Studies of Phosphorylation. III. Selective Phosphorylation of Unprotected Nucleosides

During an investigation of the factors affecting the reaction of 2′,3′-O-isopropylidene nucleosides with phosphoryl chloride, trialkyl phosphates were found to be powerful accelerators and useful solvents for the phosphorylation. The addition of a trialkyl phosphate to a large excess of phosphoryl chloride was markedly effective in promoting the phosphorylation; further, the lower alkyl phosphates, such as trimethyl and triethyl phosphates, were able to replace the excessive amount of phosphoryl chloride as solvents. When unprotected nucleosides were treated with phosphoryl chloride in trimethyl phosphate, the corresponding 5′-phosphates were mainly produced, together with a small amount of highly-phosphorylated products, in good yields. On the contrary, the treatment of 5′-O-acetyl nucleosides in a similar manner gave the corresponding 2′(or 3′)-nucleotides in low yields. The formation of the 2′(or 3′)-phosphate was strongly inhibited by the addition of a small amount of water to the reaction mixture. Th...