Process for preparing a compound of general formula III, wherein R 1 denotes -tetrahydrofuran-3-yl or -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, and:: 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: reacting the organometallic compound obtained in step 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, and: reacting the adduct obtained in step 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 and/or to compound I employed in step does not exceed 40 ppm. wherein R 1 denotes -tetrahydrofuran-3-yl or -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;: 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: reacting the organometallic compound obtained in step 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, and: reacting the adduct obtained in step 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,