On the Chemistry of Pyrrole Pigments. 461 Phytochrome Model Studies: The Tautomerism at N22—N23 of Unsymmetrically Substituted Bilatrienes‐abc and 2,3‐Dihydrobilatrienes‐abc

A set of model compounds substituted with methyl groups in positions 7,8,12 and 13 were prepared to elucidate the tautomerism within the pyrromethene moiety of unsymmetrical bilatrienes and of 2,3‐dihydrobilatrienes‐abc. Configurational and 1H‐NMR signal assignments were achieved using NOE difference spectra, coupling phenomena and comparison to the spectra of compounds substituted with ethyl groups in strategic positions. By application of resolution enhancement and the COSY‐technique a characteristic coupling pattern between the methyl groups of a pyrrole ring which is different from the one observed for the pyrrolenin ring could be documented using derivatives with fixed tautomeric structure (N‐methylation). All unsymmetrical bilatrienes and 2,3‐dihydrobilatrienes‐abc investigated are shown to be single tautomers — the position of the proton (N22, N23) obviously is governed by an optimisation of intramolecular hydrogen bonding.