Effect of solvent on racemization in carbodiimide mediated solid phase fragment condensations

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The Izumiya tripeptide was used to assess racemization in solid-phase fragment condensations. Boc-Gly-Ala-OH 1 was coupled to Leu-PAM resin DIC in a variety of solvents, both with and without HOBt. After cleavage from the resin, the extent of racemization was determined using C18 RP-HLPC to separate the epimers. Solvents used were DMF, NMP, TFE and each of these as mixtures with DCM. Also tested was a mixture of NMP and DMSO (85:15). Couplings in DMF/DCM (1:1) and NMP/DCM (1:1) in the presence of HOBt were in excess of 98% and racemization was undetectable. (<0.1%).