A solution of (1R,2R)-4-oxo-cyclopentane-1,2-dicarboxylic acid monoethyl ester (example 1, step 1, 6.0 g) in an aqueous buffer (600 mL, 30 mM; 2-(N-morpholino)ethanesulfonic acid; 0.5 M D-glucose [10 equ.]; 2 mM Magnesium chloride) was adjusted to pH 6.4 at 35° C. Under stirring the reduction was started by the addition of the cofactor NADP (120 mg), the cofactor regeneration enzyme-glucose dehydrogenase (120 mg GDH 102 [Codexis]) and the reductase (1.2 g KRED-NADP-174 [Codexis]). During the 69 h reaction time at 35° C. the pH was kept at pH 6.5 by the addition of 1M NaOH. Afterwards the enzymes were denatured by adjusting to pH 2, adding a total of 300 mL ethyl acetate and a total of 60 g filter aid—Dicalite under stirring of at least 1.5 h. After filtration, two further extractions with 200 mL ethyl acetate, treatment of the combined organic phases with magnesium sulfate, evaporation and drying for 4 h on a high vacuum, 4.1 g of the title compound was isolated. MS: 201.07 (M−H)−; chiral GC: d.r.: 1.4:97.3:1.4: 0.0 [BGB-172 (BGB Analytik AG), 30 m*0.25 mm; 0.25 μm; H2; 2° C./min, 100° C. to 200° C.]. An additional extraction of the aqueous phase with further 200 mL ethyl acetate yielded after treatment with magnesium sulfate, evaporation and drying for 4 h on a high vacuum in 1.62 g of the title compound. MS: 201.07 (M−H)−; chiral GC: d.r.: 1.3:96.3:1.6:0.4. The summing up gave a total yield of 5.72 g (93.9%).
