Synthesis of Functionalized Pentalenes via Carbonyl-Ene Reaction and Enzymatic Kinetic Resolution

Abstract

The synthesis of functionalized pentalene derivatives is described. Bicyclo[3.3.0]octane-3,7-dione 5 (Weiss diketone) was converted in six steps into the silyl-protected 3-methylbicyclo[3.3.0]octenol 6, which was submitted to Lewis acid catalyzed carbonyl-ene reactions with trioxane yielding the primary alcohol 7 with exocyclic double bond in high yield. By subsequent kinetic resolution with lipases compound 7 was enantiomerically enriched (up to 94% ee). It was also demonstrated that compound 7 could be functionalized by hydroboration and oxidative workup to give the trihydroxy pentalene 8 as well as by chain extension to the pentalene 23.