Stereochemical study of a Lewis acid-promoted reaction of 2-silyloxypyrrole with aliphatic and aromatic aldehydes

In the presence of BF3 OEt2, the reaction of 1-t-butoxycarbonyl-2-t-butyldi methylsiloxypyrrole with aliphatic and aromatic aldehydes in ether occurred stereoselectively to give the corresponding erythro and threo isomers, res pectively, while a similar reaction in the presence of SnCl4 showed complet ely opposite selectivity. The transition states leading to the major isomer s are discussed.