Methods of making a compound according to the following scheme:wherein:R1 is a side chain of an amino acid residue selected from Ala, Ser, Thr and LeuR2 is a side chain of an amino acid residue selected from Asp, Glu, Gln and Asn[Aaa] is an amino acid residue selected from Ser, Asp, Ala, Ile, Phe, Trp, Lys, Glu and ThrR3 is hydrogen or alkylEach of R4 and R4′ independently is hydrogen or alkylBoth Ra and Ra′ are hydrogen or together are an oxo(═O) groupBoth Rb and Rb′ are hydrogen or together are an oxo (═O) groupL isX is CH2, O or S R5 is hydrogen or alkyl m is an integer from 1 to 3 and n is an integer from 2 to 20.