Formation and degradation of urea derivatives in the azide method of peptide synthesis. Part 1. The curtius rearrangement and urea formation
Abstract
The azide method of peptide synthesis (R1CON3+ R2NH2→ R1CO·NHR2) has been investigated with respect to the effect of reaction conditions especially on the formation of urea derivatives (R1NH·CO·NHR2) as side products. Z-Gly-Phe-N3 and H-Gly-OBut were used as model compounds for R1CON3 and R2NH2, respectively, and the rate of formation of peptide was compared with that of urea under various conditions. Peptide formation was estimated from the consumption of R2NH2 and urea formation by the extent of the rate-determining Curtius rearrangement (R1CON3→ R1NCO + N2). Data have also been obtained for other azides and amines. The results show that the conditions currently used in azide couplings (ca. 0.1 M-solutions and 0–5 °C) are generally adequate for minimising the side reaction. On the other hand, some urea derivatives, including Boc-Gly-Tyr-Ser-NH·CH(CH2·CH2·SMe)·NH·CO-Glu(OBut)-His-Phe-Arg-Trp-Gly-OH (6) and Z-Lys(Boc)-Pro-Val-Gly-NH·CH-([CH2]4·NH-Boc)·NH·CO-Lys(Boc)-Arg-Arg-NH2(5) can be synthesised from R1NCO and R2NH2. These ureas have been compared with the corresponding peptides in terms of chromatographic behaviour; the results suggest that when R1and R2are fairly large and complex as in (5) and (6) the separation of peptide from urea will present considerable difficulties.