Total synthesis of (-)- and ent-(+)-vindoline

Article abstract of DOI:10.1021/ol051975x

(Chemical Equation Presented) Two exceptionally concise total syntheses of (-)- and ent-(+)-vindoline are detailed enlisting a diastereoselective tandem [4 + 2]/[3 + 2] cycloaddition of a 1,3,4-oxadiazole. The unique reaction cascade assembles the fully functionalized pentacyclic ring system of vindoline in a single step that forms four C-C bonds and three rings while introducing all requisite functionality and setting all six stereocenters within the central ring including three contiguous and four total quaternary centers.

Full text of DOI:10.1021/ol051975x

ORGANIC
LETTERS
2005
Vol. 7, No. 20
4539-4542
Total Synthesis of (
ent-( )-Vindoline
−)- and
+
Younggi Choi, Hayato Ishikawa, Juraj Velcicky, Gregory I. Elliott,
Michael M. Miller, and Dale L. Boger*
Department of Chemistry and Skaggs Institute for Chemical Biology, The Scripps
Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037
boger@scripps.edu
Received August 15, 2005
ABSTRACT
Two exceptionally concise total syntheses of (
cycloaddition of a 1,3,4-oxadiazole. The unique reaction cascade assembles the fully functionalized pentacyclic ring system of vindoline in a
single step that forms four C C bonds and three rings while introducing all requisite functionality and setting all six stereocenters within the
−)- and ent-(+)-vindoline are detailed enlisting a diastereoselective tandem [4 + 2]/[3 + 2]
−
central ring including three contiguous and four total quaternary centers.
Vinblastine (1) and vincristine (2) constitute the most widely
recognized members of the class of bisindole alkaloids as a
result of their clinical use as antineoplastic drugs (Figure
1).1 Originally isolated in trace quantities from Cantharan-
thus roseus (L.) G. Don,2 their biological properties were
among the first to be shown to arise from inhibition of
microtubule formation and mitosis, which today is still
regarded as one of the more successful drug targets for the
treatment of cancer.3 In addition to being among the first
natural products whose structures were determined by X-ray
crystallography of a derivative, they were among the first
for which X-ray was used to establish their absolute
configuration.4 Vindoline (3),4,5 a major alkaloid of Cantha-
ranthus roseus, constitutes the most complex half of vin-
blastine and serves as both a biosynthetic2 and synthetic6
precursor to the natural product.7
Herein, we detail two first generation total syntheses of
vindoline that complement past efforts,7-9 enlisting a unique
tandem intramolecular [4 + 2]/[3 + 2] cycloaddition cascade
of 1,3,4-oxadiazoles.10 In these studies, key issues regarding
the scope and stereochemical features of the cycloaddition
cascade were defined and its potential for use in the
construction of the vindoline structure was established. Thus,
two concise routes to 3 are detailed enlisting the tandem
(1) Neuss, N.; Neuss, M. N. In The Alkaloids; Brossi, A., Suffness, M.,
Eds.; Academic: San Diego, 1990; Vol. 37, p 229.
(2) (a) Noble, R. L.; Beer, C. T.; Cutts, J. H. Ann. N.Y. Acad. Sci. 1958,
76, 882. (b) Noble, R. L. Lloydia 1964, 27, 280. (c) Svoboda, G. H.; Nuess,
N.; Gorman, M. J. Am. Pharm. Assoc. Sci. Ed. 1959, 48, 659.
(3) (a) Owellen, R. I.; Hartke, C. A.; Dickerson, R. M.; Haines, F. O.
Cancer Res. 1976, 36, 1499. (b) Pearce, H. L. In The Alkaloids; Brossi, A.,
Suffness, M., Eds.; Academic: San Diego, 1990; Vol. 37, p 145.
(4) Moncrief, J. W.; Lipscomb, W. N. J. Am. Chem. Soc. 1965, 87, 4963.
(5) Gorman, M.; Neuss, N.; Biemann, K. J. Am. Chem. Soc. 1962, 84,
1058.
Figure 1. Natural product structures.
10.1021/ol051975x CCC: $30.25
© 2005 American Chemical Society
Published on Web 08/27/2005

cycloaddition reactions of either 1,3,4-oxadiazole 4a or 4b,
each of which assembles the fully functionalized pentacyclic
ring system of 3 with formation of four C-C bonds and
three rings in a single step, setting all six stereocenters within
the central ring of vindoline including three contiguous and
four total quaternary centers (Figure 2).
Scheme 1
and 5b, respectively, as single diastereomers (Figure 2). The
reaction leading to 5 is initiated by an intramolecular inverse
electron demand Diels-Alder cycloaddition of the 1,3,4-
oxadiazole with the tethered enol ether. Loss of N2 from the
initial cycloadduct A provides the carbonyl ylide B (eq 1,
Table 1), which undergoes a subsequent established13 1,3-
dipolar cycloaddition with the tethered indole. Importantly,
the diene and dienophile substituents complement and
reinforce the [4 + 2] cycloaddition regioselectivity dictated
by the linking tether, the intermediate 1,3-dipole is stabilized
by the complementary substitution at the dipole termini, and
the tethered 1,3-dipolarophile (indole) complements the [3
+ 2] cycloaddition regioselectivity that is dictated by the
linker tether. The relative stereochemistry in each cycload-
duct is controlled by a combination of (1) the dienophile
geometry and (2) an exclusive endo indole [3 + 2] cycload-
dition13 sterically directed to the R-face opposite the newly
formed fused lactam. This endo diastereoselection for the
1,3-dipolar cycloaddition may be attributed to a conforma-
tional (strain) preference dictated by the dipolarophile tether
since it mirrors the relative energy of the four possible
Figure 2. Key cycloaddition cascade.
(8) Racemic total syntheses: (a) Ando, M.; Bu¨chi, G.; Ohnuma, T. J.
Am. Chem. Soc. 1975, 97, 6880. (b) Kutney, J. P.; Bunzli-Trepp, U.; Chan,
K. K.; De Souza, J. P.; Fujise, Y.; Honda, T.; Katsube, J.; Klein, F. K.;
Leutwiler, A.; Morehead, S.; Rohr, M.; Worth, B. R. J. Am. Chem. Soc.
1978, 100, 4220. (c) Andriamialisoa, R. Z.; Langlois, N.; Langlois, Y. J.
Org. Chem. 1985, 50, 961. Formal racemic total syntheses: (d) Ban, Y.;
Sekine, Y.; Oishi, T. Tetrahedron Lett. 1978, 2, 151. (e) Takano, S.;
Shishido, K.; Sato, M.; Yuta, K.; Ogasawara, K. J. Chem. Soc., Chem.
Commun. 1978, 943. Takano, S.; Shishido, K.; Matsuzaka, J.; Sato, M.;
Ogasawara, K. Heterocycles 1979, 13, 307. (f) Danieli, B.; Lesma, G.;
Palmisano, G.; Riva, R. J. Chem. Soc., Chem. Commun. 1984, 909. Danieli,
B.; Lesma, G.; Palmisano, G.; Riva, R. J. Chem. Soc., Perkin Trans. 1
1987, 155. (g) Zhou, S.; Bommeziijn, S.; Murphy, J. A. Org. Lett. 2002, 4,
443.
(9) Enantioselective total syntheses: (a) Feldman, P. L.; Rapoport, H.
J. Am. Chem. Soc. 1987, 109, 1603. (b) Kuehne, M. E.; Podhorez, D. E.;
Mulamba, T.; Bornmann, W. G. J. Org. Chem. 1987, 52, 347. (c) Kobayashi,
S.; Ueda, T.; Fukuyama, T. Synlett 2000, 883. Formal enantioselective total
syntheses: Cardwell, K.; Hewitt, B.; Ladlow, M.; Magnus, P. J. Am. Chem.
Soc. 1988, 110, 2242.
The preparation of the 1,3,4-oxadiazole precursors 4 is
summarized in Scheme 1. Treatment of N-methyl-6-meth-
oxytryptamine (6)11 with phenyl carbonate followed by
hydrazine provided 7 (64%), which was coupled with methyl
oxalate (EDCI, DMAP, CH2Cl2, 23 °C, 61%) to provide 9
(see Supporting Information). Alternatively, 6 was first
treated with carbonyldiimidazole (CDI) to form 8 (90%) and
subsequently treated with methyl oxalylhydrazide12 to furnish
9 (79%). Cyclization via dehydration of 9 (1.0 equiv of TsCl,
2.5 equiv of Et3N, CH2Cl2, 67%) provided 10, which was
coupled with isomerically pure (Z)- or (E)-11 to provide 4a
(96%) and 4b (88%).
Both 4a and 4b undergo the tandem [4 + 2]/[3 + 2]
cycloaddition cascade to give the pentacyclic products 5a
(10) (a) Wilkie, G. D.; Elliott, G. I.; Blagg, B. S. J.; Wolkenberg, S.;
Soenen, D. R.; Miller, M. M.; Pollack, S.; Boger, D. L. J. Am. Chem. Soc.
2002, 124, 11292. (b) Yuan, Z. Q.; Ishikawa, H.; Boger, D. L. Org. Lett.
2005, 7, 741.
(11) Takano, S.; Shishido, K.; Matsuzaka, J.; Sato, M.; Ogasawara, K.
Heterocycles 1979, 13, 307. An improved preparation is detailed in
Supporting Information.
(12) Christl, M.; Lanzendoerfer, U.; Groetsch, M. M.; Ditterich, E.;
Hergmann, J. Chem. Ber. 1990, 123, 2031.
(13) Padwa, A.; Price, A. T. J. Org. Chem. 1998, 63, 556; 1995, 60,
6258. Interestingly, intermolecular [3 + 2] variants proceed with indole
exo cycloaddition directed to an analogous R-face, see: Muthusamy, S.;
Gunanathan, C.; Babu, S. A. Tetrahedron Lett. 2001, 42, 523.
(6) (a) Mangeney, P.; Andriamialisoa, R. Z.; Langlois, N.; Langlois, Y.;
Potier, P. J. Am. Chem. Soc. 1979, 101, 2243. (b) Kutney, J. P.; Choi, L.
S. L.; Nakano, J.; Tsukamoto, H.; McHugh, M.; Boulet, C. A. Heterocycles
1988, 27, 1845. (c) Kuehne, M. E.; Matson, P. A.; Bornmann, W. G. J.
Org. Chem. 1991, 56, 513. (d) Magnus, P.; Mendoza, J. S.; Stamford, A.;
Ladlow, M.; Willis, P. J. Am. Chem. Soc. 1992, 114, 10232. (e) Yokoshima,
S.; Ueda, T.; Kobayashi, S.; Sato, A.; Kuboyama, T.; Tokuyama, H.;
Fukuyama, T. J. Am. Chem. Soc. 2002, 124, 2137. Kuboyama, T.;
Yokoshima, S.; Tokuyama, H.; Fukuyama, T. Proc. Natl. Acad. Sci. U.S.A.
2004, 101, 11966.
(7) Review: Kuehne, M. E.; Marko, I. In The Alkaloids; Brossi, A.,
Suffness, M., Eds.; Academic: San Diego, 1990; Vol. 37, p 77.
4540
Org. Lett., Vol. 7, No. 20, 2005

a bimolecular 1,3-dipolar cycloaddition reaction of 4 may
compete with the intramolecular cycloaddition cascade at the
higher reaction concentrations. More significantly, we ob-
served that the [4 + 2] cycloaddition is now the fast step in
the reaction cascade and that the subsequent [3 + 2]
cycloaddition is the slow step for 4a, a reversal of what is
observed with 4b and other typical substrates.10 The net
consequence is that longer reaction times and/or more
vigorous reaction conditions, in conjunction with the use of
a dilute reaction concentration, led to substantial improve-
ments in the conversion of 4a. The origin of the distinctions
in the slow step of the cycloaddition cascade 4a versus 4b
is not yet clear. The 1,3-dipolar cycloaddition transition
state for 4b embodies the more serious steric interactions
disfavoring the indole endo approach on the R-face of the
1,3-dipole (Figure 2), yet it progresses with a greater facility
than that of 4a. It appears that the transition state for the
1,3-dipolar cycloaddition derived from 4a suffers a desta-
bilizing electrostatic interaction of its central oxygen with
the (Z)-OBn substituent (Figure 2) that decelerates the
reaction and/or that the (E)-OBn substituent of 4b stabilizes
its transition state (transition state anomeric effect). However,
it is also possible that the preferred stereochemistry of the
corresponding cyclobutene epoxide C or their relative
stability, a potential intermediate and reversible source of
the 1,3-dipole, may dictate the relative ease of the 1,3-dipolar
cycloaddition and studies to probe such questions are in
progress.
Table 1. Cycloaddition Concentration Dependence
compound
concn (mM)
% yield 5a or 5b
4aa
4aa
4aa
4aa
4bb
4bb
4bb
4bb
4bb
5
1
27-34
43
50
53
38
58
86
90
99
0.1
0.05
100
50
10
5
1
a Condition: 230 °C, TIPB, 90 h. b Condition: 230 °C, TIPB, 20 h.
Although unanticipated, we found that the enantiomers of
5a could be easily separated on a semipreparative Chiralcel
OD column (30% i-PrOH/hexanes, 2 cm × 25 cm) with a
remarkable efficiency (R ) 1.70, tR ) 15.1 and 25.6 min,
10 mL/min), providing access to either enantiomer on a
preparatively useful scale (40-100 mg/injection). R-Hy-
droxylation of 5a was achieved best by treatment of the
lactam enolate with TMSO-OTMS and was followed by a
direct quench with TIPSOTf providing 12 (56-64%),
Scheme 2 (natural enantiomer shown). The stereochemistry
products.10 The stereochemical assignments for 4a and 4b
were based initially on their 1H NMR spectroscopic proper-
ties and unambiguously established by X-ray.14 Thus, the
cycloaddition cascade of either 4a and 4b provides complete
control of the intrinsic stereochemistry found in the penta-
cyclic skeleton of the Aspidosperma alkaloids, establishing
all six stereocenters about the central six-membered ring and
was designed to introduce essentially all the functionality
found in 3. The distinction in the two cycloaddition cascades
is that 4a permits the direct introduction of the naturally
occurring C4 OAc â-stereochemistry, whereas 4b provides
the C4 epimer, requiring a subsequent inversion of config-
uration at this center. Although substrate 4a directly provides
the preferred cycloadduct 5a for the use in the synthesis of
3, it proved to be more difficult to implement. The cyclization
of 4a occurs in excellent yield when warmed in triisopro-
pylbenzene (TIPB), affording 5a as a single diastereomer
(Figure 2). Initial attempts to cyclize 4a provided 5a in low
yield (<30% at >5 mM), and was in contrast to the
cyclization of the 4b, which provides the C4 diastereomer
(84-99%). However, simple dilution of the reaction mixture
led to improved yields (up to 53%). A study of this
concentration effect is illustrated in Table 1 for both 4a and
4b, where a rather dramatic relationship between concentra-
tion and yield was found, especially for 4a, suggesting that
Scheme 2
(14) X-ray crystal structures of 5a (CCDC 253429), 5b (CCDC 253430),
the C7 p-bromobenzoate of 12 (CCDC 255976), and the C7 free alcohol
of 18 (CCDC 253428) have been deposited with the Cambridge Crystal-
lographic Data Centre.
Org. Lett., Vol. 7, No. 20, 2005
4541

1
of the R-hydroxylation was clear from the H NMR of 12
(C7-H, J ) 5.5, 11.7 Hz) and unambiguously determined
by X-ray of the corresponding p-bromobenzoate.14 Conver-
sion of 12 to thioamide 13 (70%) with Lawesson’s reagent,
reductive desulfurization with Raney Ni conducted under
conditions (15 h, 23 °C) that also served to cleave the benzyl
ether provided 14 (91%), and subsequent acetylation of the
resulting secondary alcohol afforded 15 (97%). Diastereo-
selective reductive cleavage of the oxido bridge upon cata-
lytic hydrogenation (45 psi H2, PtO2, 50% MeOH-EtOAc)
with reduction of the intermediate imminium ion from the
R-face provided 16 (98%), analogous to observations first
made by Padwa,13 and silyl ether cleavage (Bu4NF, 89%)
and subsequent secondary alcohol activation and elimination
of 17 (Ph3P, DEAD, THF, 23 °C) provided either (-)- or
ent-(+)-vindoline (75%) identical in all respects with au-
thentic material. The final conversion of 17 to 3 enlisting a
Mitsunobu activation of the secondary alcohol for elimination
may proceed via an intermediate aziridinium cation and relies
on the reagent-derived base (EtO2CNHN--CO2Et) to promote
the regioselective elimination. This protocol proved much
more effective and reproducible than alternative procedures
that enlist reagents (e.g., Ph3P-CCl4)15 that generate reactive
nucleophiles (e.g., Cl-) that can compete with the elimination
reaction.
Scheme 3
diastereoselective oxido bridge cleavage of 22 (H2, PtO2,
MeOH, H+)13 to furnish 23 (82%). TIPS ether cleavage (Bu4-
NF, 98%) and treatment of 24 with Ph3P-CCl4 led to
activation and elimination of the C7 alcohol to provide 25
(62%). Following the protocol of Bu¨chi,8 diastereoselective
C4 carbonyl reduction of 25 (Redal-H, AlCl3) and subsequent
acetylation (Ac2O, 83%) of deacetylvindoline (26) furnished
vindoline (3).
A second total synthesis of vindoline was accomplished
utilizing cycloadduct 5b and was conducted while efforts to
secure 5a were underway (Scheme 3). The advantage is that
the key cycloaddition cascade proceeds with greater facility
under milder conditions and in higher conversions (>95%),
albeit in a route requiring inversion of the C4 stereochem-
istry. Thus, R-hydroxylation of 5b (LDA, TMSO-OTMS)
followed by in situ treatment with TIPSOTf provided 18 in
superb yield (75%). The stereochemistry of the intermediate
alcohol, also obtained by a less effective reaction of the
lactam enolate with Davis’ reagent (40-50%), was estab-
lished by 1H NMR (C7-H, J ) 5.5, 12.1 Hz) and confirmed
by X-ray.14 C4-Alcohol deprotection (H2, Pd/C, 90%) and
oxidation of 19 with TPAP/NMO provided 20 (81%).
Thiolactam formation (Lawesson’s reagent, toluene, 81%)
and Raney Ni desulfurization of 21 (25 °C, 78%) preceded
15
Acknowledgment. We gratefully acknowledge the fi-
nancial support of the National Institute of Health (CA42056)
and the Skaggs Institute for Chemical Biology and wish to
thank Y. Li for providing material at an intermediate stage
of the studies.
Supporting Information Available: Full experimental
details. This material is available free of charge via the
Internet at http://pubs.acs.org.
(15) Kuehne, M. E.; Okuniewicz, F. J.; Kirkemo, C. L.; Bohnert, J. C.
J. Org. Chem. 1982, 47, 1335.
OL051975X
4542
Org. Lett., Vol. 7, No. 20, 2005