Highly enantioselective Rh2(S -DOSP)4-catalyzed cyclopropenation of alkynes with styryldiazoacetates

Article abstract of DOI:10.1021/ja106509b

Dirhodium tetrakis((S)-N-(dodecylbenzenesulfonyl)prolinate) (Rh ₂(S-DOSP)₄) is an effective catalyst for highly enantioselective cyclopropenation reactions between terminal alkynes and arylvinyldiazoacetates. The resulting vinylcyclo

Full text of DOI:10.1021/ja106509b

Published on Web 11/16/2010
Highly Enantioselective Rh2(S-DOSP)4-Catalyzed
Cyclopropenation of Alkynes with Styryldiazoacetates
John F. Briones,† Jørn Hansen,† Kenneth I. Hardcastle,† Jochen Autschbach,‡ and
Huw M. L. Davies*,†
Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322, United States,
and Department of Chemistry, UniVersity at Buffalo, SUNY, Buffalo, New York 14226, United States
Received July 28, 2010; E-mail: hmdavie@emory.edu
Abstract: Dirhodium tetrakis((S)-N-(dodecylbenzenesulfonyl)prolinate) (Rh2(S-DOSP)4) is an effective
catalyst for highly enantioselective cyclopropenation reactions between terminal alkynes and arylvinyldia-
zoacetates. The resulting vinylcyclopropenes can undergo rhodium-catalyzed regioselective rearrangement
to cyclopentadienes. Computational studies indicate that the high enantioselectivity of the process is
governed by the specific orientation of the alkyne during its approach to the carbenoid through a relatively
late transition state. The specific orientation occurs due to the presence of a hydrogen bonding interaction
between the alkyne hydrogen and a carboxylate ligand on the dirhodium catalyst.
of this approach are not abundant.7 An alternative way to prepare
cyclopropenes is through the cyclopropenation of alkynes by
Introduction
Chiral cyclopropenes are valuable intermediates in organic
synthesis.1 Their versatility has been demonstrated in a number
of synthetically important transformations, including addition
across the strained double bond,2 transition metal-catalyzed
cycloisomerization to heterocycles,3 cycloadditions,4 and meta-
lation reactions.5 Traditionally, methods utilized for cyclopro-
pene synthesis have relied on elimination reactions from
available cyclopropane precursors,6 but enantioselective variants
metal carbenoids. Doyle et al. have demonstrated that, the chiral
dirhodium(II) carboxamidate complex Rh2(S-MEPY)4, is an
effective catalyst for enantioselective cyclopropenation reactions
with diazoacetates.8 Corey and co-workers have developed a
mixed carboxylate/carboxamidate catalyst, Rh2(OAc)(DPTI)3,
that was efficiently applied to similar cyclopropenation reac-
tions.9 Furthermore, a convenient approach to chiral cyclopro-
pene synthesis via cyclopropenation, followed by diastereomeric
and parallel kinetic resolution strategies, has been reported by
Fox and co-workers.10
† Emory University.
‡ University at Buffalo.
(1) (a) Rubin, M.; Rubina, M.; Gevorgyan, V. Chem. ReV. 2007, 107,
3117. (b) Bach, R. D.; Dmitrenko, O. J. Am. Chem. Soc. 2006, 128,
4598. (c) Khoury, P. R.; Goddard, J. D.; Tam, W. Tetrahedron 2004,
60, 8103. (d) Fattahi, A.; McCarthy, R. E.; Ahmad, M. R.; Kass, S. R.
J. Am. Chem. Soc. 2003, 125, 11746. (e) Marek, I.; Simaan, S.;
Masawara, A. Angew. Chem., Int. Ed. 2007, 46, 7364.
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Liao, L. J. Am. Chem. Soc. 2002, 124, 14322. (c) Ferjancic, Z.;
Cekovic, Z.; Saicic, R. N. Tetrahedron Lett. 2000, 16, 2979. (d)
Rubina, M.; Rubin, M.; Gevorgyan, V. J. Am. Chem. Soc. 2003, 125,
7198. (e) Nakamura, M.; Hirai, A.; Nakamura, E. J. Am. Chem. Soc.
2000, 122, 978.
We have previously demonstrated that the dirhodium tetra-
carboxylate complex Rh2(S-DOSP)4 (1) is an effective chiral
catalyst for enantioselective cyclopropenation reactions with
aryldiazoacetates, generating cyclopropenes containing a qua-
ternary stereocenter (Scheme 1).11 Related studies by Chang et
al. have shown that copper-catalyzed reactions of aryldiazoac-
etates with arylalkynes generate indenes.12 During our earlier
studies, the reactions between vinyldiazoacetates and phenyl-
acetylene were unsuccessful.11 In this paper, we describe a
(3) (a) Cho, S. H.; Liebeskind, L. S. J. Org. Chem. 1987, 52, 2361. (b)
Padwa, A.; Kassir, J. M.; Xu, S. L. J. Org. Chem. 1991, 56, 6971. (c)
Muller, C.; Granicher, C. HelV. Chim. Acta 1993, 76, 521.
(4) (a) Rubin, M.; Rubina, M.; Gevorgyan, V. Synthesis 2006, 1221. (b)
Diev, V. V.; Kostikov, R. R.; Gleiter, R.; Molchanov, A. P. J. Org.
Chem. 2006, 71, 4006. (c) Pallerla, M. K.; Fox, J. M. Org. Lett. 2005,
7, 3593. (d) Pallerla, M. K.; Fox, J. M. Org. Lett. 2007, 9, 5625. (e)
Deem, M. L. Synthesis 1982, 701. (f) Padwa, A.; Fryxell, G. E.
Cyclization Reactions of Cyclopropenes, In AdVances in Strain in
Organic Chemistry, Vol 1; Halton, B., Ed.; Jai Press: Greenwich, CT,
1991.
(7) (a) Schollkopf, U.; Hupfeld, B.; Kuper, S.; Egert, E.; Dyrbusch, M.
Angew. Chem., Int. Ed. Engl. 1988, 27, 433. (b) Muller, P.; Imogai,
H. Tetrahedron: Asymmetry 1998, 9, 4419. (c) Doyle, M. P.; Hu, W.
Tetrahedron Lett. 2000, 41, 6265.
(8) (a) Doyle, M. P.; Winchester, W. P.; Hoorn, J. A. A.; Lynch, V.;
Simonsen, S. H.; Ghosh, R. J. Am. Chem. Soc. 1993, 115, 9968. (b)
Doyle, M. P.; Protopopova, M.; Muller, P.; Ene, D.; Shapiro, E. A.
J. Am. Chem. Soc. 1994, 116, 8492.
(5) (a) Tarwade, V.; Liu, X.; Yan, N.; Fox, J. M. J. Am. Chem. Soc. 2009,
131, 5382. (b) Yan, N.; Liu, X.; Fox, J. M. J. Org. Chem. 2008, 73,
563. (c) Liu, X.; Fox, J. M. J. Am. Chem. Soc. 2006, 128, 5600. (d)
Nakamura, I.; Bajracharya, G. B.; Yamamoto, Y. J. Org. Chem. 2003,
68, 2297.
(9) Lou, Y.; Horikawa, M.; Kloster, R. A.; Hawryluk, N. A.; Corey, E. J.
J. Am. Chem. Soc. 2004, 126, 8916.
(10) (a) Fox, J. M.; Yan, N. Curr. Org. Chem. 2005, 9, 719. (b) Liao,
L. A.; Zhang, F.; Yan, N.; Golen, J. A.; Fox, J. M. Tetrahedron 2004,
60, 1803.
(6) (a) Baird, M. S. In Houben-Weyl: Methods of Organic Chemistry,
4th ed.; de Meijere, A., Ed.; Thieme: Stuggart, 1996; Vol. E17d; pp
2695-2744.
(11) Davies, H. M. L.; Lee, G. H. Org. Lett. 2004, 6, 1233.
(12) Park, E. J.; Kim, S. H.; Chang, S. J. Am. Chem. Soc. 2008, 130, 17268.
9
10.1021/ja106509b  2010 American Chemical Society
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A R T I C L E S
Briones et al.
Scheme 1. Cyclopropenation with Aryldiazoacetates
Table 1. Synthesis of Cyclopropenes 11a
Scheme 2. Formation of Furan 5
Scheme 3. Cyclopropenation and Cyclopentadiene Formation
detailed study that re-examines this reaction because the
resulting 3-vinylcyclopropenes would be useful intermediates.
Results and Discussion
The studies began by re-examining the reaction of vinyldia-
zoacetates with phenylacetylene. 1H NMR analysis of the crude
reaction mixture from the Rh2(S-DOSP)4-catalyzed reaction of
vinyldiazoacetate 2 with phenylacetylene (3) revealed that the
cyclopropene 4 was indeed formed (Scheme 2). However, 4
was unstable on silica gel and readily rearranged to the furan
derivative 5, which was also unstable and decomposed upon
standing at room temperature. The structure of 5 was tentatively
assigned on the basis of NOE studies. These observations
demonstrated that a cyclopropene could be formed from the
reaction of vinyldiazoacetates with alkynes, but for the reaction
to be broadly useful, more stabilized vinylcyclopropenes would
need to be formed.
The study was extended to alkyl acetylenes, which would be
expected to generate more stable cyclopropenes since they lack
the phenyl donor-group. The Rh2(S-DOSP)4-catalyzed reaction
of 1-pentyne with methyl styryldiazoacetate 6 at room temper-
ature gave a mixture of the cyclopropene 7 (16%) and the
cyclopentadiene 8 (81%) (Scheme 3). Cyclopropene 7 was
formed in high enantiomeric excess (98% ee) and appeared to
be relatively stable. However, upon stirring overnight in the
presence of Rh2(S-DOSP)4, rearrangement to 8 in 82% yield
was observed. These results suggest that cyclopentadiene 8 is
formed through a rhodium-catalyzed rearrangement of the
initially formed cyclopropene 7.
a All reactions were performed by addition of the diazo compound
(1.0 mmol) in 10 mL of pentane over 2 h to a stirred solution of alkyne
(0.5 mmol) and catalyst (0.01 mmol) in 1 mL of pentane. b Methyl
(p-trifluoromethyl)phenylvinyldiazoacetate was used. c Methyl (p-bromo-
phenyl)vinyldiazoacetate was used.
Milder reaction conditions were examined next in order to
avoid the ring-expansion of the cyclopropene to the cyclopen-
tadiene. Conducting the reactions at -45 °C was optimal, and
under these conditions, cyclopropenes 11a-q were formed in
generally good yields (45-89%) and in very high enantiomeric
excess (95-99% ee) (Table 1). Increasing the length of the alkyl
chain or the bulkiness of the alkyne substituent had virtually
no effect on the enantioselectivity of the cyclopropenation
reaction. Relatively low yield, however, was obtained when 2,2-
dimethylbutyne was used (entry 9). Benzylic C-H insertion
was not observed in the case of entries 10-12, suggesting that
the cyclopropenation is a very favorable reaction. Monocyclo-
propenation of the diynes used in entries 13 and 14 was a viable
process. Clean cyclopropenation and no C-H insertion adjacent
to an electron-donating siloxy group was observed in the case
of entries 15 and 16. These studies demonstrate that the
cyclopropenation reaction can occur in a highly selective manner
with a variety of alkyl acetylenes. In contrast to the reaction
with mono-substituted alkynes, no cyclopropene product was
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Enantioselective Cyclopropenation of Terminal Alkynes
A R T I C L E S
Table 2. Synthesis of Cyclopentadienes 12a-ha
Table 3. Rh2(S-DOSP)4-Catalyzed Reactions of 15a
a Reactions were performed by addition of 1.0 mmol of 2 in 10 mL
of 2,2-dimethylbutane, 0.5 mmol alkyne, and 0.01 mmol catalyst.
b When the reaction was conducted using dirhodium(II) tetraoctanoate as
catalyst, the yield of 12a was 59%.
a Reactions were performed by addition of 1.0 mmol of diazo
compound in 10 mL of pentane, 0.5 mmol alkyne and 0.01 mmol
catalyst.
Scheme 4. Proof of Absolute Configuration of 4
Figure 1. Extreme orientations of incoming substrate in cyclopropenation
chemistry.
Other arylvinyldiazoacetates (15) were also tested in the
cyclopropenation reaction (Table 3). In the case of relatively
electron-deficient arylvinyldiazoacetates (entries 1-3), cyclo-
propenes 16a-c were obtained in good yields (79-88%) and
high enantiomeric excess (96-98% ee). In contrast, the reaction
of the more electron-rich arylvinyldiazoacetates (entries 4 and
5) resulted in ring-expansion and consequent formation of
cyclopentadienes 17a and 17b in 87% and 78% yields,
respectively. The structure of 17a was confirmed by X-ray
crystallography.14
The high enantioselectivity observed in the cyclopropenation
reactions suggests that the alkyne approaches the vinylcarbenoid
intermediate in a highly organized manner. There are two
extreme orientations proposed for the approach of the alkyne
during the cyclopropenantion event (Figure 1).11 The first has
the alkyne approaching side-on,13 while the second has the
alkyne approaching end-on to the rhodium carbenoid.15 The
observation that no cyclopropenantion occurs with disubstituted
alkynes would be evidence to support the end-on approach, as
such an approach would be highly unfavorable for disubstituted
alkynes since the second substituent will clash with the rhodium
catalyst structure.11 However, an end-on approach does not give
an obvious explanation of why Rh2(S-DOSP)4 gives such high
generated from reactions with disubstituted alkynes, such as
1-phenyl-1-propyne and diphenylacetylene.
As cyclopentadienes represent an interesting class of reagents
in organic synthesis, efforts were made to optimize their
formation. Upon heating the reaction mixture to reflux, using
2,2-dimethylbutane (DMB) as solvent, a range of cyclopenta-
dienes (12a-h) was formed in moderate to good yields
(61-83% yield) (Table 2).
The absolute configuration of the cyclopropenes was assigned
in the following manner (Scheme 4). The in situ formed
cyclopropene 4 from the reaction of 2 and 3 was trapped by
addition of LiAlH4 to afford alcohol 13 as a single diastereomer.
Chiral HPLC analysis of 13 (92% ee) confirmed that it was the
(1S, 2S)-enantiomer based on comparison of the retention time
with 13 obtained from the reduction of cyclopropane 14 derived
from 2 and styrene.13 The same absolute configuration was
observed with the silyl-substituted cyclopropene 11q, which was
determined by X-ray crystallography.14 The absolute configura-
tions of the other cyclopropenes are tentatively assigned by
analogy.
(13) Davies, H. M. L.; Bruzinski, P. R.; Lake, D. H.; Kong, N.; Fall, M. J.
J. Am. Chem. Soc. 1996, 118, 6897–6907.
(14) The crystal structure of 11q and 17d have been deposited at the
Cambridge Crystallographic Data Centre, and the deposition numbers
CCDC 765701 and 765700 have been allocated.
(15) Nowlan, D. T.; Gregg, T. M.; Davies, H. M. L.; Singleton, D. A.
J. Am. Chem. Soc. 2003, 125, 15902–15911.
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A R T I C L E S
Briones et al.
Figure 3. Predictive model for asymmetric induction in cyclopropenation
chemistry with Rh2(S-DOSP)4.
Figure 2. (a) Model reaction system, (b) transition state TS-I for
cyclopropenation reaction.
Scheme 5. Mechanistic Hypothesis for formation of 22
asymmetric induction. The chiral influence of Rh2(S-DOSP)4
has been proposed to be due to steric influence located, not
directly in front of the rhodium-carbon bond, but slightly to
the side of the carbenoid.13,15 An end-on approach is unlikely
to be influenced by steric groups located to the side of the
carbenoid.
Density Functional calculations were conducted in order to
obtain a better understanding of the mechanism of the cyclo-
propenation reaction. The study was conducted using the B3LYP
functional16 in the reaction between methyl styryldiazoacetate
(6) and propyne, catalyzed by dirhodium tetrakisformate, as a
model reaction system (Figure 2a). Only the reaction step
involving the rhodium carbenoid complex and propyne was
considered, as the pathway leading to the rhodium carbenoid
complex has been described in detail in previous works.15,17
The cyclopropenation step displayed a potential energy activa-
tion barrier of +11.6 kcal/mol, and was exothermic by -14.6
kcal/mol. These observations suggest that a much later transition
state occurs in cyclopropenation chemistry in comparison with
cyclopropanation reactions of alkenes.15,17a Starting from either
an end-on or side-on trajectory, the alkyne ultimately obtained
approximately the same orientation in all the transition state
optimizations. The favored transition structure (TS-I) is shown
in Figure 2b (see Supporting Information for more details). A
most striking feature of this transition state is the close proximity
and directionality of the terminal alkyne hydrogen to a car-
boxylate ligand (1.956 Å), which indicates a hydrogen bonding
interaction.18 The alkyne is furthermore tilted 18.2° from the
ideal end-on approach. These observations imply that a disub-
stituted alkyne could not have a suitable orientation to undergo
the cyclopropenation reaction.
DOSP)4 would be involved in the asymmetric induction. As
this complex is considered to adopt a D2-symmetric conforma-
tion, the chiral carbenoid complex can be represented with
blocking groups from the catalyst occupying the front left-hand
and the back right-hand quadrants (Figure 3).13,15 The substrate
approaches in a tilted orientation so that attack occurs to the
front face (Re-face) of the carbenoid preferentially. Approach
to the Si-face is disfavored by the blocking group in the back,
as it is on the side of the vinyl group. In the transition state, the
terminal alkyne carbon is involved in C-C bond formation with
the carbenoid carbon, while positive charge buildup occurs at
the internal sp-carbon. The second C-C bond forms with
inversion at the carbenoid center, consistent with previous
observations in cyclopropanation and C-H functionalization
chemistry.15,17 This model successfully predicts the sense of
asymmetric induction in these reactions.
The proposed mechanism for the formation of the cyclopen-
tadiene product is based on previous studies by Padwa and co-
workers.3b The rhodium catalyzed ring-opening of unsymmet-
rically substituted vinylcyclopropene 18 can potentially lead to
two regioisomeric rhodium-carbenoids 19 and 20, depending
on which bond is cleaved (Scheme 5).3b,19 Literature precedence
indicates that further reaction occurs preferentially from the least
substituted carbenoid 20.20 This may be due to preferential
cleavage of bond a to form 20, or rapid equilibration between
The tilted substrate orientation observed in TS-I is of critical
importance, as it ensures that the chiral influence of Rh2(S-
(16) See Supporting Information for details and references. The geometry
optimizations and frequency calculations were carried out using the
B3LYP functional as implemented in Gaussian’09. The Stuttgart 1997
relativistic small-core ECP and corresponding basis set augmented
with a 4f-function was used for Rh (abbreviated [Rh-RSC+4f]) and
6-31G* was used for C, H and O. Single point energies were
furthermore calculated at the B3LYP/6-311+G(2d,2p)[Rh-RSC+4f]//
B3LYP/6-31G*[Rh-RSC+4f] level of theory. The discussion is based
on these single point energies corrected with zero-point energies from
the B3LYP/6-31G*[Rh-RSC+4f] level calculations.
(18) (a) For an example of hydrogen bonding in rhodium carbenoid
chemistry between hydroxyl ylides and the carboxylate ligand, see:
Liang, Y.; Zhou, H.; Yu, Z.-X. J. Am. Chem. Soc. 2009, 131, 17783-
17785. (b) General reference for hydrogen bonding characteristics:
Jeffrey, G. A. An Introduction to Hydrogen-Bonding; Oxford Uni-
versity Press, Inc: New York, 1997.
(17) (a) Hansen, J.; Autschbach, J.; Davies, H. M. L. J. Org. Chem. 2009,
74, 6555–6563. (b) Nakamura, E.; Yoshikai, N.; Yamanaka, M. J. Am.
Chem. Soc. 2002, 124, 7181–7192.
(19) Muller, P.; Pautex, N.; Doyle, M. P.; Bagheri, V. HelV. Chim. Acta
1990, 73, 1233.
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Enantioselective Cyclopropenation of Terminal Alkynes
A R T I C L E S
1.5 Hz), 6.04 (d, 1H, J ) 15.9 Hz), 3.72 (s,1H), 2.49 (td, 2H, J )
7.5, 1.5 Hz), 1.62 (s, 2H, J ) 7.5 Hz), 0.98 (t, 3H, J ) 7.5 Hz);
13C NMR (100 MHz) δ 176.2 (CdO), 137.7 (C), 131.7 (CH), 128.7
(CH), 127.9 (CH), 127.1 (CH), 126.3 (CH), 116.7 (C), 93.3 (CH),
52.2 (CH3), 30.6 (C), 25.7 (CH2), 20.6 (CH2), 13.9 (CH3). HRMS
(ESI) m/z calcd for C16H18O2 242.1307, found 242.13014.
the isomeric vinylcarbenoids, with cyclopentadiene formation
occurring from 20. The initially formed cyclopentadiene 21
would then need to undergo a [1,5]-sigmatropic rearrangement
to form the observed product 22.
Conclusions
Representative Example for Synthesis of Cyclopentadienes. To
a two-neck, round-bottom flask equipped with condenser was added
1-pentyne (0.5 mmol), Rh2(S-DOSP)4 (0.01 mmol), and 1 mL of
2,2-dimethylbutane (DMB). The solution was stirred under an
atmosphere of argon then heated to reflux. Methyl phenylvinyl-
diazoacetate 6 (1.0 mmol) in 10 mL of DMB was then added to
former solution via syringe pump over 2 h. After addition, the
mixture was allowed to warm to room temperature. After stirring
for additional 20 min, the crude reaction mixture was concentrated
in Vacuo. The residue was purified on silica using 15:1 hexane/
diethyl ether as solvent system to give cyclopentadiene 12a in 61%
In summary, arylvinyldiazoacetates were found to be effective
systems for highly enantioselective Rh2(S-DOSP)4 -catalyzed
cyclopropenation reactions with terminal alkynes. A new class
of chiral vinylcyclopropenes with quaternary carbons is now
readily accessible. Under forcing conditions, the vinylcyclo-
propenes undergo a rhodium(II)-catalyzed ring expansion to
stable cyclopentadienes. Density functional calculations have
demonstrated that the exact transition state for the cyclopro-
penation event involves a tilted end-on approach that also
displays a favorable hydrogen-bonding interaction with a
carboxylate ligand on the catalyst. These observations provide
valuable insights for future catalyst design and expansion of
this methodology to more elaborate systems.
1
yield (75 mg) as yellow oil. IR (neat) 1696, 1205, 717 cm-1; H
NMR (400 MHz) δ 7.57 (d, 2H, J ) 7.2 Hz), 7.25-7.37 (m, 3H),
6.84 (s, 1H), 3.78 (s,1H), 3.74 (d, 2H, J ) 1.2 Hz), 2.84 (t, 2H, 7.6
Hz), 1.64 (s, 2H, J ) 7.6 Hz), 0.98 (t, 3H, J ) 7.6 Hz; 13C NMR
(100 Hz) δ 165.5 (CdO), 161.0 (C), 150.9 (C), 135.1 (C), 130.7
(CH), 128.9 (CH), 128.3 (CH), 127.4 (C), 125.8 (CH), 51.0 (CH3),
42.3 (CH2), 31.4 (CH2), 29.1 (CH2), 22.9 (CH2), 14.2 (CH3). HRMS
(ESI) m/z calcd for C16H18O2 242.1307, found 242.13004.
Experimental Section
Representative Example for Cyclopropenation of Alkynes.
A mixture of 1-pentyne (0.5 mmol) and Rh2(S-DOSP)4 (0.01 mmol)
was dissolved in 1 mL of pentane and stirred at -45 °C under an
atmosphere of argon. Methyl phenylvinyldiazoacetate 6 (1.0 mmol)
in 10 mL of pentane was then added to former solution via syringe
pump over 2 h. After addition, the mixture was stirred for additional
20 min, then concentrated in Vacuo. The residue was purified on
silica using 15:1 hexane/diethylether as solvent system to give
cyclopropene 11a in 81% yield (99 mg) as yellow oil. 99% ee
(determined by HPLC: (R,R)-Whelk, 5% i-PrOH in hexanes tR )
Acknowledgment. Support of this work by the National Science
Foundation (CHE-0750273 for H.M.L.D., CHE-952253 for J.A.)
is gratefully acknowledged. Support was also provided by the Center
for Computational Research at the University at Buffalo.
Supporting Information Available: Complete Gaussian’09
reference. Experimental details and characterization data are
available for all compounds. Cartesian coordinates and a
summary of calculated parameters are available for theoretical
structures reported. This material is available free of charge via
the Internet at http://pubs.acs.org.
8.37 (major) tR ) 9.82 (minor)). IR (neat) 1716, 1245, 743 cm-1
;
1H NMR (400 MHz) δ 7.15-7.38 (m, 6H), 6.33 (app t, 1H, J )
(20) Bishop, K. C. Chem. ReV. 1976, 76, 461. (b) Padwa, A.; Krumpe,
K. E.; Zhi, L. Tetrahedron Lett. , 1989, 2633. (c) Padwa, A.; Chiacchio,
U.; Gareau, Y.; Kassir, J. M.; Krumpe, K. E.; Schoffstall, A. M. J.
Org. Chem. 1991, 56, 2523.
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