Cyclisation reactions of some pyridazinylimidoylketenes

Abstract

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)].

Synthesis and Reactivity of Dithienopyrazines**

“…Structural proofs came from 1 H NMR and IR data. In this respect, thiophene 1,1‐dioxide units show much larger coupling constants (7–7.5 Hz) for the thiophene α,β‐protons [62] than thiophene units itself (5.6–5.9 Hz) [40,63] . Furthermore, in IR spectra of sulfones a characteristic symmetric stretching band arises in the regime of 1135–1170 cm −1 , an asymmetric band between 1300–1335 cm −1 [64] .…”

“…In this respect, thiophene 1,1-dioxide units show much larger coupling constants (7-7.5 Hz) for the thiophene α,βprotons [62] than thiophene units itself (5.6-5.9 Hz). [40,63] Furthermore, in IR spectra of sulfones a characteristic symmetric stretching band arises in the regime of 1135-1170 cm À 1 , an asymmetric band between 1300-1335 cm À 1 . [64] In contrast, for aromatic N-oxides a stretching band in the regime of 1200-1300 cm À 1 is expected.…”

Synthesis and Reactivity of Dithienopyrazines

“…In this respect, thiophene 1,1-dioxide units show much larger coupling constants (7-7.5 Hz) for the thiophene -protons [62] than thiophene units itself (5.6-5.9 Hz). [40,63] Furthermore, in IR spectra of sulfones a characteristic symmetric stretching band arise in the regime of 1135-1170 cm -1 , an asymmetric one between 1300-1335 cm -1 . [64] In contrast, for aromatic N-oxides a stretching band in the regime of 1200-1300 cm -1 is expected.…”