Racemization free coupling of peptide segments. Synthesis of an insect neuropeptide.

The total synthesis of the insect neuropeptide derivative Z-Gly-Gly-Ser-Leu-Tyr-Ser-Phe-Gly-Leu-NH2 has been carried out by a convergent solid phase strategy. For the coupling of the N-terminal pentapeptide to the C-terminal tetrapeptide, three different methods were assayed. Racemization of the acyl activated amino acid during the fragment condensation reaction was monitored by HPLC. Best results were obtained by enzymatic coupling in a low water containing media using adsorbed alpha-chymotrypsin. An optically pure product was obtained in 82% yield after 1 h of reaction. Chemical methods such as DIC/HOBt and BOP/HOBt/NMM always rendered highly optically impure products containing 10-20% of the D-epimer.