The possibility of carbon-carbon double bond formation within a ß-lactam ring is suggested by the known tendency of penicillins with electron-withdrawing substituents, such as phthalimido, at the 6-position to undergo base-promoted rearrangement to thiazetinones, [6] a reaction for which the azetone 11 is reasonabily proposed as an intermediate. [7]

Moreover, a 3,4-unsaturaded ß-lactam has been detected spectroscopically, [8] and azetinones fused to benzene or pyridine rings are well-known compounds. [9]

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