((9-FLUORENYLMETHYL)OXY)CARBONYL AMINO-ACID CHLORIDES IN SOLID-PHASE PEPTIDE-SYNTHESIS

FMOC amino acid chlorides, previously shown to be rapid-acting coupling agents in two-phase systems or in homogeneous solution, did not prove to be directly applicable to solid-phase peptide synthesis due to sluggish reactivity. The problem was traced to prior conversion of the acid chlorides by necessary basic co-reactants (DIEA, NMM, etc.) to the corresponding oxazolones. If, in their place, 1:1 mixtures of these bases and HOBt were used, rapid solid-phase acylations were possible via the intermediate formation of the corresponding HOBt esters. The 1:1 base-HOBt mixtures also were shown to enhance generally the reactivity of FMOC amino acid active esters over that caused by HOBt alone. Peptides assembled from acid chlorides by using these techniques included leucine enkephalin, the ACP decapeptide (65-74), prothrombin (1-9), several substance P analogues, and eledoisin. Where particular acid chlorides could not be used due to their instability, the corresponding pentafluorophenyl esters or mixtures of the acids and BOP reagent were used.