Scalable Synthesis of Cyclocitrinol.

A 10-step synthesis of the C25 steroid natural product cyclocitrinol from inexpensive, commercially available pregnenolone is reported. This synthesis features a biomimetic cascade rearrangement to efficiently construct the challenging bicyclo[4.4.1] A/B ring system, which enabled a gram-scale synthesis of the bicyclo[4.4.1] enone intermediate 18 in only nine steps. This work also provides experimental support for the biosynthetic origin of cyclocitrinol.