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 beta 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 (Delta(4)-2), dankasterone B (2), and periconiastone A (3).