Homochiral ketals in organic synthesis. Diastereoselective cyclopropanation of medium and large ring .alpha.,.beta.-unsaturated ketals derived from 1,4-Di-O-benzyl-L-threitol
Diastereomeric excesses as high as 84% were achieved in [2 + 2] photoadditions of methyl 3-oxo-1-cyclohexene-1-carboxylate with α,β-unsaturated homochiral ketals prepared from 2-cyclopentenone and 2R,3R-tartrate esters.