Cyclization reactions of nitriles. 50. Regioselectivity in the reactions of quaternized pyridin-3-ylidene malonitrile and cyanoacetate ester derivatives with 1,3-dicarbonyl compounds. Crystal structure of 2-amino-3,5-dicarboethoxy-6-methyl-4-(1-methylpyridinium-3-yl)-4H-pyran iodide