SYNTHESIS OF NUCLEOSIDE 3'-ALKYLPHOSPHONATES - INTERMEDIATES FOR ASSEMBLY OF CARBON-BRIDGED DINUCLEOTIDE ANALOGS

Several 3'-modified nucleosides have been prepared, through an initial Wittig or Horner-Wadsworth-Emmons condensation with an adenosine 3'-k etone, followed by catalytic hydrogenation of the resulting olefin. Su bsequent reaction of the 3'alpha-methylene carboxylate with the lithiu m salt of diethyl ethylphosphonate gave a beta-keto phosphonate, while reaction of the methylene carboxylate with LDA, diethyl chlorophosphi te, and O2 gives the corresponding alpha-phosphono ester. These new nu cleoside phosphonates can be viewed as analogues of natural phosphates and also can serve as synthetic intermediates for preparation of carb on-bridged dinucleotide analogues. To give the first such example, the beta-keto phosphonate 13 was allowed to react with a nucleoside 5-ald ehyde, affording the dinucleoside enone 23.