Vitamin B12a (1.00 g; 0.723 mmol) was dissolved in dry trifluoroethanol (70 ml) and 1-chloro-4-(1-fluoro-vinyl)-benzene (6.50 g; 41.5 mmol) and ethyl diazoacetate (6.30 g; 50 mmol; 90% purity) was added. The solution was stirred, under reflux, under nitrogen atmosphere for 18 h. After cooling the solvent was evaporated and the residue purified by flash chromatography using 9:1 hexane/ethyl acetate. 6.9 g of 2-(4-chlorophenyl)-2-fluoro-cyclopropane carboxylic acid ethyl ester was obtained in the form of an oil (81% of theory) as a 7:3 mixture of cis/trans isomers.
