Oligoribonucleotide Synthesis: The Silyl-Phosphoramidite Method

In 1968, one of us began a quest to develop a general method for the synthesis of RNA sequences. The major problems associated with the chemical assembly of ribonucleotide chains were already well established. RNA sequences are very sensitive to chemical and enzymatic degradation. Consequently, all procedures involved in the assembly of RNA chains must respect the delicacy of the assembled chain. Further, the assembly of RNA sequences is complicated by the presence of a 2′-hydroxyl group in the monomeric ribonucleosides that form the starting materials for the chemical assembly of RNA chains. The 2′-hydroxyl must be blocked (or “protected”) with a protecting group that remains stable throughout the many steps of a chain assembly yet that is labile enough to be removed at the end of chain assembly without leading to cleavage of the assembled chain. Finally, it was clear even in the late 1960s that in order to be convincing the method must be capable of producing an RNA chain of the length of a tRNA in order to overcome the inherent skepticism of chemical synthesis of RNA.