Preparation and pyrolysis of 1-(pyrazol-5-yl)-1,2,3-triazoles and related compounds

1-(Pyrazol-5-yl)-1,2,3-triazoles 8a, 9a and 10 are prepared by cycloaddition of 5-azidopyrazoles with methyl prop-2-ynoate. The regiochemistry of the process is confirmed by synthesis of 9a using an authentic route form ethyl 2-fromyl-2-diazoacetate 13. Flash vacuum pyrolysis of 8a, 9a and 10 gives 5-methoxypyrazolo[1,5-a]pyrimidin-7-ones 16-18 by a mechanism involving an unexpected oxoketenimine-imidoyl ketene rearrangement as the key step. The mechanism is supported by a C-13 labelling experiment. A general route to pyrazolo[1,5-a]pyrimidin-7-ones from pyrazolylaminomethylene Meldrum's acid derivatives (e.g. 30-32) is also reported.