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Reaction of  N -alkyl-2-imino-2 H -chromene-3-carboxamides with dimethyl acetylenedicarboxylate (DMAD) in the presence of sodium carbonate as catalyst in refluxing ethanol gave new tricyclic products identified as methyl 3-alkyl-2-(2-methoxy-2-oxoethyl)-4-oxo-3,4-dihydro-2 H -chromeno[2,3- d ]pyrimidine-2-carboxylates. In the absence of sodium carbonate, dimethyl 2-(( E )-3-(alkylcarbamoyl)-2 H -chromen-2-ylideneamino)fumarates were isolated as intermediates. These intermediates could be successfully converted to the same new tricyclic products by heating in ethanol containing sodium carbonate. All new synthetic compounds were characterized on the basis of their FT-IR,  1 H and  13 C NMR spectra, and microanalytical data. To identify the correct stereoisomer of the intermediates, in one case a 2D nuclear Overhauser effect (2D-NOESY) spectrum together with density functional theory (DFT) calculation at the B3LYP/6-311+G(d,p) level of theory was used.

Isolation of intermediates in the synthesis of new 3,4-dihydro-2 H -chromeno[2,3- d ]pyrimidines