Unexpected cis selectivity in the Pictet–Spengler reaction

Whilst cis:trans selectivity of about 4:1 can be obtained from Pictet–Spengler reactions between tryptophan methyl esters and aldehydes using conditions of kinetic control, much higher cis selectivity (>95:5) can be obtained when both the tryptophan derivative and the aldehyde possess a suitable π-system; preliminary results on the scope and limitations of this exceptional stereocontrol are presented in this Letter.