Ring–chain tautomerism. Part 9.1 2-Acylbenzamides, 8-acyl-1-naphthamides and 5-acyl-4-phenanthramides

Abstract

Ring–chain tautomerism in N-methyl- and N-(substituted phenyl)-2-formylbenzamides, 2-benzoylbenzamides, 8-formyl-1-naphthamides, 8-benzoyl-1-naphthamides and 5-formyl-4-phenanthramides has been investigated by IR and ¹H NMR spectroscopy and the measurement of their pK_a values in 30% (v/v) dimethyl sulfoxide–water. The tautomeric pair synthesised, i.e. hydroxylactam–acylamide or aminolactone–iminocarboxylic acid, appears to be a function of the method of preparation and/or the basicity of amine used. The cyclic tautomer, i.e. hydroxylactam or aminolactone, is predominant in all cases studied. However, equilibrium constants have been estimated from the pK_a values for aminolactone–iminocarboxylic acid tautomerisation in which the effect of the links follow similar trends as found for the corresponding hydroxylactone–acylcarboxylic acid tautomeric systems.