The reaction between a" alcohol and a large excess of PC13 generally gives a dichlorophosphite, which on hydrolysis affords a phosphite. Thus, treatment of a" Ip* ribonucleoside with PC13 should give, after hydrolysis and de-blocking the protective group, a ribonucleoside-5'-phosphite. Contrary to these expectations, however, when Ip inosine wss allowed to react with PC13 in acetone in an open vessel and the reaction mixture Was poured into ice water, the main product, after rsmoval of the protective group, was 5'-IMP (yield 91%) (Table I, NO. 2). The structure of this 5'-IMP was confirmed by its ultraviolet absorption spectrum, its paper electrophoretic*' and chromstographic*** behavior, and by the liberation of phosphoric acid with bull semen 5'-nucleotidase. Levene and Tipson (1) had reported that Ip inosine was phosphorylsted with POC13 in the presence Of pyridine in low yields. The present phosphorylation has been achieved with PC13 in the absence of base. I" general, purine nucleosides are not very * Ipr 2',3'-0-isopropylidene; 5'-1Mp, inosine-5'-monophosphate; 5'-GMP, guanosine-5'-monophosphate. ** 0.05 M phosphate buffer (pH 7.5). 22 V/cm. *** n-PrOH : 2% sq.NH3 : H20 (10 : 6 : l), ascending method.