Chemoselective aldol type condensation of silyl enol ethers and acetals in 5 mol dm(-3) lithium perchlorate-diethyl ether

Acetals are efficiently and chemoselectively converted into the corresponding aldol ethers upon treatment with 1-trimethylsilyloxycyclohexene 2 in 5 mol dm(-3) lithium perchlorate-diethyl ether (LPDE) at ambient temperature with moderate diastereoselectivity, whereas under the same conditions aldehydes and ketals fail to react with 2. The present method allows the acetal substitution to be carried out under neutral reaction and work up conditions. A mechanism involving the formation of an oxocarbenium ion intermediate from the acetal followed by nucleophilic addition of the silyl enol ether is proposed. The observed chemoselectivity is attributed to the mild Lewis acidity of the lithium ion in diethyl ether.