2) The reaction of 3-pyridylcarbonyl chloride (I) with (3S,4R)-2-[3-[1(R)-(allyloxycarbonyloxy)ethyl]-2-oxo-4-sulfanylazetidin-1-yl]-2-(triphenylphosphoranylidene)acetic acid allyl ester silver salt (VII) in dichloromethane containing pyridine gives the substituted phosphorane (VIII), which is cyclized in refluxing toluene to afford (5R,6S)-6-[1(R)-(allyloxycarbonyloxy)ethyl]-2-(3-pyridyl)-2-penem-3-carboxylic acid allyl ester (IX). The deprotection of (IX) with tetrakis(triphenylphosphine)palladium, tributyltin hydride and NaHCO3 in THF gives the free acid (X) as the sodium salt, which is finally esterified with bromomethyl acetate.
1) The reaction of 3-pyridylcarbonyl chloride (I) with (3S,4R)-2-[3-[1(R)-hyroxyethyl]-2-oxo-4-sulfanylazetidin-1-yl]-2-(triphenylphosphoranylidene)acetic acid allyl ester silver salt (II) in dichloromethane containing pyridine gives the substituted phosphorane (III), which is cyclized in refluxing benzene to afford (5R,6S)-6-[1(R)-hydroxyethyl]-2-(3-pyridyl)-2-penem-3-carboxylic acid allyl ester (IV). The deprotection of (IV) by means of triphenylphosphine, tetrakis(triphenylphosphine)palladium and acetic acid yields the free acid (V), which is finally esterified with bromomethyl acetate (VI) in DMF.