CYCLOCONDENSATION REACTIONS OF OPTICALLY-ACTIVE ALPHA-ALKOXY ALDEHYDES TO KETENE IMINES - SYNTHESIS OF CHIRAL 2-IMINOOXETANES

The Lewis acid promoted hetero-cycloaddition of a series of ketene imines to stereogenic (S)-alpha-alkoxy aldehydes is regiospecific and gives diastereomeric mixtures of the corresponding optically active 2-iminooxetanes. The possibility of achieving a ''chelation'' or ''non-chelation'' controlled facial selectivity is investigated. The non-chelating Lewis acids (BF3.Et2O and ZnCl2.Et2O) govern the selectivity of the cycloaddition, anti-Cram products being preferentially formed with low and medium size ketene imines. In contrast, syn-Cram configured isomers are preferentially formed whith the sterically demanding tert-butylketene N-p-methoxyphenylimine in the presence of catalytic amounts of lanthanide catalysts.