Diethoxymethane → Methyl 5(S)-ethoxymethoxy-5-(4-fluorophenyl)pentanoate

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40.6 g of phosphorus pentoxide are added to a vigorously stirred solution of 10.5 g (42.7 mmol) of methyl 5-(4-fluorophenyl)-5-(S)-hydroxypentanoate with a purity of 92% area % (Example 1) in 220 ml of dichloromethane and 152 ml (1200 mmol) of 99% pure diethoxymethane in a 4-neck flask with mechanical stirrer, the internal temperature rising from initially 23° C. to 30° C. There is initial formation of a white suspension whose particles quickly agglomerate and form a viscous mass which partly adheres to the bottom of the flask and changes color via yellow to brown. HPLC analysis (system as in Example 5) shows complete conversion of the precursor (tret9.8 min.) into the product (tret 13.4 min.) after only 5 min. The reaction mixture is stirred at 29° C. without external heating (i.e. continuing exothermic reaction) for 0.5 h, but the only effect of this was to increase an impurity (tret 13.2 min.). The reaction mixture is cooled to +10° C. in an ice bath, and 100 g of ice are added to the mixture. The viscous solid mass dissolves only slowly even with vigorous stirring. The mixture is transferred into a separating funnel and diluted with 200 ml of diethyl ether. The organic phase is separated off, and the aqueous phase is extracted again with 100 ml of diethyl ether. The combined extracts are washed with 100 ml of saturated aqueous sodium bicarbonate solution and with 100 ml of saturated aqueous sodium chloride solution, and then concentrated in vacuo, and the residue is dried under HV. 13.83 g of brown oil are obtained. The crude product has a GC purity of 84 area %. It is distilled through a silvered 15 cm vacuum-jacketed Vigreux column under HV (0.005 mbar). After a fore-run (1 g, b.p. 33-88° C., colorless mobile oil), the product distills (9.5 g, b.p. 102-103° C., colorless oil) at a bath temperature of 160-170° C., and 3 g of higher-boiling impurity remain as bottom product. Yield: 9.5 g (33.4 mmol), 78% of theory.

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