N-Boc-N-Methyl-D-phenylalaninol (I) was oxidized to aldehyde (II) using SO3-pyridine complex. Reductive amination of (II) with benzylamine to yield (III), followed by hydrogenolytic debenzylation then furnished the monoprotected diamine (IV). This was condensed with methanesulfonyl chloride to yield sulfonamide (V). After deprotection of the Boc group of (V) with trifluoroacetic acid, amine (VI) was coupled with N-Boc-N-methyl-D-naphthylalanine (VII) using EDC and HOAt. The resulting amide (VIII) was further deprotected with trifluoroacetic acid, and then coupled with 5-(tert-butoxycarbonyl)amino-5-methyl-2-hexenoic acid (IX) to yield (X). Finally, acid deprotection of the Boc group of (X) furnished the target compound.