pi-facial selectivity in Diels-Alder cycloadditions

Diels-Alder reactions between pi -facially differentiated dienes and/or pi -facially differentiated dienophiles frequently proceed with remarkable pi -facial selectivity. Experimental and theoretical studies have been underta ken in an effort to gain insight into the fundamental origins of this pheno menon. Reactions of interest in this connection include thermal [4 + 2] cyc loadditions between (i) various dienophiles and cage-annulated 1,3-cyclohex adienes (i.e., systems 1, 4, 6, and 9) and (ii) various dienes and cage-ann ulated dienophiles (i.e., systems 1a, 11a, and 14). The results of relevant molecular mechanics, semiempirical, and ab initio molecular orbital calcul ations generally are consistent with experiment.