Stannanes from Cholic Acid as Enantioselective Free-Radical Reducing Agents

7a-(Dimethylstannyl)-24-nor-5ß-cholane (12) and 3ß-(dimethylstannyl)-24-nor-5ß-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.