THE PYROLYTIC REARRANGEMENT OF 1-ALKYNOYL-3-METHYLPYRAZOLES - SYNTHESIS OF PYRAZOLO[1,5-A]PYRIDIN-5-OLS AND RELATED-COMPOUNDS

Flash vacuum pyrolysis of 1-(alkyn-2'-oyl)-3-methylpyrazoles at 650-de grees/0.03 mm forms pyrazolo[1,5-a]pyridin-5-ols, often in high yield, which may bear substituents at C2, C3 or C7. In the absence of a 3-me thyl group in the precursor, N-ethynylpyrazoles are formed in low yiel d. The formation of both types of product is interpreted as involving 3-(N-pyrazolyl)propadienones formed by N1 --> N2 migration of the N-al kynoyl group with inversion of the three-carbon chain. The fused-ring structure of 2-methylpyrazolo[1,5-a]pyridin-5-ol (25) was established by X-ray crystallography of the O-benzoyl derivative (27).