Synthesis of Morpholino Monomers, Chlorophosphoramidate Monomers, and Solid‐Phase Synthesis of Short Morpholino Oligomers

Abstract Phosphorodiamidate morpholino oligomers (PMOs) are a highly capable class of synthetic antisense oligonucleotides that are used to study gene functions in in vitro and in vivo models. This unit describes the synthesis of exocyclic‐amine‐protected 7′‐hydroxy and 7′‐chlorophosphoramidate‐activated morpholino monomers of A, T, G, and C, together with their incorporation into short PMO oligomers by solid‐phase synthesis. Starting from ribonucleosides, the exocyclic‐amine‐protected 7′‐hydroxy monomers are prepared following a modified Summerton protocol, which consists of a periodate cleavage/Schiff base formation/reduction cycle. The exocyclic amine protections are installed at a later stage (except G) to avoid the use of costly exocyclic‐amine‐protected counterparts that give control over protecting group manipulation. The 7′‐hydroxy monomers with N ‐Trit/ N ‐MMTr are then converted to the 7′‐chlorophosphoramidate morpholino monomers in one step employing a combination of lithium bromide and DBU. These chlorophosphoramidate monomers are finally assembled by solid‐support synthesis to obtain the short PMO oligomers. © 2015 by John Wiley & Sons, Inc.