The synthesis of SoM phosphoramidite monomers is shown in Scheme 3. Compounds 9a-c were synthesized using reported procedures. Details are provided in the Supporting Information. Coupling of 9a-c with 8a using DIC or DCC as the activator gave compounds 10a-c in good to excellent yields. The reactions were performed under similar conditions except that the amino group of 9a was more reactive and the reaction could proceed at room temperature, while the amino groups of 9b-c were less reactive, and higher temperatures were needed for the reactions to proceed with acceptable rates. Converting 10a-c to 11a-c were achieved using pyriding-3HF complex in DCM at 0 °C. Good yields could be obtained in all the cases. Under these conditions, the 2′-OTBS group was stable. Installation of the 5′-ODMTr groups to give 12a-c, and converting 12a-c to the target SoM phosphoramidites 1a-c were achieved under the conditions we used frequently in our lab for various projects.