A number of N-hydroxysuccinimide esters of acylamino acids have been synthesized. These compounds are crystalline solids which react readily with amino acids or custom peptide synthesis or their esters. Peptide formation under aqueous conditions goes well. Because of the water solubility of N-hydroxysuccinimide, these esters appear to be more generally useful than the analogous esters of N-hydroxyphthalimide.
After the epoch-making discovery of the still unsurpassed carbobenzoxy groups by Bergmann and Zervas (1932), the first major breakthrough in custom peptide synthesis came in the early 1950s with the introduction of mixed anhydrides by Wieland and Bernhard (1951), Boissonnas (1951), and Vaughan and Osato (1952). The impetus provided by the availability of an easily removable aminoprotecting group and efficient carboxyl activation culminated in the synthesis of oxytocin by du Vigneaud and his coworkers (du Vigneaud et al., 1953, 1954; Katsoyannis and du Vigneaud, 1954; Ressler and du Vigneaud, 1954; Swan and du Vigneaud, 1954). For even more ambitious endeavors, however, carboxyl activation in the form of mixed anhydrides seemed to be not entirely satisfactory.
Unsymmetrical anhydrides yield—at least in principle—two acylation products, the desired peptide (A) and an acyl derivative (B) of the amino component (Fig. 1). With the proper choice of the “activating” acid in the mixed anhydride, the formation of the undesired byproduct B can be kept at a minimum, but it is unlikely that the relative electrophilicities of the two carbonyl carbons could be so different that the main product, peptide A, would be completely uncontaminated by some small amount of the byproduct B. [A combination of electron release and of steric hindrance, such as in pivaloyl mixed anhydrides (Zaoral, 1962), might approach the ideal of a single acylation product.]