Stannanes from Cholic Acid as Enantioselective Free-Radical Reducing Agents

Abstract

7α-(Dimethylstannyl)-24-nor-5β-cholane (12) and 3β-(dimethylstannyl)-24-nor-5β-cholane (11) have been prepared from cholic and lithocholic acid, respectively, as free-radical reducing agents. Limited enantioselectivity-testing data indicate that these compounds show promise as enantioselective free-radical reducing agents. For example, reaction of ethyl (rac)-2-bromo-2-cyclopentyl-2-phenylacetate with one equivalent of either (11) or (12) in the presence of one equivalent of (S,S)-(–)-N,N’-bis(3,5-di-tert-butylsalycidene)-1,2-cyclohexanediaminomanganese(III) chloride (7) in toluene at –78˚C provides ethyl (S)-2-cyclopentyl-2-phenylacetate, obtained with 62 and 90% ee, respectively.