A process for preparing a compound of formula IIIa: wherein: R denotes an alkyl group with 1 to 5 carbon atoms selected from methyl, ethyl, 1-propyl or isopropyl; X denotes a protecting group for an alcohol selected from benzyl (Bn), methoxymethyl, 2-methoxyethoxymethyl, methylthiomethyl, tetrahydropyranyl, benzyloxymethyl, p-Methoxyphenyl, p- methoxybenzyl, p-methoxybenzyloxymethyl (PMBM), triisopropylsilyl (TIPS), tert-butyldimethylsilyl (TBDMS), 2-(trimethylsilyl)ethoxymethyl and (phenyldimethylsilyl)methoxymethyl (SMOM); Y denotes a protecting group for an amine selected from tert-butyl carbamate (Boc), 9-fluoroenylmethyl carbamate (Fmoc), methyl carbamate, ethyl carbamate, 2-chloro-3-indenylmethyl carbamate (Climoc), benz[f]inden-3-ylmethyl carbamate (Bimoc), 2,2,2-trichloroethyl carbamate (Troc), 2-chloroethyl carbamate, 1,1-dimethyl-2,2-dibromoethyl carbamate (DB- t -BOC), 1,1-dimethyl-2,2,2-trichloroethyl carbamate (TCBOC), benzyl carbamate (Cbz) and diphenylmethyl carbamate, and wherein the process comprises a work-up process comprising the steps of: i) providing a crude reaction product comprising said compound of formula IIIa; ii) adding ethyl acetate to the crude reaction product of step i) to prepare a suspension; iii) adding water to the suspension of step ii) to form a biphasic system comprising an aqueous phase and an organic phase, and discarding the aqueous phase; iv) adding an acid to the organic phase of step iii) to form a biphasic system comprising an acidic aqueous phase and an organic phase, and discarding the acidic aqueous phase; v) washing the organic phase of step iv) with water. R denotes an alkyl group with 1 to 5 carbon atoms selected from methyl, ethyl, 1-propyl or isopropyl; X denotes a protecting group for an alcohol selected from benzyl (Bn), methoxymethyl, 2-methoxyethoxymethyl, methylthiomethyl, tetrahydropyranyl, benzyloxymethyl, p-Methoxyphenyl, p- methoxybenzyl, p-methoxybenzyloxymethyl (PMBM), triisopropylsilyl (TIPS), tert-butyldimethylsilyl (TBDMS), 2-(trimethylsilyl)ethoxymethyl and (phenyldimethylsilyl)methoxymethyl (SMOM); Y denotes a protecting group for an amine selected from tert-butyl carbamate (Boc), 9-fluoroenylmethyl carbamate (Fmoc), methyl carbamate, ethyl carbamate, 2-chloro-3-indenylmethyl carbamate (Climoc), benz[f]inden-3-ylmethyl carbamate (Bimoc), 2,2,2-trichloroethyl carbamate (Troc), 2-chloroethyl carbamate, 1,1-dimethyl-2,2-dibromoethyl carbamate (DB- t -BOC), 1,1-dimethyl-2,2,2-trichloroethyl carbamate (TCBOC), benzyl carbamate (Cbz) and diphenylmethyl carbamate, and wherein the process comprises a work-up process comprising the steps of: i) providing a crude reaction product comprising said compound of formula IIIa; ii) adding ethyl acetate to the crude reaction product of step i) to prepare a suspension; iii) adding water to the suspension of step ii) to form a biphasic system comprising an aqueous phase and an organic phase, and discarding the aqueous phase; iv) adding an acid to the organic phase of step iii) to form a biphasic system comprising an acidic aqueous phase and an organic phase, and discarding the acidic aqueous phase; v) washing the organic phase of step iv) with water. i) providing a crude reaction product comprising said compound of formula IIIa; ii) adding ethyl acetate to the crude reaction product of step i) to prepare a suspension; iii) adding water to the suspension of step ii) to form a biphasic system comprising an aqueous phase and an organic phase, and discarding the aqueous phase; iv) adding an acid to the organic phase of step iii) to form a biphasic system comprising an acidic aqueous phase and an organic phase, and discarding the acidic aqueous phase; v) washing the organic phase of step iv) with water.