Application of the Criegee rearrangement in singlet oxygen reaction: A high yield of diosone from the photooxygenation of pseudodiosgenin diacetate

Photochemically generated singlet oxygen adds selectively to C-20-C-22 rather than C-5-C-6 double bond of pseudodiosgenin diacetate 1 to give diosone 4, an important intermediate for preparation of many steroidal drugs, and ene product 3 at room temperature. 3 was separated and characterized by H-1 and C-13 NMR as well as IR. When pyridine and acetic anhydride were presented, the Criegee rearrangement of 3 can yield 4, as a result, leading to a high collection yield (80% in acetonitrile) of 4.