The regiospecific N-derivatization of histidine side chains: reinvestigation of a supposed N tau to N pi migration.
A report claiming that AcHisOMe reacts regiospecifically with 4-fluoronitrobenzene to give AcHis[tau Ph(NO2)]OMe, which on treatment with H2/Pd(C) undergoes a partial tau-pi shift to give some AcHis[pi Ph(NH2)]OMe, cannot be substantiated. 4(5)-Methylimidazole, a model for AcHisOMe, gives on reaction with 4-fluoronitrobenzene a 4:1 mixture of the regioisomers 1-(4-nitrophenyl),4-methylimidazole, corresponding to tau-substitution, and 1-(4-nitrophenyl),5-methylimidazole, corresponding to pi-substitution, each of which has been isolated and fully characterized, including proof of orientation. In both cases, treatment with H2/Pd(C) gives a single product, without any change of orientation in either case.
Jones
J H
JH
Dyson Perrins Laboratory, Oxford, UK.
Walker
V L
VL
D006639:Histidine / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy
D006639:Histidine