Nickel-catalyzed preparation of bicyclic heterocycles: Total synthesis of (+)-allopumiliotoxin 267A, (+)-allopumiliotoxin 339A, and (+)-allopumiliotoxin 339B

Article abstract of DOI:10.1021/ja001440t

A new method for the reductive cyclization of ynals involving a Ni(COD)₂/PBu₃ catalyst system to produce allylic alcohols was developed. The triethylsilane-mediated procedure allows preparation of functionally rich pyrrolizidine, ind

Full text of DOI:10.1021/ja001440t

6950
J. Am. Chem. Soc. 2000, 122, 6950-6954
Nickel-Catalyzed Preparation of Bicyclic Heterocycles: Total
Synthesis of (+)-Allopumiliotoxin 267A, (+)-Allopumiliotoxin 339A,
and (+)-Allopumiliotoxin 339B
Xiao-Qing Tang and John Montgomery*
Contribution from the Department of Chemistry, Wayne State UniVersity, Detroit, Michigan 48202-3489
ReceiVed April 25, 2000
Abstract: A new method for the reductive cyclization of ynals involving a Ni(COD)2/PBu3 catalyst system to
produce allylic alcohols was developed. The triethylsilane-mediated procedure allows preparation of functionally
rich pyrrolizidine, indolizidine, and quinolizidine alkaloid frameworks. The method allows the direct introduction
of an allylic alcohol moiety with completely stereoselective creation of an exocyclic double bond and highly
diastereoselective alcohol introduction relative to preexisting chirality. The total syntheses of (+)-allopumiliotoxin
267A, (+)-allopumiliotoxin 339A, and (+)-allopumiliotoxin 339B were accomplished utilizing an ynal
cyclization as the key step. These syntheses provide short and efficient entries to the allopumiliotoxins and
highlight the utility of nickel-catalyzed ynal cyclizations in complex synthetic strategies.
Introduction
Scheme 1
The allopumiliotoxin alkaloids are the most complex members
of the pumiliotoxin class of indolizidine alkaloids.1,2 The
allopumiliotoxins, which are isolated from the skin of Dendro-
batid frogs, possess a characteristic 7,8-dihydroxy-8-methyl-6-
alkylideneindolizidine ring system (Scheme 1). The character-
istic E-alkylidene unit varies widely in complexity among the
members of this alkaloid class, and both epimers at C-7 have
been found in naturally occurring members of this group. The
unique structural features of these alkaloids, their scarcity from
natural sources, and their potent cardiotonic and myotonic
activity have made them important targets for synthesis.
Following the initial isolation of members of this natural product
class by Daly and co-workers,1 Overman pioneered the synthesis
of the pumiliotoxins.3 Several generations of synthetic ap-
proaches have emerged from the Overman laboratories, includ-
ing amine/methoxyallene cyclizations, iminium ion/vinylsilane
cyclizations, and iodide-promoted iminium ion/alkyne cycliza-
tions. The latter of these strategies was particularly attractive
since it allowed construction of the indolizidine skeleton and
the 6-alkylidene stereochemistry in a single step. An alternate
approach to the allopumiliotoxins involving two key palladium-
catalyzed processes was developed by Trost.4 In this approach,
the indolizidine skeleton was assembled by a palladium-
catalyzed amine/diene coupling, and the 6-alkylidene side chain
was introduced by a palladium-catalyzed vinyl epoxide/allylic
sulfone coupling. Kibayashi reported an effective nickel-
catalyzed vinyl iodide/aldehyde Nozaki-Kishi coupling5 to
prepare the allopumiliotoxin framework.6 This approach ef-
ficiently allowed introduction of the 6-alkylidene unit from a
stereodefined vinyl iodide with simultaneous stereoselective
creation of the C-7 stereocenter. Most recently, an extremely
short approach to the allopumiliotoxins involving a titanium-
mediated alkyne/ester reductive cyclization was reported by
Sato.7 This approach is characterized by the single-step creation
of the indolizidine skeleton and 6-alkylidene stereochemistry.
Furthermore, the rapid preparation of the cyclization precursor
is notable in the Sato approach. Despite the efficiency of these
numerous approaches, we envisioned that an aldehyde/alkyne
reductive cyclization could potentially provide the most direct
approach to the allopumiliotoxins since the indolizidine frame-
work, the 7-hydroxy stereocenter, and the 6-alkylidene double
bond would all be constructed in a single step. Herein, we
provide a full account of the discovery and development of a
nickel-catalyzed ynal cyclization procedure, its application in
the synthesis of various nitrogen heterocycles, and the total
syntheses of (+)-allopumiliotoxin 267A (1), (+)-allopumil-
iotoxin 339A (2), and (+)-allopumiliotoxin 339B (3).8
(1) (a) Daly, J. W.; Garraffo, H. M.; Spande, T. F. In Alkaloids: Chemical
and Biological PerspectiVes; Pelletier, S. W., Ed.; Wiley: New York, 1999;
Vol. 13, Chapter 1. (b) Tokuyama, T.; Daly, J. W.; Highet, R. J. Tetrahedron
1984, 40, 1183.
(2) Franklin, A.; Overman, L. E. Chem. ReV. 1996, 96, 505.
(3) (a) Goldstein, S. W.; Overman, L. E.; Rabinowitz, M. H. J. Org.
Chem. 1992, 57, 1179. (b) Overman, L. E.; Robinson, L. A.; Zablocki, J.
J. Am. Chem. Soc. 1992, 114, 368. (c) Caderas, C.; Lett, R.; Overman, L.
E.; Rabinowitz, M. H.; Robinson, L. A.; Sharp, M. J.; Zablocki, J. J. Am.
Chem. Soc. 1996, 118, 9073 and references therein.
(5) (a) Jin, H.; Uenishi, J.; Christ, W. J.; Kishi, Y. J. Am. Chem. Soc.
1986, 108, 5644. (b) Takai, K.; Tagashira, M.; Kuroda, T.; Oshima, K.;
Utimoto, K.; Nozaki, H. J. Am. Chem. Soc. 1986, 108, 6048.
(6) (a) Aoyagi, S.; Wang, T.; Kibayashi, C. J. Am. Chem. Soc. 1993,
115, 11393. (b) Aoyagi, S.; Wang, T.; Kibayashi, C. J. Am. Chem. Soc.
1992, 114, 10653.
(7) Okamoto, S.; Iwakubo, M.; Kobayashi, K.; Sato, F. J. Am. Chem.
Soc. 1997, 119, 6984.
(8) Portions of this work have previously been communicated: Tang,
X. Q.; Montgomery, J. J. Am. Chem. Soc. 1999, 121, 6098.
(4) Trost, B. M.; Scanlan, T. S. J. Am. Chem. Soc. 1989, 111, 4988.
10.1021/ja001440t CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/11/2000

Synthesis of (+)-Allopumiliotoxins 267A, 339A, and 339B
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 6951
Table 1
Results and Discussion
Development of an Ynal Reductive Cyclization Procedure.
Transition metal-catalyzed reductive cyclizations of dienes,
enynes, and diynes have been widely developed with numerous
catalyst systems.9 The corresponding cyclizations of carbonyls
with alkenes and alkynes, however, are less well developed.
Early metals are effective at promoting the coupling of carbonyls
with alkenes and alkynes via the formation of oxametallacycles,
but catalytic turnover in these systems is often difficult due to
the strength of the early metal-oxygen bond.10 While very
efficient stoichiometric procedures have been developed that
are quite broad in scope,7 the corresponding catalytic procedures
have been somewhat elusive. This limitation was partially
overcome by Buchwald11 and Crowe,12 who demonstrated that
titanocene-catalyzed reductive cyclizations of enones and ynones
were effective when silanes were employed as the reducing
agent. However, the titanium-catalyzed methods were unsuc-
cessful in the formation of rings larger than five-membered and
in couplings involving terminal alkynes. A number of late-metal-
catalyzed reductive cyclizations involving carbonyls have also
been reported. Of particular note are the studies from Mori and
Tamaru that described the efficient nickel-catalyzed coupling
of diene-aldehydes with triethylsilane,13 triethylborane,14 or
diethylzinc14 as the reducing agent. The scope of this procedure
is quite broad, and several complex synthetic applications have
been reported.15
suited for preparation of the allopumiliotoxin indolizidine
framework. However, we quickly found that direct addition of
the organozinc to the aldehyde was impossible to suppress as
the ynal substrate complexity increased.
We then examined triethylsilane as a less nucleophilic
reducing agent.17 In contrast to the diethylzinc-promoted process,
reductive cyclizations employing triethylsilane were very ef-
ficient across a broad range of substrate classes. The quinolizi-
dine ring system18 6 was first targeted, via cyclization of ynals
5a-d which possess a piperidine ring in the tether chain (Table
1). Cyclizations proceeded cleanly in several hours at 45 °C in
THF to afford high yields and very good diastereoselectivities.
Typical catalyst loadings were 10-20 mol % with a 2:1 to 4:1
phosphine-nickel ratio, and substrate concentrations typically
ranged from 0.02 to 0.05 M. Upon lowering the reaction
temperature to 0 °C, cyclizations required 18-48 h under
otherwise identical conditions. At 0 °C, diastereoselectivities
were uniformly outstanding across the range of substrates
examined. The quinolizidine framework was efficiently pro-
duced from ynals that possessed a variety of acetylenic
substituents, including aromatic (entry 3) and aliphatic (entry
4) internal alkynes, terminal alkynes (entry 5), and alkynylsilanes
(entry 6). The equatorial hydroxyl was selectively produced in
all cases. No evidence was obtained for competing reduction
of the aldehyde or alkyne without cyclization.
Our group recently reported a procedure for nickel-catalyzed
aldehyde-alkyne reductive and alkylative cyclizations involving
organozincs.16 In cyclizations involving a simple ynal and
diethylzinc, Ni(COD)2 catalyzed formation of the alkylative
cyclization product 4a, whereas Ni(COD)2/PBu3 catalyzed
formation of the reductive cyclization product 4b (eq 1). The
The [3.3.0] bicyclic framework of pyrrolizidine alkaloids19
was next examined (Table 2). The requisite ynals were quite
unstable, and attempts to purify the adducts from Swern
oxidation led to extensive decomposition. However, Swern
oxidation of alcohols 8a-c followed by simple extraction and
concentration afforded aldehydes of sufficient purity to ef-
ficiently participate in the nickel-catalyzed cyclizations. The
overall two-step procedure of oxidation and cyclization cleanly
Ni(COD)2/PBu3 catalyzed procedure appeared to be ideally
(9) (a) For an extensive review, see: Ojima, I.; Tzamarioudaki, M.; Li,
Z. Y.; Donovan, R. J. Chem. ReV. 1996, 96, 635-662. (b) For an early
example of this reaction class, see: Trost, B. M.; Rise, F. J. Am. Chem.
Soc. 1987, 109, 3161.
(10) Hewlett, D. F.; Whitby, R. J. J. Chem. Soc., Chem. Commun. 1990,
1684.
(11) (a) Kablaoui, N. M.; Buchwald, S. L. J. Am. Chem. Soc. 1995, 117,
6785-6786. (b) Kablaoui, N. M.; Buchwald, S. L. J. Am. Chem. Soc. 1996,
118, 3182-3191.
(12) Crowe, W. E.; Rachita, M. J. J. Am. Chem. Soc. 1995, 117, 6787-
6788.
(13) (a) Sato, Y.; Takimoto, M.; Hayashi, K.; Katsuhara, T.; Takagi,
K.; Mori, M. J. Am. Chem. Soc. 1994, 116, 9771. (b) Sato, Y.; Takimoto,
M.; Mori, M. Tetrahedron Lett. 1996, 37, 887. (c) Takimoto, M.; Hiraga,
Y.; Sato, Y.; Mori, M. Tetrahedron Lett. 1998, 39, 4543.
(14) (a) Kimura, M.; Ezoe, A.; Shibata, K.; Tamaru, Y. J. Am. Chem.
Soc. 1998, 120, 4033. (b) Kimura, M.; Fujimatsu, H.; Ezoe, A.; Shibata,
K.; Shimizu, M.; Matsumoto, S.; Tamaru, Y. Angew. Chem., Int. Ed. 1999,
38, 397.
(17) For other transition metal-catalyzed cyclizations involving hydrosi-
lanes, see: (a) Ojima, I.; Zhu, J. W.; Vidal, E. S.; Kass, D. F. J. Am. Chem.
Soc. 1998, 120, 6690. (b) Trost, B. M.; Rise, F. J. Am. Chem. Soc. 1987,
109, 3161-3163. (c) Tamao, K.; Kobayashi, K.; Ito, Y. J. Am. Chem. Soc.
1989, 111, 6478. (d) Takacs, J.; Chandramouli, S. Organometallics 1990,
9, 2877. (e) Widenhoefer, R. A.; DeCarli, M. A. J. Am. Chem. Soc. 1998,
120, 3805. (f) Molander, G. A.; Nichols, P. J. J. Am. Chem. Soc. 1995,
117, 4415.
(18) (a) Michael, J. P. Nat. Prod. Rep. 1999, 16, 675. (b) Kinghorn, A.
D.; Balandrin, M. F. In Alkaloids: Chemical and Biological PerspectiVes;
Pelletier, S. W., Ed.; Wiley: New York, 1984; Vol. 2, Chapter 3. (c) Herbert,
R. B. In Alkaloids: Chemical and Biological PerspectiVes; Pelletier, S.
W., Ed.; Wiley: New York, 1985; Vol. 3, Chapter 6. (d) Pearson, W. H.;
Suga, H. J. Org. Chem. 1998, 63, 9910.
(15) (a) Sato, Y.; Saito, N.; Mori, M. Tetrahedron Lett. 1997, 38, 3931.
(b) Sato, Y.; Saito, N.; Mori, M. Tetrahedron 1998, 54, 1153. (c) Sato, Y.;
Takimoto, H.; Mori, M. Synlett 1997, 6, 734.
(16) (a) Oblinger, E.; Montgomery, J. J. Am. Chem. Soc. 1997, 119,
9065-9066. (b) Montgomery, J. Acc. Chem. Res. In press.
(19) (a) Liddell, J. R. Nat. Prod. Rep. 1999, 16, 499. (b) Hartmann, T.;
Witte, L. In Alkaloids: Chemical and Biological PerspectiVes; Pelletier,
S. W., Ed.; Wiley: New York, 1995; Vol. 9, Chapter 4. (c) Broggini, G.;
Zecchi, G. Synthesis 1999, 6, 905. (d) Li, Y. W.; Marks, T. J. J. Am. Chem.
Soc. 1998, 120, 1757.

6952 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Tang and Montgomery
Table 2
ments were unambiguously determined by an analysis of
coupling constants and NOE spectra (Scheme 2).
Scheme 2
afforded pyrrolizidines 9a-c in good yield with very good
diastereoselectivities. With this particular substitution pattern,
the exo hydroxyl was selectively produced in each case. Both
aromatic and aliphatic alkynes participated cleanly in the
cyclizations.
Indolizidines18c,19c,d,20 that are isomers of the pumiliotoxin
skeleton were then produced upon cyclization of the one-carbon
homologues of the substrates described above. Again, the
requisite aldehydes were unstable, but the crude aldehyde from
a Swern oxidation of 11 could be directly cyclized to afford
the indolizidine alkaloid 12 as a single diastereomer in good
yield (eq 2). Consistent with observations made in the quino-
The mechanistic basis for this remarkably clean reversal of
stereochemistry is unclear; however, we propose the following
mechanistic scheme. In the cyclization of substrate 14, initial
complexation of Ni(0) to the alkyne and aldehyde π-systems
may be followed by oxidative cyclization to oxametallacycle
20 (Scheme 3).21 Orientation of the initial π-complex in a trans-
hydrindane conformation would ultimately lead to production
of the equatorial silyloxy substituent. σ-Bond metathesis22 of
the silane and Ni-O bond would afford intermediate 21 which
would undergo C-H reductive elimination to afford the
observed product 16. With substrate 17, inversion at nitrogen
could lead to cis-hydrindane conformation 22 (Scheme 4). The
corresponding trans-hydrindane conformation of 22 would be
destabilized by electrostatic repulsions involving the ring
nitrogen, benzyl ether, and aldehyde. From cis-hydrindane
conformation 22, the identical oxidative cyclization/σ-bond
metathesis mechanism would afford product 18 with the axial
C-7 silyloxy substituent. This proposed mechanism is directly
analogous to that proposed by Buchwald and Crowe in titanium-
catalyzed reductive cyclizations of enones.11,12
It is interesting to compare the selectivities of the nickel-
catalyzed ynal cyclizations to the nickel-catalyzed Nozaki-Kishi
couplings described by Kibayashi on very closely related
systems.6 Direct comparisons are difficult since the Nozaki-
Kishi couplings5 were carried out in DMF (vs THF in our study)
and in the presence of chromium salts that could alter confor-
mational preferences. In the fully functionalized pumiliotoxin
framework, very high selectivities for formation of the C-7 axial
hydroxyl were observed in both methods. However, in the
preparation of less-functionalized quinolizidines 6b and 26, the
ynal cyclization was highly selective for formation of the
equatorial hydroxyl, whereas the Nozaki-Kishi coupling pro-
ceeded in a nonselective fashion (Scheme 5).
lizidine series described above (Table 1), the equatorial hydroxyl
of 12 was exclusively produced.
At this juncture, we proceeded to attempt preparation of the
allopumiliotoxin indolizidine ring system. On the basis of the
results described in Table 1, and eq 2, we suspected that the
equatorial hydroxyl, found in (+)-allopumiliotoxin 339B, should
be produced in the allopumiliotoxin series. The axial hydroxyl
found in allopumiliotoxins 267A and 339A is the more prevalent
orientation within the known allopumiliotoxins. As suspected,
cyclization at 0 °C of ynals 13 and 14, which are models of the
allopumiliotoxin ring system, led exclusively to indolizidines
15 and 16 in excellent yield and diastereoselectivity (eq 3).
Total Synthesis of (+)-Allopumiliotoxin 267A, (+)-Allo-
pumiliotoxin 339A, and (+)-Allopumiliotoxin 339B. With the
above methodological studies completed, we proceeded to apply
the ynal cyclization method in the preparation of allopumil-
However, cyclization of more highly functionalized model
compound 17 that possesses the C-8 tertiary hydroxyl protected
as the benzyl ether afforded a surprising result (eq 4). A single
diastereomer was obtained, but product 18 possessed the
epimeric axial hydroxyl at C-7! These stereochemical assign-
(21) Tsuda, T.; Kiyoi, T.; Saegusa, T. J. Org. Chem. 1990, 55, 2554.
(22) For examples of late-metal-catalyzed σ-bond metatheses, see ref
17e and: (a) LaPointe, A. M.; Rix, F. C.; Brookhart, M. J. Am. Chem. Soc.
1997, 119, 9066. (b) Hartwig, J. F.; Bhandari, S.; Rablen, P. R. J. Am.
Chem. Soc. 1994, 116, 1839.
(20) Elbein, A. D.; Molyneux, R. J. In Alkaloids: Chemical and
Biological PerspectiVes; Pelletier, S. W., Ed.; Wiley: New York, 1987;
Vol. 5, Chapter 1.

Synthesis of (+)-Allopumiliotoxins 267A, 339A, and 339B
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 6953
Scheme 3
Scheme 6a
Scheme 4
a Conditions: (a) I. KOH, EtOH; Ii. 28, i-Pr2NEt, THF, 74 % for
Two Steps. (b) BnBr, KH, THF, 83 %. (c) NBu4F, Molecular Sieves,
THF, 94 %. (d) Cl2(CO)2, DMSO, Et3N, 93 %. (e) Et3SiH, Ni(COD)2,
PBu3, THF, 95 %. (f) HF‚Pyridine, THF, 92 %. (g) Li°, NH3, THF, 88
%.
equiv) in THF at 0 °C for 18 h to afford bicycle 33 as a single
diastereomer in 95% yield. Careful inspection of the crude 500
1
MHz H NMR spectrum revealed no trace of the C-7 epimer
or C-6 Z-alkylidene. Deprotection of the triethylsilyl ether with
HF‚pyridine and the benzyl ether with Lio/NH3 afforded (+)-
allopumiliotoxin 267A (1) that was identical in all respects with
synthetic material kindly provided by Overman.3c
The identical strategy was pursued in the syntheses of
allopumiliotoxins 339A and 339B (Scheme 7). Propargyl
bromide 35 was prepared by bromination of the corresponding
propargyl alcohol6a with carbon tetrabromide and triphenylphos-
phine in CH2Cl2 in 93% yield. Hydrolysis of oxazolidinone 27
as before followed by N-propargylation with bromide 35
proceeded cleanly. Benzylation of the tertiary hydroxyl followed
by primary hydroxyl deprotection and oxidation under Swern
conditions led to cyclization substrate 39 in 67% yield for the
three-step conversion. Cyclization of substrate 39 with trieth-
ylsilane (5.0 equiv), Ni(COD)2 (0.2 equiv), and tributylphos-
phine (0.8 equiv) proceeded in a completely diastereoselective
fashion in THF at -10 to 0 °C in 18 h to afford silyl ether 40
in 93% isolated yield. Silyl ether 40 was deprotected with HF‚
pyridine to afford alcohol 41, which serves as the branchpoint
for the preparation of allopumiliotoxins 339A (2) and 339B (3).
Further deprotection6a of alcohol 41 with 3 N HCl and then
Liï/NH3 afforded synthetic (+)-allopumiliotoxin 339A (2) that
exhibited melting point, optical rotation, and NMR data
consistent with that previously reported.3c,6a
Scheme 5
iotoxin 267A (Scheme 6). For our studies, we relied on the
excellent procedures developed by Overman,3 Kibayashi,6 and
Sato7 in the preparation of the requisite cyclization precursors.
Accordingly, oxazolidinone 273c was prepared from L-proline
methyl ester. Oxazolidinone hydrolysis with KOH in ethanol
followed by propargylation with enantiopure bromide 287
afforded substrate 29 in 74% yield for the two-step conversion.
Benzylation of the tertiary hydroxyl followed by primary
hydroxyl deprotection and oxidation led to cyclization substrate
32 in high yield. Aldehyde 32 was then treated with triethylsilane
(5 equiv), Ni(COD)2 (0.2 equiv), and tributylphosphine (0.4
Alcohol 41 was then oxidized under Swern conditions to
enone 42, and reduction with CeCl3/NaBH4 in methanol
23
afforded equatorial hydroxyl 43 in 95% yield as a single
diastereomer. The procedure for this selective reduction had
previously been reported by Overman in a simpler system.3a
Deprotection of this material with 3 N HCl and then Liï/NH3
afforded synthetic (+)-allopumiliotoxin 339B (3) that exhibited
optical rotation and NMR data consistent with that previously
reported.3a,4
(23) Luche, J. L. J. Am. Chem. Soc. 1978, 100, 2226.

6954 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Tang and Montgomery
Scheme 7a
a Conditions: (a) I. KOH, EtOH; Ii. 35, i-Pr2NEt, THF, 92% for Two Steps. (b) BnBr, KH, THF, 82 %. (c) NBu4F, Molecular Sieves, THF, 92
%. (d) Cl2(CO)2, DMSO, Et3N, 89 %. (e) Et3SiH, Ni(COD)2, PBu3, THF, 93 %. (f) HF‚Pyridine, THF, 87 %. (g) I. 3N HCl/THF, 92 %; Ii. Li°,
NH3, THF, 80 %. (h) Cl2(CO)2, DMSO, Et3N, 86 %. (i) CeCl3‚7H2O, NaBH4, MeOH, 95 %. (j) I. 3N HCl/THF, 91 %; Ii. Li°, NH3, THF, 82 %.
Conclusions
Acknowledgment. The author thank the National Institutes
of Health (R01 GM57014) for support of this research. J.M.
acknowledges receipt of a Camille Dreyfus Teacher Scholar
Award and a National Science Foundation CAREER Award.
Professor Larry Overman is thanked for providing compounds
34 and 1 for comparison.
The studies described herein detail the development of a
nickel-catalyzed procedure for the reductive cyclization of ynals.
The method is particularly useful for generating cyclic allylic
alcohols with a stereodefined exocyclic double bond. With a
variety of heterocyclic templates, the diastereoselectivites with
respect to preexisting stereocenters were shown to be uniformly
excellent. The total syntheses of (+)-allopumiliotoxin 267A,
(+)-allopumiliotoxin 339A, and (+)-allopumiliotoxin 339B were
completed in a highly efficient and stereoselective fashion
utilizing the ynal cyclization procedure as a key step. These
studies underscore the utility of nickel-catalyzed ynal cycliza-
tions in the preparation of highly functionalized heterocycles.
Supporting Information Available: Full experimental
details and copies of 1H NMR spectra of all compounds (PDF).
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA001440T