The Supertristeroids:  Large, Chiral, Molecular Bowls Prepared by Trimerization of Pentacyclic Steroidal Ketones

Robinson annulation of coprostanone (1) at the 2,3- and 3,4-positions gave two pentacyclic enones (7 and 10) that contain A/B-cis-fused ring junctions. Reduction of these enones gave the pentacyclic steroidal ketones 2α,3β- (8) and 2α,3α-(3‘-oxocyclohexano)-5β-cholestane (9) and 4α,3β- (11) and 4α,3α-(3‘-oxocyclohexano)-5β-cholestane (12). The structures of compounds 8, 9, and 11 were unambiguously established by X-ray analysis. TiCl₄-promoted trimerization of compounds 8 and 11 gave the “supertristeroids” 4 and 5, respectively:  large (C₉₃) chiral, hydrocarbon clefts with C₃-symmetric pockets approximately 12 Å in diameter.