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Dynamic stereochemistry transfer in a transannular aldol reaction: total synthesis of hypocrellin A.

shiraiachrome A, and hypocrellin B are isolates of Shiraia bambusicola. Interestingly, a different source, Hypocrella bambusae, gave rise to hypocrellin (4), the enantiomer of hypocrellin A (1), along with the racemic hypocrellin B. The perylenequinones have attracted considerable attention because of their potent biological activity, including lightinduced activity against tumor cell lines, antiviral activity, and immunotherapeutic properties, and elegant approaches to the calphostins have been reported. The more architecturally complex perylenequinones are exciting targets for photodynamic cancer therapy (PDT) 8] but have not succumbed to total syntheses. Notably, hypocrellin A (1) with a seven-membered ring that contains two stereogenic centers, one of which is quaternary, remains a challenging synthetic target. Herein, we report the first total synthesis of hypocrellin A (1) through a dynamic stereochemistry transfer reaction. There has been much confusion in the literature regarding the naming of these structurally distinct members of the perylenequinone natural products. One contributing factor lies in the tautomerization available within the perylenequinone moiety (1 and 1·taut ; Scheme 1). The major tautomer of each perylenequinone varies depending on the planarity of the naphthalene core and the strength of the intramolecular hydrogen bonds. For hypocrellin A (1), the two tautomers are present in almost equal amounts. A second issue centers on atropisomerization of the helical configuration in the perylenequinone. Although the structurally related calphostins are atropisomerically stable, the additional seven-membered ring in hypocrellin A lowers the atropisomerization barrier. As a result, hypocrellin A exists as an equilibrium mixture of atropisomers (1 and 1·atrop), with 1 favored (Scheme 2). Scheme 1. Hypocrellin A and related perylenequinone natural products. M and P are the helical configurations, taut= tautomer.