Nuclear analogs of β-lactam antibiotics. I. Synthesis of O-2-isocephams

The preparation by total synthesis of a saturated cephalosporin analog 7-β-phenoxyacet-amido-3-ethoxy-O-2-isocepham-4-α-carboxylic acid 30, is described. Compound 30 was prepared via cycloaddition of azidoacetyl chloride to the cinnamylidene Schiff base of ethyl 2-amino-3, 3-diethoxypropionate 13b to give the cis-3-azido-4-styryl β-lactam 15b. Ozonolysis of 15b followed by sodium borohydride reduction gave the alcohol 18b. Boron trifluoride treatment of 18b gave ethyl 7-β-azido-3- β-ethoxy-O-2-isocephem-4-carboxylate 27. Reduction of the azido group followed by coupling with phenoxyacetic acid and saponification of the ester gave 30. The mechanism of the cycloaddition reaction and the stereochemical assignments are also discussed.