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. Reactions of 4,5-dihydrothieno[3,2-b]pyrrol-6-one with diazonium salts, with isatin, and with dimethyl acetylenedicarboxylate take place at the methylene position to provide a hydrazone, an indirubin analogue, and a succinate derivative, respectively. Oxidation of 4,5-dihydrothieno[3,2-b]pyrrol-6-one gives a heteroindigotin, which shows a hypsochromic shift in the UV spectrum, relative to indigotin itself.
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. FVP of the corresponding acetyltetrazolo[1,5-a]quinoline 19 was much more complex, with 2-(cyanophenyl)acetonitrile 30 (rather than a heteroindoxyl) the major product at 750 °C. FVP of 3-acetyl-4-azidoquinoline 24 at 400 °C gave 3-methylisoxazolo[4,3-c]quinoline 33, but rearrangement to the heteroindoxyl was not observed at higher temperatures.
Flash vacuum pyrolysis (FVP) of aminopyridazinone derivatives of Meldrum's acid at 600 1C (0.02 Torr) results in generation of an imidoylketene intermediate followed by cyclisation. In the case of the 5-amino derivatives, the products are pyrido[2,3-d]pyridazines, whereas the 4-amino compounds lead to mixtures of pyrido[2,3-d]pyridazines and pyrrolo[3,2-c]pyridazines. The feasibility of the 1,5-sigmatropic shift of a chlorine atom, required for the formation of two of the pyrido[2,3-d]pyridazines, was supported by the corresponding reaction of a corresponding 2,6-dichloroaniline derivative. The feasibility of the decarboxylation mechanism required for the formation of the pyrrolo[3,2-c]pyridazines, was supported by related processes in the FVP reactions of model compounds and by DFT calculations. Scheme 1 Reagents and conditions: (i) heat [either in solution (e.g. Dowtherm, 220 1C) or by FVP (e.g. 600 1C, 0.02 Torr)].
Testing performed by ramped and isothermal differential
scanning
calorimetry has demonstrated a previously unreported effect wherein
the thermal stability of nitro-containing compounds can be significantly
reduced by the presence of low levels of a secondary nitro-containing
compound, where the impurity is of lower thermal stability than the
bulk material. This effect is believed to be due to the autocatalytic
decomposition of nitro-containing species being catalyzed by a common
catalyst, such that the catalyst generated from an impurity can cross-catalyze
the decomposition of the bulk material. Accordingly, this effect does
not occur when the impurity is of similar or greater thermal stability
than the bulk. This effect reiterates the importance of performing
thermal stability testing on samples whose impurity profile is representative
of that to be utilized or produced.
Pure indoxyl can be obtained in 75% yield by flash vacuum pyrolysis (FVP) of 2¢-azidoacetophenone at 650 °C. Reaction of indoxyl with methoxymethylene Meldrum's acid takes place at the 1-position, and FVP of the resulting derivative provides 1-hydroxy-9H-pyrrolo[1,2-a]indol-9-one (54%). FVP of the isomeric 2methylene compound gives pyrano[3,2-b]indol-2(5H)-one (42%).
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