α-Pyrones. Part V. Structure effects on the intramolecular cyclization of functionalized 6-pyronylacetamides: synthesis of new 2,5,7-trioxo-pyrano[3,2-c]pyridines

Reaction of arylisocyanates 2 with methyl 2-oxo-2H-pyran-6-acetate 1 and with ylide 4 gave two classes of pyronylacetamides 3 and 5, respectively. Phosphoranes 5 were reduced to the corresponding acetamides 6 with zinc and acetic acid. Compounds 6 were alkylated under solid-liquid PTC conditions using anhydrous potassium carbonate as a base to give the Cα-alkyl derivatives 7 in good yields. Intramolecular cyclization with different bases and solvents of acetamides 3,5-7 to give the new 2,5,7-trioxo-pyrano[3,2-c]pyridines has been studied.