New tetrahydroisoquinolines were synthesized by the Pictet‐Spengler reaction. Influence of a wide range of aryl and heteroaryl aldehydes, was investigated in the cyclization step with 3,4‐dimethoxyphenylethylamine 1 , L‐DOPA 2 and L‐3,4‐dimethoxyphenylalanine methyl ester 3 . Compounds 2 and 3 served as probes to assess the efficiency of two Pictet‐Spengler reactions with respect to their diastereoselectivity, in order to obtain optically active diastereoisomers. Cis‐ and trans‐diastereoisomers were obtained in short reaction times with moderate to good isolated yields (20‐79%). Both nmr and X‐ray analyses confirmed the expected diastereoisomer configurations.
