Studies on the construction of glycosidic linkage in guanofosfocins. Glycosylation of 8-oxoinosine and 8-oxoguanosine derivatives with mannopyranosyl bromide

In model studies directed to the total synthesis of guanofosfocins, a unique glycosydic bond formation between the 8-oxoinosine derivatives and mannopyranosyl donor is described. The reaction employing previously reported conditions resulted in the formation of two regioisomers, the 8-O- and N7-mannosylated inosine derivatives. However, the use of tetrabutylammonium iodide as a catalyst was found to improve the chemoselectivity towards O-glycosylation to afford the desired 8-O-mannosylated products in satisfactory yields. The established procedure was successfully applied for the synthesis of 8-(mannopyranosyloxy)guanosine derivative.