joined at glycine, which is achiral, or proline, which is resistant to oxazolone formation. Enantiomerization of cysteine can occur during coupling and during piperidine-mediated removal of N™-Fmoc, when the cysteine residue is anchored to the resin via an ester bond, and may involve a p-elimination-type mechanism; these problems are discussed in Chapter 4. With histidine, it is the proximity of the basic Tr-nitrogen of the imidazole side-chain to the ahydrogen is thought to account for the facile enantiomerization of carboxyactivated histidine derivatives. This is not normally a problem during routine chain extension using Fmoc-His(Trt) derivatives, but may become an issue with aggregated sequences where slow reactions are encountered. In such cases, the use of derivatives in which the ir-nitrogen is blocked, such as FmocHis(Bum)-OH (58), is recommended.