Facial Differentiation in Diels-Alder Reactions to Dissymmetric Cyclohexa-1,3-dienes

Article abstract of DOI:10.1021/jo00007a051

Reactions of 2a-c with maleic anhydride (MA) and benzoquinone (BQ) show strong preference for addition to the "carbonyl" face of the diene.For dimethyl acetylenedicarboxylate (DMAD), attack from this face decreases with successive methylidene substitution while for N-phenyl-1,2,4-triazolinedione (PTAD) the reverse occurs.The consequence of orbital tilting and transition-state steric and torsional interactions cannot alone account for the facial selectivity for the reactions with DMAD and PTAD.Unfavorable orbital interaction of the closed shells of the carbonyl(s) and methylidene(s) syn to the incoming orthogonal ? orbital of DMAD is considered to be important.