Chemistry of cephalosporin antibiotics. Part XVI. Configurational and conformational analysis of deacetoxy-Δ2- and -Δ3-cephalosporins and their corresponding sulphoxide isomers by nuclear magnetic resonance
The results of solvent-shift and hydrogen-bonding studies indicate that the (S)-sulphoxide configuration can be assigned to the major sulphoxide oxidation products of Δ2- and Δ3-deacetoxycephalosporin (III) and (VI) respectively. From a comparison of sulphoxide-bond solvent and anisotropy shifts recorded for analogous penicillin and cephalosporin systems, we are able to assign the conformation of the dihydrothiazine ring in these cephalosporin derivatives. These conformations are supported by results obtained from a study of internal nuclear Overhauser effects (NOE) as well as from the magnitude of observed long-range and geminal coupling constants. Additional evidence is presented in support of the previously assigned stereochemistry at C-4 in Δ2-deacetoxycephalosporin (VI).