Stereo- and spinselectivity of primary (singlet) and secondary (triplet) Norrish type II reactions

Abstract

The influence of reactive conformation and substitution pattern on the Norrish Type II reactivity and selectivity of singlet-excited phthalimides was investigated. Only the cis-diasteroisomer of the 4-tert-butyl cyclohexylamine derivatives 1 underwent Yang cyclization. The phthaloyl leucine esters 3a and 3b both gave primarily Yang cyclization with subsequent ring expansion. As a secondary photoreaction, 3a gave Norrish II cleavage solely, wheras photolysis of the tert.-butyl ester 3b resulted in a 1:1 mixture of Norrish II cleavage and Yang cyclization product 4 and 5.