Efficient conversion of 2-(7-oxo-2,6-diaza-4-thiabicyclo left bracket 3. 2. 0 right bracket hept-2-en-6-yl)-3-chloromethyl-3-butenoates (4), derived from natural pencillins, into 3-methylenecephams (1) has been performed by a simple two-step operation, which comprises replacement of the allylic chlorine atom of 4 with iodine and subsequent acid-catalyzed hydrolysis, leading to 1 in 70-54% overall yields. In the latter step, the ring opening of the thiazoline moiety and the intramolecular substitution of the iodide with the thiol group proceed simultaneously.
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