Catalytic hydrogenation of acylaminocinnamic acids proceeds cis with high stereoselectivity and provides a route to aromatic amino-acids labelled stereoselectively with deuterium or tritium in the β-methylene groups; [βR-³H]- and [βS-³H]-tyrosine, prepared in this way, were used to show that a hydroxylation step in the biosynthesis of haemanthamine proceeded with retention of configuration.