Enhanced formation of inverted housane through steric effects by rotationally unsymmetric bridgehead substituents in the ring closure of triplet cyclopentane-1,3-diyl diradicals, generated photolytically from 2,3-diazabicyclo[2.2.1]heptene(DBH)-type azoalkanes

The photolysis of the cyclopentene-annelated DBH-type azoalkanes 1c,d with the n-propyl and acetoxymethyl substituents at the bridgehead positions affords under singlet conditions (high-temperature direct photolysis) predominantly the retained housanes anti-2c,d, while under triplet conditions (low-temperature direct or benzophenone-sensitized photolysis) the inverted diastereomer syn-2c,d is favored. The latter novel stereochemical feature of the triplet pathway is rationalized in terms of the unsymmetric nature of the n-propyl and acetoxymethyl substituents in regard to rotation about the bridgehead position of the planar cyclopentane-1,3-diyl triplet diradicals (DR)-D-3(c,d). In their lowest-energy conformation (AM1 calculations), the rotationally unsymmetric substituents prefer to be located opposite to the annelated cyclopentene ring. The ratio of the diastereomeric syn and anti housanes is determined by the equilibrium distribution of the bridgehead-substituent conformers for the triplet diradicals on ring closure after ISC.