Finally, resin and protecting group removal were accomplished using hydrofluoric acid. In addition, replacement of 2,2'-dipyridyl disulfide with the corresponding N-oxide, 2,2'dithiopyridine-1,1'-dioxide was found to enhance yields and minimize racemization in the socalled oxidation-reduction condensation coupling reaction. At the end of the twentieth century, strategies were investigated employing 2-chloro trityl anchors to avoid the strong acid conditions that were used in the cleavage of the cross-linked polystyrene carbamate resin (Scheme 5). Two different amino esters were independently examined. Allyl esters (e.g., 36) were removed using catalytic tetrakis(triphenylphosphine) palladium(0) and phenylsilane in dichloromethane. Fluorenyl methyl esters (e.g., 37) were taken off using 20% piperidine in DMF. The application of the 2-chloro trityl anchor enabled In the case of couplings using DIC activation in the presence of hydroxybenzotriazole (HOBt) as additive, impurities from failed couplings were identified by mass spectrometric analyses, which revealed molecular ions corresponding to products from intramolecular cyclization: dioxopiperazine and oxazolone. Epimerization and yield varied contingent on coupling agent.