Pyrido[1,2-a]pyrimidinones and Thiazolo[3,2-a]pyrimidinones: Precursors for Pyridyl- and Thiazolyliminopropadienones, R - N=C=C=C=O

2-Pyridyliminopropadienone 6 is formed together with 2-aminopyridine on flash vacuum thermolysis (FVT) of 2-pyridylamino-pyridopyrimidinone 16a and observed by Ar matrix IR spectroscopy, but the two products recombine on warming to room temperature to regenerate the starting material. FVT of the picolinylamino-pyridopyrimidinones 16b and 16c generated mixtures of pyridyliminopropadienone 6 and picolinyliminopropadienones 19b and 19c, respectively. These reactions can be understood in terms of fragmentation of the open-chain bis(2-pyridylamino)methyleneketene intermediates 20 or the thermal interconversion of pyridopyrimidinones 16 and mesoionic pyridopyrimidinium olates 21. 2-Thiazoyliminopropadienone 28 was obtained in an analogous manner by FVT of the 2-(methylthio)thiazolopyrimidinone 24. However, the corresponding dihydro derivative 31 yielded cyanoketene 36 as the major product.

Thioketenes and Iminopropadienethiones RN=C=C=C=S from Isoxazolones

Isoxazolones 6 undergo thermal elimination of propene and isopropylthiol to produce thioketenes 7 at 500–600°C under flash vacuum thermolysis conditions. At 700–900°C further fragmentation occurs to produce iminopropadienethiones, RNCCCS 8. In addition, 3-alkylisoxazolones 6d–e rearrange to cyanothioketenes 10d–e. Compounds 7, 8, and 10 were characterized by Ar matrix IR spectroscopy and comparison with density functional theory-calculated spectra. Thioketenes 7 reacted with amines to afford thioamides 11. Reaction of aryliminopropadienethiones 8 with amines caused cyclization to 2-aminoquinoline-4(1H)-thiones 16.