3-(Nitrobenzylidene)-2,4(3H,5H)-furandione in the Hantzsch-pyridine synthesis 2. 6,7-Dihydrophthalide

Melting the 3-(o- and p-nitrobenzylidene)tetronic acids 2a and 2c with the enaminocarbonyl compounds 1 at 70-90 degrees C yields the hexahydro-7a-hydroxyfuro3,4-b]-pyridines 3, while with the m-nitro compound 2b at 100 degrees C the 6,7-dihydrophthalides 4 are obtained. Dehydrogenation of the dihydro derivatives 4 with activated manganese dioxide affords the phthalides 5.