Face Selection in Thermal Cycloaddition and -reversion

Article abstract of DOI:10.1021/jo00020a041

The isomer distribution has been determined in several cycloaddition reactions involving 5-fluoro- and 5-phenyladamantane derivatives in which C₂ serves as one of the trigonal termini.The reactions include a <2+2> cycloaddition with dichloroketene, a nucleophilic addition followed by cyclization, a <2+2> cycloreversion of a β-lactone, and two Diels-Alder reactions; in the latter case, the adamantane moiety was incorporated in the diene in one instance and in the dienophile in the other.In all these reactions, the reagent attacking the adamantane substrate was found to do so preferentially at the zu face, by modest but clear margins.This observation is in accord with the concept of transition-state hyperconjugation.This explanation is extented to several literature examples involving substrates with faces that are not isosteric because the polarizing group is placed closer to the site of attack; the ratios are substantial and in favor of the more hindered product in many of these examples.