Oxidation of aldehyde (I) by means of sodium chlorite gave carboxylic acid (II), which was converted to the corresponding Weinreb amide (III) via activation with isobutyl chloroformate and N-methylmorpholine. Subsequent condensation of (III) with 2-lithiobenzothiazole (IV) in cold THF formed the benzothiazolyl ketone (V). In order to minimize racemization, ketone (V) was reduced to alcohol (VI) using NaBH4. After deprotection of the Boc group of (VI) with HCl in dioxan, the resulting amino alcohol (VII) was coupled with the protected dipeptide (VIII) to furnish (IX). Further acid deprotection of the Boc group of (IX) afforded amine (X).

The coupling of (X) with 5-methylisoxazole-3-carbonyl chloride (XI), yielded amide (XII). The alcohol group of (XII) was then oxidized to ketone (XIV) employing Dess-Martin periodinane (XIII). Finally, the 2,4-dimethoxybenzyl group (DMB) of (XIV) was cleaved by treatment with DDQ in hot acetonitrile.