Dolastatin 3 (1) has been reported to be a cyclic pentapeptide containing two thiazole amino acids,(g1y) Thz (2) and (g1n) Thz (3). The syntheses of (R)-and (S)-(g1u) Thz (8a, b) and (g1u) Thz-(g1y) Thz (lla, b) derivatives suitable for elaboration to dolastatin 3 are described. A key feature of the (g1u) Thz syntheses is the selective thiation of a primary amide with Lawesson's reagent in the presence of ester and urethane functionalities.