Preparation Of Ceph-3-Em Esters Unaccompanied By δ³ To δ² Isomerization Of The Cephalosporin Derivatives

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• Analysis of the crude product invariably showed the mixture consist of excess alkyl halide and the δ3-cephem as the sole product. To a subsequent crystallization is usually sufficient to remove the alkyl halide from the desired product
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• The absence of δ2-isomer was confirmed by HPLC and 1H-NMR analysis of crude reaction products. The δ3-and δ2-isomer of cephalosporins have very distinct resonances. The C-7 hydrogen multiplet and the C-6 hydrogen doublet are separated by over 0.8ppm for the δ3-isomer and by only 0.4ppm for the δ2-isomers. Additionally, A2-isomer exhibit singlets at 5.0∼5.3ppm and 6.4∼6.7ppm for the C-2 and the C-4 hydrogens, respectively, which replace the C-2 methylene signals of the δ3-isomers
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• The isomer ratios were determined by integration of the 1H-NMR spectra of the crude reaction products prior to purification and crystallization
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