Synthesis, Structure, and Reactivity of Secosteroids Containing a Medium-Sized Ring. Part 36. Transannular Photocyclization of Some Unsaturated 5,10-Secosteroidal Ketones: A Reevaluation.

The structures 9 and 8 are proposed for the single isolated irradiation product of 5-oxo-5,10-secocholest-1(10)-en-3α-yl acetate (6) [2] and for the minor product of irradiation of 5-oxo-5,10-secocholest-1(10)-en-3β-yl acetate (1) [3], respectively. These compounds are formed in an alternative reaction with respect to the originally observed intramolecular Paterro-Buchi photoprocess (transformation of 1 to oxetane 2). The formerly postulated ‘active’ conformations for 1 and 6 still allow explanation of their generation.