ON THE SYNTHESIS OF ZWITTERIONIC HETEROPOLYCYCLIC PYRAZOLES BY A 3-COMPONENT REACTION - SOME MECHANISTIC CONSIDERATIONS

The novel three-component reaction involving a heterocyclic iminophosphorane, an isocyanate, and a hetarene component is applied to an aromatic pyrazole iminophosphorane 1 and to an analogous pyrazolone derivative 13. The hitherto unknown zwitterionic pyrazolo[3,4':4,5]pyrimido[6,1-alpha]-isoquinolines 11a,b and 16a,b, pyrazolo[3',4':4,5]pyrimido[6,1-alpha]phthalazine 6b, and pyrazolo[3',4':4,5]pyrido[6,1-alpha]pyrimidines 15a,b are obtained. Additionally, the novel cycloaddition products with triazolinediones, the pyrazolo[3,4-e][1,2,4]triazolo[1,2-alpha] 1,2,4]triazinediones 9a,b, are described. The isolation of the dihydro compound 4 and the phthalazinium salt 5 supports a stepwise mechanism for the zwitterion formation. While the influence of the aromaticity of the iminophosphorane component appears to be negligible, the aromaticity of the hetarene component determines the limitations of this versatile reaction. X-ray structures of 11a, 5b, and 16b as well as UV-, MS-, and NMR-spectra and semiempirical calculations confirm the zwitterionic character of the reaction products.