M. Since we have shown4 that dehydroamino acid residues confer enzyme stability on some peptides, we have investigated the synthesis of the dehydro analogues of la, 2a and 2b, as potential ACE inhibitors, expecting that the thio compound 2b would bind more tightly to the enzyme active site than 2a since it is well-known that ACE is a zinc-containing enzyme. 2eis