Instead of environmentally toxic chromium oxidant, singlet oxygen generated photochemically was used as environmentally friendly and benign oxidizing agent to accomplish the transformation of pseudodiosgenin diacetate to diosone efficiently in a low toxic and less expensive solvent acetone. Accordingly 16-dehydropregnenolone acetate (16-DPA), an important intermediate for preparation of steroidal drugs, was prepared in good yield (75%) when photoreaction was run in acetone/acetic anhydride/pyridine system. The mild reaction condition as well as simple and environmentally friendly process made the method commercially viable and important for production of 16-DPA in industrial scale.

Photochemically generated singlet oxygen adds selectively to C20–C22 rather than C5–C6 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 1H and 13C 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.