π-Facial Selectivity in Diels-Alder Cycloadditions

Diels-Alder reactions between π-facially differentiated dienes and/or π-facially differentiated dienophiles frequently proceed with remark-able π-facial selectivity. Experimental and theoretical studies have been undertaken in an effort to gain insight into the fundamental origins of this phenomenon. Reactions of interest in this connection include thermal [4 + 2] cycloadditions between (i) various dienophiles and cage-annulated 1,3-cyclohexadienes (i.e. systems 1, 4, 6, and 9) and (ii) various dienes and cage-annulated dienophiles (i.e. systems 1a, 11a, and 14). The results of relevant molecular mechanics, semiempirical, and ab initio molecular orbital calculations generally are consistent with experiment