Diazotization of D-asparagine (I) in the presence of KCl afforded (R)-2-chlorosuccinamic acid (II) with retention of the configuration. Subsequent displacement of the chlorine atom of (II) with tert-butyl mercaptan provided thioether (III). Reduction of the carboxyl group of (III) by means of borane in THF gave alcohol (IV), which was converted to thioester (V) by esterification with thioacetic acid under Mitsunobu conditions. The chiral bromo acid (VII) (prepared by diazotization of L-leucine (VIII) in the presence of KBr) was then condensed with the thiol liberated from thioacetate (V) in the presence of NaOEt, yielding the corresponding thioether (VIII). Coupling of N-methyl-L-phenylalaninamide (IX) with carboxylic acid (VIII) using DCC and HOBt gave rise to amide (X). The S-tert-butyl group of (X) was then removed by means of 2-nitrobenzenesulfenyl chloride (XI) in HOAc, generating the disulfide (XII). Finally, the disulfide bond of (XII) was reductively cleaved with 2-mercaptoethanol to furnish the title thiol.