The condensation of N-(tert-butoxycarbonyl)-L-isoleucine (I) with thiazolidine (II) by means of isobutyl chloroformate and N-ethylmorpholine (NEM) in THF gives 3-[N-(tert-butoxycarbonyl)-L-isoleucyl]thiazolidine (III), which is deprotected with HCl and thioanisole in acetic acid.