1-(Fmoc)-4-Cbz-piperazine-2-carboxylic acid

Various 3-substituted chiral 1,2,4-oxadiazole-containing Fmoc-beta(3)- and -alpha-amino acids were synthesized from Fmoc-(l or d)-Asp(OtBu)-OH and Fmoc-l-Asp-OtBu, respectively, in three steps (i.e., condensation of an aspartyl derivative with differentially substituted amidoximes, formation of the 1,2,4-oxadiazole, and cleavage of the tert-butyl ester). These compounds represent new series of nonnatural amino acids, which could be used in combinatorial synthesis. A simple protocol has been developed to generate the 1,2,4-oxadiazole ring. Indeed, common methods resulted in cleavage of the Fmoc group or required long reaction times. We found that sodium acetate in refluxing ethanol/water (86 degrees C) was a convenient and efficient catalyst to promote conversion of Fmoc-amino acyl amidoximes to 1,2,4-oxadiazoles, and this procedure proved to be compatible with Fmoc protection. It is shown that these compounds can be prepared without significant loss of enantiomerical purity. Furthermore, the alkaline conditions used to cleave the Fmoc protecting group from these compounds did not induce epimerization of their chiral center.

Despite numerous reports on catalytic amide bond formation (ABF), these methods have thus far had a minimal impact on the universal fluorenylmethoxycarbonyl (Fmoc)/t-Bu solid-phase peptide synthesis (SPPS) methodology. We now report a proof-of-principle Fmoc/t-Bu SPPS in which both couplings and Fmoc deprotections were catalyzed by readily available reagents in an inexpensive green solvent. Couplings were carried out with >99% stereoselectivity, employing 1.1 equiv of Fmoc amino acids (AAs), using diisopropylcarbodiimide (DIC) as a coupling agent and 1-hydroxy-1,2,3-triazole-5-carboxylic acid ethyl ester (HOCt) (TON ~ 30) as a catalyst, while Fmoc deprotections were carried out using 1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) (TON ~ 7), facilitating synthesis of a model pentapeptide in 95% HPLC purity while also enabling minimization of solvent washing.

Today, Fmoc SPPS is the method of choice for peptide synthesis. Very-high-quality Fmoc building blocks are available at low cost because of the economies of scale arising from current multiton production of therapeutic peptides by Fmoc SPPS. Many modified derivatives are commercially available as Fmoc building blocks, making synthetic access to a broad range of peptide derivatives straightforward. The number of synthetic peptides entering clinical trials has grown continuously over the last decade, and recent advances in the Fmoc SPPS technology are a response to the growing demand from medicinal chemistry and pharmacology. Improvements are being continually reported for peptide quality, synthesis time and novel synthetic targets. Topical peptide research has contributed to a continuous improvement and expansion of Fmoc SPPS applications.