Stereoselective Addition of a Wittig Reagent To Give a Single Nucleoside Oxaphosphetane Diastereoisomer. Synthesis of 2′(and 3′)-Deoxy-2′(and 3′)-methyleneuridine (and cytidine) Derivatives from Uridine Ketonucleosides

Treatment of 3′,5′(or 2′,5′)-bis-O-silyl-protected 2′(or 3′)-ketouridine derivatives with methyltriphenylphosphonium bromide and sodium 2-methyl-2-butoxide in diethyl ether/benzene at 0-4°C resulted in the slow formation of the corresponding 2′(or 3′)-deoxy-2′(or 3′)-methylene analogues. ¹H- and ³¹P-NMR spectra were in harmony with formation of a single oxaphosphetane diastereoisomer during early stages of the Wittig reaction. Conversions of protected deoxymethyleneuridine to deoxymethylenecytidine derivatives were effected smoothly via 4-(1,2,4-triazol-1-yl) intermediates. Deprotection with tetrabutylammonium fluoride gave 2′(or 3′)-deoxy-2′(or 3′)-methyleneuridine and cytidine nucleosides.