The Effect of Additives on the Peptide Synthesis by Azide Method

Abstract

The effect of bases and additives in the azide method were investigated on the yields in the fragment condensation on a soluble polymer support. The effects of them on the racemization and side reactions were also investigated. The amount of racemization was determined by the Izumiya method. Z-Gly-NHCH(CH₃)NHCO-Leu-OBzl [1], [Z-Gly-NHCH(CH₃)NH]₂CO [2], and Z-Gly-Ala-NH₂ [3] were synthesized by different methods and hydrogenated to obtain the standards for determination of the amounts of by-products by use of ion-exchange chromatography.
When triethylamine (TEA) was used as a base in 0-6 equivalent excess, increase in base contents increased the amount of recemization. On the other hand, N-methylmorphorine (NMM) was used as a base, no racemization occurred even in the case of 6 equivalent excess. Addition of 1-hydroxy-1 H-benzotriazole (HOBt) depressed the racemization fairly, and addition of N-hydroxysuccimide (HOSu) slightly. The amounts of by-products [ 1'] and [ 3'] were less in the case of TEA used as a base than that of NMM, regardless of an additive. The addition of HOBt and HOSu had no effect on side reactions. In the coupling reactions of BocAla-Gly-Ala-N₃, Boc-Ala₂-Leu₃-N₃, or Boc-Ala₂-Leu₃-Pro₂-Leu₃-N₃ with Leu or Ile bound to a soluble polymer support, almost quantitative yields were achieved by the use of TEA or NMM as a base in 6 equivalent excess and HOBt as an additive in 12 equivalent.