The synthesis of XI has as its starting material the readily available 1, 2-O-isopropylidene-~- xylofuranosea (I). This was converted to crystalline 1, 2-0-isopropylidene-5-0- methoxycarbonyl-D-xylofuranose (11) by reaction with an excess of methyl chloroformate in pyridine, a procedure that takes advantage of the large difference in rate of