Racemization studies in peptide chemistry. Re-investigation of the “β-elimination–readdition” mechanism of N-benzyloxycarbonyl-S-benzylcysteine derivatives

3-(1-Piperidinyl)alanine formation during the preparation ofC-terminal cysteine peptides with the Fmoc/t-Bu strategy

“…Despite numerous attempts using various gradients on reversed- A plausible mechanism to account for impurities in the synthesis of C-terminal cysteine-containing peptides is a base-catalyzed elimination of the sulfhydryl-protected side chain to afford the dehydroalanine derivative followed by a nucleophilic addition to the alkene (Scheme 1). A similar mechanism had been proposed to explain the racemization of protected cysteine active esters in the presence of a base [14], although later this could not be confirmed [15]. If N~-Fmoc deprotection is accomplished with a solution of piperidine in N,N-dimethylformamide (DMF), a +51 Da impurity would be expected.…”

New synthesis of isoxazoles from 1,4-dianions of oximes having an .alpha. hydrogen. Mass spectrometry

“…All analyses were performed by M-H-W Laboratories, Garden City, Mich. Infrared spectra were obtained on Perkin-Elmer Model 137 and 237 spectrometers. The nmr spectra were ob- [ii + 117 (8, 117 (2 89 (5.5); 81 (9.5); 77 (7.7); 75 (5.9); 73 (5.2); 69 (18.3); 57 (7.3); 55 (7.0); 51 (6); 43 (7.4); 41 (7.3). 3c: [i + 2]; 138 (7.5); 137 (10); 97 (5.7); 95 (6.2); 83 (7.5); 82 (6); 81 (19); 73 (9.5); 71 (9.5); 70 (6.5); 69 (48); 68 (6.5); 67 (5); 60 (8.5); 57 (17); 56 (5.5); 55 (14); 43 (16.5); 41 (17.6).…”

“…d Recrystallized from benzene. 6 Satisfactory analyses (±0.30%) for C, , N, and Cl (where applicable) were obtained for all new compounds reported (Editor). / C. Goldschmidt, Chem.…”

Isoracemization of N-carbobenzoxyamino acid ester derivatives

“…Syn., 46,113, 120 (1966); R. B. Silverman and R. A. Olofson, Chem. Commun., 1313(1968; S. Kabuss, Angew. Chem., Int.…”