Stereochemical Effect of the Small Ring Unit in 7-R-3,4-Cyclopropano- and 7-R-3,4-Epoxy-bicyclo[4.2.0]octan-1,5-Dienes on Conrotatory Ring Opening Torquoselectivity

Application of the differential activation energy approach suggests that the strong inward opening tendency of the formyl group in 3,4-cyclopropano-7-formyl-bicyclo[4.2.0]octan-1,5-diene and also 3,4-epoxy-7-formyl-bicyclo[4.2.0]octan-1,5-diene is either counter balanced to a level that the reaction turns torquo-neutral or it rotates outward, depending upon the stereo-relationship of the small ring unit with the formyl substituent. The NBO interaction approach, however, predicts inward selectivity throughout. These molecules, therefore, can be used as excellent test examples to judge the validity of one approach over the other.