Syntheses of 5a′-homo-vinblastine and congeners designed to establish structural determinants for isolation of atropisomers

Article abstract of DOI:10.1021/jo000249z

The syntheses of 5a′-homo-vinblastine (3a) and its C-20′ methyl congener 62a were achieved. In contrast to vinblastine, these compounds did not allow isolation of atropisomers because of their lower conformational inversion barrier. However, annelation of a six-membered ring to the conformationally mobile D′-piperidine ring provided an isolated atropisomer 81a, which could be converted to its lower energy conformation 65a on heating. The 5a′-homo-vinblastine congeners 3a, 62a, and 65a showed vinblastine-like inhibition of tubulin polymerization and cytotoxicity to L1210 leukemia cells, albeit at lower potency for the latter activity, than that found with the corresponding compounds in the vinblastine series.

Full text of DOI:10.1021/jo000249z

J . Org. Chem. 2001, 66, 5303-5316
5303
Syn th eses of 5a ′-h om o-Vin bla stin e a n d Con gen er s Design ed to
Esta blish Str u ctu r a l Deter m in a n ts for Isola tion of Atr op isom er s
Martin E. Kuehne,*,† Scott D. Cowen,† Feng Xu,† and Linda S. Borman‡
Departments of Chemistry and Pharmacology, University of Vermont, Burlington, Vermont 05405
mkuehne@zoo.uvm.edu
Received February 22, 2000 (Revised Manuscript Received May 11, 2001)
The syntheses of 5a′-homo-vinblastine (3a ) and its C-20′ methyl congener 62a were achieved. In
contrast to vinblastine, these compounds did not allow isolation of atropisomers because of their
lower conformational inversion barrier. However, annelation of a six-membered ring to the
conformationally mobile D′-piperidine ring provided an isolated atropisomer 81a , which could be
converted to its lower energy conformation 65a on heating. The 5a′-homo-vinblastine congeners
3a , 62a , and 65a showed vinblastine-like inhibition of tubulin polymerization and cytotoxicity to
L1210 leukemia cells, albeit at lower potency for the latter activity, than that found with the
corresponding compounds in the vinblastine series.
In tr od u ction
In 1991, we reported a practical synthesis of vinblas-
tine (VLB, 1).1 In its course it was possible to generate
and to isolate a vinblastine atropisomer 2 in which the
piperidine ring D′ of the cleavamine moiety is in a
conformationally inverted chair form relative to that
found in natural vinblastine (Figure 1). This atropisomer
2 lacks the cytotoxicity and inhibition of tubulin polym-
erization that is the basis of vinblastine’s anti-cancer
activity. Since the atropisomer 2 could be converted to
vinblastine (1) on heating, it might be considered, in prin-
ciple, as a noncytotoxic vinblastine pro-drug that could
be thermally activated. However, the thermal barrier for
this conformational inversion (100 °C) precludes a practi-
cal in vivo thermal activation. While variations of the
piperidine ring D′ substitution did have some influence
on the conformational inversion barrier of this ring,2,3
a
more pronounced attenuation of the thermal barrier was
required. Interestingly, this thermal barrier is lowered
considerably (by 40 °C) for the piperidine inversion in
the cleavamine lacking vindoline as a C-16′ substituent,4
but such a compound does not have any anti-cancer
potential.
Consequently, we decided to see if a 5a′-homo-vinblas-
tine (3a ), in which the nine-membered ring C′ of the VLB
cleavamine moiety is expanded by one methylene unit,
could be synthesized. Would a 10-membered ring C′, with
an expected lower conformational inversion barrier for
the piperidine ring D′, still allow isolation of an atropi-
somer that might be used as a noncytotoxic pro-drug?
Of course, the requisite tubulin activity and cytotoxicity
of the final homo-vinblastine would also have to be
attained. In this context, one might note that 5a′-nor-
F igu r e 1. Established thermal conformational inversion of
vinblastine atropisomer 2 to natural atropisomer 1 and
projected syntheses of 5a′-homo-vinblastine (3a ) and 5a′-homo-
20′-epi-vincovaline (3b).
anhydrovinblastine (Navalbine) is in clinical use as an
anti-cancer agent.
† Department of Chemistry.
‡ Department of Pharmacology.
Resu lts a n d Discu ssion
(1) Kuehne, M. E.; Matson, P. A.; Bornmann, W. G. J . Org. Chem.
1991, 56, 513.
(2) Kuehne, M. E.; Zebovitz, T. C.; Bornmann, W. G.; Marko, I. J .
Org. Chem. 1987, 52, 4340.
(3) Kuehne, M. E.; Bornmann, W. G. J . Org. Chem. 1989, 54, 3407.
(4) Kuehne, M. E.; Zebovitz, T. C. J . Org. Chem. 1987, 52, 4331.
P oten tia l Rou tes to In d oloa zocin es 4, 22. To see
if our previous vinblastine synthesis could be extended
to 5a′-homo-vinblastine (3a ), a synthesis of the indolo-
azocine 4 was required. While the corresponding in-
10.1021/jo000249z CCC: $20.00 © 2001 American Chemical Society
Published on Web 07/07/2001

5304 J . Org. Chem., Vol. 66, No. 16, 2001
Kuehne et al.
Sch em e 1a
Sch em e 2a
a
Key: (a) ref 5; (b) Me Cl-pyruvate, MeOH, HCl or tol. TsOH,
reflux; (c) MeO(CO)2Cl, Et3N, CH2Cl2; (d) Lawesson’s reagent,
THF, rt, 80%; (e) MeI, THF, 48 h reflux; POCl3, tol. reflux 3 h.
doloazepine 5 is readily made by condensation of
tryptamine with methyl chloropyruvate, followed by
rearrangement of the chloromethyltetrahydrocarboline 6,
and reduction of the unsaturated azepine 7 (Scheme 1),5
this sequence failed with homo-tryptamine (8), prepared
from indole and acrylonitrile, followed by reduction.6
Under a variety of Pictet-Spengler condensation condi-
tions, this process did not proceed past the initial
formation of an iminium salt 9, or else it resulted in as
many as eight products, of which the tetracyclic product
10 could be identified. These results support previous
observations that closure to form a seven-membered ring
on condensation of homo-tryptamine (8) with aldehydes
is problematic (0.4%, ref 7).7
Attempted alternative Bischler-Napieralski cycliza-
tion of an oxalamide ester, for subsequent methylene
addition,8 and of its thioimonium derivative,9 also did not
give an indole annelation product (Bischler-Napieralski
cyclizations are known to fail with amides bearing an
adjacent electron-withdrawing function).10 Therefore,
construction of the indoloazocine 4 was explored through
a Mannich cyclization.
a
Key: (a) ref 5; (b) t-BUOCl, THF, -20 °C; (c) Tl Me2 malo-
nate, THF/Et2O, -10 °C to rt, 76% 16; or N-BOC analogue of 15,
1 M ZnCl2, THF, Li Me2 malonate, -60 °C to rt; MeOH/HCl, 88%
17; (d) MeOH, CH2O, 93%; (e) tol. reflux 71%; (f) POCl3, NaBH4,
91%; (g) Lawesson’s reagent, tol., reflux, 76%; (h) R/Ni, EtOH, refl.
81%; (i) NaBH4, NiCl2; (j) NaCNBH4, HOAc, 90%; (k) LiCl, DMA,
Et3N-HCl, 120-130 °C, 82%; (l) Pd/C, H2, HOAc; (m) chroma-
tography SiO2.
chloroindolenine 15 with thallium dimethylmalonate
gave the 2-indolylmalonate 16. A significant improve-
ment to this earlier methodology for formation of 2-in-
dolylmalonates5 was found in the reaction of the N-BOC-
N-benzyl compound corresponding to 15 with lithium
dimethylmalonate and ZnCl2. Hydrolysis of the BOC
derivative, or hydrogenolysis of the dibenzyl compound
16, and condensation of the resulting secondary amine
17 with formaldehyde readily provided the spiroindoline
alkene 13; but in contrast to the lower homologue 11, its
rearrangement for formation of an indoloazocine could
not be accomplished.
Formation of the azocine ring was, however, achieved
by thermal cyclization of the malonate 17 to the lactam
ester 18. An attempt to selectively reduce the lactam
function by its reaction with POCl3, followed by reduction
with sodium borohydride, yielded only the lactam alcohol
19 (this method was developed for the selective reduction
We had previously found that the spiropyrrolidine
indoline alkene 11 rearranged quantitatively, at 60 °C
in THF, to form the indoloazepine 12 (Scheme 2).5 To
attempt the homologous rearrangement, the spiropiperi-
dine indoline 13 was synthesized.
Chlorination of N,N-dibenzyl-homo-tryptamine 14 with
tert-butyl hypochlorite and reaction of the resulting
(5) Kuehne, M. E.; Bohnert, J . C.; Bornmann, W. G.; Kirkemo, C.
L.; Kuehne, S. E.; Seaton, P. J .; Zebovitz, T. C. J . Org. Chem. 1985,
50, 919.
(6) (a) Misztal, S.; Boska, J .; Chojnacka-Wojcik, E.; Tatarczynska,
E. Pol. J . Pharmacol. Pharm. 1979, 31, 149; (b) Murphy, H. W. J .
Pharm. Sci. 1964, 53, 272.
(7) Szantay, C.; Danseo, A.; Kajtar-Peredy, M. J . Heterocycl. Chem.
1989, 26, 1867.
(8) Kametani, T.; Hirata, S.; Satoh, F.; Fukumoto, K. J . Chem. Soc.,
Perkin Trans. 1 1974, 2509.
(9) Ishida, A.; Tohru, N.; Hunihiko, I.; Tokuro, O. Chem. Pharm.
Bull. 1982, 30, 4226.
(10) Whately, W. M.; Govindachari, T. R. Org. React. VI 1951, 74.

Syntheses of 5a′-homo-Vinblastine and Congeners
J . Org. Chem., Vol. 66, No. 16, 2001 5305
of amides in the presence of ester functions,11 though not
for malonamide esters). Conversion of the lactam ester
18 to a thiolactam 20 with Lawesson’s reagent and its
treatment with Raney nickel provided the 1,2,3,4-tet-
rahydroazocine 21, which could be further reduced to the
saturated indoloazocine 22 with cyanoborohydride in
acetic acid. A mixture of indoloazocines 21 and 22 was
obtained by reduction of the thiolactam 20 with nickel
boride, generated in situ from sodium borohydride and
nickel chloride.
Sch em e 3a
The indoloazocine 22 is not stable to chromatography
on silica gel, and it readily undergoes acid-catalyzed
rearrangement to the spirocyclic isomer 23, which was
independently synthesized by condensation of formalde-
hyde with the amino ester 25 (obtained from the diester
16, through its mono-decarbomethoxylation with LiCl/
DMF to N,N-dibenzyl ester 24 and mono-debenzylation
of the monoester 24 by hydrogenolysis).
Gen er a tion of Tetr a cyclic In ter m ed ia tes 27a ,b
a n d Th eir Cou p lin g to Vin d olin e. Condensation of the
indoloazocine 22 with the aldehyde 26,1 in refluxing
toluene, provided an anticipated diastereomeric mixture
of tetracyclic products 27a ,b in 97% isolated yield, after
chromatography (Scheme 3). Thus, the higher homologue
22 of our Nb-benzyl indoloazepine underwent fragmenta-
tion and the intramolecular Diels-Alder-type reaction
as readily as the earlier fragmentation and biogenetic
secodine analogue reaction.1
For chromatographic separation of diastereomers, we
had hydrolyzed the acetonides corresponding to ac-
etonides 27a ,b to diols in the VLB synthesis.1 When this
was attempted in the present sequence, a new structural
product type was formed. It was isomeric with the target
diols 28a ,b but it lacked UV and IR spectroscopic
characteristics of an aminoacrylate, and it was instead
indolic.
We had previously found that the pentacyclic ami-
noacrylate alcohol alkaloid ibophyllidine, under acidic
conditions, gave an indolic hemiaminal cyclic ether.12
Therefore, in the present case, liberation of two hydroxyl
functions on acid hydrolysis of the acetonides 27a ,b, and
intramolecular formation of hemiaminal ethers by alter-
facial attack of the hydroxyl groups on an intermediate
iminium function 29 could possibly lead to eight potential
stereoisomeric hemiaminal ethers 30 (16 diastereomers
if epimeric esters are considered). This complexity did
not bode well for a separation of diastereomers arising
from hydrolysis of the pair of acetonides 27a ,b.
Since an indoline vinylogous urethane structure, as in
the diols 28a ,b, is key to stereoselectivity in the antici-
pated coupling reaction with vindoline,1 and since the
hemiaminal ether products 30 did not show any tendency
to revert to that ring system, a coupling reaction had to
be performed directly on the diastereomeric mixture of
acetonides 27a ,b.
Chlorination of the mixture of acetonides 27a ,b with
tert-butyl hypochlorite, followed by reaction of the result-
ing chloroalkyl indoline imine with silver tetrafluorobo-
rate and vindoline, provided the indoline imines 31a ,b
(Scheme 4). Reductive cleavage of the C-3′-C-7′ bond
with potassium borohydride in acetic acid then afforded
the indoles 32a ,b, showing four NMR methyl singlets for
these acetonides. Hydrolysis of the acetonides 32a ,b with
a
Key: (a) tol. reflux 12 h, 96%; (b) MeOH, aqueous HCl, reflux
30 min.
HCl in aqueous methanol gave primarily two diols 33a
and 33b, which were separated by chromatography and
derivatized with p-toluenesulfonic anhydride to furnish
the primary tosylates 34a and 34b.
F or m a tion of 5a ′-h om o-Vin bla stin e (3a ) a n d Its
C-14′epi,C-16′epi-Dia ster eom er 3b. Heating of each of
the tosylates 34a and 34b might allow cyclization to two
diastereomeric quaternary salts 35a , 36a , and 35b, 36b,
respectively, while cyclization of the tosylate- derived
epoxides 37a and 37b should be stereoelectronically
directed to the unique quaternary salts 35a and 35b.1-4
On debenzylation by hydrogenolysis, these compounds
would give the (high energy) atropisomers 38a of 5a′-
homo-vinblastine and 38b of 20′-epi-5a′-homo-vincova-
line, in analogy to the corresponding compounds in the
VLB series.1
To achieve selective epoxide formation, the tosylate 34a
was treated with sodium hydride and the resulting
product 37a was heated in methanol to provide a
(11) Kuehne, M. E.; Shannon, P. J . J . Org. Chem. 1977, 42, 2082.
(12) Kuehne, M. E.; Pitner, J . B. J . Org. Chem. 1989, 54, 4553.

5306 J . Org. Chem., Vol. 66, No. 16, 2001
Kuehne et al.
Sch em e 4a
Sch em e 5a
a
Key: (a) t-BUOCl, CH2Cl2, 0 °C; vindoline, HBF4-Et2O,
AgBF4, acetone; (b) KBH4, HOAc; (c) 1 N HCl, reflux 1 h; (d) Ts2O,
CH2Cl2, 0 °C, 63-66%.
a
Key: (a) tol. reflux 8 h; (b) Pd/C, H2, MeOH, 3 h; 68-73%
overall; (c) NaH, THF; (d) MeOH, reflux.
quaternary salt 35a (Scheme 5). Its hydrogenolysis gave
a compound, albeit in low yield, which fit the expected
NMR and TLC characteristics of a 5a′-homo-vinblastine
low energy atropisomer 3a , rather than the pronounced
characteristics expected for the corresponding high en-
ergy atropisomer 38a .1 The same product was also
obtained by direct cyclization of the tosylate 34a in
refluxing toluene (that is expected to pass primarily
through the epoxide 37a ),1 followed by debenzylation.
Alter n a tive Sequ en ces P r ovid in g Tetr a cyclic Di-
ols 28a ,b or 45a ,b a n d Ep oxid es 37a ,b. To improve
access to the epoxides 37a ,b, two reaction sequences were
examined. In the first, the acetonide protecting group of
aldehyde 26 was replaced by a carbonate function 39
(Scheme 6), since that would allow eventual deprotection
of the diol under nonacidic conditions and avoid ring
fragmentation to the hemiaminal derivatives 30. Deriva-
tization of the diol 40 with phosgene and pyridine,
followed by ozonolysis, provided the aldehyde 39 for
condensation with the indoloazocine 22. Analogously, the
lower homologue (see below) methyl diol 41 and its
aldehyde carbonate derivative 42 were also prepared. On
condensation of the indoloazocine 22 with the aldehydes
39 and 42, formation of two diastereomeric pairs of
tetracyclic carbonate derivatives 43a ,b and 44a ,b was
achieved in high yields. Their hydrolysis with 1 N sodium
hydroxide gave the separable diols 28a ,b and 45a ,b.
While these diols underwent the aforementioned rear-
rangement on silica gel chromatography, this could be
suppressed by column pretreatment with triethylamine.
But, the desired monotosylation of these diols proved to
be impossible and resulted in rearrangement to the
hemiaminal cyclic ethers (i.e., 30), found before.
The second alternative sequence to the epoxides 37a ,b
was based on an initial functionalization of the starting
aldehyde by synthesis of the trimethylsilyl ether tosylate
46 from the olefinic diol 40 by its reaction with tosyl
anhydride and triethylamine, followed by TMS triflate,
and ozonolysis (Scheme 7). While we were concerned that
there could be an undesired N-alkylation on formation
of the tetracyclic intermediates 47a ,b in refluxing tolu-
ene, this was not problematic and a 93% yield of these
products was obtained. Chromatographic separation of

Syntheses of 5a′-homo-Vinblastine and Congeners
J . Org. Chem., Vol. 66, No. 16, 2001 5307
Sch em e 6a
Sch em e 8a
a
Key: (a) pyd, C6H6, COCl2, 84%, 96%; (b) O3, 73%, 75%; (c)
tol. reflux 79%, 88%; (d) 0.5 N NaOH, 1 h, rt; (e) CDCl3 rt.
Sch em e 7a
a
Key: (a) BH3-Me2S, 91%; (b) (t-BUO)2CO, NaOH, 96%; (c)
t-BUOCl, Et3N; (d) Tl Me2 malonate, c + d 87%; or ZnCl2, Li Me2
malonate, c + d 91%; (e) MeOH, HCl, reflux, 98%; (f) tol. reflux
85%; (g) Lawesson’s reagent, tol. reflux 64%; (h) R/Ni, MeOH, 81%;
(i) NaCNBH4, HOAc, 62%; (j) tol. reflux 89%; (k) t-BUOCl, CH2Cl2,
0 °C; vindoline, HBF4-Et2O, AgBF4, acetone; KBH4, HOAc, 62%
overall.
objective of introducing diastereoselectivity in generation
of our key tetracyclic intermediates (e.g., 47a ,b) and in
order to have a better chance of separation of subsequent
diastereomeric intermediates, the indoloazocines 49 with
a chiral R-phenethyl Nb-substituent were chosen for
condensation with an aldehyde (Scheme 8). They were
prepared by formation of the indole 3-propionic acid
amide 50 of S-(-)-R-methylbenzylamine and its reduction
with lithium aluminum hydride to the corresponding
amine 51. In a better alternative, this compound was
obtained from indole-3-propionitrile, its reduction with
DIBALH to the corresponding aldehyde, a subsequent
condensation with S-(-)-R-methylbenzylamine, and final
reduction with sodium borohydride. Installation of a
t-BOC amide 52 allowed conversion to a chloroindolenine
with tert-butylhypochlorite and its reaction with thallium
dimethyl malonate to form the malonylindole 53. Re-
moval of the BOC protecting group and cyclization of the
resulting amine 54 afforded the lactams 55. Their con-
version to thiolactams 56 with Lawesson’s reagent, desul-
furization with R/Ni, and reduction with NaBH4/NiCl2,
gave the indoloazocines 49 (mixture of ester epimers).
For formation of tetracyclic intermediates to be used
in the following reaction sequence, the lower homologue
a
Key: (a) Ts2O, Et3N, 83%; (b) TMSOTf, DMAP, Et3N, 91%;
(c) O3, 93%; (d) tol. reflux, 97%; (e) t-BUOCl, CH2Cl2, 0 °C;
vindoline, HBF4-Et2O, AgBF4, acetone; KBH4, HOAc, 51% overall;
(f) TBAF, THF, 25 °C, 90%.
the diastereomers was, however, not possible at this
stage, or even after the coupling reaction with vindoline
and imine reduction to tosylates 48a ,b. Thus, a diaster-
eomeric mixture of epoxides 37a ,b was made by treat-
ment of the silyl ethers 48a ,b with fluoride. Their
cyclization and hydrogenolysis gave the separable 5a′-
homo-vinblastine (3a ) and 20′-epi-5a′-homo-vincovaline
(3b) shown in Scheme 5. Again, no atropisomer of these
products (38a or 38b) was detected.
Dia ster eoselective F or m a tion of Tetr a cyclic In -
ter m ed ia tes 58a ,b, Th eir Cou p lin g to Vin d olin e,
a n d Con ver sion of th e Ma jor Dia ster eom er P r od u ct
59a to 18′-n or -5a ′-h om o-vin bla stin e (62a ). With the

5308 J . Org. Chem., Vol. 66, No. 16, 2001
Kuehne et al.
Sch em e 9a
aldehyde 57 rather than aldehyde 46 was chosen for two
reasons: We had previously found that the anti-leukemic
cytotoxicity (L1210 cells) of VLB congeners with C-20
modified substituents follows the order of CH3 > C2H5 >
H, C3H7 and we hoped to maximize this activity in the
homo-VLB case. Also, a C-20′ methyl substituent would
simplify NMR spectra and thus facilitate analysis of
atropisomeric products, if they were obtained. Finally, a
TBDMS substituent in aldehyde 57, rather than the TMS
substituent in aldehyde 46, was selected to ensure better
hydrolytic stability during the C-3′-C-7′ reductive cleav-
age reaction of the vindoline coupling product in acetic
acid.
Condensation of the chiral indoloazocines 49 with the
aldehyde 57 produced a 2:1 mixture of diastereomers,
analogous to that found in the VLB synthesis.1,13 Cou-
pling of the tetracyclic condensation products 58a ,b to
vindoline, followed by reductive cleavage of the C-3′-C-
7′ bond proceeded, as described above, to give the seco-
products 59a ,b. Here, the chiral Nb-substituent allowed
chromatographic separation of diastereomers.
The major diastereomer 59a was directed along two
pathways (Scheme 9). Its conversion to a unique epoxide
60a on treatment with fluoride was followed by cycliza-
tion of the epoxide in refluxing methanol. A quaternary
salt, which was expected to have the configuration
leading to the higher energy atropisomeric conformation
of 18′-nor-5a′-homo-vinblastine, was obtained and con-
verted to its chloride 61a on an ion-exchange resin. On
hydrogenolysis of this salt, 18′-nor-5a′-homo-vinblastine
62a with the lower energy atropisomeric conformation
was obtained.
Alternatively, the tosylate 59a was subjected to cy-
clization by direct Nb-alkylation in benzene at 150 °C,
and the quaternary product, after cleavage of the silyl
ether, was converted to its chloride 63a by ion exchange.
On the basis of the analogous reaction sequence in the
VLB synthesis, a quaternary salt with a configuration
leading to the lower energy atropisomer of the amine is
expected here.1
a
Key: (a) TBAF, THF, 74%; (b) MeOH, reflux; Amberlite IRA
400; (c) Pd/C, H2, b + c 81%; (d) C6H6, 150 °C; (e) TBAF, THF;
Amberlite IRA 400.
Unfortunately, a definitive NMR comparison of the
two quaternary salts 61a and 63a (i.e., NOE signals for
the C-20′ methyl substituent) was not possible due to
intense signal broadening.14 However, a comparison of
the CD spectra of the quaternary salts 61a and 63a , and
their comparison with CD spectra of the VLB atropiso-
mers,1 clearly showed that the expected two configura-
tions 61a and 63a had been obtained by the two
cyclization routes. Yet, both salts 61a and 63a gave rise
to the same product 62a on hydrogenolysis. Therefore,
it must be concluded that a higher energy atropisomer
of 18′-nor-5a′-homo-vinblastine (64a ) is converted to its
lower energy atropisomer 62a at ambient temperatures
by inversion of the bridged piperidine ring (in contrast
to the conformational stability of the corresponding VLB
atropisomer).1
In ver sion to 65a ,b. Our goal of lowering the atropiso-
meric inversion barrier of vinblastine (100 °C) had been
achieved, but exceeded. Some conformational restraint
was required to bring this barrier back up and into a
range where it might be clinically exploited. Therefore,
we set out to synthesize a 5a′-homo-vinblastine congener
65a with a cis-fused bridged perhydroisoquinoline (Scheme
9) in place of the bridged piperidine ring.
Condensation of the racemic aldehyde acetate 6615 with
the indoloazocine 22 led to a 1:1 diastereomeric mixture
of cyclohexyl-substituted tetracyclic vinylogous urethanes
67 and 68 (Scheme 10) Their relative stereochemistry is
consistent with the later cyclization results (below).
Methanolysis of their primary acetate function did not
give the corresponding primary alcohols but, instead, the
now expected hemi-aminal cyclic ethers derived from
C-3-C-7 cleavage of the tetracyclic ring moiety. A
mixture of two C-16 epimeric ester pairs, 69a ,b and
70a ,b, was formed with sodium methoxide over 4 h. The
esters 70a ,b, which in molecular modeling showed their
C-16 H directed toward the Nb lone pair, were distin-
Syn th eses of P er h yd r oisoqu in olin e Con gen er s
65a ,b of 5a ′-h om o-Vin bla stin e (3a ), Th eir Isola ble
Atr op isom er s 81a ,b, a n d Th er m a l Con for m a tion a l
(13) Use of a chiral 2-ferrocenyl auxiliary N-substituent, which gives
complete diastereoselectivity in the vinblastine synthesis, was devel-
oped later: Kuehne, M. E.; Bandarage, U. K. J . Org. Chem. 1996, 61,
1175.
(14) We had already seen that in contrast to vinblastine in its
normal lower energy atropisomeric conformation, the higher energy
atropisomer of VLB gives a very poorly defined 1H NMR spectrum.1
(15) Prepared and used by Dr. I. Marko in syntheses of the
corresponding VLB congeners with two distereomeric cis-fused per-
hydroisoquinoline moieties; manuscript in preparation.

Syntheses of 5a′-homo-Vinblastine and Congeners
J . Org. Chem., Vol. 66, No. 16, 2001 5309
Sch em e 10a
Sch em e 11a
a
Key: (a) tol. reflux 69%; (b) 2 N NaOH, rt, 40 min, 98%; (c)
tol. reflux 91%; (d) NaOMe, rt, 4 h, 71%; (e) CHCl3, rt, 10 h, 94%;
(f) Ts2O, DMAP, Et3N, 71%.
1
guished by a strong downfield H NMR shift for C-16 H
(to δ 5.07-5.13), in analogy to corresponding C-16
a
epimeric carbomethoxycleavamine spectra.16
Key: (a) t-BUOCl, CH2Cl2, 0 °C; vindoline, HBF4-Et2O,
AgBF4, acetone; KBH4, HOAc, 66% overall; (b) tol. reflux; (c) Pd/
Separation of the tetracyclic diastereomeric series (67
and 68) was achieved through the corresponding 18-
imidazolyl urethanes 71 and 72, which were obtained by
condensation of the indoloazocine 22 with the p-nitro-
phenyl carbonate aldehyde 73 and subsequent displace-
ment of the p-nitrophenol substituent with imidazole.
Careful hydrolysis of the separated urethanes 71 and 72
provided the alcohols 74 and 75. These sensitive com-
pounds underwent rearrangement to the cyclic ethers
69a ,b (without epimerization at C-16) in CDCl3 at room
temperature during their NMR characterization, or on
storage at 0 °C. However, they could be trapped as their
tosylate derivatives 76 (71%) and 77 (68%).
C, H2, MeOH, b + c 49% 65a , 39% 81a ; (d) 91 °C.
A kinetic study of this atropisomerism at 110, 100, and
91 °C gave ∆Gq ) 29 kcal/mol, ∆Hq ) 25 kcal/mol, ∆Sq )
11 ( 2 eu.
Analogously, the diastereomeric tosylate 78b furnished
the atropisomeric indole-indolines 81b (37%) and 65b
(52%, Scheme 12). Here, the thermal energy barrier,
determined in toluene at 101, 90, and 75 °C, was found
to have ∆Gq ) 28 kcal/mol, ∆Hq ) 20 kcal/mol, ∆Sq
)
20 ( 5 eu, with 50% conversion of 81b to 65b in 5 h at
75 °C.
As in vinblastine and related examples,1 the higher
energy atropisomers 81a and 81b gave characteristic
Coupling of the racemic tosylate 76 with vindoline, and
reduction according to the above protocol, provided the
chromatographically separated diastereomeric tosylates
78a and 78b (Scheme 11). Cyclization of the former
tosylate to the corresponding quaternary salts 79a and
80a and debenzylation of these salts by hydrogenolysis,
furnished the binary indole-indoline compound with
C-14′, C-16′ VLB-type stereochemistry in two atropiso-
meric forms 65a (49%) and 81a (39%). On heating in
toluene, the latter atropisomer was converted to the
former (with natural VLB-type conformation), with 50%
conversion in 4 h at 91 °C.
1
very diffuse, poorly resolved H NMR spectra.
The coupling products 82a ,b, which were obtained from
the other racemic cyclohexyl cis-substituted tosylate 77
on coupling to vindoline (Scheme 13), could not be
separated. However, their cyclization and debenzylation
allowed chromatographic isolation of two binary indoline-
indole products 83a (27%) and 83b (24%), to which the
lower (natural VLB-type) atropisomer conformation could
(16) Kuehne, M. E.; Kirkemo, C. L.; Matsko, T. H.; Bohnert, J . C.
J . Org. Chem. 1980, 45, 3259.

5310 J . Org. Chem., Vol. 66, No. 16, 2001
Kuehne et al.
Sch em e 12a
cytotoxicity ED50 for 5a′-homo-vinblastine (3a ) was only
5 × 10-8 and 2 × 10-7 M, respectively, compared to a 1
× 10-9 M L1210 cytotoxicity for vinblastine.
We had discovered that in the vinblastine series (in
contrast to the present 5a′-homo-vinblastine congeners)
the cis-fusion of a six-membered ring to the D′-piperidine
ring resulted in an L1210 leukemia cell cytotoxic potency
500-1000 times greater than that of natural vinblastine
(ED50 ) 8 × 10-13 M for the lower homologue correspond-
ing to 65a and 3 × 10-13 M for lower homologue
corresponding to 83a ). Therefore, we hoped that this
structural element would also lead to a greatly increased
activity in the higher homologues 65a and 83a , but this
was not found (L1210 ED50 > 1 × 10-7M, Sarcoma 180
ED ) 1 × 10-7 M for 65a ). As expected, the diastere-
50
omeric 5a′-homovinblastine- congeners 3b and 62b did
not inhibit tubulin polymerization, nor did they show
L1210 cytotoxicity in a measured significant range.
Con clu sion . The syntheses of 5a′-homo-vinblastine
(3a ) and its 18′-nor-congener 62a demonstrated that, in
contrast to vinblastine, separate atropisomers derived
from a ring D′-piperidine chair-chair conformational
inversion, cannot be isolated here. However, cis-fusion
of the D′-piperidine ring to a cyclohexane ring raises the
energy barrier sufficiently to allow isolation of two
atropisomers, and permits a conformational inversion at
75-90 °C. Thus, introduction of an additional conforma-
tional barrier (compare 13.0 kcal/mol for cis-decalin vs
10.8 kcal/mol for cyclohexane inversions) has provided a
yardstick for the design of other ring D′-substituted 5a′-
homo-vinblastine congeners that will fall into a poten-
tially practical range for clinical, thermal pro-drug
activation.
a
Key: (a) tol. reflux; (b) Pd/C, H2, MeOH, a + b 52% 65b, 37%
81b; (c) 75 °C.
Sch em e 13a
Exp er im en ta l Section
A complete Experimental Section containing full NMR, IR,
UV, mass spectrometry, and elementary analyses/HRMS data,
as well as original NMR spectra, is provided in the Supporting
Information. Procedures for preparation of compounds 8, 19,
27a ,b, 28a ,b, 30, 39, 42, 43a ,b, 44a ,b, 45a ,b, 69a ,b, 70a ,b,
and good alternative preparations for 14, 17, 53, are given
there.
3-[3-[(Nb,Nb-Diben zyl)am in o]pr opyl]in dole (14). Meth od
B. Indole-3-propionic acid (10.25 g, 54.2 mmol) and dibenzyl-
amine (10.69 g, 54.2 mmol) in 250 mL of CH2Cl2 were stirred
at 0 °C, and 11.8 g (54.2 mmol) of dicyclohexyl carbodiimide
in 50 mL of CH2Cl2 was added in a stream. The mixture was
allowed to warm to 25 °C and stirred for 4 h. The solvent was
evaporated and 50 mL of Et2O added, causing dicyclohexylurea
to crystallize. Filtration and concentration afforded a yellow
oil, which was purified by chromatography, eluting with 1%
MeOH-CH2Cl2. Trituration with hexane induced crystalliza-
tion of the product indole-3-(N,N-dibenzyl)propanamide. Re-
crystallization from methanol provided 12.8 g (34.7 mmol, 64%)
of product, which was free of both DCU and N-acyl urea
rearrangement product: mp 89 °C; TLC (Et2O) Rf ) 0.40 (CAS,
tan).
a
Key: (a) t-BUOCl, CH2Cl2, 0 °C; vindoline, HBF4-Et2O,
AgBF4, acetone; KBH4, HOAc; (b) tol. 50 °C, 4 h; Pd/H2, MeOH, 3
h, a + b overall 27% 83a , 24% 83b.
LiAlH4 (10.4 mL, 1.0 M in THF) in 10 mL of THF was cooled
to 0 °C, and indole-3-(N,N-dibenzyl)propanamide (3.83 g, 10.4
mmol) in 20 mL of THF was added to the stirred solution. After
1
be assigned on the basis of their well-defined H NMR
spectra. The corresponding more polar, higher energy
atropisomers were not found in this more complex
reaction mixture.17
Biological evaluation of 5a′-homo-vinblastine (3a )
showed that, like vinblastine, it inhibits tubulin polym-
erization at the same concentration characteristic for very
active VLB congeners (ED50 ) 1 × 10-7 M). With L1210
leukemia cells, and with Sarcoma 180 cells, however, the
(17) In contrast to the cyclization of tosylate 78a leading to 79a (with
three 1,3 diaxial C/C repulsions relieved by a twist boat conformation)
and formation of 80a (no 1,3 diaxial C/C repulsions), the cyclization of
tosylate 82a to the quaternary salt precursor of amine 83a generates
no 1,3-diaxial C/C interactions. The same difference is found for
cyclization of tosylate 78b vs 82b. Thus, lack of these 1,3-diaxial
interactions on cyclization of 82a ,b would not promote cyclization to a
quaternary salt precusor of a higher energy atropisomer corresponding
to 81a ,b.

Syntheses of 5a′-homo-Vinblastine and Congeners
J . Org. Chem., Vol. 66, No. 16, 2001 5311
the addition was complete, the reaction mixture was heated
at reflux for 2-3 h and then cooled in ice. The reaction was
quenched by slow addition of 5 mL of water, followed by 20
mL of 2 N NaOH. Extraction with 5 × 30 mL of Et2O, washing
of the combined organic phases with brine, drying (MgSO4),
filtration, and concentration furnished a yellow oil, which was
purified by flash chromatography (SiO2, CH2Cl2). The amine
was triturated with hexane to afford 3.28 g (89%) of the title
compound as a white powder. An analytical sample was
recrystallized from methanol: mp 80 °C; TLC (ethyl acetate-
hexane) Rf ) 0.58 (CAS, yellow-orange).
Dim eth yl 3-[3-[(Nb,Nb-Diben zyl)a m in o]p r op yl]in d ole-
2-p r op a n ed ioa te (16). N,N-Dibenzyl-homo-tryptamine (14,
17.1 g, 48.4 mmol), in 250 mL of dry Et2O, under argon, was
cooled to -20 °C, and then 8.1 mL (58.1 mmol) of triethylamine
and 6.58 mL (58.1 mmol) of tert-butyl hypochlorite in 20 mL
of THF were added sequentially. The mixture was stirred for
1 h at -15 to -10 °C and then filtered through a plug of Celite
into a stirred suspension of thallium dimethyl malonate in 100
mL of 1:1 THF/Et2O, maintained at -10 °C. After 5 h at -10
°C, the reaction mixture was warmed to room temperature,
filtered to remove thallium chloride, then washed with brine,
dried (MgSO4), concentrated, and then triturated with 50 mL
of 1:1 ether-hexane to give 17.8 g (76%) of the title compound
as a white powder: mp 95-96 °C; TLC (1:1 ether-hexane) Rf
) 0.36 (CAS blue).
Dim eth yl 3-[3-[(Nb-Ben zyl)a m in o]p r op yl]-in dole-2-pr o-
p a n ed ioa te (17). Meth od B. Dimethyl 3-[3-(N,N-dibenzyl-
amino)propyl]-indole-2-propanedioate (16, 11.68 g, 24.13 mmol)
and 3.5 g of 10% palladium on carbon, in 80 mL of wet acetic
acid, was vigorously stirred under a hydrogen atmosphere for
3 h, purged with nitrogen, and filtered through Celite. The
filtrate was poured over crushed ice, basified with concentrated
NH4OH, and then extracted with methylene chloride (3 × 50
mL). The organic layers were dried (MgSO4), and the solvent
was removed to yield 7.1 g (75%) of the monobenzyl product
17 as a yellow oil: TLC (5% MeOH-CHCl3) Rf ) 0.14 (CAS,
blue).
Dim eth yl 1′-Ben zyl[sp ir o(3H-in d ole-3,3′-p ip er id in o)]-
2-m eth ylen ed ica r boxyla te (13). A 100 mL, two-necked
round-bottom flask, containing 866 mg (2.19 mmol) of mono-
N-benzyl diester homotryptamine 17 in 20 mL of methanol,
was fitted with a reflux condenser and an adapter, connecting
it, via a glass tube, to a second flask, containing about 500
mg of paraformaldehyde. The paraformaldehyde container was
immersed in a 140 °C oil bath for 1-2 min; an aspirator
vacuum was then applied to the first vessel. The gaseous
formaldehyde was bubbled into the stirred amine solution until
the solid paraformaldehyde was used up. Then the aspiration
was discontinued, and the solution was heated at reflux for 4
h. The solution was condensed to ∼3 mL, diluted with 20 mL
of diethyl ether, and washed with water to remove excess
paraformaldehyde. Concentration of the organic layers fur-
nished a white foam, which was purified by flash chromatog-
raphy (SiO2, 3:2 Et2O-hexane) to give 827 mg (93%) of the
title compound: TLC (3:2 Et2O-hexane) Rf ) 0.46 (CAS, blue).
g (76%) of the title compound: mp 197-198 °C; TLC (CH2Cl2)
Rf ) 0.29 (CAS, yellow).
Meth yl 3-Ben zyl-4,5,6,11-tetr a h yd r o-3H-a zocin o[4,5-b]-
in dole-1-car boxylate (21). Raney nickel (500 mg) was washed
repeatedly with anhydrous ethanol and added to a stirred
solution of 3.19 g (8.44 mmol) of methyl 2-thioxo-2,3,4,5,6,11-
hexahydro-1H-azocino[4,5-b]indole-1-carboxylate (20). The mix-
ture was heated at reflux for 3.5 h, cooled, filtered and
condensed to give a yellow solid, which was purified by flash
chromatography on a 3 × 12 cm silica gel column, eluting with
1% MeOH-CH2Cl2, to give 2.36 g (81%) of the title compound.
An analytically pure sample was obtained by recrystallization
from methanol: mp 177-178 °C; TLC (7:3 ether-hexane) Rf
) 0.35 (CAS, yellow).
Met h yl 3-Ben zyl-2,3,4,5,6,11-h exa h yd r o-1H -a zocin o-
[4,5-b]in d ole-1-ca r boxyla te (22). Unsaturated N-benzyl azo-
cine (21, 1.62 g, 4.68 mmol), was dissolved in a minimum of
glacial acetic acid (∼ 3 mL) and, with stirring, 2 eq. of sodium
cyanoborohydride was added over 10 min.The reaction mixture
was stirred for an additional 15 min, then cooled to 0 °C and
basified with 10 mL of ice-cold concentrated ammonium
hydroxide. The mixture was extracted with Et2O and the
extract washed with brine, dried, and filtered. The solvent was
removed in vacuo and the resulting yellow gum purified by
flash chromatography, eluting with 7:3 Et2O-hexane, to give
1.48 g (90%) of the title compound as a white foam. Addition
of HCl to the reaction mixture prior to workup resulted in
significantly reduced yields: TLC (7:3 ether-hexane) Rf ) 0.50
(CAS, blue).
1′-Ben zyl-2-m eth oxycar bon ylm eth ylen [spir o(3H-in dole-
3,3′-p ip er id in e)] (23). Dibenzyl diester homo-tryptamine 16
(905 mg, 1.87 mmol) was combined with 103 mg (2.43 mmol,
1.3 equiv) of lithium chloride and 77 mg (506 mmol, 0.3 equiv)
of triethylamine hydrochloride in 10 mL of DMA. The stirred
solution was then heated at 120-130 °C for 2 h. After cooling,
the contents of the reaction vessel were diluted with 20 mL of
Et2O and were washed repeatedly with water. Chromatogra-
phy of the crude product provided 650 mg (82%) of dibenzyl
monoester homo-tryptamine 24. A 330 mg sample of this
monoester was subjected to the monodebenzylation and con-
densation of the product 25 with formaldehyde, under the
same conditions described for the preparation of the spiro
product 13 from the diester 17, above. The product was filtered
through a plug of silica gel and crystallized upon trituration
with 1:1 Et2O-hexane: TLC (5% MeOH-CHCl3) Rf ) 0.74
(CAS, blue).
(6R,8S)- a n d (6S,8R)-Meth yl 4-Ben zyl-2,3,4,4a ,5,6,8,9-
octah ydo-6-[(2S)-2-eth yl-2,3-(isopr opyliden edioxy)pr opyl]-
8-(15-vin d olin yl)-1H -a ze cin o-[5,4-b]in d ole -8-ca r b oxy-
la te (32a a n d 32b). A solution of 858 mg (1.66 mmol) of
acetonide tetracycles 27a ,b and 254 µL (1.83 mmol, 1.1 equiv)
of triethylamine in 30 mL of CH2Cl2, under argon, was cooled
to 0 °C. Dropwise addition of 220 µL (1.83 mmol) of tert-butyl
hypochlorite and stirring for 5 min gave a solution that by TLC
was free of starting material (CAS, blue) and contained a more
polar compound (CAS, green). The solution was washed with
water, dried (MgSO4), and concentrated under reduced pres-
sure to give a white foam that was used in the next step.
The chloroindolenine was taken up in 30 mL of dry acetone,
and 756 mg (1.66 mmol) of vindoline was added, followed by
640 µL (3.32 mmol) of tetrafluoroboric acid-diethyl ether
complex. After 5 min, 646 mg (3.32 mmol) of silver tetrafluo-
roborate in acetone was added, causing the mixture to become
heterogeneous. After being stirred for an additional 5 min, this
mixture was basified by addition of 20 mL of 10% aqueous
ammonium hydroxide and then extracted with CH2Cl2. Drying
and solvent removal gave the coupling product 31a ,b as a
white foam.
Meth yl 3-Ben zyl-2-oxo-2,3,4,5,6,11-h exa h yd r o-1H-a zo-
cin o[4,5-b]in d ole-1-ca r boxyla te (18). N-Benzyl diester ho-
motryptamine 17 (7.1 g, 18.0 mmol) in 50 mL of toluene was
heated under a Dean-Stark trap at reflux for 5 h.The toluene
was removed in vacuo, and the resulting oil was chromato-
graphed (silica gel, 7:3 ether-hexane), to furnish a yellow oil,
which crystallized on trituration with petroleum ether. The
product lactam was recrystallized from methanol (4.64 g, 12.8
mmol, 71%): mp 112-115 °C; TLC (5% MeOH-CH2Cl2) Rf )
0.75 (CAS, blue).
Met h yl 2-Th ioxo-2,3,4,5,6,11-h exa h yd r o-1H -a zocin o-
[4,5-b]in d ole-1-ca r boxyla te (20). A solution of 4.47 g (12.3
mmol) of methyl 3-benzyl-1-hydroxymethyl-2-oxo-2,3,4,5,6,11-
hexahydro-1H-azocino[4,5-b]indole-1-carboxylate (18) and 6.49
g (16.0 mmol, 1.3 equiv) of Lawesson’s reagent in toluene was
heated for 12 h. Cooling to 25 °C, filtration through a plug of
silica gel, and concentration gave a yellow oil, which crystal-
lized on cooling in 10 mL of anhydrous methanol to give 3.54
This material was dissolved in 10 mL of acetic acid and
reduced by slow addition of 432 mg (8.3 mmol) of potassuim
borohydride. The solution was stirred for 10 min and then
poured into crushed ice and made basic with concentrated
ammonium hydroxide. The mixture was then extracted with
CH2Cl2. The organic phases were washed with brine, dried
(MgSO4), and concentrated in vacuo. Chromatography, eluting

5312 J . Org. Chem., Vol. 66, No. 16, 2001
Kuehne et al.
with ethyl acetate, gave the title compounds as an inseparable
mixture of diastereomers. The diols 33a ,b, resulting from acid
hydrolysis of the acetonide functionality, were also isolated.
For 32a , 32b: TLC (ethyl acetate) Rf ) 0.28 (CAS, purple-
tan).
equiv) of trimethylsilyl trifluoromethanesulfonate, and about
10 mg of DMAP. After 30 min, the mixture was poured into
75 mL of saturated sodium bicarbonate. Extraction with ether,
washing of the extract with saturated sodium bicarbonate and
brine, drying over magnesium sulfate, and concentration gave
1.86 g (91% yield) of the TMS ether.
A solution of 1.86 g (4.91 mmol) of (2S)-2-ethyl-1-(p-
tolylsulfonyloxy)-2-(trimethylsiloxy)hex-5-ene in CH2Cl2 was
cooled to -78 °C and ozone was bubbled through the stirred
solution until a blue tint persisted (∼15 min). Excess ozone
was removed by purging the reaction vessel with argon for 5
min. Then 1.14 g (5.4 mmol, 1.1 equiv) of triphenylphosphine
was introduced as a solution in CH2Cl2, and the mixture was
allowed to warm to 25 °C. After being stirred for 6 h, the
solution was concentrated and the residue was disolved in 1:1
Et2O-hexane. The triphenylphosphine oxide, which precipi-
tated, was filtered off and the filtrate was purified by flash
chromatography, eluting with 1:1 Et2O-hexane. The aldehyde
was obtained as 1.71 g (93%) of a colorless oil: TLC (1:1 Et2O-
hexane) Rf ) 0.35 (DNP).
(4a R,5R,12bS)- a n d (4a S,5S,12bR)-Meth yl 4-Ben zyl-
1,2,3,4,4a ,5,6,8-octa h yd r o-5-[(2S)-2-eth yl-2-[(tr im eth ylsi-
lyl)oxy]-3-[(p-tolylsu lfon yl)oxy]p r op yl]p yr id o[2,3-d ]ca r -
ba zole-7-ca r boxyla te (47a a n d 47b). In a reaction vessel
fitted with a Dean-Stark trap, 1.25 g (3.58 mmol) of N-benzyl
indoloazocine 22 and 1.42 g (3.83 mmol, 1.1 equiv) of TMS-
tosyl aldehyde 46 in 50 mL of toluene were heated at reflux
for 18 h. The solvent was removed in vacuo, and the resulting
foam was purified by flash chromatography, eluting with 2:1
Et2O-hexane, to give 2.46 g (97%) of the title compound as
an inseparable mixture: TLC (1:1 Et2O-hexane) Rf ) 0.29
(CAS, blue-green).
(6R,8S)- a n d (6S,8R)-Meth yl 4-Ben zyl-2,3,4,4a ,5,6,8,9-
octa h ydr o-6-[(2S)-2-eth yl-2,3-d ih yd r oxyp r op yl]-8-(15-vin -
d olin yl)-1H-a zecin o[5,4-b]in d ole-8-ca r boxyla te (33a a n d
33b). A mixture of 1.48 g (1.52 mmol) of the acetonide dimers
32a and 32b was dissolved in 50 mL of THF, and 30 mL of 1
N HCl was added. The solution was heated at reflux for 1 h,
cooled to 0 °C, and basified by addition of concentrated
ammonium hydroxide. The mixture was then extracted with
1:1 ethyl acetate-Et2O, and the organic extracts were washed
with brine, dried over MgSO4, and concentrated in vacuo. The
crude diols were purified by successive chromatography,
eluting first with ethyl acetate, and then with 5% MeOH-
CH2Cl2.
Data for the higher Rf isomer 33b : TLC (5% MeOH-
CH2Cl2) Rf ) 0.34 (CAS, purple). Data for lower Rf isomer
33a : TLC (5% MeOH-CH2Cl2) Rf ) 0.16 (CAS, tan).
(6R,8S)-Met h yl 4-Ben zyl-2,3,4,4a ,5,6,8,9-oct a h yd o-6-
[(2S)-2-eth yl-3-(p-tolylsu lfon yl)oxy-2-h yd r oxyp r op yl]-8-
(15-vin d olin yl)-1H -a zecin o[5,4-b]in d ole-8-ca r b oxyla t e
(34a ). A solution of 150 mg (0.161 mmol) of the more polar
diol 33a and 42 mL (1.5 equiv) of diisopropylethylamine in 15
mL of CH2Cl2was cooled to 0 °C, and then 80 mg (0.241 mmol,
1.5 equiv) of p-toluenesulfonic anhydride was added to the
stirred solution. After 24 h, TLC indicated that the starting
material had been consumed and a new, less polar, product
was present. Additionally, a small amount of a very polar
material (4° salt?) was evident. The mixture was washed with
10 mL of water and 10 mL of brine, dried (MgSO4), and
concentrated in vacuo. The crude product was purified by
radial chromatography on a 2 mm plate to give 110 mg (63%)
of the tosylate as a white foam: TLC (5% MeOH-CH2Cl2) Rf
) 0.33 (CAS, purple).
(6S,8R)-Met h yl 4-Ben zyl-2,3,4,4a ,5,6,8,9-oct a h yd o-6-
[(2S)-2-eth yl-3-(p-tolylsu lfon yl)oxy-2-h yd r oxyp r op yl]-8-
(15-vin d olin yl)-1H-a zecin o[5,4-b]in d ole (34b). A solution
of 328 mg (0.0.351 mmol) of the less polar diol dimer 33b was
subjected to the same conditions as described for the prepara-
tion of 34a . After chromatography, 250 mg (66%) of the
tosylate was obtained as a white foam: TLC (5% MeOH-
CH2Cl2) Rf ) 0.53 (CAS, purple).
5a ′-Hom ovin bla stin e (3a ). Tosylate 34a (25 mg, 0.023
mmol) was heated in toluene for 8 h, after which time TLC
indicated complete conversion to a new, very polar compound
(36a ), with Rf ) 0.02 (10%MeOH-CH2Cl2). The solvent was
removed by rotary evaporation, and the resulting white residue
was dissolved in 5 mL of MeOH and hydrogenated in the
presence of 5 mg of 10% palladium on carbon. After 3 h, a new
product Rf ) 0.72 (10%MeOH-CH2Cl2) had been formed.
Chromatography (SiO2, 5% MeOH-CH2Cl2), afforded 19 mg
(68%) of the title compound as the sole product: TLC (5%
MeOH-CH2Cl2) Rf ) 0.44 (CAS, light purple).
(6R,8S)- a n d (6S,8R)-Meth yl 4-Ben zyl-2,3,4,4a ,5,6,8,9-
oct a h yd r o-6-[(2S)-2-et h yl-2-[(t r im et h ylsilyl)oxy]-3-[(p -
tolylsu lfon yl)oxy]p r op yl]-8-(15-vin d olin yl)-1H-a zecin o-
[5,4-b]in d ole-8-ca r boxyla te (48a a n d 48b). Following the
procedure for formation of the binary compounds 32a ,b, 437
mg (0.622 mmol) of TMS-tosyl tetracycles 47a ,b was coupled
to 269 mg (0.591 mmol) of vindoline. Chromatography, eluting
with ethyl acetate, gave 361 mg (51%) of the title compounds
as an inseparable mixture of diastereomers, along with
another very polar material resulting from hydrolysis of the
TMS group: TLC (ethyl acetate) Rf ) 0.54 (CAS, purple).
(6R,8S)- a n d (6S,8R)-Meth yl 4-Ben zyl-2,3,4,4a ,5,6,8,9-
octa h yd r o-6-[(2S)-2-eth yl-2,3-ep oxyp r op yl]-8-(15-vin d oli-
n yl)-1H-azecin o[5,4-b]in dole-8-car boxylate (37a an d 37b).
A solution of 57 mg (0.0492 mmol) of the TMS-tosyl products
48a and 48b in 20 mL of THF was stirred at 25 °C, and 0.147
mL of TBAF (3 equiv, 1 M in THF) was added dropwise. After
25 min, TLC indicated complete conversion to a new, more
polar product. The reaction mixture was diluted with 20 mL
of Et2O and then washed with aqueous saturated sodium
bicarbonate, followed by brine. Drying and concentration gave
a yellow foam, which was purified by radial chromatography
on a 2 mm plate, eluting with ethyl acetate. The products were
obtained as 40 mg (90%) of the title epoxides as a mixture of
diastereomers: TLC (ethyl acetate) Rf ) 0.28 (CAS, purple).
[Nb-(S)-r-Meth lyben zyl](3-in d olyl)-3-p r op a n a m id e (50)
A stirred solution of 9.1 g (48.1 mmol) of indole-3-propionic
acid and 6.41 g (52.9 mmol, 1.1 equiv) of S-(-)-R-methylben-
zylamine in 250 mL of 5:1 CH3CN-CH2Cl2 was cooled to 0
°C. A solution of 10.9 g (52.9 mmol, 1.1 equiv) of dicyclohexyl-
carbodiimide in 40 mL of CH2Cl2was added via addition funnel.
After the addition was complete, the reaction was stirred at
25 °C for 3 h. The solution was filtered to remove dicyclohexy-
lurea and was then transferred to a separatory funnel and
washed successively with 50 mL portions of 1 N HCl, water,
1 N NaOH, and brine. The product mixture was dried (MgSO4)
and concentrated by rotary evaporation. The residue was then
taken up in Et2O. The product crystallized from solution as
8.5 g of white prisms. The mother liquor was concentrated and
chromatographed, eluting with 1:1:1 ethyl acetate-Et2O-
hexane to furnish an additional 1.3 g of product (70% overall
yield): mp 121-122 °C; TLC (3% MeOH-CH2Cl2) Rf ) 0.20
20′-epi-5a ′-Hom ovin cova lin e (3b). When a 40 mg sample
of tosylate 34b was subjected to the same procedure that
was applied to product 34a , above, 33 mg (73%) of 20′-epi-
5a′-homovincovaline 3b was obtained: TLC (10% MeOH-
CH2Cl2) Rf ) 0.29 (CAS, purple).
(4S)-4-Eth yl-5-(p-tolylsu lfon yloxy)-4-(tr im eth ylsilyloxy)-
p en ta n a l (46). A solution of 0.835 g (5.76 mmol) of (2S)-2-
ethyl-2-hydroxyhex-5-enol (40) and 0.965 mL of triethylamine
(6.92 mmol) was cooled to 0 °C, and 2.26 g (6.92 mmol, 1.2
equiv) of toluenesulfonic anhydride in 1 mL of dichloromethane
was added. After being stirred at room temperature for 12 h,
the solution was washed with water and brine, dried (MgS04),
and concentrated to furnish a yellow oil. The crude product
was purified by column chromatography, eluting with 1:1
Et2O-hexane, to furnish 1.50 g (83%) of the tosylate as a clear
colorless oil.
To a solution of 1.50 g (5.4 mmol) of the above alcohol
tosylate in 100 mL of dry THF, stirring at 0 °C under nitrogen,
was added 1.42 mL (1.5 equiv) of triethylamine, 1.79 g (1.5
(CAS, yellow orange); [R]25 -33.4 (c 10, MeOH).
D

Syntheses of 5a′-homo-Vinblastine and Congeners
J . Org. Chem., Vol. 66, No. 16, 2001 5313
3-(3-In d olyl)p r op a n a l. A solution of 14.1 g (82.7 mmol) of
3-(2-cyanoethyl)indole in 100 mL of CH2Cl2 was stirred vigor-
ously at -60 °C. Then 91 mL of DIBALH (1.0 M in CH2Cl2)
was added by addition funnel. After being stirred for 30 min
at -60 °C, the mixture was stirred an for an additional 1 h at
25 °C, cooled to 0 °C, and quenched by very slow addition of
20 mL of methanol, followed by 25 mL of saturated aqueous
sodium potassium tartrate. Vigorous stirring was continued
for 40 min (a gelatinous material may form which can be
dissolved by addition of 1:1 MeOH-CH2Cl2). The product
mixture was washed with water and brine, then dried and
concentrated. The resulting yellow oil was filtered through a
plug of silica gel, rinsing with Et2O, to give 10.45 g (73%) of
the aldehyde, which was used directly in the next step. Note:
the Rf of the product matches that of the starting material:
TLC (CH2Cl2) Rf ) 0.55 (CAS, orange).
additional 5 min, 1.22 g of lithium dimethyl malonate was
added as a solution in THF. The reaction mixture was allowed
to warm to room temperature and stirring was continued for
6 h. Methanol (2 mL) was added, followed by 20 mL of water,
and then the mixture was extracted with Et2O and the
combined organic extracts were washed with 50 mL of brine.
Solvent removal, followed by flash chromatography, eluting
with CH2Cl2, furnished 2.89 g (91%) of 53 as a colorless gum,
which was free of tert-butyl alcohol: TLC (1:1 Et2O-hexane)
Rf ) 0.28 (CAS, blue); [R]25 -50.6 (c 10, MeOH).
D
Dim eth yl 3-[3-[Nb-(S)-r-Meth ylben zyla m in o]p r op yl]in -
d ole-2-p r op a n ed ioa te (54). Anhydrous methanol, saturated
with HCl, was added to a stirred solution of t-BOC derivative
53 (2.85 g, 5.6 mmol) in 10 mL of THF. The clear, colorless
solution was heated to near reflux for 3 h, cooled to 23 °C and
concentrated by rotary evaporation to ∼3 mL. The residue was
basified (10% NH4OH) and extracted with Et2O, (3 × 30 mL).
The organic extracts were washed with brine and dried
(MgSO4). Concentration, followed by flash chromatography (5%
MeOH-CHCl3), gave 2.24 g (5.5 mmol, 98%) of the title
compound as a colorless oil: TLC (5% MeOH-CHCl3) Rf )
0.19 (CAS, blue).
3-[3-[(Nb-(S)-r-Meth ylben zyl)a m in o]p r op yl]in d ole (51).
Meth od A. From 10.45 g (60.4 mmol) of indole-3-propional-
dehyde, 60 mL of benzene, and 7.8 mL (60.4 mmol) of S-(-)-
R-methylbenzylamine the solvent was distilled until the
volume was reduced by half and the distillate was clear (20
min). The reaction mixture was cooled, and the remaining
solvent was removed in vacuo. The residue was taken up in
40 mL of anhydrous methanol and cooled to 0 °C, and sodium
borohydride (4.0 g, 110 mmol) was added in several portions
with vigorous stirring. After 30 min, 5 mL of concentrated HCl
was added dropwise. Concentration to ∼10 mL gave 8.93 g of
the amine hydrochloride as a white crystalline solid, which
was collected by suction filtration. The filtrate was basified
with 10% aqueous ammonium hydroxide and extracted with
CH2Cl2. Solvent removal provided an additional 2.3 g of the
title compound as the free amine. An analytical sample was
obtained by flash chromatography (SiO2, 5% MeOH in
CH2Cl2), followed by trituration of the concentrated eluate with
hexane.
(1R a n d 1S)-Meth yl 3-[(S)-r-Meth ylben zyl]-2-oxo-2,3,4,-
5,6,11-h e xa h yd r o-1H -a zocin o[4,5-b]in d ole -1-ca r b oxy-
la tes (55). A solution of dimethyl 3-[3-[(N-(S)-R-methylbenzyl)-
amino]propyl]indole-2-propanedioate (54, 2.3 g, 5.6 mmol) in
50 mL of toluene was heated at reflux for 12 h using a Dean-
Stark apparatus filled with 4 Å molecular sieves. The solvent
was removed in vacuo, and the crude products were chromato-
graphed on a 3 × 15 cm flash silica column, eluting with 1:1
ethyl acetate-hexane. The isolated yield of the products was
85% (1.78 g): TLC (1:1 ethyl acetate-hexane) Rf ) 0.33, 0.41-
(CAS, blue).
(1R a n d 1S)-Meth yl 3-[(S)-r-Meth ylben zyl]-2-th ioxo-
2,3,4,5,6,11-h exa h yd r o-1H-a zocin o[4,5-b]in d ole-1-ca r box-
yla te (56). A solution of 1.70 g (4.51 mmol) of chiral lactams
55 and 0.6 equiv of Lawesson’s reagent was heated in toluene
at 110 °C for 6 h. The cooled reaction mixture was diluted with
Et2O, washed with 10 mL of dilute acetic acid and 10 mL of
brine, and then dried and concentrated. The resulting yellow/
brown oil was chromatographed, eluting with 3:2 ether-
hexane (CHCl3 was used to facilitate solution) to furnish 1.13
g (64%) of the title compound as a mixture of C-1 epimers:
TLC (3:2 ether-hexane) Rf ) 0.40, 0.47 (CAS, yellow).
(1R a n d 1S)-Met h yl 3-[(S)-r-Met h ylben zyl]-2,3,4,5,6,-
11-h exa h yd r o-1H -a zocin o-[4,5b]-in d ole-1-ca r b oxyla t es
(49). Raney nickel (200 mg) was washed repeatedly with
anhydrous methanol and added to a stirred solution of 800
mg (2.22 mmol) of chiral thiolactam 56 in methanol. The
mixture was warmed for 3.5 h, cooled, filtered and concen-
trated to give a yellow solid, which was purified by flash
chromatography on a 3 × 12 cm silica gel column, eluting with
1% MeOH-CHCl3, to give 2.36 g (81%) of methyl 3-[(S)-R-
methylbenzyl]-4,5,6,11-tetrahydro-3H-azocino[4,5b]indole-1-
carboxylate. TLC (7:3 Et2O-hexane) Rf ) 0.49 (CAS, yellow).
To a stirred solution of 616 mg (1.32 mmol) of methyl 3-[(S)-
R-methylbenzyl]-4,5,6,11-tetrahydro-3H-azocino-[4,5b]-indole-
1-carboxylate in 5 mL of glacial acetic acid was added 200 mg
(3.31 mmol, 2.5 equiv) of sodium cyanoborohydride in several
portions over 10 min. The reaction mixture was allowed to stir
for an additional 15 min, then cooled to 0 °C, and basified with
10 mL of ice-cold concentrated ammonium hydroxide. The
mixture was extracted with Et2O, and then the extract was
washed with brine, dried (MgSO4), and filtered. The solvent
was removed in vacuo, and the resulting foam was purified
by flash chromatography, eluting with 7:3 Et2O-hexane, to
give 382 mg (62%) of the title compound as a white foam.
Addition of HCl to the reaction mixture prior to workup
resulted in significantly reduced yields: TLC (7:3 Et2O-
hexane) Rf ) 0.49, 0.40 (CAS, blue).
Meth od B. A solution of 9.47 g (32.4 mmol) of chiral indole-
3-propanamide 50 in 100 mL of dry THF was heated to reflux,
and 48.6 mL of borane-methyl sulfide solution (2.0 M in THF)
was added dropwise. After the addition was complete, the
reaction mixture was allowed to stir at reflux for 1 h. The
solvent and dimethyl sulfide were distilled until the volume
had been reduced by half. The reaction mixture was then
cooled to 0 °C, and 60 mL of HCl-saturated methanol was
slowly added with vigorous stirring. The acidified solution was
then heated to reflux for 30 min to decompose the boron-
amine complex. The solvent was removed in vacuo, and the
residue was dissolved in a minimum amount of methanol,
basified with concentrated ammonium hydroxide, and ex-
tracted with CH2Cl2. The combined organic extracts were
dried, concentrated, and chromatographed, eluting with 5%
MeOH-CH2Cl2. The product was obtained as 8.22 g (91%) of
a yellow oil, which solidified on standing, or on trituration with
hexane: mp 83 °C; TLC (5% MeOH-CH2Cl2) Rf ) 0.26 (CAS,
yellow).
3-[3-[(Nb-(S)-r-Meth ylben zyl-Nb-ter t-bu toxyca r bon yl)-
a m in o]p r op yl]in d ole (52). (S)-R-Methlybenzyl-homo-tryp-
tamine 51 (8.6 g, 27 mmol) was stirred in 100 mL of THF at
0 °C, and 80 mL of 1.0 N NaOH was added, followed by 7.2 g
(1.1 equiv) of di-tert-butyl dicarbonate. The mixture was stirred
for 1 h and then allowed to warm to 25 °C. Diethyl ether (50
mL) was added, and the organic phase was separated, washed
twice with 30 mL portions of brine, dried over MgSO4, and
concentrated in vacuo. The product was chromatographed (1:1
Et2O-hexane) to furnish 9.9 g (96%) of the title compound as
a colorless oil: TLC (CH2Cl2) Rf ) 0.56 (CAS, yellow); [R]25
-60.6 (c 10, MeOH).
D
Dim eth yl 3-[3-[N-(S)-r-Meth ylben zyl-N-ter t-bu toxyca r -
bon yla m in o]pr opyl]in d ole-2-pr op a n ed ioa te (53). Meth od
B. A solution of 2.37 g (5.75 mmol) of chiral benzyl BOC
homotryptamine 52 and 0.882 mL (6.33 mmol) of triethylamine
in 30 mL of THF was cooled to -60 °C. Dropwise addition of
0.717 mL (6.33 mmol) of tert-butyl hypochlorite and stirring
for 20 min resulted in complete conversion to the more polar
chloroindolenine (CAS, colorless). A solution of 1.3 mL of zinc
chloride (1.0 M in Et2O, 0.2 equiv) was added, and after an
(4S)-4-Met h yl-5-(p -t olylsu lfon yloxy)-4-(ter t-b u t yld i-
m et h ylsilyloxy)p en t a n a l (57). A solution of 8.14 g (62.6

5314 J . Org. Chem., Vol. 66, No. 16, 2001
Kuehne et al.
mmol) of distilled (2S)-2-methyl-2-hydroxyhex-5-enol (41)18 and
14 mL (10.0 mmol) of triethylamine in 50 mL of CH2Cl2 was
cooled to 0 °C, and 19.1 g (10.0 mmol) of p-toluenesulfonyl
chloride, as a solution in 15 mL of CH2Cl2, was added from an
addition funnel. After being stirred at 25 °C for 12 h, the
solution was washed with water and was then dried and
concentrated. Flash chromatography, eluting with Et2O, fur-
nished 7.35 g (86%) of 5-(S)-5-methyl-5-hydroxy-6-[(p-tolylsul-
fonyl)oxy]hexene: TLC (1:1 Et2O-hexane) Rf ) 0.3 (PMA).
A solution of 2.10 g (7.38 mmol) of the tosylate and 1.6 mL
(11.4 mmol) of triethylamine in 30 mL of CH2Cl2 was stirred
at 25 °C, and 2.54 mL (11.1 mmol) of tert-butyldimethylsilyl
triflate was added dropwise. After the addition was complete,
45 mg (0.37 mmol) of DMAP was added. The reaction was
allowed to proceed for 12 h, and the mixture was then washed
with saturated sodium bicarbonate (2 × 30 mL), followed by
brine. The organic solution was then dried over MgSO4 and
concentrated. The orange residue was purified by flash chro-
matography, eluting with 1:1 Et2O-hexane, to furnish 2.49 g
(87%) of the TBDMS tosylate: TLC (1:2 Et2O-hexane) Rf )
0.8 (PMA).
All of the preceding alkene was taken up in 30 mL of dry
CH2Cl2 and the solution cooled to -78 °C. Ozone was bubbled
through the stirred solution until a blue color was present.
The ozone flow was discontinued, and the solution was purged
with argon until the blue color faded. A solution of 2.0 g (7.53
mmol) of triphenylphosphine was added as a solution in 5 mL
of CH2Cl2, and the stirred reaction mixture was allowed to
warm to 25 °C. After 8 h the solution was then concentrated
to a yellow oil, from which triphenylphosphine oxide solidified.
The residue was triturated with hexane and the solid oxide
was filtered off. The filtrate was concentrated and purified by
flash chromatography to give 2.46 g (98%) of aldehyde: TLC
(1:1 Et2O-hexane) Rf ) 0.58 (PMA).
(4a R,5R,12bS)- a n d (4a S,5S,12bR)-Meth yl 4-[(S)-R-
Meth ylben zyl]-1,2,3,4,4a ,5,6,8-octa h yd r o-5-[(2S)-2-m eth -
yl-2-[(ter t-bu tyldim eth ylsilyl)oxy]-3-[(p-tolylsu lfon yl)oxy]-
p r op yl]p yr id o[2,3-d ]ca r ba zole-7-ca r boxyla te (58a a n d
58b). Under a Dean-Stark trap, 511 mg (1.41 mmol) of chiral
N-benzyl azocine 49 and 657 mg (1.64 mmol) of (4S)-4-methyl-
5-(p-tolylsulfonyloxy)-4-(tert-butyldimethylsilyloxy)pentanal (57)
in 40 mL of toluene were heated at reflux for 20 h. The solvent
was removed in vacuo and the resulting yellow foam was
purified by flash chromatography, eluting with 1:1 ethyl
acetate-hexane, to give 931 mg (89%) of tosyl TBDMS
tetracycles 58a and 58b: TLC (1:1 Et2O-hexane) Rf ) 0.29
(CAS, blue-green).
(6R,8S a n d 6S,8R)-Meth yl 4-[(S)-R-Meth ylben zyl]-2,3,4,-
4a ,5,6,8,9-oct a h yd r o-6-[(2S )-2-m e t h yl-2-[(t er t -b u t yld i-
m et h ylsilyl)oxy]-3-[(p -t olylsu lfon yl)oxy]p r op yl]-8-(15-
vin d olin yl)-1H -a zecin o[5,4-b]in d ole-8-ca r b oxyla t e (59a
a n d 59b). Following the above procedure for formation of
compounds 32a ,b, 803 mg (1.08 mmol) of tetracycles 58a ,b
was coupled to 467 mg (1.03 mmol) of vindoline to provide 301
mg (23%) of the higher Rf (minor) isomer 59b and 505 mg
(39%) of the lower Rf isomer 59a .
(6S,8R)-Meth yl 4-[(S)-R-Meth ylben zyl]-2,3,4,4a ,5,6,8,9-
oct a h yd r o-6-[(2S)-2-m et h yl-2,3-ep oxyp r op yl]-8-(15-vin -
d olin yl)-1H-a zecin o[5,4-b]in d ole-8-ca r boxyla te (60b). A
solution of 80 mg (0.0667 mmol) of TBDMS toslyate 59b was
subjected to the conditions described for the epoxidation of 59a .
The product was purified by flash chromatography, eluting
with ethyl acetate, to give 54 mg (89%) of epoxide 60b: TLC
(ethyl acetate) Rf ) 0.27 (CAS, purple).
20′-Deseth yl-20-m eth yl-5a ′-h om ovin bla stin e (62a ). A
solution of 25 mg (0.027 mmol) of epoxide 60a was heated in
5 mL of methanol for 24 h. The solution was cooled to 25 °C
and ∼5 mg of 10% palladium on carbon was added, then the
stirred mixture was purged with hydrogen. After 2 h, TLC
indicated complete debenzylation of the quaternary salt. The
solution was filtered, concentrated and purified by flash
chromatography to give 17 mg (81%) of 62a : TLC (ethyl
acetate) Rf ) 0.32 (CAS, purple-tan).
Qu a r ter n a r y Sa lt (63a ). A 50 mg portion of (6R,8S)-methyl
4-[(S)-R-methylbenzyl]-2,3,4,4a,5,6,8,9-octahydro-6-[(2S)-2-
methyl-2-[(tert-butyldimethylsilyl)oxy]-3-[(p-tolylsulfonyl)oxy]-
propyl]-8-(15-vindolinyl)-1H-azecino[5,4-b]indole-8-carboxy-
late (59a ) was dissolved in 10 mL of benzene and heated at
150 °C in a sealed tube, under an argon atmosphere, for 24 h.
After cooling to 25 °C, the solvent was removed by rotary
evaporation. The crude product was taken up in THF and
treated with 125 mL (0.042 mmol, 3 equiv) of TBAF. The
reaction mixture was stirred at 25 °C for 4 h and then
concentrated. This material was disolved in ∼2 mL of metha-
nol and applied to a 8 × 1 cm column of Amberlite IRA 400
(chloride) ion-exchange resin, and eluted with methanol. CD
data: λmax 256, 316 nm, q ) +31, 18.
Qu a ter n a r y Sa lt (61a ). A 45 mg (0.049 mmol) sample of
(6R,8S)-methyl 4-[(S)-R-methylbenzyl]-2,3,4,4a,5,6,8,9-octahy-
dro-6-[(2S)-2-methyl-2,3-epoxypropyl]-8-(15-vindolinyl)-1H-
azecino[5,4-b]indole-8-carboxylate (60a ) was heated at reflux
in 10 mL of methanol for 18 h. The solution was concentrated
to ∼2 mL and passed through an 8 × 1 cm column of Amberlite
IRA 400 (chloride) ion-exchange resin. Solvent removal af-
forded the “unnatural” quaternary salt as a white film. CD
data: λmax 261, 300 nm, q ) +39, -8.
cis-(2-(2,5-Dith iola n ylm eth yl)cycloh exyl)m eth yl (p-
Nitr op h en oxy)for m a te. A solution of cis-(2-(2,5-dithiolanyl-
methyl)cyclohexyl)methan-1-ol (1.0 g, 4.3 mmol)15 and p-
nitrophenyl chlorocarbonate (984 mg, 4.73 mmol) in 20 mL of
pyridine was stirred at room temperature for 24 h. The solvent
was evaporated under reduced pressure. The residue was
dissolved in ether and washed with water and brine. The
residue, obtained on concentration, was purified on a silica
gel column, eluting with ether-hexane (3:7), to afford 1.7 g of
product as a colorless oil (99% yield): TLC Rf ) 0.29 (Et2O/
Hex, 3:7).
cis-2-(2-Oxoeth yl)cycloh exyl)m eth yl (p-Nitr oph en oxy)-
for m a te (73). To a solution of HgO (217 mg, 1.0 mmol) and
HBF4 (48%, 393 µL, 2.15 mmol) in 4 mL of THF and 1 mL of
water, at room temperature, was added dropwise cis-(2-(2,5-
dithiolanylmethyl)cyclohexyl)methyl (p-nitrophenoxy)formate
(200 mg, 0.5 mmol) in 2 mL of THF. The solution was stirred
for an additional 5 min, dichloromethane was added, and the
mixture was filtered through a Celite pad. The filtrate was
washed with 10% NaI and saturated NaHCO3. Chromatogra-
phy on a silica gel column, eluting with ether-hexane (2:3),
gave 140 mg of product as an oil (87% yield): TLC Rf ) 0.31
(Et2O/Hex, 2:3).
(4a R,S,5R,S,12bS,R)-Meth yl 4-Ben zyl-1,2,3,4,4a ,5,6,8-
octa h yd r o-5-((1′S,R, 2′S,R, a n d 1′R,S,2′R,S)-2-((1-im id a -
zole)ca r bon yloxym eth yl)cycloh exyl)p yr id o[2,3-d ]ca r ba -
zole-7-ca r boxyla te (71, 72). A solution of indoloazocine 22
(100 mg, 0.287 mmol) and cis-(2-(2-oxoethyl)cyclohexyl)methyl
(p-nitrophenoxy)formate (73, 111 mg, 0.344 mmol) in 5 mL of
dry toluene was heated at reflux for 5 h. The solvent was
removed under reduced pressure and the residue taken into
4 mL of THF and 1 mL of water. Imidazole (40 mg, 0.574
mmol) was added at room temperature, and the solution was
stirred for an additional 2 h. The product was extracted with
dichloromethane. The combined organic phase was washed
Lower Rf isomer 59a : TLC (3% MeOH-CHCl3) Rf ) 0.23
(CAS, purple).
Higher Rf isomer: 59a : TLC (3% MeOH-CHCl3) Rf ) 0.43
(CAS).
(6R,8S)-Meth yl 4-[(S)-R-Meth ylben zyl]-2,3,4,4a ,5,6,8,9-
oct a h yd r o-6-[(2S)-2-m et h yl-2,3-ep oxyp r op yl]-8-(15-vin -
d olin yl)-1H-a zecin o[5,4-b]in d ole-8-ca r boxyla te (60a ). A
solution of 185 mg (0.154 mmol) of tosyl TBDMS dimer 59a
in 10 mL of THF was stirred at 25 °C and 478 mL of TBAF
(1.0 M in THF), was added dropwise. The reaction was allowed
to proceed for 4 h and was then diluted with 10 mL of Et2O
and washed with 10 mL of saturated sodium bicarbonate. The
organic phase was dried and concentrated to give a white solid
that was purified by radial chromatography, eluting with ethyl
acetate. The product was obtained as 103 mg (74%) of a white
foam: TLC (ethyl acetate) Rf ) 0.25 (CAS, brown).
(18) Leboulluec, K. L. Ph.D. Thesis, University of Vermont, 1990.
Manuscript in preparation.

Syntheses of 5a′-homo-Vinblastine and Congeners
J . Org. Chem., Vol. 66, No. 16, 2001 5315
with saturated sodium bicarbonate and brine. The residue was
chromatographed on a silica gel column, eluting with EtOAc-
Hex (3:7), to give 55 mg of tetracycle 72 (33% yield) and 60
mg of tetracycle 71 (36% yield) as a white foam.
was removed under reduced pressure, and the residue (79a ,
80a ) was hydrogenolyzed in 10 mL of MeOH in the presence
of 7.7 mg of 10% Pd-C for 3 h. Filtration through Celite and
concentration gave the crude products, which were dissolved
in dichloromethane and shaken with 10% Na2CO3. The aque-
ous layer was extracted with dichloromethane. The residue
was chromatographed on a silica gel column, eluting with
EtOAc-MeOH-Et3N (90:10:1), followed by MeOH-Et3N (9:
1), to give 30 mg of “natural” conformation product 65a (49%
yield) and 25 mg of its atropisomer 81a (39% yield) as a white
solid.
For 72: TLC Rf ) 0.14 (EtOAc/Hex, 3:7; CAS, green).
For 71: TLC Rf ) 0.08 (EtOAc/Hex, 3:7; CAS, green).
(4a R,S,5R,S,12bS,R)-Meth yl 4-Ben zyl-1,2,3,4,4a ,5,6,8-
oct a h yd r o-5-((1′S,R,2′S,R, a n d 1′R,S,2′R,S)-2-(h yd r oxy-
m et h yl)cycloh exyl)p yr id o[2,3-d ]ca r b a zole-7-ca r b oxy-
la te (74, 75). Method a. A solution of indoloazocine 22 (500
mg, 1.435 mmol) and cis-(2-(2-oxoethyl)cyclohexyl)methyl (p-
nitrophenoxy)formate (73, 553 mg, 1.72 mmol) in 20 mL of
dry toluene was heated at reflux for 5 h. The solvent was
removed under reduced pressure, and the residue was taken
into 20 mL of THF and 5 mL of 2 N NaOH. The solution was
stirred at room temperature for 40 min and then extracted
with dichloromethane. The residue, obtained on concentration,
was chromatographed on a silica gel column (EtOAc-Hex, 35:
65) to afford 210 mg of tetracycle 75 (30% yield) and 220 mg
of tetracycle 74 (32% yield) as a white foam.
For 75: TLC Rf ) 0.33 (EtOAc/Hex, 35:65; CAS, green).
For 74: TLC Rf ) 0.21 (EtOAc/Hex, 35:65; CAS, green).
Meth od b. A solution of imidazole derivative 72 (60 mg,
0.103 mmol) in 3 mL of THF and 0.5 mL of 2 N NaOH was
stirred at room temperature for 40 min. The product was
extracted with dichloromethane. The residue, obtained on
concentration, was chromatographed on a silica gel column
(EtOAc-Hex, 1:1) to afford 49 mg of product 75 as a white
foam (98% yield).
(4a R,S,5R,S,12bS,R)-Meth yl 4-Ben zyl-1,2,3,4,4a ,5,6,8-
oct a h yd r o-5-((1′S,R,2′S,R)-2-(t osyloxym et h yl)cycloh ex-
yl)p yr id o[2,3-d ]ca r ba zole-7-ca r boxyla te (76). To a solution
of the tetracyclic alcohol 74 (160 mg, 0.329 mmol), DMAP (4
mg, 0.033 mmol), and Et3N (91 µL, 0.658 mmol) in 15 mL of
dichloromethane, at 0 °C, was added Ts2O (172 mg, 0.526
mmol) in one portion. The solution was stirred at 0 °C for 1.5
h, and then the solvent was evaporated in a vacuum at 15 °C.
The residue was chromatographed on a silica gel column
(Et2O-Hex, 2:3) to afford 150 mg of the title product as a white
foam (71% yield): TLC Rf ) 0.20 (Et2O/Hex, 2:3; CAS, blue/
green).
For 65a : TLC Rf ) 0.95 (CH2Cl2/MeOH, 98:2, SiO2 plate
deactivated with Et3N; CAS, light blue/gray).
For 81a : TLC Rf ) 0.5 (CH2Cl2/MeOH, 9:1, SiO2 plate
deactivated with Et3N; CAS, light blue/gray).
(3R,18R,23S)-Meth yl 5,16-Dia za p en ta cyclo[14.7.1.04,12
.
06,11.018,23]tetr a cosa -4(12),6(11),7,9-tetr a en e-3-vin d olin yl-
3-ca r boxyla te (65b) a n d Its High er En er gy Atr op isom er
(81b). A solution of 70 mg of tosylate 78b (0.064 mmol) in 20
mL of dry toluene was heated at reflux for 4 h. The solvent
was removed under reduced pressure, and the residual qua-
ternary salts were hyrogenolyzed for 3 h in 10 mL of MeOH
in the presence of 6.8 mg of 10% Pd-C. Filtration through
Celite and concentration gave the crude ammonium salts,
which were dissolved in dichloromethane and shaken with 10%
Na2CO3. The aqueous layer was extracted with dichlo-
romethane. The residue was chromatographed on a silica gel
column, eluting with EtOAc-MeOH-Et3N (90:10:1), followed
by MeOH-Et3N (9:1), to give 30 mg of the lower energy
conformer 65b (52% yield) and 18 mg of its higher energy
atropisomer 81b (37% yield) as white solids.
For 65b: TLC Rf ) 0.96 (CH2Cl2/MeOH, 9:1, SiO2 plate
deactivated with Et3N; CAS light blue/grey).
For 81b: TLC Rf ) 0.34 (CH2Cl2/MeOH, 9:1, SiO2 plate
deactivated with Et3N; CAS, light blue/gray).
(4a R,S,5R,S,12bS,R)-Meth yl 4-Ben zyl-1,2,3,4,4a ,5,6,8-
oct a h yd r o-5-((1′R,S,2′R,S)-2-(t osyloxym et h yl)cycloh ex-
yl)p yr id o[2,3-d ]ca r ba zole-7-ca r boxyla te (77). To a solution
of alcohol 75 (190 mg, 0.39 mmol), DMAP (5 mg, 0.041 mmol),
and Et3N (108 µL, 0.78 mmol) in 15 mL of dichloromethane
at 0 °C was added Ts2O (204 mg, 0.62 mmol) in one portion.
The solution was stirred at 0 °C for 1.5 h. The solvent was
evaporated in a vacuum at 15 °C, and the residue was
chromatographed on a silica gel column (CH2Cl2-EtOAc-Hex,
1:1:8) to afford 180 mg of the tosylate 77 as a white foam (72%
yield): TLC Rf ) 0.19 (EtOAc/CH2Cl2/Hex, 1:1:8, CAS, blue/
green).
(6R,8S)-Meth yl 4-Ben zyl-2,3,4,4a ,5,6,8,9-octa h yd r o-6-
(1′S,2′S)-2-(t osyloxym et h yl)cycloh exyl]-8-(15-vin d olin -
yl)-1H-a zecin o[5,4-b]in d ole-8-ca r boxyla te (78a ) a n d Its
(6S,8R,1′R,2′R)-Dia ster eom er (78b). To a solution of tetra-
cyclic tosylate 76 (70 mg, 0.11 mmol) and Et3N (19 µL, 0.132
mmol) in 6 mL of dichloromethane, at 0 °C, was added
dropwise t-BuOCl (16 µL, 0.132 mmol). After being stirred for
5 min at 0 °C, the mixture was partioned between water and
dichloromethane. The dried organic phase was concentrated
under vacuum at 20 °C. The residue was dissolved in 6 mL of
dry acetone and cooled to 0 °C. Vindoline free base (48 mg,
0.104 mmol) was added, followed by dropwise addition of
HBF4‚Et2O complex (42 µL, 0.219 mmol). The solution was
stirred for 5 min. AgBF4 (43 mg, 0.219 mmol) was added in
one portion. After the solution was stirred in the dark for 15
min, 10% NH4OH in saturated brine was added and the
solution was extracted several times with dichloromethane.
The dried organic phase was concentrated under vacuum. The
residue was dissolved in 4.5 mL of HOAc, and KBH4 (59 mg,
1.1 mmol) was added at 20 °C in several portions. After 10
min of further stirring, the reaction was quenched by adding
water-ice and then basified to pH ∼10 with concentrated
NH4OH, while the flask was well cooled in an ice bath. The
aqueous phase was extracted with dichloromethane. The
residue, obtained on concentration, was chromatographed on
a silica gel column, eluting with EtOAc-Hex (4:1), to obtain
40 mg of 74a (Rf ) 0.5, EtOAc/Hex, 4:1; CAS blue/green; 35%
yield) and 35 mg of 74b (Rf )0.3, EtOAc/Hex, 4:1; CAS, blue/
green; 31% yield) as a white solid.
(3R,18S,23R)-Meth yl 5,16-Dia za p en ta cyclo[14.7.1.04,12
.
06,11.018,23]tetr a cosa -4(12),6(11),7,9-tetr a en e-3-vin d olin yl-
3-ca r boxyla te (83a ) a n d Its (3S,18R,23S)-Dia ster eom er
(83b). To a solution of tosylate 77 (65 mg, 0.086 mmol) and
Et3N (15 µL, 0.103 mmol) in 5 mL of dichloromethane, at 0
°C, was added t-BuOCl (13 µL, 0.103 mmol) dropwise. After
being stirred for 5 min at 0 °C, the mixture was partioned
between water and dichloromethane. The dried organic phase
was concentrated under vacuum at 20 °C and the residue was
dissolved in 6 mL of dry acetone, and cooled to 0 °C. Vindoline
free base (37 mg, 0.081 mmol) was added, followed by dropwise
addition of HBF4‚Et2O complex (30 µL, 0.081 mmol). The
solution was stirred for 5 min, and AgBF4 (39 mg, 0.172 mmol)
was added in one portion. After stirring in the dark for 15 min,
10% NH4OH in saturated brine was added, and the solution
was extracted with dichloromethane several times.The dried
organic phase was concentrated under vacuum. The residue
was dissolved in 4 mL of HOAc, and KBH4 (47 mg, 0.86 mmol)
was added at 20 °C in several portions. After 10 min of further
stirring, the reaction was quenched by adding water-ice, and
the mixture was basified to pH ∼10 with concentrated
NH4OH, while the flask was well cooled in an ice bath. The
aqueous phase was extracted with dichloromethane. The
residue (82a , 82b), obtained on concentration, was dissolved
in 40 mL of dry toluene and the solution was heated at 50 °C
for 4 h. Concentration and chromatography (CH2Cl2-MeOH,
9:1) gave 55 mg of the white, solid quaternary salts, which
were hyrogenolysed in 20 mL of MeOH, in the presences of 4
(3S,18S,23R)-Meth yl 5,16-Dia za p en ta cyclo[14.7.1.04,12
.
06,11.018,23]tetr a cosa -4(12),6(11),7,9-tetr a en e-3-vin d olin yl-
3-ca r boxyla te (65a ) a n d Its High er En er gy Atr op isom er
(81a ). A solution of 80 mg of the tosylate 78a (0.073 mmol) in
20 mL of dry toluene was heated at reflux for 4 h. The solvent

5316 J . Org. Chem., Vol. 66, No. 16, 2001
Kuehne et al.
mg of 10% Pd-C, for 3 h. Filtration through Celite and
concentration gave the crude products, which were dissolved
in dichloromethane and shaken with 10% Na2CO3. The aque-
ous layer was extracted with dichloromethane. The residue
was chromatographed on a silica gel column, eluting with
CH2Cl2-Hex-Et3N (100:100:1), to give 18 mg of natural
conformation product 83a (27% yield) and 16 mg of its
diasteromer 83b (24% yield) as white solids.
protocols previously used in our laboratories with vinblastine
congeners.19,20
Ack n ow led gm en t. We thank Mr. Robert Bennett
of our group for low-resolution mass spectra and the
Emory University Mass Spectrometry Center and Dr.
Michael Gross of the Washington University Mass
Spectrometry Research Resource for high-resolution
mass spectra. These studies were supported by the
National Cancer Institute through NIH Grant No. CA
RO1 12010.
For 83a : TLC Rf ) 0.19 (CH2Cl2/Hex, 1:1, SiO2 plate
deactivated with Et3N; CAS, light blue/grey).
For 83b: TLC Rf ) 0.12 (CH2Cl2/Hex, 1:1, SiO2 plate
deactivated with Et3N; CAS, light blue/grey).
Su p p or tin g In for m a tion Ava ila ble: A complete Experi-
mental Section containing UV, IR, 1H and 13C NMR, MS data,
and elemental analyses/HRMS molecular ion values is pro-
vided. Also, full NMR spectra, 1H and 13C, except where
marked by (), for compounds 3a ,b, 8, 13, 14, 16-23, 27a ,b,
(28b), (30), (33a ), (33b), (34b), (37a and 37b), (39), 42, 44a ,b,
(46), (47a + b), (48a + b), 49, 50-55, 56 u p p er , 56 low er ,
(57), (58a + b), 59a ,b, 60a , (60b), 62a , (69a + b), 69b, (70a
+ b), 71, (72), 73, 74, (75), (76), 77, 83a ,b, and NMR spectra
for unnumbered intermediates indicated in the Experimental
Section, are reproduced. This material is available free of
charge via the Internet at http://pubs.acs.org.
Atr op isom er ic In ver sion s. Rates of conformational inver-
sions of the atropisomers 81a and 81b to the corresponding
binary indole-indolines 65a and 65b were measured by heating
1 mg samples in 2.5 mL of toluene at 110 °C (k ) 2.73 ×
10-4s-1), 100 °C (k ) 9.82 × 10-5s-1), 91 °C (k ) 4.62 × 10-5s-1
and at 101 °C (k ) 4.88 × 10-4s-1), 90 °C (k ) 1.52 × 10-4s-1
)
)
and 75 °C (k ) 4.18 × 10-5s-1), respectively, taking 10-18
conversion points. The progress of the conformational inversion
was followed by HPLC on a Rainin Rabbit-HP instrument with
an Si column, eluted with CH2Cl2/MeOH/Et3N, 85:15:3, at 580
psi and 300 mL/min (refill 8, comp rate 4 settings). The relative
retention times were 65a : 2.27 min, 81a : 8.44 min, 65b: 2.29
min, 81b: 11.11 min. Thermodynamic constants were calcu-
lated from ∆Gq ) RT(23.76 + ln T - ln k)/1000; ∆Gq ) ∆Hq -
T∆Sq.
J O000249Z
(19) Borman, L. S.; Kuehne, M. E. Biochem. Pharmacol. 1989, 38,
715.
(20) Borman, L. S.; Bornmann, W. G.; Kuehne, M. E. Cancer
Chemotherapy Pharmacol. 1993, 31, 343.
Biologica l Da ta . Tubulin polymerization inhibition and
L-1210 or S-180 cytotoxicities were determined according to