 24 h) and a similar
experiment with CsF as the fluoride donor resulted in 95%
conversion within 20 h. A simpler procedure using CsF in 2,2,2-
trifluoroethanol (TFE) solution was also tested, and returned
61% of the desilylated aziridine 45 on preparative scale (2 d
reaction time). However, only 14% desilylation was observed in
trifluoroethanol without any added fluoride under similar
conditions in an NMR experiment. These observations suggest
that a proton source is necessary to activate aziridine nitrogen
for N−Si cleavage by nucleophilic attack, and that trifluor-
oethanol is not sufficiently nucleophilic for a convenient rate of
desilylation in the absence of fluoride. Thus, the combination of
an anhydrous fluoride source with the relatively non-
nucleophilic trifluoroethanol offered some hope that desilyla-
tion of the sensitive aziridinomitosene 36 might be faster than
heterolysis at C(10). However, the same nitrogen activation
event (presumably, N-protonation) required for desilylation
might also promote heterolysis at C(1).
b
2
3
38
3.41 d
2.25 s
3.53 dd
2.25 s
3935
4.08 d
3.41 d
3.80 d
3.40 dd
4.44 d
4.10 dd
5.50 s
4
5
41
2.28 d
3.04 d
2.13 dd
3.01 dd
6.29 d
5.47 d
4.89 d
4.73 d
42
a
b
Chemical shifts in ppm, CDCl3. CD3OD solution
entry 1, with Table 1, entry 7). More surprisingly, the C(10)
AB quartet was missing, but two vinylic protons at δ = 5.54 and
6.28 ppm were present. The nominal mass (ESMS; m/z = 514
amu) corresponded to loss of the carbamate and incorporation
of OCD3. Based on the above data and comparison with the
natural product mitomycin K (39; Table 2, entry 3),35 the
exocyclic olefin structure 37 of the stable product was
tentatively assigned. Further support for the structure by 13C
NMR was precluded by the small quantity obtained and the
inability to completely purify 37.
To support the structure of 37 by analogy and to better
understand the methanolysis process, the relatively well-
behaved N-Tr aziridinomitosene 4015 (Scheme 4) was exposed
to the basic solvolysis conditions (Cs2CO3 in deuterated
1
methanol, monitoring by H NMR spectroscopy). Analogous
changes appeared in the NMR spectrum as observed starting
with 36, and two major products were observed in a 1:1 ratio
after 3 d. One of the products (41) correlated well with
Aziridinomitosene 36 was subjected to the reoptimized
desilylation conditions (1 equiv of CsF in 8:1TFE:THF), but
no reaction was observed after 2 h. Conversion did occur after
1 d in the presence of excess CsF, and the major product
1
mitomycin K (39) and also with 37 in terms of H NMR
chemical shifts (Table 2, entry 4). Furthermore, the presence of
two alkenyl carbons (δ 140.2 and 114.4 ppm) and a quaternary
hemiaminal carbon (δ 106.1 ppm) were confirmed by 13C
NMR spectroscopy. The second product was also isolated, and
the molecular ion observed by ESMS indicated loss of the
C(10) carbamate and incorporation of OCD3 (M + Na, m/z
1
diastereomer detected by H NMR spectroscopy was stable
enough for chromatographic isolation (eq 2). A second
1
540 amu). Inspection of the H NMR spectrum showed a pair
of AB doublets at δ = 4.89 and 4.73 ppm, similar to C(10)−O
derivatives such as the alcohol 32 (Table 1, entry 3). Although
the other signals were initially puzzling, they proved to be
identical to signals of the methyl ether 43 (unexpectedly
obtained from alcohol 29 among other products after treatment
with trichloroacetyl isocyanate, followed by alumina and
quenching with methanol), excepting the absence of the singlet
of the C(10) methoxy group (Table 2, entry 5). Thus, the
“unknown” product is 42, and differs from 43 only by the
presence of OCD3 in place of OCH3. By analogy, the transient
compound observed upon reaction of 36 with Cs2CO3 in
deuterated methanol was assigned as 38. Although the
contrasting outcome of methanolysis in the presence or
absence of carbonate with these functionally dense heterocycles
is intriguing in the context of DNA cross-link formation, the
diastereomer was also detected. Although it could not be
fully separated, meaningful NMR data were obtained. Major
changes were evident in the NMR spectra of both
diastereomers compared to the starting 36, and it was clear
that the N-TBDPS group had survived the CsF/TFE treatment
while the aziridine ring had not. Thus, both products showed a
new doublet near δ = 4.6 ppm, coupled to a multiplet near δ =
3.9 ppm, consistent with aziridine ring cleavage by attack at
C(1) involving an oxygen nucleophile.26b Also, the C(10)
methylene protons of both products had moved upfield from δ
5.4−5.5 to ca. 5 ppm, suggesting that the carbamate had been
replaced by an alkoxy group. Together with molecular ion data
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Scheme 5. Solvolysis of Aziridinomitosenes
corresponding to incorporation of 2 equiv of trifluoroethanol
(ESMS, M + Na, m/z = 701 amu), the products were assigned
as diastereomers of the ring opened mitosene 46. Further
support for this structure was provided by the observation of
two triplets in the 19F NMR spectrum for each diastereomer,
and by homonuclear and heteronuclear (19F) decoupling
experiments. Although the incorporation of 2 equiv of the
non-nucleophilic TFE was not desired, more concerning was
the survival of the N-TBDPS protecting group on the nitrogen.
Clearly, nitrogen activation had occurred, and just as clearly,
heterolysis at C(1) was faster than nucleophilic attack at silicon.
The aziridinomitosene 36 had proven to be sensitive to
nucleophilic attack at both C(1) and C(10) under the mildly
acidic protic conditions, and undesired heterolysis had taken
place faster than desilylation.
Egbertson and Danishefsky reported that treatment of the
hydroquinone corresponding to N-methylaziridinomitosene A
with potassium ethyl xanthate in aqueous pyridine affords two
nucleophilic substitution products, one resulting from attack at
C(1) as well as C(10) (5%), and the other from attack
exclusively at C(10) (8%).27b This latter product is noteworthy
because nucleophilic substitution in the fully aromatic substrate
occurs at C(10) without prior attack at aziridinomitosene C(1),
in contrast to virtually all of the literature reports describing
reductive activation of mitomycins.
In the FK317 series, structures such as 36 or 40 are also fully
aromatic. Although they are presumably stabilized somewhat by
the electron-withdrawing C(6) formyl carbonyl, the effect
should be small compared to that of the mitomycin quinone
carbonyls. Reactivity in 36 or 40 is dominated by the donor
indole nitrogen, and the activating effect is felt most strongly at
C(10). The first indications of this regioselectivity were seen
earlier when attempted reductive detritylation of 40 with
MsOH/triisopropyl silane gave a product of hydride capture at
C(10).15 In the current study, the same preference for C(10)
heterolysis has been demonstrated in the methanolysis of 36
and 40. Heterolysis at C(10) was also seen upon CsF/
trifluoroethanol treatment of 36. Selective attack at C(10) can
be understood by invoking the formation of an iminium ion 52
starting from the FK317 derivatives 36 or 40 as the first step in
the heterolysis sequence (Scheme 6). Although these
DISCUSSION
■
Extensive prior literature indicates that the C(10) carbamate of
mitomycin C (6) is generally less reactive than the C(1)
aziridine, although there are several competing pathways and
both C(10) and C(1) are involved in the DNA cross-linking
mechanism under conditions of reductive activation.39 Under
modified conditions, monoalkylation of DNA by mitomycin C
is also known where alkylation occurs at C(1) but not at
C(10).40 Kohn has shown that the key aziridinomitosene C
(47) is an intermediate in one of the activation pathways, and
that preformed 47 monoalkylates DNA.41 In the presence of
exogenous nucleophiles, 47 reacts at C(1) and not C(10) to
afford diastereomeric substitution products (Scheme 5).19,26b
Similar regioselectivity has been observed with a synthetic
quinone derivative of aziridinomitosene A (50), generated by
methanolysis of 49.29 Thus, treatment of 50 under reductive
detritylation conditions gave 51 by heterolysis and nucleophilic
attack at C(1). In contrast to 36, the C(10) carbamate of 50
survived the K2CO3/MeOH procedure as well as the acidic
conditions in the attempted detritylation.29 The lower reactivity
at C(10) vs C(1) in the quinones 47, 49, and 50 reflects the
influence of delocalization involving the stabilizing quinone
carbonyls. Only one prior investigation has explored the
solvolytic behavior of a well-defined synthetic substrate that
does not contain the stabilizing quinone carbonyls. Thus,
Scheme 6. Heterolysis of C(10) Carbamates 36 and 40
observations are directly relevant only to several trans-
formations conducted in organic solvents, they do raise the
possibility that the first heterolytic event in DNA alkylation by
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aziridinomitosene intermediates derived from FK31742 and
related structures also involves alkylation at C(10) and not at
C(1).
(3H, s), 1.71 (1H, ddd, J = 18.9, 10.6, 6.8 Hz), 1.56 (1H, t, J = 6.1
Hz), 1.53−1.40 (6H, m), 1.36 (3H, t, J = 7.1 Hz), 1.34−1.24 (7H, m),
1.06−0.96 (6H, m), 0.87 (9H, t, J = 7.3 Hz).
N-Trityl Tetracyclic Ester 18 and N-Trityl Tetracyclic
Aldehyde 7. The procedure reported earlier15 was modified as
follows. A three-neck 100 mL round-bottom flask was fitted with a
mechanical stirrer and thermocoupler. The flask was charged with a
solution of 1515 (2.7 mmol, 2.68 g) 40 mL THF, and the solution was
cooled to an internal temperature of −76 °C. A solution of MeLi in
Et2O (10.8 mmol, 1.51 M, 7.2 mL) was added slowly via syringe (max
internal temp −74 °C). After 5 min of stirring, the solution was
warmed to −65 °C and stirred for 20 min. A solution of PhSeCl in 14
mL THF (12.4 mmol, 2.37 g) was added slowly via cannula such that
the reaction temperature did not exceed −58 °C, and the solution
color became orange and then brown. The brown solution was
removed from the cooling bath after 20 min and allowed to warm.
When the temperature reached −18 °C, the flask was opened to air
and stirred an additional 2 h. Opening of the reaction vessel to air was
found to facilitate formation of 18 by oxidation and elimination of the
selenium by product. The reaction was quenched with H2O, extracted
with CHCl3, dried over Na2SO4 and allowed to sit overnight with
exposure to air. 1H NMR spectroscopy of the crude residue showed no
noneliminated selenium tetracycle. The major product matched that of
Kim.15 The crude residue was filtered through a short plug of silica gel
with 400 mL hexanes/2% Et3N to remove excess PhSeCl, and then the
product was eluted with 400 mL ethyl acetate. Limited exposure of this
material to silica gel was necessary in order to retain a significant
quantity of material. The material was immediately taken on to the
next step without additional purification.
SUMMARY
■
The fully functionalized N−H aziridinomitosene of FK317 4
remains an elusive target. Selective removal of the N-TBDPS
group could not be accomplished at the stage of 36. Based on
model studies as well as observations made with the substrates
36 and 40, protonation at aziridine nitrogen is a prerequisite for
desilylation as well as for detritylation. At this stage of our
understanding, deprotection strategies that rely on activation by
aziridine N-protonation are plausible only for aziridinomito-
senes that contain stabilizing conjugated electron-withdrawing
substituents, as in the methanolysis from N-TBDPS aziridine
30 to the N−H aziridine 7. Although it has not been possible so
far to detect hypersensitive aziridinomitosene analogues 5 that
lack stabilizing substituents, the survival of 36−38 and 40−42
under carefully controlled conditions suggests that spontaneous
heterolysis at C(1) is not a major hazard, contrary to our
expectations. Stability of the C(10)−O bond is more
problematic, although detection (and in some cases, isolation)
of structures such as 29, 35, 38, 42, and 43 indicates that
detection of 5 should also be possible.
EXPERIMENTAL SECTION
■
General Methods. Solvents and reagents were purified as follows:
diethyl ether and tetrahydrofuran (THF) were distilled from sodium/
benzophenone or purified using an Anhydrous Engineering solvent
purification system using columns packed with A-2 Alumina;
dichloromethane (CH2Cl2) was distilled from P2O5 or purified using
an Anhydrous Engineering solvent purification system using columns
packed with A-2 Alumina; CH3CN was stirred over molecular sieves
for 24 h, then distilled from P2O5; benzene, toluene, triethylamine, i-
PrEt2N, TMEDA, and DMPU were distilled from CaH2; methanol was
distilled over activated magnesium turnings, the purified reagents and
solvents were used immediately or stored under nitrogen. Alkyl and
aryllithiums were titrated using the procedure of Kofron43 before use.
Unless otherwise noted, all chemicals were used as obtained from
commercial sources and all reactions were performed under nitrogen
atmosphere in glassware dried in an oven (140 °C) or flame-dried and
cooled under a stream of nitrogen. All reactions were stirred
magnetically unless otherwise noted and liquid reagents were
dispensed with all PP/PE plastic syringes or Hamilton Gastight
microsyringes. Preparatory layer chromatography was performed using
Whatman Partisil K6F silica gel 60 Å 200 or 1000 μm plates. Flash
chromatography was performed with 230−400 mesh silica gel 60.
Isolation of Tetracyclic Alcohol 19. To a solution of indole 11
(0.69 mmol, 280 mg) in 1.8 mL THF and 0.9 mL DMPU (2:1) cooled
to −78 °C was added a suspension of NaHMDS (0.759 mmol, 139.2
mg) in 0.75 mL THF cooled to −78 °C via cannula (1 drp/sec). After
5 min, aziridine 13 in 0.45 mL THF cooled to −78 °C was added
dropwise into deprotonated indole via cannula (1 drp/sec). The
cooling bath was removed and the reaction was warmed to rt while
being concentrated to half the original volume under a stream of N2.
The reaction vessel was fitted with a reflux condenser and the solution
was heated to 60 °C. After 3 d, the solution was cooled to rt and
quenched with pH 3 phosphate buffer. The mixture was extracted 3 ×
Et2O, the combined organics were washed with brine and dried over
MgSO4. Concentration under rotary evaporation provided 573 mg of a
yellow oil. Purification by flash column chromatography (3:1 hexanes:
ethyl acetate with 2% Et3N) provided 124 mg product 14 pure by 1H
NMR and 36 mg (17%) desilylated 19. The 1H NMR spectrum of 14
matched that of Kim.15 Spectroscopic data for 19: 500 MHz 1H NMR
(CDCl3, ppm) δ 7.67 (1H, s), 7.40−7.36 (6H, m), 7.21−7.13 (9H,
m), 6.80 (1H, s), 6.68 (1H, s), 4.73 (2H, d, J = 6.1 Hz), 4.30 (2H, m),
4.23 (1H, dd, J = 14.0, 6.5 Hz), 4.09 (1H, dd, J = 14.0, 4.8 Hz), 3.98
To the residue of 15 in 27 mL THF at 0 °C was added TBAF (1.0
M, 3.2 M, 3.2 mmol). The cooling bath was removed, and after 1 h 20
min of stirring at rt, the solution was cooled to 0 °C, and sat NaHCO3
was added. The mixture was extracted 3 × Et2O, the combined
organics were washed with brine and dried over pulverized Na2SO4.
Concentration under reduced pressure provided a brown oil consisting
of tetracyclic alcohol product with minor impurities. The crude residue
was filtered through a short silica gel plug with 400 mL 9:1 hexanes:
ethyl acetate with 2% Et3N and then 800 mL ethyl acetate. The residue
obtained after solvent removal was immediately taken on to the next
step without further purification. To a mixture of tetracyclic alcohol
and 4 Å MS in 27 mL CH2Cl2 was added NMO (4.0 mmol, 469 mg)
and TPAP (0.14 mmol, 49 mg). The black slurry was vigorously
stirred for 1 h and then filtered through silica gel with ethyl acetate.
Solvent removal provided an orange solid, which was dissolved in
acetone and precipitated by adding hexanes. Washing of the recovered
amorphous gray solid with hexanes provided 475 mg (33% over 3
1
steps, 69% average per step) of tetracycle 7. The H NMR spectrum
matched that previously reported by Kim.15
(2R,3R)-3-Iodomethyl-2-tributylstannyl-N-triphenylmethyla-
ziridine 20. To a solution of PPh3 (0.56 mmol, 147 mg) in 2 mL
toluene at 0 °C was added diisopropyl azodicarboxylate neat (0.56
mmol, 0.12 mL) via syringe, and the reaction mixture turned yellow.18
After 5 min of stirring, aziridinol 1214 as a solution in toluene (0.37
mmol, 1.7 mL) was added to the PPh3/DIAD mixture dropwise via
cannula, and the mixture was stirred 5 min. Neat iodomethane (0.52
mmol, 0.03 mL) was added to the reaction mixture via syringe, and the
mixture turned cloudy white. The reaction vessel was pulled out of the
cooling bath, warmed to rt, fitted with a reflux condenser and heated
to 70 °C. After 6 h, the reaction mixture was cooled to rt, and the
solvent was removed under reduced pressure. The cloudy residue was
purified immediately by flash column chromatography on silica gel (2
× 15 cm, 15:1 hexanes/ethyl acetate with 2% Et3N, Rf = 0.82).
Fractions 7−11 provided 257 mg (97%) of a clear colorless oil.
Molecular ion (M − C4H9) calculated for C30H37INSn = 658.0993,
1
found (EI) m/z = 658.1004, error = 2 ppm; 500 MHz H NMR
(CDCl3, ppm) δ 7.47 (6H, m), 7.27−7.24 (6H, m), 7.21−7.20 (3H,
m), 3.57 (1H, dd, J = 10.0, 5.0 Hz), 3.00 (1H, dd, J = 9.5, 8.0 Hz), 1.62
(1H, ddd, J = 8.0, 6.8, 5.0 Hz), 1.5−1.4 (6H, m), 1.32 −1.24 (6H, m),
1.08−0.94 (7H, m), 0.86 (9H, t, J = 7.3 Hz); 100 MHz 13C NMR
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(CDCl3, ppm) δ 144.1, 129.5, 127.3, 126.6, 76.0, 39.0, 29.8, 29.2, 27.3,
13.6, 10.8, 10.4.
partly overlapping singlets), 1.04 (8.9H, m, ethyl ester and residual
Et3N). The presence of Et3N was confirmed by a quartet at δ 2.56.
Two diastereomers were expected and confirmed by the presence of
two OCH3 singlets (δ = 3.94 and 3.93 ppm) and two t-Bu singlets at δ
= 1.14 and 1.13 ppm. All other signals of the diastereomers could not
be distinguished by 1H NMR spectroscopy; relative integral values are
based on the clean doublet at 4.17 ppm = 1.0H (both diastereomers).
Conversion of 31 to Tetracyclic Carbamate Aldehyde 36. To
a solution of pyrrole hemiaminal 31 (48 mg, 0.08 mmol) in 1.3 mL
Et2O at 0 °C was added LAH in a solution of THF (0.42 mL, 1.0 M,
0.40 mmol, 5 mol equiv) dropwise via syringe. The cloudy
nonhomogeneous mixture was stirred vigorously at 0 °C. After 50
min, 0.5 mL EtOAc was added slowly at 0 °C, the mixture was allowed
to warm to rt, and was stirred for an additional 1.5 h under air. The
crude mixture was then filtered through Celite with ethyl acetate (4 ×
10 mL), and the combined organic washes were concentrated under
reduced pressure to provide 25 mg of alcohol 33 and 32 (14:1 33:32)
as a clear green oil (63% mass recovery). In preliminary experiments
the stirring time at rt prior to Celite treatment was shorter, and
resulted in ratios of 33:32 in the range of 1−3:1. For characterization
purposes, the mixture of 33 and 32 and byproducts was purified on an
analytical TLC plate on silica gel 60 Å (20 cm × 20 cm × 100 μm)
pretreated with Et3N vapors for ≥1 h with 1:1 hexanes: ethyl acetate
but most of 32 decomposed during separation and an enriched sample
could not be obtained. Selected data for 32 (observed in the mixture
Alkylation of 11; Preparation of Aziridinyl Indole 14. To a
solution of 1115 (3.75 mmol, 1.52 g) in 9 mL THF and 4.6 mL DMPU
(2:1) at −78 C, was added 1.0 M solution of NaHMDS (3.75 mmol,
3.75 mL) in THF via cannula dropwise (1 drop/sec). After 5 min of
stirring at −78 °C, iodostannyl aziridine 20 in 2 mL THF was added
dropwise via cannula. The cooling bath was removed and the reaction
was warmed to rt while being concentrated by half under an N2
stream. The reaction vessel was fitted with a reflux condenser and
heated to 60 °C. After 3 h, the reaction was cooled in a 0 °C bath and
quenched with satd NH4Cl solution. The mixture was poured into
water and extracted 3× with Et2O, washed 2× with H2O and then 1×
with EtOAc, the combined organics were washed with brine and dried
over MgSO4. The organic solvent was removed under rotary
evaporation to provide a dark brown oil as a mixture of product 14
and desilylated byproduct 19 (3:1 ratio) and minor unidentified
byproducts. Purification by flash column chromatography (9:1 hex/
EtOAc, 2% Et3N) provided 860 mg (65%) of 14. The NMR spectra of
14 matched those reported earlier.15 An unknown contaminant
multiplet was observed at 1.10 ppm.
Preparation of N-TBDPS Tetracyclic Aldehyde Ester 30. To a
solution of 7 in 1.9 mL CH2Cl2 was added i-Pr2EtN (0.02 mL, 0.14
mmol). The solution was cooled to −78 °C and TBDPSOTF28 as a
solution in CH2Cl2 (0.62 M, 0.16 mL, 0.096 mmol) was added
dropwise via syringe (1 drop/5 s). After 30 min at −78 °C, the mixture
was poured into pH 9.6 carbonate buffer, extracted 3 x with CH2Cl2
(10 mL), and the combined organic phases were washed with brine
and dried over Na2SO4. Removal of the solvent by rotary evaporation
provided a yellow oil, purified by flash chromatography on silica gel (2
× 15 cm, 2:1 hexanes/EtOAc with 2% Et3N). Fractions 22−39
provided 37 mg (72%) of 30 as a pale yellow oil. Analytical TLC on
silica gel 60 Å with 1:1 hexanes/EtOAc, Rf = 0.58. Molecular ion (M +
Na) calculated for C32H34N2NaO4Si: 561.2186; found (electrospray)
m/z = 561.2179, error = 1 ppm; IR (neat, cm−1) 1721, 1690 CO;
400 MHz 1H NMR (CDCl3, ppm) δ 9.98 (1H, s), 7.64−7.58 (4H, m),
7.48−7.32 (7H, m), 7.18 (1H, s), 4.29 (1H, dq, J = 12.0, 7.2 Hz), 4.27
(1H, d, J = 11.6), 4.08 (1H, m), 4.07 (1H, dd, J = 11.6, 3.6), 4.06 (3H,
s), 3.62 (1H, d, J = 3.6 Hz), 3.38 (1H, dd, J = 3.6, 3.6 Hz), 1.15 (9H,
s), 1.07 (3H, t, J = 7.2 Hz) 100 MHz NMR 13C (CDCl3, ppm) δ
191.8, 163.4, 154.7, 153.2, 135.85, 135.78, 134.7, 132.7, 131.9, 130.9,
129.8, 127.9, 127.7, 123.7, 107.6, 103.0, 101.3, 60.0, 56.0, 48.1, 40.6,
35.5, 27.6, 19.3, 14.0.
1
prior to separation): 400 MHz H NMR (CDCl3, ppm) δ 4.93 (1H,
ABX dd, J = 12.4, 5.3 Hz), 4.73 (1H, ABX dd, J = 12.4, 8.2 Hz), 3.28
(1H, dd, J = 3.4, 3.4 Hz), 3.22 (1H, d, J = 4.0 Hz), 2.99 (1H, ABX dd,
J = 8.2, 5.3 Hz). Characterization data for 33 (purified; Rf = 0.44): IR
1
(neat, cm−1) 3539 OH, 1681 CO; 400 MHz H NMR (CDCl3,
ppm) δ 9.95 (1H, s), 7.62−7.58 (4H, m), 7.48−7.36 (6H, m), 7.32
(1H, s), 7.08 (1H, s), 4.95 (1H, ABX dd, J= 12.8, 5.6 Hz), 4.76 (1H,
ABX dd, J = 12.8, 8.0 Hz), 4.26 (1H, d, J = 11.2 Hz), 4.06 (3H, s), 4.03
(1H, ABX dd, J = 11.2, 3.2 Hz), 3.4 (1H, ABX dd, J = 3.6, 3.2 Hz),
3.29 (1H, d, J = 3.6 Hz), 2.88 (1H, ABX dd, J = 8.0, 5.6 Hz), 1.15 (9H,
s).
Because purification resulted in considerable material loss due to
decomposition, the crude 14:1 mixture of alcohols 32 and 33 (25 mg,
0.05 mmol) was used directly in the following step. Thus, 32+33 in 1
mL CH2Cl2 was cooled to −78 °C, charged with Et3N (0.72 M, 0.06
mmol) in 0.08 mL of CH2Cl2 and Fmoc-NCO31 (0.47 M, 0.10 mmol)
in 0.21 mL of CH2Cl2 was added via syringe. After 20 min of stirring at
−78 °C, the yellow green solution was warmed to rt and stirred for 1 h
(the mixture turned dark yellow upon warming). The CH2Cl2 was
Pyrrole Hemiaminal 31. To a solution of pyrrole (0.10 mL, 1.44
mmol) in 3 mL THF at −78 °C was added n-BuLi as a solution in
hexanes (0.87 mL, 1.5 M, 1.3 mmol) via syringe.21,22 The resulting
C4H4NLi was stirred at −78 °C for 20 min. To a solution of aldehyde
30 (0.020 mmol, 11 mg) in 0.40 mL THF at −78 °C was added
C4H4NLi (1.3 M, 0.041 mmol) as a solution in 0.12 mL THF/
hexanes. The resulting yellow solution was stirred at −78 °C for 15
min, and then (CF3)2CHOH as a solution in Et2O (0.05 mL, 0.95 M,
0.048 mmol) was added. The cooling bath was removed and while
warming the solution was diluted with 2 mL cold Et2O, and a small
spatula tip of Celite was added to absorb insolubles. The organic phase
was decanted from the insoluble material, and the solvent was
removed under N2. The residue was immediately purified by
preparative TLC on silica gel 60 Å (20 cm ×20 cm x 1000 μm)
pretreated with Et3N vapors for ≥2 h to give 31 after extraction and
solvent removal as a 1:1 mixture of diastereomers containing a trace of
aldehyde 30 (6%) and residual Et3N. Attempted removal of Et3N
under vacuum increased the conversion to aldehyde 30 due to loss of
pyrrole hemiaminal. Characterization of 31: 2:1 hexanes/EtOAc, Rf =
removed under N2, and the crude residue was dissolved in 1 mL THF
32,33
and cooled to 0 °C. Next, TBAF on Al2O3
(436 mg, 0.25 mmol)
was added in one portion and the mixture was stirred and warmed to
rt in the cooling bath. After 1.5 h, the crude reaction mixture was
filtered through Celite with EtOAc and the solvent was removed under
reduced pressure to provide 22 mg of residue. The residue was purified
twice by preparative TLC on silica gel 60 Å (20 cm ×20 cm x 250 μm)
pretreated with Et3N vapors for ≥30 min, providing 36 (20 mg, 74%)
as a white amorphous powder; 1:1 hexanes/EtOAc; Rf = 0.28;
Molecular ion calculated for (M − CO2NH2) C30H31N2O2Si:
479.2149; found (electrospray) m/z = 479.2168, error = 4 ppm; IR
(neat, cm−1) 3473, 3348, CONH2; 400 MHz 1H NMR (CDCl3, ppm)
δ 9.94 (1H, s), 7.63−7.61 (4H, m), 7.43−7.38 (6H, m), 7.31 (1H, s),
7.05 (1H, s), 5.47 (2H, ABq, J = 12.2 Hz), 4.41(2H, br s), 4.23 (1H, d,
J = 11.2 Hz), 4.02 (1H, ABX dd, J = 11.2, 3.2 Hz), 3.97 (3H, s), 3.41
(1H, d, 4.0 Hz), 3.36 (1H, ABX dd, J = 4.0, 3.2 Hz), 1.15 (9H, s); 100
MHz 13C NMR (CDCl3, ppm) δ 192.0, 156.9, 154.6, 146.5, 135.9,
135.8, 132.1, 132.0, 130.0, 127.9, 127.8, 125.1, 104.5, 98.2, 59.6, 55.5,
47.5, 41.4, 33.2, 27.5, 19.3.
ttempted Conversion Tetracyclic Alcohol 33 to 36 using
Trichloroacetyl Isocyanate. To a solution of 33 (0.0046 mmol, 2.28
mg) in 0.30 mL CH2Cl2 at −78 °C was added trichloroacetyl
isocyanate as a solution in CH2Cl2 (0.28 M, 0.0051 mmol, 18 μL) via
microsyringe. The dark yellow solution was stirred at −78 °C for 1 h,
and then allowed to warm to rt. After 1 h, pH 9.6 carbonate buffer was
added and the mixture was stirred for 5 min, extracted 3× with
1
0.28. IR (neat, cm−1) 3411, OH; 1696, CO. 500 MHz H NMR
(CDCl3, ppm) δ 7.64−7.58 (4 H, m, Si-Ph2), 7.48−7.32 (6H, m, Si-
Ph2), 6.88−6.82 (2.7H, m, pyrrole + C(7)H), 6.68−6.60 (1.9H, m,
C(OH)H + C(5)H), 6.21 (1.7H, m, pyrrole), 4.31−4.22 (1.4H, m,
OEt), 4.17 (1.0H, d, J = 11.2 Hz, C(3)HaHb), 4.10−4.02 (1.2H, m,
OEt), 3.95−3.93 (1H, m, C(3)HaHb), 3.94−3.93(3H, two partly
overlapping singlets, OCH3), 3.56 (0.95H, d, J = 3.6 Hz, C(1)H), 3.31
(1H, dd, J = 3.6 Hz, 3.2 Hz, C(2)H), 1.14−1.13 (8.8H, Si-t-Bu, two
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CH2Cl2, washed 1× with brine, and dried over pulverized Na2SO4.
Solvent removal provided a yellow film. Analysis of the residue by H
9 were observed; however, the presence of a tert-butyl
signal at 1.01 ppm is consistent with desilylation to form
TBDPSOCD3. After 2 days, the sample showed signals
near 4.6 ppm and 3.8 ppm, consistent with aziridine ring-
opening.
1
NMR spectroscopy (CDCl3, ppm) showed an AB doublets at δ = 5.66
(J = 12.0 Hz) and 5.56 (J = 11.5 Hz) in a complex spectrum. The
crude residue was dissolved in 0.30 mL MeOH and 0.30 mL K2CO3
(5% w/v) in MeOH29 and stirred vigorously for 2.5 h. The mixture
was extracted with ethyl acetate and solvent removal under reduced
(b) Cs2CO3/CD3OD; detection of 37 and 38: Tetracycle 36
(ca. 5 mg, crude residue contaminated with FMOC-NH2
and dibenzofulvene) was dissolved in a mixture of
Cs2CO3 (5 mg) and 0.5 mL CD3OD and the sample was
monitored by 1H NMR spectroscopy every 15 min for 10
h. After 1 h, four new methine signals began to appear (δ
3.5−3.4 ppm and 2.6−2.4 ppm) and a new ABX pattern
was observed (δ 4.7−4.9 ppm). The intensity of these
signals continued to increase during a 10 h period, and at
17 h, all four methine signals were still present. However,
after 6 d, the methine protons near δ = 3.5−3.4 ppm had
disappeared, while the signals near δ = 2.6−2.4 ppm were
still present. After evaporation, the material was applied
to an analytical TLC plate, silica gel 60 Å (20 cm × 20
cm × 250 μm) pretreated with Et3N fumes (≥30 min)
and immediately developed sufficient to advance the
mobile zone beyond poorly defined polar zones streaking
up from the origin. The silica gel containing the mobile
zone was collected rapidly, and immediately extracted to
give material containing the highly sensitive 37, FMOC-
NH2, dibenzofulvene, and other unknown contaminants.
Partial data for 37: Molecular ion (M + H) calcd for
C31H32D3N2O3Si = 514.3, found (electrospray, nominal
mass) m/z = 514.3; 500 MHz 1H NMR (CDCl3, ppm) δ
9.92 (1H, s), 7.45−7.30 (m), 6.89 (1H, s), 6.78 (1H, s),
6.29 (1H, d J = 1.0 Hz), 5.55 (1H, d J = 1.0 Hz), 4.01
(3H, s), 3.55 (1H, d, J = 12.0 Hz), 3.34 (1H, d, J = 12.0,
1.4 Hz), 2.66 (1H, d, J = 3.1 Hz), 2.43 (1H, dd, J = 3.1,
1.4 Hz), 0.84 (9H, s); singlet at δ = 6.08 ppm due to
dibenzofulvene; doublet at δ = 4.42 ppm due to FMOC-
NH2. Partial data for 38 (based on signals in the mixture
1
pressure provided an orange film. Analysis by H NMR spectroscopy
showed loss of the characteristic AB doublets in a complex spectrum
lacking diagnostic signals.
(a)
Attempted Desilylation of Tetracycle 30 using Weak
Silaphiles. Treatment with D2O in CD2Cl2: a residue of
30 (2.3 mg) was dissolved in CD2Cl2 saturated with
D2O, and the sample was monitored by 1H NMR
spectroscopy. After 3 h no reaction was observed, so the
NMR tube was heated to 35 °C (oil bath); no
deprotected 7 was detected after 2 h of heating.
(b) Treatment with 2,2,2-trifluoroethyl alcohol in CDCl3: a
solution of 30 (2.3 mg) in CDCl3 was treated with excess
1
TFE (ca. 30 μL), and the sample was monitored by H
NMR spectroscopy. No change was seen after 3 h. The
sample was gently heated with an oil bath to 35 °C; no
deprotected 7 was observed after 20 h.
(c) CD3OD: 30 (2.3 mg) was dissolved in CD3OD and
monitored by 1H NMR spectroscopy. New peaks
consistent with formation of 7 were apparent between
δ = 4.4 and 3.4 ppm after 1.5 h, and the relative
intensities continued to increase at 3 and 5 h. Also, two
new signals appeared upfield: a triplet at δ = 1.43 ppm
corresponding to the CH3 signal of the ethyl ester, and a
singlet at δ = 1.01 ppm corresponding to a new TBDPS
species (TBDPSOCD3). After 20 h, no starting material
1
remained by H NMR, and the spectrum showed only
the known NH aziridine 715 with chemical shifts as
follows: 400 MHz 1H NMR (CD3OD, ppm) δ 9.94 (1H,
s), 7.72−7.66 (4H, m), 7.56 (1H, s), 7.48−7.38 (6H, m),
7.21 (1H, s), 4.37 (2H, q, J = 7.2 Hz), 4.28 (1H, br s),
4.04−3.96 (4H, m), 3.75 (1H, br s), 1.43 (3H, t, J = 7.2).
(d) Cs2CO3/CD3OD: A residue of TBDPS aziridinomito-
sene 30 was dissolved in a solution of Cs2CO3 (5 mg) in
1
prior to chromatography; not isolable): 500 MHz H
NMR (CDOD3, ppm) δ 4.79 (2H, ABq, J = 11.3 Hz),
3.50 (1H, m), 3.40 (1H, d, J = 3.5 Hz).
Heterolysis of Tetracycle 40 with CD3OD/Cs2CO3; Detection
of 41 and 42. Deuterated methanol (0.50 mL) was stirred with
Cs2CO3 (10 mg, 0.03 mmol) and the mixture was allowed to settle.
The supernatant liquid was decanted away from insoluble material,
and then added to 40 (<15 mg).15 The solution was monitored
0.5 mL CD3OD.44 The sample was monitored by H
1
NMR spectroscopy over 5 h. No change was observed
after 5 min, but after 15 min, a triplet, corresponding to
the ethyl ester group of 7 was observed at δ = 1.43 ppm
and a singlet, corresponding to a new TBDPS species,
was observed at δ = 1.01 ppm. After 1 and 4 h, the signal
intensity of both the triplet and singlet had increased. At
5 h, the relative amount of 7 was determined to be 12%
by integration of the unobscured triplet at 1.43 ppm.
1
repeatedly at rt by H NMR spectroscopy. After 1.5 h, the ratio of
40:41:42 was 75:11:14 according to integration, and 12:42:46 after 26
h. After 67 h, 3:40:57 of 40:41:42, the solvent was removed under N2
flow and the residue was purified on an analytical TLC plate on silica
gel 60 Å (20 cm × 20 cm × 250 μm) pretreated with Et3N fumes
(≥30 min) with 2:1 pentane/ethyl acetate providing small amounts of
41 and 42 as yellow oils. The less polar fraction was identified as 41:
molecular ion (M + Na) calcd for C34H27D3N2NaO3 = 540.2342,
found (electrospray with formic acid, for optimum ionization) m/z =
540.2360, error = 3 ppm; 500 MHz (CDCl3, ppm) δ 9.97 (1H, s),
7.26−7.20 (6H, m), 7.18−7.12 (9H, m), 6.96 (1H, d, J = 0.5 Hz), 6.93
(1H, s), 6.29 (1H, d, J = 1.0 Hz), 5.47 (1H, d, J = 0.5 Hz), 4.11 (3H,
s), 3.80 (1H, d, J = 12.5), 3.40 (1H, dd, J = 12.5, 1.5 Hz), 2.28 (1H, d,
J = 4.5 Hz), 2.13 (1H, dd, J = 4.5, 1.5) 125 MHz 13C (CDCl3, ppm) δ
192.1, 157.1, 156.8, 144.1, 140.2, 138.8, 129.3, 127.4, 126.6, 120.9,
114.4, 106.1, 104.0, 103.9, 73.8, 55.6, 51.5, 45.3, 37.0, the OCD3
carbon was not detected. The polar fraction was identified as 42:
molecular ion (M + Na) calcd for C34H27D3N2O3 = 540.2342, found
(electrospray with formic acid) m/z = 540.2341, error = 0.2 ppm; 500
MHz 1H NMR (CDCl3, ppm) δ 9.97 (1H, s), 7.49 (6H, d, J = 7.5 Hz),
7.38 (3H, d, J = 14.0 Hz), 7.30 (6H, t, J = 7.5 Hz), 7.25 (1H, m), 7.07
(a)
Attempted Desilylation of Tetracycle 36 using Oxy-
gen Silaphiles. CD3OD: Tetracycle 36 was dissolved in
CD3OD and monitored by 1H NMR spectroscopy over a
4 h period. After 15 min, the aziridine C(1) and C(2)
protons at δ = 3.53 and 3.49 ppm, as well as the C(10)
methylene protons at δ = 5.41 and 5.34 ppm began to
decrease in intensity. The singlet at 1.09 ppm
corresponding to the tert-butyl of the TBPDS group
was less intense, while a singlet at 1.01 ppm began to
appear. At 45 min, the intensity of the C(1), C(2) and
C(10) peaks decreased further, but no new aziridine
methine peaks appeared. By 2 h, the C(1) and C(2)
peaks were no longer visible, and after 4 h, the singlet at
1.09 ppm had disappeared. No signals corresponding to
1053
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The Journal of Organic Chemistry
Article
(1H, s), 4.81 (2H, ABq, J = 11.5 Hz), 4.44 (1H, d, J = 11.0 Hz), 4.10
(1H, dd, J = 11.0, 3.5 Hz), 3.04 (1H, d, J = 5.0 Hz), 3.01 (1H, dd, 5.0,
3.5 Hz); 125 MHz 13C NMR (CDCl3, ppm) δ 192.1, 154.8, 145.3,
144.1, 133.8, 131.8, 129.3, 128.3, 127.8, 127.1, 125.7, 109.0, 107.1,
98.1, 74.5, 66.2, 55.5, 47.4, 42.5, 35.1.
by flash chromatography with silica gel (2 × 15 cm, 3:1
hexanes/ethyl acetate, with 2% Et3N in the eluent).
Fractions 25−32 provided 42 mg (61%) of 45.
Attempted Desilylation of 36 with CsF in TFE; Isolation of
46. To a solution of 36 (0.0019 mmol, 1 mg) in 0.05 mL THF was
added TFE (5.6 mmol, 0.40 mL). The cloudy solution cleared and
turned bright yellow. A solution of CsF in TFE (0.11 M, 0.0022 mmol,
20 μL) was then added via microsyringe. After no apparent reaction by
TLC, another equiv of CsF was added at 2 and 5 h, and 2 equiv of CsF
was added at 6.5 h for a total of 5 equiv of CsF. At 24 h, the solvent
was removed under a flow of N2, and the orange residue was purified
by preparatory plate TLC on silica gel 60 Å (20 cm × 20 cm × 250
μm) pretreated with Et3N fumes (≥30 min) with 100% ethyl acetate
to provide one band consisting of a mixture of two products in
approximately a 1:1 ratio contaminated with unknown by products.
Repeated purification by preparatory plate TLC on silica gel 60 Å (20
cm × 20 cm × 250 μm) pretreated with Et3N fumes (≥30 min) with
5:1 hexanes/ethyl acetate provided a zone at Rf = 0.27 consisting of
the major diastereomer 46b containing unknown contaminants;
molecular ion (M + Na) calculated for C34H36F6N2NaO4Si = 701.2,
found (electrospray, nominal mass) m/z = 701.3; 500 MHz partial 1H
NMR (CDCl3, ppm) δ 9.91 (1H, s), 7.70 (2H, dd, J = 8.1, 1.4 Hz),
7.65 (2H, dd, J = 8.1, 1.4 Hz), 7.46−7.37 (7H, m), 7.08 (1H, s), 5.09
(1H, d, J = 11.8 Hz), 4.99 (1H, d, J = 11.8 Hz), 4.74 (1H, s), 4.34 (1H,
dd, J = 10.5, 5.5 Hz), 4.06−4.00 (1H, m), 4.00 (3H, s), 3.93 (2H, q,
JH,F = 8.8 Hz), 3.89 (1H, dd, J = 10.5, 2.0 Hz), 3.56−3.44 (2H, dq, J =
12.3 Hz, JH,F = 8.6), 1.01 (9H, s); 376 MHz 19F (CDCl3, ppm) δ
−74.0 (t, JF,H = 8.8 Hz), −74.4 (t, JF,H = 8.6 Hz). Impurity signals were
present δ 4.12 (EtOAc), 2.25−2.20, 2.04 (EtOAc), 2.04−1.98, 1.67−
1.60, 1.54 (H2O), 1.40−1.20 (EtOAc), 1.01 (s), 0.91−0.78 ppm. The
minor diastereomer 46a (tentative assignment), Rf = 0.42 could not be
obtained free of 46a and unknown contaminants; molecular ion (M +
Na) calculated for C34H36F6N2NaO4Si = 701.2, found (electrospray,
nominal mass) m/z = 701.3; 500 MHz partial 1H NMR (CDCl3, ppm)
δ 9.94 (1H, s), 5.11 (1H, d, J = 12.0 Hz), 4.85 (1H, d, J = 12.0 Hz),
4.56 (1H, d, J = 4.5 Hz), 4.09 (1H, m), 3.98−3.88 (4H, m), 3.84 (1H,
m); 376 MHz 19F NMR (C6D6, ppm) δ −73.7 (dd, J = 9.0, 8.6 Hz),
−73.9 (dd, J = 9.7, 8.3 Hz).
(a)
Desilylation Attempts with 44 by Treatment with
Silaphiles. TBAF: To a solution of aziridine 44 (0.24
mmol, 108 mg) in 2.4 mL DMF at rt was added TBAF as
a solution in THF (1.0 M, 0.24 mL). After 3 h of stirring
at rt, saturated NaHCO3 (1.0 mL) was added. The
mixture was extracted 3× with Et2O, the combined
organics were washed with brine, and then dried over
MgSO4. Removal of the solvent by rotary evaporation
provided an oil, and flash chromatography on silica gel (2
× 15 cm silica gel, 1:1 hexanes/Et2O, Rf = 0.18) provided
28 mg (56%) of desilylated aziridine 45.36
(b) TBAT: Aziridine 44 was dissolved in ca. 1 mL DMF-d7,
and a spatula tip of TBAT was added to the solution.
Only 44 and TBAT were observed by 1H NMR
spectroscopy after 3 d. To a sample of 44 (0.58 mmol,
25.9 mg) in 1 mL DMF-d7 was added TFE (0.58 mmol,
4.2 μL) and TBAT (0.58 mmol, 32 mg). The sample was
1
monitored by H NMR spectroscopy. At 1 h, peaks
corresponding to 45 had appeared; at 2 h, 27%
conversion was determined by integration. After 24 h,
75% of 45 was present.
(c) CsF and TFE: An oven-dried NMR tube was charged
with CsF (0.10 mmol, 15 mg) and capped with a rubber
septum under nitrogen. A solution of aziridine 44 in
DMF-d7 (0.06 mmol, 27.5 mg, 0.5 mL) was then added
via syringe. Neat trifluoroethanol (TFE, 0.06 mmol, 4.3
μL) was then added, the NMR tube was shaken for 30 s,
and the insoluble material was allowed to settle. The
sample was monitored by 1H NMR spectroscopy without
spinning. After 10 min, 45 began to appear, and after 2 h,
48% of 45 was present. At 20 h, 93% conversion to 45
was observed.
(d) CsF + TFE/CH3CN: To a nonhomogenous solution of
44 (0.26 mmol, 114.1 mg) in 1.8 mL CH3CN was added
CsF (0.30 mmol, 47 mg) as a solution in CH3CN (0.8
mL) and TFE (0.10 mL, 1.39 mmol). The resulting
white nonhomogenous mixture was stirred vigorously.
The solution cleared after 3 d of stirring, and a saturated
solution of NaHCO3 was added. The mixture was
extracted 3 x with Et2O, the combined organics were
washed with brine, and dried over MgSO4. Removal of
the solvent under rotary evaporation provided a clear
yellow oil. The residue was purified by preparatory plate
TLC on silica gel 60 Å (20 cm × 20 cm × 1000 μm)
pretreated with Et3N fumes (≥30 min) with 9:1 hexane/
ethyl acetate. Elution of the bands with ethyl acetate
provided 13 mg (11%) of recovered 44 and 22 mg (41%)
of 45.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra of new compounds and experimental details for
24-28 and 44 are reported in the Supporting Information. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Email: edved@umich.edu.
Present Address
†Pacific Northwest National Laboratory, 902 Battelle Blvd,
Richland, WA 99352.
ACKNOWLEDGMENTS
■
This work was supported by the National Institutes of Health
(CA17918).
(e) CsF/TFE: To a solution of aziridine 44 (0.33 mmol, 147
mg) in 1.3 mL TFE was added CsF as a solution in TFE
(0.17 M, 2.0 mL, 0.33 mmol). Aziridine 44 was insoluble
in TFE, so the reaction vessel was placed in a sonicator
to break up the insoluble material. Then the suspension
was stirred vigorously at rt. After 2 d of stirring, 2 mL of
satd NaHCO3 was added, and the mixture was extracted
3× with Et2O, washed 1× with brine, and dried over
MgSO4. Solvent removal under rotary evaporation
provided an oily residue. The crude residue was purified
REFERENCES
■
(1) Kiyoto, S.; Shibata, T.; Yamashita, M.; Komori, T.; Okuhara, M.;
Terano, H.; Kohsaka, M.; Aoki, H.; Imanaka, H. J. Antibiot. 1987, 594.
(2) Terano, H.; Takasa, S.; Hosada, J.; Kohsaka, M. A. J. Antibiot.
1989, 42, 145.
(3) (a) Shimomura, K.; Manda, T.; Mukumoto, S.; Masuda, K.;
Nakamura, T.; Mizota, T.; Matsumoto, S.; Nishigaki, F.; Oku, T.;
Mori, J.; Shibayama, F. Cancer Res. 1988, 48, 1166. (b) Masuda, K.;
1054
dx.doi.org/10.1021/jo202286a | J. Org. Chem. 2012, 77, 1045−1055

The Journal of Organic Chemistry
Article
Nakamura, T.; Mizota, T.; Mori, J.; Shimomura, K. Cancer Res. 1988,
48, 5172.
(28) (a) Bassindale, A. R.; Stout, T. J. Organomet. Chem. 1984, 271, 1.
(b) Vloon, W. J.; Van den Bos, J. C.; Koomen, G. J.; Pandit, U. K. Recl.
Trav. Chim. Pays-Bas 1991, 110, 414.
(29) Bobeck, D. R.; Warner, D. L.; Vedejs, E. J. Org. Chem. 2007, 72,
8506.
(30) Kocovsky, P. Tetrahedron Lett. 1986, 45, 5521.
̌
́
(31) Vedejs, E.; Klapars, A.; Naidu, B. N.; Piotrowski, D. W.; Tucci,
F. C. J. Am. Chem. Soc. 2000, 122, 5401.
(32) Ueki, M.; Amemiya, M. Tetrahedron Lett. 1987, 28, 6617.
(33) Ando, T.; Yamawaki, J.; Kawate, T.; Sumi, S.; Hanafusa, T. Bull.
Chem. Soc. Jpn. 1982, 55, 2504.
(34) Scheidt, K. A.; Chen, H.; Follows, B. C.; Chemler, S. R.; Coffey,
S.; Roush, W. R. J. Org. Chem. 1998, 63, 6436.
(35) (a) Urakawa, C.; Tsuchiya, H.; Nakano, K. J. Antibiot. 1981, 243.
(b) Urakawa, C.; Tsuchiya, H.; Nakano, K. J. Antibiot. 1981, 1152.
(c) Kono, M.; Kasai, M.; Shirahata, K. Synth. Commun. 1989, 19, 2041.
(d) Benbow, J. W.; McClure, K. F.; Danishefsky, S. J. J. Am. Chem. Soc.
1993, 115, 12305.
(36) Kitahonoki, K.; Kotera, K.; Matsukawa, Y.; Miyazaki, S.; Okada,
T.; Takahashi, H.; Takano, Y. Tetrahedron Lett. 1965, 1059.
(37) Cox, D. P.; Terpinski, J.; Lawrynowicz, W. J. Org. Chem. 1984,
49, 3216.
(4) (a) Naoe, Y.; Inami, M.; Kawamura, I.; Nishigaki, F.; Tsujimoto,
S.; Matsumoto, S.; Manda, T.; Shimomura, K. Jpn. J. Cancer Res. 1998,
89, 666. (b) Naoe, Y.; Inami, M.; Matsumoto, S.; Nishigaki, F.;
Tsujimoto, S.; Kawamura, I.; Miyayasu, K.; Manda, T.; Shimomura, K.
Cancer Chemother. Pharmacol. 1998, 42, 31.
(5) Rajski, S. R.; Williams, R. M. Chem. Rev. 1998, 98, 2723.
(6) (a) Masuda, K.; Nakamura, T.; Shimomura, K. J. Antibiot. 1988,
1497. (b) Huang, H.; Rajski, S. R.; Williams, R. M.; Hopkins, P. B.
Tetrahedron Lett. 1994, 35, 9669. (c) Williams, R. M.; Rajski, S. R.;
Rollins, S. B. Chem. Biol. 1997, 4, 127. (d) Paz, M. M.; Hopkins, P. B.
Tetrahedron Lett. 1997, 38, 343.
(7) Fukuyama, T.; Goto, S. Tetrahedron Lett. 1989, 30, 6491.
(8) Woo, J.; Sigurdsson, S. T.; Hopkins, P. B. J. Am. Chem. Soc. 1993,
115, 1199.
(9) (a) Shimomura, K.; Hirai, O.; Mizota, T.; Matsumoto, S.; Mori,
J.; Shibayama, F.; Kikuchi, H. J. Antibiot. 1987, 600. (b) Hirai, O.;
Shimomura, K.; Mizota, T.; Matsumoto, S.; Mori, J.; Kikuchi, H. J.
Antibiot. 1987, 607.
(10) Naoe, Y.; Inami, M.; Matsumoto, S.; Takagaki, S.; Fujiwara, T.;
Yamazaki, S.; Kawamura, I.; Nishigaki, F.; Tsujimoto, S.; Manda, T.;
Shimomura, K. Jpn. J. Cancer Res. 1998, 89, 1306.
(38) Pilcher, A. S.; Ammon, H. L.; DeShong, P. J. Am. Chem. Soc.
1995, 117, 5166.
(11) Reviews: (a) Danishefsky, S. J.; Schkeryantz, J. M. Synlett 1995,
475. (b) Andrez, J.-C. Beilstein J. Org. Chem. 2009, 5 (No. 33), 1.
(12) Total Syntheses: (a) Fukuyama, T.; Xu, L.; Goto, S. J. Am.
Chem. Soc. 1991, 114, 383. (b) Schekeryantz, J. M.; Danishefsky, S. J. J.
Am. Chem. Soc. 1995, 117, 4722. (c) Katoh, T.; Nagata, Y.; Yoshino,
T.; Nakatani, S.; Terashima, S. Tetrahedron 1997, 53, 10253
andreferences cited therein. (d) Suzuki, M.; Kambe, M.; Tokuyama,
H.; Fukuyama, T. Angew. Chem., Int. Ed. 2002, 41, 4686. (e) Ducray,
R.; Ciufolini, M. A. Angew. Chem., Int. Ed. 2002, 41, 4688. (f) Judd, T.
C.; Williams, R. M. Angew. Chem., Int. Ed. 2002, 41, 4683. (g) Judd, T.
C.; Williams, R. M. J. Org. Chem. 2004, 69, 2825. (h) Trost, B. M.;
O’Boyle, B. M.; Torres, W.; Ameriks, M. K. Chem.−Eur. J 2011, 17,
7890.
(39) Tomasz, M.; Chawla, A. K.; Lipmin, R. Biochemistry 1988, 27,
3182.
(40) Tomasz, M.; Lipman, R. Biochemistry 1981, 20, 5056.
(41) Li, V.-S.; Choi, D.; Tang, M.-S.; Kohn, H. J. Am. Chem. Soc.
1996, 118, 3765.
(42) Williams, R. M.; Ducept, P. Biochemistry 2003, 42, 14696.
(43) Kofron, W. G.; Baclawski, L. M. J. Org. Chem. 1976, 41, 1879.
(44) Beckers, J. L.; Ackermans, M. T.; Boce
24, 1544.
̌
k, P. Electrophoresis 2003,
(13) Vedejs, E.; Little, J. D. J. Am. Chem. Soc. 2002, 124, 478.
(14) Vedejs, E.; Little, J. D.; Seaney, L. M. J. Org. Chem. 2004, 69,
1788.
(15) Kim, M.; Vedejs, E. J. Org. Chem. 2004, 69, 7262.
(16) Vedejs, E.; Klapars, A.; Warner, D. L.; Weiss, A. H. J. Org. Chem.
2001, 66, 7542.
(17) Kunz, H.; Schmidt, P. Liebigs. Ann. Chem. 1982, 1245.
(18) Vedejs, E.; Naidu, B. N.; Klapars, A.; Warner, D. L.; Li, V.; Na,
Y.; Kohn, H. J. Am. Chem. Soc. 2003, 125, 15796.
(19) Han, I.; Kohn, H. J. Org. Chem. 1991, 56, 4648.
(20) Little, J. D. Synthetic Studies Toward Aziridinomitosenes and
their Derivatives. Ph.D. Dissertation; University of Wisconsin-
Madison: Madison, WI, 2001.
(21) Evans, D. A.; Borg, G.; Scheidt, K. A. Angew. Chem., Int. Ed.
2002, 41, 3188.
(22) Dixon, D. J.; Scott, M. S.; Luckhurst, C. A. Synlett 2003, 2317.
́ ́
(23) Begue, J.-P.; Bonnet-Delpon, D.; Crousse, B. Synlett 2004, 18.
(24) Warner, D. L.; Hibberd, A. M.; Kalman, M.; Klapars, A.; Vedejs,
E. J. Org. Chem. 2007, 72, 8519.
(25) For recent reviews discussing reductive activation and the
consequences of heterolysis at mitomycin C(1) and C(10), see:
(a) Tomasz, M.; Palom, Y. Pharmacol. Ther. 1997, 76, 73.
(b) Bargonetti, J.; Champeil, E.; Tomasz, M. J. Nucleic Acids 2010, 1.
(26) (a) Zein, N.; Kohn, H. J. Am. Chem. Soc. 1986, 108, 296.
(b) Schiltz, P.; Kohn., H. J. Am. Chem. Soc. 1993, 115, 10510. (c)
Reference 19 describes solvolytic reactions of an isolated aziridinomi-
tosene involving C(1) heterolysis.
(27) For pioneering studies of the exceptionally sensitive reduced
(leucoaziridinomitosene) hydroquinone derivative of N-methylmito-
mycin A, see: (a) Danishefsky, S. J.; Egbertson, M. J. Am. Chem. Soc.
1986, 108, 4648. (b) Egbertson, M.; Danishefsky, S. J. J. Am. Chem.
Soc. 1987, 109, 2204.
1055
dx.doi.org/10.1021/jo202286a | J. Org. Chem. 2012, 77, 1045−1055