O=C1CCCC1C(OC)=O.[F:11][C:12]1([C:18]([O:20][CH3:21])=[O:19])[CH2:16][CH2:15][CH2:14][C:13]1=[O:17].B(F)(F)F.CCOCC.C(=O)(O)[O-].[Na+]>COCCOCCOC>[F:11][C:12]1([C:18]([O:20][CH3:21])=[O:19])[CH2:16][CH2:15][CH2:14][C:13]1=[O:17].[F:11][C:12]1([C:18]([O:20][CH3:21])=[O:19])[CH2:16][CH2:15][CH2:14][CH:13]1[OH:17]First, methyl 1-fluoro-2-oxocyclopentanecarboxylate was prepared from methyl 2-oxocyclopentanecarboxylate referring to the process described in Tetrahedron Letters 27 (37), 4465-4468 (1986). Then, 2.3 g (0.032 mol) of trimethylamine-borane complex and 5.0 g (0.031 mol) of the methyl 1-fluoro-2-oxocyclopentanecarboxylate were added to 50 ml of dry diglyme, the resulting solution was cooled to 0° C., and while it as stirred in the stream of nitrogen, 0.8 ml of boron trifluoride etherate was added dropwise. The resulting solution was stirred at room temperature for 1 hour and then poured into a saturated aqueous sodium bicarbonate solution, followed by extraction with ether. The ether layer was washed with water and dried over anhydrous magnesium sulfate, and the solvent was distilled away to obtain 4.5 g (yield: 89%) of the objective methyl 1-fluoro-2-hydroxycyclopentanecarboxylate in cis form (compound number 2a; the substance such that the substituent at the 2-position is supposed to take cis configuration when the methoxycarbonyl group at the 1-position is taken as a standard) which contains almost no the same in trans form.
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