Synthesis of Highly Functionalized Pentalenes via IntermolecularPauson–Khand Reaction

A concise synthetic route to highly substituted pentalenones 4 and 5 , respectively, is reported. The key step is a Pauson–Khand reaction of norbornadiene ( 7 ) with the functionalized acetylenes 8 to give the methanoindenone derivatives 6 . 1,4‐Addition of organocuprates resulted in clean formation of 2,3‐ trans ‐disubstituted methanoindenones 9 . After ozonolysis and reductive workup, the bis(hydroxymethyl)pentalenones 10 were obtained in moderate to good yields. A differentiation of the hydroxy groups became possible by lipase‐mediated acetylation with vinyl acetate, giving highly selectively monoacetates 4 with the acetyl group at C‐6 of the pentalene system. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)