The reaction of cyclopentylamine (I) with tertbutyl bromoacetate (II) by means of NH3 in acetonitrile gives N-cyclopentylglycine tert-butyl ester (III), which is condensed with 3-(acetylthio)-2-methylpropionyl chloride (IV) [prepared from the corresponding acid (V) with SOCl2] by means of triethylamine in dioxane to yield the acylated glycine (VI). Deacetylation of (VI) with dry ammonia in methanol affords the mercapto derivative (VII), which is acylated with pivaloyl chloride (VIII) in the usual way giving the corresponding thio ester (IX). Finally, this compound is hydrolyzed partially with trimethylsilyl iodide in methylene chloride.