Treatment of (Boc)-beta-methyltryptophan (I) with SOCl2 and MeOH produced beta-methyltryptophan methyl ester (II). 3,3-Diphenylpropylamine (III) was protected with Boc2O and then alkylated with allyl bromide in the presence of potassium hexamethyldisilazide to afford (IV). Further ozonization of the allyl double bond of (IV) generated the aminoaldehyde (V). Amino ester (II) was reductively alkylated with aldehyde (V) in the presence of NaBH3CN to yield the protected diamine (VI). After acid deprotection of the Boc group of (VI), the resulting diamine (VII) was converted to imidazolinone (VIII) upon treatment with phosgene and diisopropylethylamine. Subsequent hydrolysis of the methyl ester of (VIII) gave carboxylic acid (IX).

Coupling of (IX) with monoprotected diamine (X) by means of EDC and HOBt to furnish amide (XI). Finally, deprotecion of the amino group of (XI) afforded the title compound.