benzhydryl (6R)-8-oxo-3-[(Z)-prop-1-enyl]-7-[[(2Z)-2-[2-(tritylamino)-1,3-thiazol-4-yl]-2-trityloxyiminoacetyl]amino]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate → (6R)-7-[[(2Z)-2-(2-amino-1,3-thiazol-4-yl)-2-hydroxyiminoacetyl]amino]-8-oxo-3-[(E)-prop-1-enyl]-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylic acid

A mixture of the crude product of Example 33 containing 20% of its E isomer (9.2 g, 8.7 mmol) in 85% HCOOH (60 ml) was stirred for 1 hr at room temperature and evaporated in vacuo. The residue was treated with 90%-TFA (60 ml) for 1 hr at room temperature and poured into ice-water (300 ml). The insolubles were filtered off. The filtrate was chromatographed on a reverse phase column (Waters, prepPAK C18, 300 ml) and the column was eluted with 20% MeOH. The polar fractions were combined, concentrated in vacuo and the residue was triturated with isopropyl ether to give 1.15 g (33%) of the Z isomer and the less polar fractions gave 143 mg (4%) of the title compound.