1-oxaspiro[4.4]nonan-6-ones. Synthetic access via oxonium ion technology, optical resolution, and conversion into enantiopure spirocyclic alpha,beta-butenolides

A general approach to the synthesis of enantiomerically pure spirocyclic al pha,beta -butenolides is presented where the fundamental framework is rapid ly elaborated by acid- or bromonium ion-induced rearrangement of the carbin ol derived by addition of 2-lithio-4,5-dihydrofuran to cyclobutanone. Subse quent resolution of the resulting ketones by either sulfoximine or mandelat e acetal technology has been applied effectively. The availability of these building blocks makes possible in turn the acquisition of the enantiomers of dihydrofurans typified by 17, 35, and 38 and lactones such as 25 and 31, as well as the targeted title compounds. Complementary reductions of the e arly intermediates provide the added advantage that the alpha- and beta -st ereoisomeric carbinol series can be obtained on demand. These capabilities have been coordinated to allow the crafting of any member of the series in relatively few steps.