INTRAMOLECULAR META PHOTOCYCLOADDITION OF CONFORMATIONALLY RESTRAINED5-PHENYLPENT-1-ENES .1. BICHROMOPHORIC CYCLOHEXANE DERIVATIVES

The cis isomers of 1-allyl-2-phenylcyclohexane and 1-benzyl-2-vinylcyc lohexane yield predominantly 1,3 addition, while the trans isomers sho w a high selectivity for 2,6 addition, which is explained by steric in teractions. Introduction of a third substituent, OH or OCH3, on the al kenyl substituted carbon of the cyclohexane ring gives almost exclusiv ely 1,3 addition. This can mostly be explained in terms of more steric hindrance in the conformations leading to 2,6 addition. From the resu lts described it can be concluded that for 2,6 addition two conformati ons are available, while the energy difference between the 1,3 conform ations is much larger and therefore a completely stereoselective 1,3 a ddition is found (except for 6).