Bioinspired Divergent Synthesis of Aspersteroids A and B.

Aspersteroids A and B are novel ergostane-type 18,22-cyclosterols with immunosuppressive and antimicrobial activities. Herein, we report the first synthesis of these two natural products, which was accomplished in 15 and 14 steps, respectively, from commercially available ergosterol by means of a bioinspired divergent approach. Key features of this synthesis include an unprecedented radical relay cyclization that was initiated by iron(II)-mediated decomposition of an alkyl hydroperoxide to construct the E ring cyclopentane motif; a titanium(III)-mediated diastereoselective radical reduction of an epoxide to install the challenging C22 stereocenter; and highly regioselective, divergent late-stage oxidations to access the highly oxidized core framework.