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A series of model DCC-mediated couplings of Z-Pro-D-Leu-OH with 2,3-benzo-10-aza-1,4,7,13-tetraoxacyclopentadeca-2-ene were studied by high-performance liquid chromatography. It was found that racemization during preparation of N-(Z-Pro-Leu-)benzoaza-15-crown-5 is promoted by HOBT, an additive usually reducing this process.
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