Stereochemistry of Internucleotide Bond Formation by the H-Phosphonate Method. 5. The Role of BrOnsted and H-Bonding Base Catalysis in Ribonucleoside H-Phosphonate CondensationChemical and Stereochemical Consequences

Efficiency and stereoselectivity of condensations of ribonucleoside 3'-H-phosphonates with ethanol promoted by pivaloyl chloride were investigated as a function of tertiary amines used. Side reactions leading to an increased demand for the condensing agent were identified as derived from an attack of the pivalate anion at carbonyl centers of reactive pivaloyl derivatives. The conditions that secured quantitative yields of H-phosphonate diester condensations were assessed. Several tertiary amines promoted condensations with stereoselectivity higher than that observed for pyridine derivatives. A correlation between diastereoselectivity of the product formation and BrOnsted and H-bonding basicities of the amine used was found.