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

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 pKa 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 pKa 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.