In peptide coupling reactions, various auxiliary reagents have been used to suppress racemization. See, e.g., N-Hydroxysuccinimide, N-Hydroxyphthalimide, and endo-N-Hydroxy-5-norbornene-2,3-dicarboximide. For use of CuCl2, see: Tetrahedron Lett., 25, 771 (1984); J. Chem. Soc., Chem. Commun., 419 (1988). For ester or thioester formation from an acid and an alcohol, the rate is greatly increased by addition of 4-(Dimethyl-amino)-pyridine: Angew. Chem. Int. Ed., 17, 522 (1978). For t-butyl esters, see: Org. Prep. Proced. Int., 20, 180 (1988); Org. Synth. Coll., 7, 93 (1990). The same combination has been found useful in macrolactonization reactions: J. Org. Chem., 50, 2394 (1985); and in formation of propynoic esters under mild conditions: Tetrahedron Lett., 30, 4575 (1989).
