Structure of chiral pyrazoles in the solid state and in solution

The X-ray molecular structures of three chiral pyrazoles bearing (4S)-4-ben zyloxazolidin-2-ones or (2R)-bornane-10,2-sultams at position 3(5) have bee n studied by single crystal X-ray diffraction and by NMR. In the solid stat e, the same pyrazole tautomer a has been found for the three compounds, tha t with the chiral group in the 3-position. Their crystal structures consist of infinite chains (catemers) formed by N-H . . . O2-X (X: C, S) hydrogen bonds. The use of solid-state C-13 NMR spectroscopy demonstrated that the p yrazole is also an a tautomer. C-13 NMR spectra in methanol solution yield average signals even at -80 degreesC; nevertheless, interpolation allows on e to estimate that in solution tautomer a also predominates.