The Reaction of Acetylenecarboxylic Acid with Amines. XV. Reaffirmation of the Enamine Structure Facilitated by Intramolecular Hydrogen Bonding Common to the Reaction Products

The reactions of dimethyl acetylenedicarboxylate (V) with p-chloroaniline, ethanolamine, ethylenediamine, o-aminophenol, and o-phenylenediamine gave dimethyl N-(p-chlorophenyl)aminofumarate(VI), 3-methoxycarbonylmethylene-3,4,5,6,-tetrahydro-2H-1,4-oxazin-2-one (VII), 3-methoxycarbonylmethylenepiperazin-2-one (VIII), 3-methoxycarbonylmethylene-3,4,-dihydro-2H-1,4-benzoxazine-2-one(IX), and 3-methoxycarbonylmethylene-3,4-dihydro-2(1H)-quinoxalinone(X), respectively. It has been shown that methoxycarbonylmethylene is a common structural element in all products, where the carbonyl is hydrogen-bonded with an amino group to form an enamine form. The spectra measured for crystals (IR) and for solutions in inert solvents (NMR) exhibited the fixation of the enamine form preferentially to the imine form, while NMR spectra for trifluoroacetic acid solutions indicated both forms in equilibrium. Tautomerization occurring in the acid is discussed.