Herein we report a modular peptide ligation methodology that couples dioxazolones, arylboronic acids, and acrylamides to construct amide bonds in a diastereoselective manner under mild conditions, facilitated by Rh(III) catalysis. By converting the C-terminus of one peptide into a dioxazolone and the N-terminus of a second peptide into an acrylamide, the two pieces can be bridged by an arylboronic acid to construct unnatural phenylalanine, tyrosine, and tryptophan residues at the junction point with diastereoselectivity for their corresponding d-stereocenters. The reaction exhibits excellent functional group tolerance with a large substrate scope and is compatible with a wide array of protected amino acid residues that are utilized in Fmoc solid phase peptide synthesis. The methodology is applied to the synthesis of six diastereomeric proteasome inhibitor analogs, as well as the ligation of two 10-mer oligopeptides to construct a 21-mer polypeptide with an unnatural phenylalanine residue at the center.