Diastereoselectivity in theLewis acid mediated aldol reaction of chiral α, β-epoxyaldehydes with a ketene silyl acetal

The Lewis acid mediated aldol reaction of chiral α, β-cis andtrans epoxyaldehydes1 and2 withtert-butyl ketene silyl acetal proceeds mainly withanti diastereofacial preference. The best results were obtained forcis epoxyaldehyde1 in the presence of catalytic amounts of BiCl3·1.5 eq. ZnI2 (anti:syn ∼ 13:1), whereas the poorest stereoselectivity was observed when an excess of LiClO4 was used (anti:syn ∼ 1:1). The more stable epoxyaldehyde conformers were determined and the diastereofacial preference was found to be in agreement with a nucleophilic attack on the energetically more favoured conformers.