A Biomimetic Approach to C-nor-D-homo-Steroids

A biomimetic three-step transformation of classical 6-6-6-5-steroids into their C-nor-D-homo-counterparts gives an easy and fast access to this highly important substructure of natural products, as it is found in cyclopamine, and nakiterpiosin. A novel reagent combination allows for the rearrangement even of 17-keto steroids with high endoselectivity. In several examples the broadness of this strategy is outlined.