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Studies on the racemization and coupling of Na,Nim-protected histidine active esters

Studies on the racemization and coupling of Na,Nim-protected histidine active esters. Kovacs, J.; Kim, S.; Holleran, E.; Gorycki, P. (Dep. Chem., St. John's Univ., Jamaica, NY 11439, USA). J. Org. Chem., 50(9), 1497-504 (English) 1985.Several reagents such as 20898-44-6 is used here. CODEN: JOCEAH. ISSN: 0022-3263. DOCUMENT TYPE: Journal CA Section: 34 (Amino Acids, Peptides, and Proteins) Section cross-reference(s): 22 Racemization and coupling rate studies were carried out on protected histidine active esters, R2-His(R1)-OR [I; R = C6Cl5 (Pcp), 2,4,5-Cl3C6H2 (Tcp), 4-O2NC6H4 (NP); R1 = PhCH2 (Bzl), PhCH2O2C (Z), tosyl (Tos), 2,4-dinitrophenyl (DNP); R2 = Z, Me3CO2C (Boc)] in THF soln. using NEt3 as a base. The racemization rate consts. (kr) decrease with increasing electron-withdrawing ability of the imidazole protecting group. 20898-44-6 which is the cas registry number is also used here. Z-His(Bzl)-OPcp underwent racemization much faster than the Z-, Tos-, or DNP-protected histidine and 103 times faster than Z-Ala-OPcp. The Pcp, Tcp, and Np esters do not follow the general trend with the electron-withdrawing ability of the ester group. However, the rate increases with increasing solvent polarity. The coupling rate consts. (kc) of I with H-Val-OMe were detd. in THF and DMF. The Kc decrease as follows: OPcp > OTcp > ONp. In general, the Kc are larger in polar solvents; in DMF the coupling rates are 4 to 6 times faster than those in THF. The values for the Pcp esters are Boc-His(DNP) 2400 > Z-His(Z) 1300 > Z-His(Tos) 640 > Z-His(Bzl) 10. Thus, the most extensive racemization is expected with Bzl and the least with DNP imidazole protection. ..

Studies on the racemization and coupling of Na,Nim-protected histidine active esters

Studies on the racemization and coupling of Na,Nim-protected histidine active esters. Kovacs, J.; Kim, S.; Holleran, E.; Gorycki, P. (Dep. Chem., St. John's Univ., Jamaica, NY 11439, USA). J. Org. Chem., 50(9), 1497-504 (English) 1985.Several reagents such as 20898-44-6 is used here. CODEN: JOCEAH. ISSN: 0022-3263. DOCUMENT TYPE: Journal CA Section: 34 (Amino Acids, Peptides, and Proteins) Section cross-reference(s): 22 Racemization and coupling rate studies were carried out on protected histidine active esters, R2-His(R1)-OR [I; R = C6Cl5 (Pcp), 2,4,5-Cl3C6H2 (Tcp), 4-O2NC6H4 (NP); R1 = PhCH2 (Bzl), PhCH2O2C (Z), tosyl (Tos), 2,4-dinitrophenyl (DNP); R2 = Z, Me3CO2C (Boc)] in THF soln. using NEt3 as a base. The racemization rate consts. (kr) decrease with increasing electron-withdrawing ability of the imidazole protecting group. 20898-44-6 which is the cas registry number is also used here. Z-His(Bzl)-OPcp underwent racemization much faster than the Z-, Tos-, or DNP-protected histidine and 103 times faster than Z-Ala-OPcp. The Pcp, Tcp, and Np esters do not follow the general trend with the electron-withdrawing ability of the ester group. However, the rate increases with increasing solvent polarity. The coupling rate consts. (kc) of I with H-Val-OMe were detd. in THF and DMF. The Kc decrease as follows: OPcp > OTcp > ONp. In general, the Kc are larger in polar solvents; in DMF the coupling rates are 4 to 6 times faster than those in THF. The values for the Pcp esters are Boc-His(DNP) 2400 > Z-His(Z) 1300 > Z-His(Tos) 640 > Z-His(Bzl) 10. Thus, the most extensive racemization is expected with Bzl and the least with DNP imidazole protection. ..