Conformational Study of SnCl4 Complexes of Aldehydes, α,β-Unsaturated Aldehydes, and Esters: Observation of Both s-Cis and s-Trans Isomers

Article abstract of DOI:10.1021/jo951625w

A variable-temperature NMR study shows that 2-methylpropanal (1a), ethyl 2-pentenoate (2), ethyl 4-methyl-2-pentenoate (3a), 2-pentenal (4), and 4-methyl-2-pentenal (5a) prefer the C-CH3 eclipsed conformations, rather than the CH-eclipsed form.The preference is enhanced in the presence of SnCl4.This is opposite to consideration based on steric interactions but is consistent with hyperconjugative interaction, in which the C-H bond is aligned with the ?*_C=C orbital.Thus, the current result suggests that the C-H bond is a better donor than the C-C bond in hyperconjugation with electron-deficient ? orbital.On the other hand, 2-ethylbutanal (1b), ethyl 4-ethyl-2-hexenoate (3b), and 4-ethyl-2-hexenal (5b) prefer the CH-eclipsed form due to steric effects.At -50 deg C, the SnCl4 complexes of 3a and 3b show two distinct sets of vinyl protons in a ratio of ca. 10:1, which are assigned to the s-trans- and s-cis isomers, respectively.

Full text of DOI:10.1021/jo951625w

J . Org. Chem. 1996, 61, 947-951
947
Con for m a tion a l Stu d y of Sn Cl4 Com p lexes of Ald eh yd es,
r,â-Un sa tu r a ted Ald eh yd es, a n d Ester s: Obser va tion of Both s-Cis
a n d s-Tr a n s Isom er s
Benjamin W. Gung* and Mathew M. Yanik
Department of Chemistry, Miami University, Oxford, Ohio 45056
Received September 6, 1995X
A variable-temperature NMR study shows that 2-methylpropanal (1a ), ethyl 2-pentenoate (2), ethyl
4-methyl-2-pentenoate (3a ), 2-pentenal (4), and 4-methyl-2-pentenal (5a ) prefer the C-CH3 eclipsed
conformations, rather than the CH-eclipsed form. The preference is enhanced in the presence of
SnCl4. This is opposite to consideration based on steric interactions but is consistent with
hyperconjugative interaction, in which the C-H bond is aligned with the π*CdC orbital. Thus, the
current result suggests that the C-H bond is a better donor than the C-C bond in hyperconjugation
with electron-deficient π orbital. On the other hand, 2-ethylbutanal (1b), ethyl 4-ethyl-2-hexenoate
(3b), and 4-ethyl-2-hexenal (5b) prefer the CH-eclipsed form due to steric effects. At -50 °C, the
SnCl4 complexes of 3a and 3b show two distinct sets of vinyl protons in a ratio of ∼10:1, which are
assigned to the s-trans- and s-cis isomers, respectively.
Ch a r t 1
In tr od u ction
Recently, we have reported the results from a system-
atic study using both ab initio molecular orbital methods
and variable-temperature (VT) NMR technique for the
conformations of chiral alkenes.1 Our study confirmed
Hehre's early predictions about the influence of the
substituent at vinyl position on the ground state confor-
mations of chiral alkenes: “...it is to be expected that
electron-withdrawing group (EWG) will favor conforma-
tions with the CO bond eclipsing πCdC, whereas electron-
releasing group (ERG) should enhance the ground-state
preference for the CH eclipsing the double bond.”2
the s-cis isomers for the SnCl4 complexes of esters 3a ,b
1
by H NMR at low temperature. This is of significant
interest since in many cases the goal of obtaining high
enantioselectivity in nucleophilic addition to R,â-unsatur-
ated ester system depends on our understanding and
control of the conformation.4-11
In a related issue, there has been a continuing debate
on the relative donor ability of C-C vs. C-H bonds in
organic molecules.12-16 Cieplak suggested that the C-H
bond is a better donor than the C-C bond on the basis
In our previous study, the VT NMR evidence also
suggested the presence of conformation III.1a We were
somewhat surprised because form III should be sterically
inhibited due to A1,3 strain.3 In order to clarify the
relative stability of III with respect to the steric bulk of
the R group, we have studied compounds 1-5 (Chart 1)
and their SnCl4 complexes. There is a clear difference
in conformational preference between the methyl (R )
Me, 1a , 2, 3a , 4, 5a ) and the ethyl (R ) Et, 1b, 3b, and
5b) derivatives. The CC-eclipsed conformation (III) was
found to be more stable than the CH-eclipsed form in the
cases of the methyl derivatives. The opposite is true for
the ethyl derivatives. In addition to these observations,
we have also been able to observe both the s-trans and
(4) Loncharich, R. J .; Schwartz, T. R.; Houk, K. N. J . Am. Chem.
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(5) Oppolzer, W.; Loher, H. J . Helv. Chim. Acta 1981, 64, 2808.
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1984, 25, 5885. (b) Oppolzer, W.; Kelly, M. J .; Bernardinelli, G.
Tetrahedron Lett. 1984, 25, 5889. (c) Oppolzer, W.; Rodriguez, I.; Blagg,
J .; Bernardinelli, G. Helv. Chim. Acta 1989, 72, 123.
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1990, 200, 77.
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X Abstract published in Advance ACS Abstracts, February 1, 1996.
(1) (a) Gung, B. W.; Wolf, M. A.; Zhu, Z. J . Org. Chem. 1993, 58,
3350-3354. (b) Gung, B. W.; Wolf, M. A. J . Org. Chem. 1993, 58, 7038-
7044. (c) Gung, B. W.; Gerdeman, M. S.; Fouch, R.; Wolf, M. A. J . Org.
Chem. 1994, 59, 4255-4261. (d) Gung, B. W.; Melnick, J . P.; Wolf, M.
A.; Marshall, J . A.; Beaudoin, S. J . Org. Chem. 1994, 59, 5609-5613.
(e) Gung, B. W.; Zhu, Z.; Fouch, R. A. J . Am. Chem. Soc. 1995, 117,
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1984, 106, 466.
(11) (a) Suzuki, I.; Kin, H.; Yamamoto, Y. J . Am. Chem. Soc. 1993,
115, 10139-10144. (b) Shida, N.; Kubato, C.; Niwa, T.; Ebata, T.;
Yamamoto, Y. J . Org. Chem. 1994, 56, 4068.
(12) (a) Cieplak, A. S.; Tait, B. D.; J ohnson, C. R. J . Am. Chem. Soc.
1989, 111, 8447. (b) Cieplak, A. S. J . Am. Chem. Soc. 1981, 103, 4540.
(13) (a) Roseboom, M. D.; Houk, K. N. J . Am. Chem. Soc. 1982, 104,
1189. (b) Dorigo, A. E.; Pratt, D. W.; Houk, K. N. J . Am. Chem. Soc.
1987, 109, 6591. (c) Broeker, J . L.; Hoffmann, R. W.; Houk, K. N. J .
Am. Chem. Soc. 1991, 113, 5006.
(2) Khan, S. D.; Pau, C. F.; Chamberlin, A. R.; Hehre, W. J . J . Am.
Chem. Soc. 1987, 109, 650.
(3) For a review, see: Hoffmann, R. W. Chem. Rev. 1989, 89, 1841.
0022-3263/96/1961-0947$12.00/0 © 1996 American Chemical Society

948 J . Org. Chem., Vol. 61, No. 3, 1996
Gung and Yanik
of the Baker-Nathan order.12 However, Houk indicated
that the C-C bond is a better donor on the basis of the
fact that the ionization potentials in a series of pip-
eridines decrease in a nonlinear fashion.13a Whether the
origin of the Baker-Nathan effect is hyperconjugative
has been disputed. Differential solvation was suggested
as the reason for the observed rate order.14 Thus, the
order of hyperconjugation ability for alkyl groups was
also questioned. The stereochemical results from some
rigid cyclic ketones also cast doubt on the relative donor
ability of CH vs. CC bonds.15
However, recent evidence seems to indicate that hy-
perconjugative electron donation by σCH bonds is larger
than that by σCC bonds. Laube has reported a theoretical
study concerning hyperconjugation in substituted acet-
aldehydes.16a On the basis of X-ray structure data and
ab initio calculations at the 4-31G* level, the potential
energy curves for rotation about the Csp2-Csp3 bond of
substituted acetaldehydes are obtained. The least-
squares adjustment of a truncated Fourier series allows
a separation of the potential energy into a 1-fold (dipolar
and steric interactions), a 2-fold (hyperconjugative in-
teractions), and a 3-fold term (bond electron repulsions).
A comparison of the V2 coefficients shows the following
order of hyperconjugation: CH > CC > CCl > CF. On
the basis of the X-ray structure of the SbCl5-complexed
5-phenyladamantanone, Laube also concludes that a CH
bond is a better donor than a Csp3-Csp2 bond.16b
We figured that the conformational preference of
compounds 1-5 can be used to judge the donor ability of
C-H/C-C bonds. Rotation around the Csp2-Csp3 bond
can lead to either the CH-eclipsed form (A) or the C-C-
eclipsed form (B). The better donor bond would prefer
to align with the π* orbital, rather than the CdC bond,
on the basis of the hyperconjugative model.13b,16a
F igu r e 1. Coupling constants (3J 12) as a function of temper-
ature for aldehyde 1a (O), SnCl4‚1a (]), 1b (-), and SnCl4‚1b
(+).
of the coupling constants will change with the popula-
tions of the conformers. As the temperature is lowered,
the more stable conformation will become more popu-
lated. Therefore, the more stable conformer is found
through examining the relationship of coupling constant
with temperature.17
Exp er im en ta l Section
Esters 2 and 3a ,b (Chart 1) are prepared from propanal,
2-methylpropanal (1a ), and 2-ethylbutanal (1b), respectively,
by treatment with Wittig reagent (Ph3P)CHCO2Et). The R,â-
unsaturated aldehydes 4 and 5a ,b are obtained through
reduction of 2 and 3a ,b with DIBAL-H, respectively, followed
by oxidation with the Swern procedure.18 The NMR experi-
ments involve the measurements of the three-bond (3J 12, 3J 23
,
3J 34) proton coupling constants of compounds 1-5 at various
temperatures and subsequent analysis using the J -averaging
method.17 The details of the VT NMR experiments follow the
1
early reports.1 Briefly, the H NMR spectra were recorded on
a Bruker 300 MHz instrument with a variable-temperature
probe. The samples were degassed before each run, and the
concentration was about 0.05 M. The variable-temperature
data were reproducible to (0.02 Hz in duplicate runs. Deu-
terated chloroform was used in these experiments. The SnCl4
complexes of compounds 1-5 were freshly prepared in a ratio
of 1:1 before each experiment under nitrogen atmosphere.
Resu lts a n d Discu ssion
A. Ster ic a n d Hyp er con ju ga tive Effects on Th e
Con for m a tion s of Com p ou n d s 1-5 a n d Th eir Sn Cl4
Com p lexes. The variable-temperature NMR data are
summarized in Figures 1-5 and Figure 8. The three-
bond coupling between the aldehydic proton and the R
proton (3J 12) of aldehydes 1a ,b (Figure 1) gives a measure
for the equilibrium shown in eq 1. Similarly, the coupling
constants between the vinylic proton and the allylic
proton (3J 23) of esters 2 and 3a ,b (Figures 2 and 3) are
3
measures for the equilibrium shown in eq 2. The J 34 of
Using the VT NMR technique, we were able to deter-
aldehydes 4 and 5a ,b (Figures 4 and 5) corresponds to
mine which bond is eclipsed with the CdC linkage by
the rotation shown in eq 3.
recording the coupling constants of the Csp2H-Csp3
H
According to the data in Figure 1, when R ) Me (1a )
the coupling constants are small and decrease as tem-
perature is lowered. This indicates a shift of population
toward conformation B at lower temperatures. There-
fore, the methyl-eclipsed form (B) is more stable than
the CH-eclipsed form (A). This is consistent with the
rotamer.1 The CH-eclipsed form (conformer A) should
give a greater coupling constant (3J 12, 3J 23, 3J 34 for eqs 1,
2, 3, respectively) than the C-C-eclipsed conformation
(B). In a rapid equilibrium mixture, a weighted average
(14) (a) Schubert, W. M.; Sweeney, W. A. J . Org. Chem. 1956, 21,
119. (b) Hehre, W. J .; McIver, Pople, J .; Schleyer, J . Am. Chem. Soc.
1974, 96, 7162.
(15) Li, H.; Mehta, G.; Padma, S.; le Noble, W. J . J . Org. Chem.
1991, 56, 2006.
(16) (a) Laube, T.; Ha, T.-K. J . Am. Chem. Soc. 1988, 110, 5511. (b)
Laube, T.; Stilz, H. U. J . Am. Chem. Soc. 1987, 109, 5876.
(17) For examples of the application of the J -averaging method,
see: (a) Bothner-By, A. A.; Castellano, S.; Ebersole, S. J .; Gunther, H.
J . Am. Chem. Soc. 1966, 88, 2466. (b) Copley, S. D.; Knowles, J . R. J .
Am. Chem. Soc. 1987, 109, 5008.
(18) Mancuso, A. J .; Swern, D. Synthesis 1981, 165.

Conformational Study of SnCl4 Complexes
J . Org. Chem., Vol. 61, No. 3, 1996 949
F igu r e 2. Coupling constants (3J 23) as a function of temper-
ature for esters 2 (O), 3a (4), and 3b (+).
F igu r e 5. Coupling constants (3J 34) as a function of temper-
ature for the SnCl4 complexes of aldehyde 4 (O), 5a (4), and
5b (+).
This divergent behavior between the methyl and the
ethyl derivatives is also observed for the R,â-unsaturated
esters 3a ,b (Figures 2 and 3) and aldehydes 5a ,b (Figures
4 and 5). The differences in the magnitude of coupling
between 3a and 3b and between 5a and 5b are even
greater, but complexation of SnCl4 did not change the
coupling constants for 3b and 5b. These facts are
consistent with the 1,3-interactions shown above for 5b
(D). Clearly, a CH-eclipsed form (A) is favored for 5b if
the pentane carbon chain in 5b is to remain in the
preferred all-anti conformation.
F igu r e 3. Coupling constants (3J 23) as a function of temper-
ature for the SnCl4 complexes of esters 2 (O), 3a (4), and 3b
(+).
On the basis of the steric effects described above, it is
expected that the ethyl derivatives (1b, 3b, and 5b) prefer
the CH-eclipsed form (A). However, it is uncertain why
the methyl derivatives (1a , 2, 3a , 4, 5a ) favor the CC
eclipsed form (B). Wiberg has indicated that the prefer-
ence for an alkyl group to be eclipsed with a CdO bond
in an aldehyde or a ketone is probably due to dipole
induced-dipole interaction.18 However the data are not
inconsistent with hyperconjugative effects,13b,16a such as
the overlap between the allylic σCH and π*CdC orbitals.
This overlap is possible only when the CH bond is in the
skew position, Figure 6. The fact that the C-C bond is
eclipsed with the CdC linkage is consistent with the
C-H bond being a better donor.
It is known that the CH-eclipsed forms of 1-butene and
3-methyl-1-butene are more stable than their methyl-
eclipsed form by ∼0.2518 and ∼0.7513c kcal/mol, respec-
tively. Although these results are in contrast to our
current observations for the R,â-unsaturated aldehydes
and esters, an argument can be made to support the
hyperconjugative concept. Hyperconjugation is most
pronounced when there is a positive charge, such as in
the case of a carbocation, next to CH bonds. 1-Butene
and 3-methyl-1-butene have an electron neutral double
bond, but the R,â-unsaturated aldehydes 5 and esters 3
are electron-deficient alkenes. Thus, different degrees
of demands for hyperconjugation exist in these alkenes.
It is reasonable to assume that the R,â-unsaturated
aldehydes 5 and esters 3 are stabilized by hyperconju-
gative interactions more than that for 1-butene and
3-methyl-1-butene.
F igu r e 4. Coupling constants (3J 34) as a function of temper-
ature for aldehyde 4 (O), 5a (4), and 5b (+).
results from previous studies on the conformations of
propanal and 2-butanone.19
When R ) Et (1b), the coupling constants (Figure 1)
are much larger than that of 1a and increase as temper-
ature is lowered, which means that the CH-eclipsed form
(A) is favored. We interpret the difference between the
methyl and the ethyl derivatives as steric effects. As
shown below (C), a 1,3-close contact between the terminal
methyl group and the aldehydic proton and between the
other methyl group and the carbonyl oxygen is present
if the CdO is eclipsed with the ethyl group in 1b. This
3
analysis is confirmed by the observation that J 12 of 1b
increases when 1b is complexed with SnCl4 (Figure 1).
The reason for this increase in magnitude of coupling is
the increased steric bulk around the carbonyl oxygen
atom, which further raises the repulsive interaction
shown below (C). Thus, the preferred conformation for
1b is the CH-eclipsed form (B).
(19) Wiberg, K. B.; Martin, E. J . Am. Chem. Soc. 1985, 107, 5035.

950 J . Org. Chem., Vol. 61, No. 3, 1996
Gung and Yanik
F igu r e 6. Stabilizing interaction between the σCH bond and
the π*CdC orbital in methyl 2-pentenoate.
In cases where 1 equiv of SnCl4 is added to the CDCl3
solution of the carbonyl compounds, a slight enhancement
in the trend is observed. For example, aldehyde 4
exhibits three-bond coupling 3J 34 from a range of 6.25 Hz
at 325 K to 5.88 Hz at 223 K (∆J /∆T ) 0.0036). Its SnCl4
complex has coupling constants of 6.11 Hz at 325 K and
5.48 Hz at 223 K (∆J /∆T ) 0.0062). Thus, the SnCl4
complex has both smaller coupling constants and greater
temperature coefficient, which indicates a greater stabil-
ity of the methyl-eclipsed conformer (B). The hypercon-
jugative effect is more pronounced in the Lewis acid
complex since SnCl4-complexation reduces the electron
density of the double bond and lowers the energy of the
π*CdC orbital.
It should be noted that the other SnCl4 complexes
examined in this study (other than 4) show negligible
changes in their coupling constants. This is probably due
to the fact that the methyl-eclipsed form (see below) of
aldehyde 4 enjoys two skew CH bonds while the confor-
mation B of 3a and 5a have only one CH bond in the
skew position to participate in hyperconjugation. The
stabilization of a conformation resulted from a CH bond
participating in hyperconjugation with the π*CdC orbital
is only slightly greater than that from a CC bond.
Currently, we prefer the hyperconjugative model since
the dipole induced-dipole theory does not give a better
explanation of the data.
B. Ob ser va t ion of t h e s-Cis a n d t h e s-Tr a n s
Isom er s. The interconversion of the s-trans and s-cis
forms of R,â-unsaturated esters has been a concern of
synthetic organic chemists for a long time because
opposite π-face selectivity results from s-trans and s-cis
forms.5-11 Illustrated below is an example recently
reported by Yamamoto.11
F igu r e 7. Vinyl region of the proton NMR spectra of ethyl
4-methyl-2-pentenoate complexed with 1 equiv of SnCl4 at
various temperatures.
theoretical4 studies, the major isomer is assigned the
s-trans conformation. The coalescence temperature for
s-trans/s-cis isomerization is ∼-25 °C (Figure 7). By
using the approximate equation (eq 4)20,21 and extrapolat-
Although X-ray structures of acrylates have been
reported,5-11 the dynamics of s-trans/s-cis isomerization
in solution have rarely been studied. One reason for the
lack of study might be the lack of proper tools.
During our work on SnCl4-complexed R,â-unsaturated
esters, it was discovered that the barrier to rotation in
complex 3a was considerably higher than that of the free
ester. At -40 °C, the s-trans and the s-cis isomers give
distinct 1H NMR spectra (Figure 7). A ratio of ap-
proximately 93:7 can be determined by integration of the
1H NMR. On the basis of previous experimental5-11 and
(20) (a) Binsch, G. In Topics in Stereochemistry; Eliel, E. L., Allinger,
N. L., Eds,; Wiley: New York, 1968; Vol. 3, p 97. (b) Sutherland, I. O.
In Annual Reports on NMR Spectroscopy; Mooney, E. F., Ed.; Academic
Press: London, 1971, 4, 71-235.
(21) (a) J ackman, L. M.; Cotton, F. A. Dynamic Nuclear Magnetic
Resonance Spectroscopyp; Academic Press: New York, 1975. (b) Binsch,
G. J . Am. Chem. Soc. 1969, 91, 1304. (c) Iwamura, M. Katoh, M.;
Iwamura, H. Org. Magn. Reson. 1980, 14, 392-397.

Conformational Study of SnCl4 Complexes
J . Org. Chem., Vol. 61, No. 3, 1996 951
almost exclusively the s-trans form. However, Marshall
has shown that the addition of chiral allylstannanes to
a chiral R,â-unsaturated aldehyde occurs most likely
through the s-cis form.24 The current results are consis-
tent with both Corey and Denmark's observations; i.e.,
the s-trans isomer is preferred in solution. Complexation
with SnCl4 enhances this preference. All three R,â-
unsaturated aldehydes (4, 5a ,b) exhibit the same trend
as that shown in Figure 8 for 5a . The significant increase
of the magnitude in coupling constant from free aldehyde
to complex is consistent with a greater population of
s-trans isomer for the complex. However, since no
coalescence was observed, the rotation around the C-C
bond must be fast. This means the s-cis isomer is
available for reaction. Thus, according to Curtin-Ham-
mett principle,25 the experimental observations of both
Corey and Marshall can be interpreted as this: the s-cis
isomer must be the more reactive conformation. In fact,
this has been suggested in the literature.4
F igu r e 8. Coupling constants (3J 12) as a function of temper-
ature for aldehyde 5a (O) and SnCl4‚5a (]).
ing the υab from several ∆υ’s at lower temperatures to
the coalescence point, the barrier to rotation for the
complex is ∼12.5 kcal/mol using the Eyring equation (eq
5).20,21
Con clu sion
πνab
kc )
) 2.22νab
(4)
By using the variable-temperature NMR technique, it
is found that aldehyde 1a , R,â-unsaturated esters 2 and
3a , and R,â-unsaturated aldehydes 4 and 5a and their
SnCl4 complexes prefer the methyl-eclipsed conformation
(B), rather than the C-H-eclipsed form (A). This phe-
nomenon is discussed in terms of hyperconjugative
interactions between the better donor C-H bond and the
π* orbital. The C-C bond is not as good a donor as the
C-H bond. Therefore, the methyl-eclipsed form is more
stable for these electron-deficient alkenes. The ethyl
derivatives (1b, 3b, and 5b) favor the CH-eclipsed form
as expected on the basis of steric effects. The predomi-
nant conformation for SnCl4-complexed R,â-unsaturated
esters is the s-trans form. The rotational barrier around
the Csp2-Csp2 bond in a SnCl4 complexed R,â-unsaturated
ester is about 12.5 kcal/mol. The rotation around this
bond is slower than the NMR time scale at or below -40
°C. On the other hand, the rotation around the same
bond is still fast for the SnCl4-complexed R,â-unsaturated
aldehydes at this temperature.
x
2
T
k
∆Gq ) 2.303RT 10.319 + log
(5)
(
)
Unlike the SnCl4-complexed acrylates, rapid equilib-
rium still occurs between the s-trans and s-cis isomers
of SnCl4-complexed R,â-unsaturated aldehydes at or
above -50 °C. No coalescence was observed for these
complexes. However, it is also of significance to uncover
the relative stability of these isomers. For example,
Corey has demonstrated the importance of understanding
the detailed structure of Lewis acid-complexed acrolein
in relation to enantioselectivity and has shown that BF3-
complexed 2-methylacrolein has the s-trans conforma-
tion, yet the enantioselective Diels-Alder cycloadditions
occur through the s-cis form.22 Denmark has shown that
the s-trans conformation is the predominant form for the
complex of 2-heptenal with SnCl4 by using nuclear
Overhauser difference spectroscopy.23
The calculated energy difference between s-trans and
s-cis acrolein is 1.8 kcal/mol at the 6-31G*//3-21G level
of theory.4 Lithium-complexed acrolein gives a difference
of 3.2 kcal/mol.4 According to these calculated numbers,
Lewis acid-complexed R,â-unsaturated aldehydes assume
Ack n ow led gm en t. This research is supported by a
grant from the National Institutes of Health (GM49745).
M.M.Y. acknowledges a summer Undergraduate Schol-
arship from Howard-Hughes Medical Institute. We
thank the National Science Foundation for Grants
(CHE-9012532 and CHE-8951897) for the purchase of
the 300 MHz Bruker FT-NMR and the HP GC-MS.
Su p p or tin g In for m a tion Ava ila ble: Variable tempera-
ture 1H NMR data for compounds 1-5 (7 pages). This material
is contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
J O951625W
(22) (a) Corey, E. J .; Loh, T.-P. J . Am. Chem. Soc. 1991, 113, 8966.
(b) Corey, E. J .; Loh, T.-P.; Roper, T. D.; Azimioara, M. D.; Noe, M. C.
J . Am. Chem. Soc. 1992, 114, 8290. (c) Corey, E. J .; Loh, T.-P.; Sarshar,
S.; Azimioara, M. Tetrahedron Lett. 1992, 33, 6945.
(23) Denmark, S. E.; Almstead, N. G. J . Am. Chem. Soc. 1993, 115,
3133-3139.
(24) Marshall, J . A.; Beaudoin, S. J . Org. Chem. 1994, 59, 7833. See
also: Wong, S. S.; Paddon-Row, M. N.; Li Y.; Houk, K. N. J . Am. Chem.
Soc. 1990, 112, 8679.
(25) Seeman, J . I. Chem. Rev. 1983, 83, 83.