Synthesis of imidazo-fused bridgehead-nitrogen 2′-deoxyribo-C-nucleosides: coupling-elimination reactions of 2,5-anhydro-3,4,6-tri-O-benzoyl-D-allonic acid

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Abstract
A short synthesis of imidazo-fused bridgehead-nitrogen 2′-deoxyribo-C-nucleosides has been developed. This is based on a coupling–elimination reaction of 2,5-anhydro-3,4,6-tri-O-benzoyl-D-allonic acid with a series of aminoalkyl-substituted heterocycles and alcohols. The intermediate α,β-unsaturated carboxamides and esters thus formed are converted into novel imidazo[1,5-a]pyridine, imidazo[1,5-b]pyridazine, and imidazo[5,1-f][1,2,4]triazine 2′-deoxyribo-C-nucleosides, including analogues of 2′-deoxyguanosine and 2′-deoxyadenosine. Assignment of the anomeric configuration of the nucleosides is made on the basis of proton n.O.e. experiments.