Diastereoselection of the addition of silyloxyfurans to five-, six- and seven-membered N-acyliminium ions

The addition of silyloxyfuran 1a to five-, six- and seven-membered N-acyliminium ions 2 afforded threo-3 as the major isomers (the structures of 3a, 3g and 3m were determined by X-ray analysis). The diastereoisomeric ratio increased with bulkier carbamate groups (Boc>Cbz>CO2Me) with the five- and seven-membered N-acyliminium ions more selective than the six-membered ones. However, erythro-4 isomers predominated when 5-methylsilyloxyfuran 1b was employed (the structures were determined by NOE studies on the corresponding bicyclic lactams 6a,b and 7a,b) and the formation of regioisomer 5 was observed for N-acyliminium ions with Boc (seven-membered series) and Cbz (six- and seven-membered series) groups.