Biomimetic total synthesis of cyanosporaside aglycons from a single enediyne precursor through site-selective p-benzyne hydrochlorination

Abstract

The cyanosporasides A-F are a collection of monochlorinated benzenoid derivatives isolated from the marine actinomycetes Salinispora and Streptomyces sp. All derivatives feature one of two types of cyanocyclopenta[a]indene frameworks, which are regioisomeric in the position of a single chlorine atom. It is proposed that these chloro-substituted benzenoids are formed biosynthetically through the cycloaromatization of a bicyclic nine-membered enediyne precursor. Herein, we report the synthesis of such a bicyclic precursor, its spontaneous transannulation into a p-benzyne, and its differential 1,4 hydrochlorination reactivity under either organochlorine or chloride-salt conditions. Our bioinspired approach culminated in the first regiodivergent total synthesis of the aglycons A/F and B/C, as well as cyanosporasides D and E. In addition, empirical insights into the site selectivity of a natural-like p-benzyne, calculated to be a ground-state triplet diradical, to hydrogen, chlorine, and chloride sources are revealed.

Keywords: benzenoids; cyclization; diradicals; enediynes; monochlorination.