USE OF N-FMOC AMINO-ACID CHLORIDES AND ACTIVATED 2-(FLUORENYLMETHOXY)-5(4H)-OXAZOLONES IN SOLID-PHASE PEPTIDE-SYNTHESIS - EFFICIENT SYNTHESES OF HIGHLY N-ALKYLATED CYCLIC HEXAPEPTIDE OXYTOCIN ANTAGONISTS RELATED TO L-365,209

Fmoc amino acid chlorides have been shown to be useful reagents in the solid-phase synthesis of hexapeptides containing up to four sequential secondary amino acids. The oxytocin antagonist cyclo-(D-Phe-Ile-D-Pip-Pip-D-(N-Me)Phe-Pro) (1) was prepared in 70% overall yield starting from Boc-L-Pro-O-(PAM)-resin. In the synthesis of 1, the high reactivity of Fmoc-L-pipecolic acid chloride used in the di- to tripeptide step averted diketopiperazine formation seen with active ester couplings. The use of Fmoc-amino acid chlorides in the subsequent couplings provided a rapid method for assembly of the linear hexapeptide. The two potent cyclic hexapeptide oxytocin antagonists L-366,682 and L-366,948 were prepared in 45-48% overall yield on a 20 mmol scale using the methodology developed for the synthesis of 1. A particularly difficult coupling was encountered that involved acylation of a sterically hindered N(delta)-Cbz-piperazic acid N-terminus with Fmoc-L-isoleucine. Excess Fmoc-L-isoleucine acid chloride in the presence of tertiary amine base gave only 30% conversion. The efficiency was improved to 76% by utilizing the acid chloride with AgCN in toluene. Further investigation revealed that this combination of reagents produces an activated form of the isoleucine 2-alkoxy-5(4H)-oxazolone derivative.