DIELS-ALDER REACTIONS OF 5-ALKYL-1,3-CYCLOPENTADIENES

Facial selectivity in the Diels-Alder reactions of 1,3-cyclopentadiene s substituted at C-5 by a variety of simple alkyl groups has been asse ssed with a number of dienophiles. The results are consistent with an explanation based on steric hindrance. Syn addition is more favored wi th sterically less demanding dienophiles. Diene 6, which is substitute d at C-5 with methoxymethyl, shows a remarkable preference for syn add ition with less encumbered dienophiles. This may indicate a conformati onal difference in its syn transition state relative to the transition states for addition syn to methyl, ethyl, or n-butyl substituents (di enes 1, 4, and 5). Dienophiles are more reluctant to add syn to the la rger C-5 group with 1,2,3,4,5-pentamethyl-1,3-cyclopentadiene (2) and derivatives (3, and 7) and conformational effects become very importan t when C-5 bears two alkyl groups, as in dienes 3 and 7.