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The coupling of Boc-Val-OH to either H-Pro-OBzl or H-Pro-Gly-Val-Gly-OBzl by the mixed anhydride method leads to the formation of a urethane by-product in yields of 40-60%. This side reaction can be suppressed by the addition of HOBt to the reaction mixture before the amino component is added. This results in a substantially increased yield of the desired peptide.
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Prasad et al. (1985) studied this question.
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