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-OBu^t 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(OBu^t)-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.