title: Diels-Alder Reactions of 5-Alkyl-1,3-cyclopentadienes.
abstract: Facial selectivity in the Diels-Alder reactions of 1,3-cyclopentadienes substituted at C-5 by a variety of simple alkyl groups has been assessed with a number of dienophiles. The results are consistent with an explanation based on steric hindrance. Syn addition is more favored with sterically less demanding dienophiles. Diene 6, which is substituted at C-5 with methoxymethyl, shows a remarkable preference for syn addition with less encumbered dienophiles. This may indicate a conformational difference in its syn transition state relative to the transition states for addition syn to methyl, ethyl, or n-butyl substituents (dienes 1, 4, and 5). Dienophiles are more reluctant to add syn to the larger C-5 group with 1,2,3,4,5-pentamethyl-1,3-cyclopentadiene (2) and derivatives (3, and 7) and conformational effects become very important when C-5 bears two alkyl groups, as in dienes 3 and 7.
authors.affiliation: Department of Chemistry, Memorial University of Newfoundland, St. John's, Newfoundland A1B 3X7, Canada.
authors.forename: Johnathon E.
authors.initials: JE
authors.lastname: Letourneau
authors.forename: Mark A.
authors.initials: MA
authors.lastname: Wellman
authors.forename: D. Jean
authors.initials: DJ
authors.lastname: Burnell