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

The six-step synthesis of the di-triethylammonium salt of 5'-O-trityl-6-N-p ivaloyladenosine-2'-(H-pho phonate)-3'-[(2-chlorophenyl) phosphate] 9 from 3',5'-O-(1,1,3,3-tetraisopropyldisiloxan-1,3-diyl)-6-N-pivaloyladenosine 1 in 68% overall yield is described. Compound 9 is converted into a branched pentaribonucleoside tetraphosphate 24 and a branched cyclic pentaribonucleo tide ('lariat') 25 by solution phase triester chemistry involving both H-ph osphonate and conventional phosphotriester coupling reactions. The monomeri c building block 9 is proposed as a universal synthon for the preparation o f branched and branched cyclic oligoribonucleotides derived from adenosine.