A method of preparing an oligonucleotide of formula, wherein V 1, V 3 and V 5 are independently in each case selected from O, S and Se; V 2, V 4 and V 6 are independently in each case selected from OH, OR 1, SH, SR 1, F, NH 2, NHR 1, N(R 1) 2 and BH 3 - M +, W 1 is O or S, W 2 is O, S, NH or NR 2, W 3 is O, S, NH, NR 2, CH 2, CHHal or C(Hal) 2, R 1, R 2 and R 3 are selected from C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 2-6 acyl or a cyclic group, each optionally substituted, or wherein two R 1 may form a ring together with an N-atom bound thereto, M + is a cation, X is NH, NR 3, O or S, Z represents a capture tag, which is a C 8-24 alkyl residue, a perfluoroalkyl entity, an azide or an alkynyl group, Y represents a bond or a linker connecting the capture tag to X, and ON represents an oligonucleotide comprising at least 4 nucleotide or nucleotide analogue building blocks, comprising the steps: (a) reacting a compound of formula (IIa) wherein V 1, V 3, V 5, V 4, V 6, W 1, W 2, W 3, and ON are as defined above, with an oxidizing agent to obtain a compound of formula (IIb) wherein V 1, V 3, V 5, V 2, V 4, V 6, W 1, W 2, W 3 and ON are as defined above, (b) reacting a compound of formula (IIb) with a capture tag agent of formula, Z-Y-XH wherein X, Z, and Y are as defined above to obtain a reaction product comprising the oligonucleotide of formula, and (c) contacting the reaction product of step (b) with a capture reagent capable of interacting with the capture tag, wherein the capture reagent is selected from the group consisting of a solid phase of a standard reverse phase chromatography such as RP-HPLC, a chromatographic material with affinity for hydrophobic groups, a chromatographic material with affinity for fluorinated groups such as a fluorous affinity support, a capture reagent containing an alkynyl moiety, and a capture reagent containing an azido moiety, wherein the contacting takes place under conditions which allow separation of the oligonucleotide from other species contained in said reaction product. wherein V 1, V 3 and V 5 are independently in each case selected from O, S and Se; V 2, V 4 and V 6 are independently in each case selected from OH, OR 1, SH, SR 1, F, NH 2, NHR 1, N(R 1) 2 and BH 3 - M +, W 1 is O or S, W 2 is O, S, NH or NR 2, W 3 is O, S, NH, NR 2, CH 2, CHHal or C(Hal) 2, R 1, R 2 and R 3 are selected from C 1-6 alkyl, C 2-6 alkenyl, C 2-6 alkynyl, C 2-6 acyl or a cyclic group, each optionally substituted, or wherein two R 1 may form a ring together with an N-atom bound thereto, M + is a cation, X is NH, NR 3, O or S, Z represents a capture tag, which is a C 8-24 alkyl residue, a perfluoroalkyl entity, an azide or an alkynyl group, Y represents a bond or a linker connecting the capture tag to X, and ON represents an oligonucleotide comprising at least 4 nucleotide or nucleotide analogue building blocks, comprising the steps: (a) reacting a compound of formula (IIa) wherein V 1, V 3, V 5, V 4, V 6, W 1, W 2, W 3, and ON are as defined above, with an oxidizing agent to obtain a compound of formula (IIb) wherein V 1, V 3, V 5, V 2, V 4, V 6, W 1, W 2, W 3 and ON are as defined above, (b) reacting a compound of formula (IIb) with a capture tag agent of formula, Z-Y-XH wherein X, Z, and Y are as defined above to obtain a reaction product comprising the oligonucleotide of formula, and (c) contacting the reaction product of step (b) with a capture reagent capable of interacting with the capture tag, wherein the capture reagent is selected from the group consisting of a solid phase of a standard reverse phase chromatography such as RP-HPLC, a chromatographic material with affinity for hydrophobic groups, a chromatographic material with affinity for fluorinated groups such as a fluorous affinity support, a capture reagent containing an alkynyl moiety, and a capture reagent containing an azido moiety, wherein the contacting takes place under conditions which allow separation of the oligonucleotide from other species contained in said reaction product. (a) reacting a compound of formula (IIa) wherein V 1, V 3, V 5, V 4, V 6, W 1, W 2, W 3, and ON are as defined above, with an oxidizing agent to obtain a compound of formula (IIb) wherein V 1, V 3, V 5, V 2, V 4, V 6, W 1, W 2, W 3 and ON are as defined above, (b) reacting a compound of formula (IIb) with a capture tag agent of formula, Z-Y-XH wherein X, Z, and Y are as defined above to obtain a reaction product comprising the oligonucleotide of formula, and (c) contacting the reaction product of step (b) with a capture reagent capable of interacting with the capture tag, wherein the capture reagent is selected from the group consisting of a solid phase of a standard reverse phase chromatography such as RP-HPLC, a chromatographic material with affinity for hydrophobic groups, a chromatographic material with affinity for fluorinated groups such as a fluorous affinity support, a capture reagent containing an alkynyl moiety, and a capture reagent containing an azido moiety, wherein the contacting takes place under conditions which allow separation of the oligonucleotide from other species contained in said reaction product.