Process for preparing a compound of general formula III, wherein R 1 denotes ( R )-tetrahydrofuran-3-yl or ( S )-tetrahydrofuran-3-yl; and wherein R 2 independently of one another denote hydrogen, (C 1-8 -alkyl)carbonyl, (C 1-8 -alkyl)oxycarbonyl, phenylcarbonyl, phenyl-(C 1-3 -alkyl)-carbonyl, phenyl-C 1-3 -alkyl, allyl, R a R b R c Si, CR a R b OR c , wherein two adjacent groups R 2 may be linked with each other to form a bridging group SiR a R b , CR a R b or CR a OR b -CR a OR b ; wherein R a , R b , R c independently of one another denote C 1-4 -alkyl, phenyl or phenyl-C 1-3 -alkyl, while the alkyl groups may be mono- or polysubstituted by halogen; while the phenyl groups mentioned in the definition of the above groups may be mono- or polysubstituted with L1, wherein L1 independently of one another are selected from among fluorine, chlorine, bromine, C 1-3 -alkyl, C 1-4 -alkoxy and nitro; and wherein R' denotes hydrogen, methyl or ethyl; comprising the steps (S1), (S2) and (S3): (S1): reacting a compound of general formula I wherein R 1 is defined as hereinbefore and X denotes Br, I or triflate; with a C 1-4 -alkyl-magnesium chloride or bromide, wherein lithium bromide and/or lithium chloride is optionally used, and (S2): reacting the organometallic compound obtained in step (S1) with a compound of general formula II wherein R 2 is defined as hereinbefore; and wherein lithium bromide and/or lithium chloride is optionally used, and wherein R 2 not being hydrogen are optionally cleaved during or at the end of (S2), and (S3): reacting the adduct obtained in step (S2) with a compound R'-OH or a mixture of compounds R'-OH, wherein R' is defined as hereinbefore, in the presence of one or more acids, characterized in that , the mole ratio of iron ions in the reaction mixtures of step (S1) and/or (S2) to compound I employed in step (S1) does not exceed 40 ppm. wherein R 1 denotes ( R )-tetrahydrofuran-3-yl or ( S )-tetrahydrofuran-3-yl; and wherein R 2 independently of one another denote hydrogen, (C 1-8 -alkyl)carbonyl, (C 1-8 -alkyl)oxycarbonyl, phenylcarbonyl, phenyl-(C 1-3 -alkyl)-carbonyl, phenyl-C 1-3 -alkyl, allyl, R a R b R c Si, CR a R b OR c , wherein two adjacent groups R 2 may be linked with each other to form a bridging group SiR a R b , CR a R b or CR a OR b -CR a OR b ; wherein R a , R b , R c independently of one another denote C 1-4 -alkyl, phenyl or phenyl-C 1-3 -alkyl, while the alkyl groups may be mono- or polysubstituted by halogen; while the phenyl groups mentioned in the definition of the above groups may be mono- or polysubstituted with L1, wherein L1 independently of one another are selected from among fluorine, chlorine, bromine, C 1-3 -alkyl, C 1-4 -alkoxy and nitro; and wherein R' denotes hydrogen, methyl or ethyl; (S1): reacting a compound of general formula I wherein R 1 is defined as hereinbefore and X denotes Br, I or triflate; with a C 1-4 -alkyl-magnesium chloride or bromide, wherein lithium bromide and/or lithium chloride is optionally used, and (S2): reacting the organometallic compound obtained in step (S1) with a compound of general formula II wherein R 2 is defined as hereinbefore; and wherein lithium bromide and/or lithium chloride is optionally used, and wherein R 2 not being hydrogen are optionally cleaved during or at the end of (S2), and (S3): reacting the adduct obtained in step (S2) with a compound R'-OH or a mixture of compounds R'-OH, wherein R' is defined as hereinbefore, in the presence of one or more acids, <EOS>