A comparison of Boc and Fmoc SPPS strategies for the preparation of C-terminal peptide α-thiolesters: NY-ESO-1 39Cys-68Ala-COSR

ABSTRACT The synthesis of a polypeptide derived from the cancer testis antigen NY‐ESO‐1 bearing a C‐terminal α‐thiolester is described. Employing tert‐butyloxycarbonyl solid phase peptide synthesis the thiolester moiety was installed on‐resin using a mercaptopropionic acid linker, thereby requiring no post synthetic manipulations and delivering the requisite α‐thiolester polypeptide after cleavage from the resin with HF. Several 9‐fluorenylmethyloxycarbonyl solid phase peptide synthesis approaches whereby the thiolester was required to be introduced in a post synthesis manner were examined concurrently. These comprised syntheses on two different “safety catch” linkers, an N‐alkyl‐N‐acyl sulphonamide and an N‐acyl benzimidazolone wherein the thiolester is generated from an activated precursor. The condensation of a mercaptan with the C‐terminal carboxylate in a direct thiolesterification reaction was also examined. When using either of the three 9‐fluorenylmethyloxycarbonyl‐based approaches, the linear polypeptide could be assembled straightforwardly on the solid phase resin; however, a thiolesterification of the C‐terminal carboxyl of the fully side chain protected peptide proved to be the most effective post‐assembly method for the installation of the C‐terminal thiolester. © 2013 Wiley Periodicals, Inc. Biopolymers (Pept Sci) 100: 356–365, 2013.