Chemistry of Substituted Pyrazolo(1,5-a)pyrimidines. Part 5. 7-Oxo-6-( pyrazol-3′(5′)-yl)-4,7-dihydropyrazolo(1,5-a)pyrimidines from 6- Ethoxycarbonyl-7-(2-dimethylaminovinyl)pyrazolo(1,5-a)pyrimidines: Elucidation of the Reaction Mechanism Through NMR Spectroscopy and X- Ray Diffraction Analysis.