Improved Protocol for the Synthesis of Flexibly Protected Morpholino Monomers from Unprotected Ribonucleosides

An inexpensive and much improved protocol has been developed for the synthesis of protected morpholino monomers from unprotected ribonucleosides in high overall yield, using oxidative glycol cleavage and reductive amination strategy. Unlike the previous methods, the present strategy allows installing the exocyclic amine protections at a later stage, and thus avoids the use of expensive, or commercially unavailable, exocyclic amine-protected ribonucleosides as starting materials. To demonstrate the flexibility of the present method in choosing protecting groups, the monomers have been protected with several such groups of different deblocking properties at the exocyclic amine position.