Page 505 - Advanced Organic Chemistry Part B - Reactions & Synthesis

Table 6.1. Endo:Exo Stereoselectivity toward 479
Cyclopentadiene
SECTION 6.1
Dienophile Endo:exo ratio
Diels-Alder Reactions
CH 2 =CHCH=O a 80:20
a
CH 2 =CHCOCH 3 82:18
b
CH 2 =CHCO 2 CH 3 73:27
b
CH 2 =C CH 3 CO 2 CH 3 30:70
b
CH 3 CH=CHCO 2 CH 3 52:48
c
CH 2 =CHSO 2 CH 3 75:25
d
CH 2 =CHPO OCH 3 2 55:45
CH 2 =CHCN e 58:42
CH 2 =C CH 3 CN e 12:88
CH 3 CH=CHCN e 34:66
a. O. F. Guner, R. M Ottenbrite and D. D. Shillady, J. Org.
Chem., 53, 5348 (1988).
b. K. N. Houk and L. J. Lusku, J. Am. Chem. Soc., 93, 4606
(1971).
c. J. C. Philips and M. Oku, J. Org. Chem., 37, 4479 (1972).
d. H. J. Callot and C. Berezra, J. Chem. Soc., Chem. Commun.,
485 (1970).
e. A. I. Konovalov and G. I. Kamasheva, Russ. J. Org Chem
(Engl. Trans.), 8, 1879 (1972)
given in Table 6.1. Most of the data pertain to dienophiles with carbonyl substituents.
Note that tetrahedral noncarbonyl EWGs such as sulfonyl and phosphonyl also exhibit
a small preference for the endo TS. The cyano group shows little endo:exo preference.
Both - and -methyl groups result in more exo product, as seen for the methyl-
substituted esters and nitriles. As we will see shortly, the use of Lewis acid catalysts
usually increases the preference for the endo TS.
Steric effects play a dominant role with more highly substituted dienes. Hexachloro-
cyclopentadiene, for example, shows a higher endo preference than cyclopentadiene
because the 5-chlorine causes steric interference with exo substituents. 13

Cl
Cl
X

Cyclic -methylene ketones and lactones, in which the syn conformation is
enforced, give predominantly exo adducts. 14

O
O
CH 2 O
X
+ X
X
X = O, CH 2

13 K. L. Williamson, Y.-F. L. Hsu, R. Lacko, and C. H. Youn, J. Am. Chem. Soc., 91, 6129 (1969).
14
F. Fotiadu, F. Michel, and G. Buono, Tetrahedron Lett., 31, 4863 (1990); J. Mattay, J. Mertes, and
G. Maas, Chem. Ber., 122, 327 (1989).