4-Nitro-benzoic acid 5-(1-ethoxycarbonyl-2-vinyl-cyclopropylcarbamoyl)-1-[hept-6-enyl-(4-methoxy-benzyl)-carbamoyl]-pyrrolidin-3-yl ester → 18-Hydroxy-14-(4-methoxy-benzyl)-2,15-dioxo-3,14,16-triaza-tricyclo-[14.3.0.0*4,6*]nonadec-7-ene-4-carboxylic acid ethyl ester

Compound 2a (1 g, 1.48 mmol) was dissolved in 1,2-dichloroethane (2 l). The mixture was degassed for 15 min using a stream of argon. Hoveyda-Grubbs catalyst (II) (50 mg, 5 mol %) was added and the mixture was refluxed for 4 h. The solvent was evaporated and the crude ester was dissolved in THF (100 ml), methanol (50 ml) and water (50 ml). The mixture was cooled to 0° C. on an ice-bath. Aqueous lithium hydroxide (20 ml, 1M) was added and the mixture was stirred at 0° C. for 4 h. The volume was then doubled by addition of water and the mixture was acidified with acetic acid. Extraction (dichloromethane) followed by flash column chromatography (methanol 1→5% in ether) gave pure title compound (450 mg, 61%).