Reversal of regioselectivity with increasing ring size of alkene component in [2+2] photoadditions

Article abstract of DOI:10.1016/S0040-4039(00)97707-4

Photoaddition of cyclohexenones 1a or 1b to methyl cyclobutene- (2), cyclopentene- (5) or cyclohexenecarboxylate (9) shows a gradual reversal of regioselectivity from head-to-head (3a/b) to head-to-tail adducts (11a/b). These results are inconsistent with the generally accepted dipolar interaction model.