An assisted solvolysis of 23-spirostanyl bromides and tosylates. A new rearrangement of spirostanes to the bisfuran systems.

Steroidal sapogenins bearing a good leaving group at C23 undergo a completely stereospecific rearrangement under a variety of conditions via a mechanism involving neighboring-group participation by the acetal oxygen atom in the departure of the nucleofuge from C23. The reactions of equatorial (23S)-23-bromo- or (23S)-23-tosyloxyspirostanes with either the alpha (25R) or beta (25S) oriented 25-methyl group lead to the bisfuran products with inversion of configuration at C23. The reactions of the starting compounds with axial substituents (23R) at C23 require drastic conditions and result in the formation of the corresponding olefin accompanied by the rearranged product (in the case of the 25S isomer only).