NMR studies of the products of hydrolysis of 3-ethyl-2-methylbenzo[d]azol-3-ium iodides

Abstract N -(2-Hydroxyphenyl)- and N -(2-mercaptophenyl)- N -ethylacetamides were found as the main reaction products of the hydrolysis of the corresponding 3-ethyl-2-methylbenzazol-3-ium iodides in boiling ethanol in the presence of piperidinium base; the same products were obtained in aq. DMSO. The thiol undergoes spontaneous or accelerated oxidative coupling to the corresponding disulfide and forms a methylene-bridged dimer in the presence of CH 2 Cl 2 . All amides displayed diastereotopicity of the N -methylene group protons due to restricted rotation around the N–aryl bond, which is additional to the normal restricted rotation around the N–CO amide bond. This hydrolysis might be of crucial importance in cyanine dye synthesis in which benzazolium salt precursors are used; the findings provide an explanation for the process of obtaining ramified compounds. Structural elucidation of all products was based on detailed NMR analysis. The revision of several dye structures has been proposed.