Stereoselectivity in the TiCl4-Catalysed Reaction of Danishefsky's Diene with a N-(acyloxy)iminium Ion: Synthesis of 5alpha versus 5beta delta1(2) -19-Nor-10-azasteroids. 4

The synthesis of 17 beta-acetyloxy-Delta(1(2))-10-azaestr-1-en-3-one has been accomplished by the TiCl4-catalysed reaction of carbamate 1 with Danishefsky's diene 2. The reaction provided 5 alpha isomer 3a or 5 beta isomer 3b depending on the reaction conditions: the addition of TiCl4 a mixture of 1 and 2 in CH2Cl2 resulted in the formation of thermodynamically favoured 5 alpha compound 3a (18%). whereas the addition of diene 2 to the mixture of the catalyst and 1 provided the 5 beta isomer 3b (23%) only