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0 Hydrolytic degradation products of cefdinir were studied in acidic (pH 1), neutral (pH 6), and basic (pH 9) solutions. Seven major degradation products were isolated by preparative and/or high-performance liquid chromatography and characterized by UV, IR, 1 H-NMR, and mass spectra. To clarify degradation pathways in each pH solution, kinetic and product analyses during hydrolysis of cefdinir were carried out along with the followup reaction of representative degradation products. Cefdinir was shown to degrade via two major degradation routes: -lactam ringopening and pH-dependent isomerizations (lactonization, epimerization at C-6 or C-7, syn−anti isomerization of N-oxime function).

Tautomeric phenomenonwas observed in the HPLCchromatogramobtained from a novel potent immunosuppressant, FK506, in ethanol solution. Two tautomeric compounds derived from FK506 were isolated and purified by HPLC.Their structures were elucidated by spectral analyses. The mechanismsfor this tautomerization were also established based on the results of structure analysis.Cis (major component1*) and trans conformational isomers of FK506 resulting from the restricted rotation of the amide bond in the pipecolic acid moiety ( Fig. 1) have been known to exist in chloroform as the equilibrium mixture1'2) which could be detected by HPLCunder low temperature column condition3*. However, this equilibrium is not detectable on the HPLCchromatogram obtained by increasing the column temperature in order to improve a peak shape, while FK506 in ethanol solution provided three peaks which suggest the existence of another kind of equilibrium phenomenon (Fig. 2). This means that the origin of the equilibrium in ethanol is different from the equilibrium associated with the rotation around the amide bond. Wereport the conclusion that a new equilibrium is explained by tautomerism of FK506; the resulting mixture consists of three species, FK506, Tautomeric compoundsI and II in ethanol solution4*. Herein wereport structures of two tautomeric compounds of FK506in comparisonwith spectral data of FK5064) and postulate a mechanism of interconversion between these three compounds. Materials and Methods Isolation of Tautomeric Compounds

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