A new approach to the synthesis of branched and branched cyclic oligoribonucleotides.

The six-step synthesis of the di-triethylammonium salt of 5prime]-O -trityl-6-N-pivaloyladenosine-2prime]-(H -phosphonate)-3'-(2-chlorophenyl) phosphate]9 from 3', 5'- O -(1,1,3,3-tetraisopropyldisiloxan-1,3-diyl)-6-N-pivaloyla denosine1in 68% overall yield is described. Compound9is converted into a branched pentaribonucleoside tetraphosphate 24 and a branched cyclic pentaribonucleotide ('lariat') 25 by solution phase triester chemistry involving both H-phosphonate and conventional phosphotriester coupling reactions. The monomeric building block 9 is proposed as a universal synthon for the preparation of branched and branched cyclic oligoribonucleotides derived from adenosine.