[GRAPHICS] The protected hydroxyethylene dipeptide isostere of Phe-Arg and the tripeptide derivative 1 were synthesized as components of potential peptidase inhibitors. Key to the success of these syntheses is selective rhodium-catalyzed hydroboration in the presence of a readily reduced lactone, A convenient one-pot conversion of azides to diprotected guanidines was developed on the basis of the Staudinger reaction.