3-Hydroxypyrrolo[2,3-b]pyridine and related compounds - indoxyl analogues with fused electron deficient rings.

Flash vacuum pyrolysis (FVP) of 4-acetyltetrazolo[1,5-a]pyridine 5 at 400 °C provides 3-methyl isoxazolo[3,4-b]pyridine 6 whose structure was confirmed by X-ray crystallography. At higher pyrolysis temperatures, the unstable heteroindoxyl 8 was obtained, which exists as the keto form (1,2-dihydropyrrolo[2,3-b]pyridin-3-one) 8K in CDCl(3) solution and the enol tautomer (3-hydroxypyrrolo[2,3-b]pyridine) 8E in DMSO. The heteroindoxyl 8 oxidatively dimerises to the heteroindigotin 9, undergoes condensation reactions at the 2-position and reacts with methoxymethylene Meldrum's acid at the 1-position.

Synthesis and chemistry of 4,5-dihydrothieno[3,2-b]pyrrol-6-one--a heteroindoxyl.

Flash vacuum pyrolysis (FVP) of 2-acetyl-3-azidothiophene gives 3-methylthieno[3,2-c]isoxazole as the major product at a furnace temperature of 350 degrees C whereas at temperatures above 550 degrees C the new heteroindoxyl 4,5-dihydrothieno[3,2-b]pyrrol-6-one is exclusively formed. The heteroindoxyl exists predominantly as the keto tautomer. It is O-protonated by TFA, N-acetylated by acetic anhydride, N-nitrosated by nitrous acid, and provides an N-methylene Meldrum's acid derivative on treatment with methoxymethylene Meldrum's acid.