Alkylation of (1s,2r,5r,6s,7r)- and (1r,2r,5r,6s,7s)-5-methyl-4-oxatricyclo-[5.2.1.0^2,6]-8-decen-3-one. Application to the Synthesis of (r)-3-Alkyl-5-Methyl-2(5h)-Furanones

The retro-Diels-Alder reaction has been extensively applied for the synthesis of natural products in the last 10 years.1 In one of the most frequent synthetic strategies, this reaction is used to protect a double bond, generated again after elimination of the diene. We have recently reported the Diels-Alder cycloaddition of (R)-angelica lactone, with cyclopentadiene to afford enantiomerically pure endo and exo adducts, 2 and 3, respectively. In the present work, we describe the alkylation of both diastereoisomeric adducts and subsequent pyrolysis to provide an easy entry to optically active (R)-3-alkyl-5-methyl-2- (5H)-furanones 12-15 in good yields. Many of such compounds have a natural origin and interesting properties, e.g., butenolide 12 is a component of mushroom flavor, 13 has fungicidal activity, and 14 is a metabolite from Streptomyces griseus. A related route has been published, based on the alkylation of adducts from maleic anhydride and cyclopentadiene, to yield racemic butenolides. On the other hand, the method reported herein provides a general pathway leading to a variety of dialkyl,β-butenolides in a single enantiomeric form, since the starting optically active 5-alkyl-2(5H)-furanones are easily available. © 1988, American Chemical Society. All rights reserved.