N-methylated 25-oxasmaragdyrins: synthesis, structure and properties

Abstract

The NN-methylated 25-oxasmaragdyrins were synthesized by reacting 25-oxasmaragdyrins with CH₃I in presence of KOH in THF at reflux temperature. Although the reactions were carried out in presence of excess CH₃I, we obtained only dimethyl-25-oxasma ragdyrins, where NN-methylations occurred at 24 and 26 “N”s of pyrrole rings of tripyrrane moiety of smaragdyrin core as confirmed by X-ray crystallography. The NN-methylated-25-oxasmaragdyrins were characterized in detail by 1D and 2D NMR spectroscopy, absorption, fluorescence and electrochemical techniques. The effect of distortion induced by methylation, clearly reflected in structural and electronic properties of the macrocycle. All the pyrrole and furan protons experienced upfield shifts in the ¹H NMR and maximum upfield shifts were noted for the NN-methylated pyrrole ring protons. The absorption spectra showed broad Soret band and four ill-defined Q-bands with significant reduction in extinction coefficients. The NN-methylated 25-oxasmaragdyrins were comparatively less fluorescent. The redox studies indicated that they are difficult to oxidize and easier to reduce.