A convenient solid-phase method for synthesis of 3 '-conjugates of oligonucleotides

We present a new procedure for the preparation of S ' -conjugates of oligon ucleotides through solid-phase synthesis. A suitable universal solid suppor t was readily prepared using a series of peptidelike coupling reactions to incorporate first a spacer and then an L-homoserine branching unit. The N-a lpha -position of the homoserine carries an Fmoc protecting group that is r emoved by treatment with piperidine to liberate an amino group suitable for attachment of the conjugate (e.g., small organic molecule, fluorescent gro up, cholesterol, biotin, amino acid, etc.) or for assembly of a short pepti de. The side-chain hydroxyl group of the homoserine carries a trityl protec ting group. After TFA deprotection, the hydroxyl group acts as the site for oligonucleotide assembly. An additional spacer, such as aminohexanoyl, may be incorporated easily between the conjugate molecule and the oligonucleot ide. A number of examples of synthesis of 3 ' -conjugates of oligonucleotid es and their analogues are described that involve standard automated oligon ucleotide assembly and use of commercially available materials. The linkage between oligonucleotide and 3 ' -conjugate is chirally pure and is stable to conventional ammonia treatment used for oligonucleotide deprotection and release from the solid support. The homoserine-functionalized solid suppor t system represents a simple and universal route to 3 ' -conjugates of olig onucleotides and their derivatives.