oxy-1,2,4-triazine6 (126 mg, 0.5 mmol) in CHClg (0.5 mL) was treated with l-pyrrolidino-l-[2-(benzyloxy)-3,4-dimethoxyphenyl]-l-propene (4c; 353 mg, 1.0 mmol, 2.0 equiv) in CHClg (0.5 mL) under nitrogen (25 °C), and the resulting dark orange solution was warmed at 45 °C for 3 h. Chromatography of the crude product (Si02, 30% ether-pentane eluant) afforded 167 mg (274 mg theoretical, 59%) of pure 6c as a viscous yellow oil:

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A new method for the synthesis of N3'-->P5' phosphoramidate oligodeoxynucleotides is demonstrated. Described herein is the synthesis of the monomers utilized in the phosphoramidite amine-exchange process and the experimental details pertaining to this new mode of chain assembly. The phosphoramidite amine-exchange method generates coupling yields in the 92-95% range per cycle and further enables the synthesis of chimeric phosphoramidate/phosphodiester or phosphoramidate/phosphorothioate oligonucleotides with no instrument modifications.

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