A process for preparing an oligonucleotide comprising a) reacting the compound of Formula (I): with the compound of Formula (II): to form a compound of Formula (III): and b) sulfurizing the compound of Formula (III) with a sulfurization agent to form a compound of Formula (IV): and from unreacted compound of Formula (I) forms a compound of Formula (V): wherein Each R 1 and R 11 are independently a nucleobase, wherein the NH 2 of the nucleobase, if present, is protected by an amine protecting group; Each R 2 and R 12 are independently selected from the group consisting of H, halo, and C 1-6 alkoxy optionally substituted with C 1-6 alkoxy; R 3 is a solid support, optionally comprising a linker; Y is an oligonucleotide comprising 2 or more nucleotides; Each R 4 is independently H or forms a ring with the alkoxy group of R 2 ; Each R 14 is independently H or forms a ring with the alkoxy group of R 12 ; R 15 is a hydroxy protecting group; R 16 is C 1-6 alkyl optionally substituted with -CN; R 17a and R 17b are independently C 1-6 alkyl; R S is a hydroxy protecting group formed from a byproduct of the sulfurization agent; wherein the sulfurization agent is recirculated for 1-20 times in step b) to convert unreacted compound of Formula (I) to the compound of Formula (V) and to convert the compound of Formula (III) to Formula (IV); wherein the sulfurization agent is added in an amount 3 to 6 equivalents relative to the amount of the linker or the first nucleotide if the first nucleotide is directly attached to the solid support to convert unreacted compound of Formula (I) to Formula (V) and to convert the compound of Formula (III) to Formula (IV); wherein no other reagent is added to cap the unreacted compound of Formula (I); and wherein the compounds of Formulas (I), (II), (III), (IV), and (V) are optionally in the form of a pharmaceutically acceptable salt. a) reacting the compound of Formula (I): with the compound of Formula (II): to form a compound of Formula (III): and b) sulfurizing the compound of Formula (III) with a sulfurization agent to form a compound of Formula (IV): Each R 1 and R 11 are independently a nucleobase, wherein the NH 2 of the nucleobase, if present, is protected by an amine protecting group; Each R 2 and R 12 are independently selected from the group consisting of H, halo, and C 1-6 alkoxy optionally substituted with C 1-6 alkoxy; R 3 is a solid support, optionally comprising a linker; Y is an oligonucleotide comprising 2 or more nucleotides; Each R 4 is independently H or forms a ring with the alkoxy group of R 2 ; Each R 14 is independently H or forms a ring with the alkoxy group of R 12 ; R 15 is a hydroxy protecting group; R 16 is C 1-6 alkyl optionally substituted with -CN; R 17a and R 17b are independently C 1-6 alkyl; R S is a hydroxy protecting group formed from a byproduct of the sulfurization agent; wherein the sulfurization agent is recirculated for 1-20 times in step b) to convert unreacted compound of Formula (I) to the compound of Formula (V) and to convert the compound of Formula (III) to Formula (IV); wherein the sulfurization agent is added in an amount 3 to 6 equivalents relative to the amount of the linker or the first nucleotide if the first nucleotide is directly attached to the solid support to convert unreacted compound of Formula (I) to Formula (V) and to convert the compound of Formula (III) to Formula (IV); wherein no other reagent is added to cap the unreacted compound of Formula (I); and wherein the compounds of Formulas (I), (II), (III), (IV), and (V) are optionally in the form of a pharmaceutically acceptable salt. <EOS>