The differentiation of π- and τ-derivatised histidines

Abstract

Two simple methods of differentiating π- and τ-derivatised histidines unambiguously are described. The first involves conversion into one of the two known im-methyl-L-histidines, which, to avoid all possible confusion arising from ambiguous nomenclature and previous work, have been correlated with N(α)-t-butoxycarbonyl-N(π)-benzyloxymethyl-L-histidine, whose structure has been established by X-ray crystallography. The second method, which is appropriate for im-substituents of type RCH₂–, involves the measurement of nuclear Overhauser effects. If the substituent is at the π-position, the CH₂ signal is enhanced if the low-field adjacent proton between the heterocyclic nitrogens is irradiated, but not if the high-field, more distant, ring proton is irradiated. If the substituent is at the τ-position, the CH, signal is enhanced whichever of the equidistant ring protons is irradiated.