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)di abeo -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)di abeo -framework of the swinhoeisterol class of natural products. The synthesis of these frameworks then allowed for efficient access to swinhoeisterol A ( 1 ), dankasterone A (Δ 4 - 2 ), dankasterone B ( 2 ), and periconiastone A ( 3 ).