Synthesis of Swinhoeisterol A, Dankasterone A and B, and Periconiastone A by Radical Framework Reconstruction

A switchable radical framework reconstruction approach
to structurally
unique 13(14 → 8),14(8 → 7)­diabeo-steroid
swinhoeisterol A was developed. The conversion of an ergostane skeleton
proceeded through the intermediacy of a 13(14 → 8)abeo-framework as present in the dankasterone and periconiastone family
of natural products and features a β scission of a 14-alkoxy
radical with concomitant generation of the C8–C13 bond. From
this intermediate, and dependent on the conditions employed, the cascade
continues with a Dowd–Beckwith rearrangement and leads to the
formation of the 13(14 → 8),14(8 → 7)­diabeo-framework of the swinhoeisterol class of natural products. The synthesis
of these frameworks then allowed for efficient access to swinhoeisterol
A (1), dankasterone A (Δ4-2), dankasterone B (2), and periconiastone A (3).