Page 546 - Advanced Organic Chemistry Part B - Reactions & Synthesis

520 One factor that is believed to contribute to the sluggishness of the reaction is that
a chairlike arrangement of the linking group causes a twist in the ester group from
CHAPTER 6 the preferred planarity. The TS also requires that the ester alkyl group be in an anti
Concerted relationship to the carbonyl group, rather than the preferred syn conformation. Several
Cycloadditions,
Unimolecular substituted systems have been studied and they react primarily through a boatlike endo
Rearrangements, and TS. 127 The size of the -substituent R controls the degree of preference for the TS.
Thermal Eliminations
O
R R O CH(CH 3 2 O CH(CH )
)
O O O O H H 3 2
(CH ) CH O CH CH O
3 2 3 H 3 O
+
H
R R
R H H H
H
CH 3
R = H R = CH R = C(CH )
3 3 3
52% yield; 76% yield; 55% yield,
6:4 cis:trans 5:1 cis:trans only cis ring
ring junction ring junction junction
This system has been studied computationally at the B3LYP/6-31+G level. 128
∗
In agreement with the experimental results, the endo boat TS was found to be the most
stable. The endo chair and exo boat were about 1.3 kcal/mol higher in energy, and the
exo chair still higher. This study confirmed that the boatlike TS allows the ester group
to stay closer to planarity. Eclipsing interactions also contribute to the higher energy
of the chairlike TS. In accordance with the idea that a bidentate Lewis acid might both
effect Lewis acid catalysis and promote a planar geometry at the ester group, it was
found that the reaction could be effectively catalyzed by a bidentate Lewis acid. 129
Use of one equivalent of the catalyst gave 95% yield after 2 h at 0 C. The catalyst is

believed to be coordinated with both the carbonyl and the ester oxygens.
CH 3
O
O H Al Cl
1.1 eq cat. O CF SO N
O 3 2
Al Cl
0°C, 2 h CH 3
CH 3 H CH 3
catalyst
Some examples of IMDA reactions are given in Scheme 6.5. In Entry 1 the
dienophilic portion bears a carbonyl substituent and cycloaddition occurs easily. Two
stereoisomeric products are formed, but both have the cis ring fusion, which is the
stereochemistry expected for an endo TS, with the major diastereomer being formed
from the TS with an equatorial isopropyl group.

O
O H O H H
H H

H

127
M. E. Jung, A. Huang, and T. W. Johnson, Org. Lett., 2, 1835 (2000); P. Kim, M. H. Nantz, M. J. Kurth,
and M. M. Olmstead, Org. Lett., 2, 1831 (2000).
128 D. J. Tantillo, K. N. Houk, and M. E. Jung, J. Org. Chem., 66, 1938 (2001).
129
A. Saito, H. Ito, and T. Taguchi, Org. Lett., 4, 4619 (2002).