RNA synthesis by reverse direction process: phosphoramidites and high purity RNAs and introduction of ligands, chromophores, and modifications at 3'-end.

We have synthesized and studied the coupling properties of 3'-DMT-5'-CE phosphoramidites. The coupling efficiency per step surpasses 99% in the reverse-direction synthesis methodology, leading to high-purity RNA in a large number of 20- to 21-mers and long-chain oligonucleotides. Our data show that 5'→3' direction synthesis has a distinct advantage compared to the conventional method. As a result, this method of RNA synthesis is expected to be a very useful and practical method of choice for therapeutic-grade RNA. The phosphoramidites, Rev-A-n-bz, Rev-C-n-bz, Rev-C-n-ac, Rev-G-n-ac, and Rev-rU are routinely produced with an HPLC purity of greater than 98% and (31)P NMR purity greater than 99.5%.