Abstract
Highly strained 2β,7β-methano-1-oxacephem esters with the thienamycin-type [(R)-1-hydroxyethyl] side chain at 7α, 16a and 16b, were successfully synthesized in a highly stereoselective manner. X-ray crystallography of 16a showed very high distortion of the oxacephem moiety in 16, as suggested by the high frequency IR absorption at 1791 cm-1, and showed an h value of 0.52 Å which slightly exceeded the h values of 0.49-0.50 Å for the carbapenem nucleus. Having this highly strained structure, the compounds were very fragile, and in particular, the pivaloyloxymethyl (POM) ester 16b decomposed within a few hours even when left standing in a refrigerator. As expected from its instability, this compound showed no antibacterial activity in an assay performed in the presence of horse serum.
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