Two (Z)-α,β-dehydropentapeptides, Boc-βAla-Δ Z Trp-Met-Asp-Phe-NH 2 1 and Boc-βAlaΔ Z -Trp-NLeu-Asp-Phe-NH 2 2, have been synthesized by enzymatic α,β-dehydrogenation, and then submitted to rhodium-catalysed asymmetric hydrogenation. The influence of the S-atom on the rate and stereoselectivity of the reduction have been investigated.