Tandem cycloadditions of N,O-ketene N-1,3-butadienyl-N-alkyo-O-silylacetals with C-60: A straightforward stereoselective synthesis of bicyclic derivatives of 1,2,3,4-tetrahydrobuckminsterfullerene

We have studied the reactivity of the N,O-ketene N-1,3-butadienyl-N-alkyl-O-silylacetals 1a-e with C-60 which proceeds through a tandem process to give the adducts 2a-e with high diastereoselectivity. The addition order of these tandem reactions has been evaluated, The structures of compounds 2a-e were determined from the H-1 and C-13 NMR shifts and from the H-H coupling patterns, while the stereochemistry was deduced from 2D T-ROESY NMR experiments. The proposed mechanism for the Michael-type addition step is a single electron transfer, and the source of the C-60-attached proton is discussed.