Addition Reactions of 3,6-Anhydro-D-Glucal

FIGURE 12.1 X-tay diffraction data for compound X. (a) Showing H-bonding between moleeules in the lattice, (b) Unit ceU showing H-bonding with other cells. 

A larger alcohol nucleophile was then used in this reaction 1,2 3,4-Di-isopropylidene-D-galactopyranose was used instead of methanol. With this bulkier nucleophile, none of the expected disaccharide was observed. Instead, the dianhy-drosugar 9 was isolated as the major product (48%). Presumably, this product was formed by an intramolecular reaction of the bridgehead alcohol with sugar iodonium ion intermediate. None of the external nucleophile was able to react because of the sterically crowded environment. 

In fact, when the anhydroglucal 7 was placed in anhydrous acetonitrile with NIS and only activated 4 A molecular sieves, an improved yield of 76% ofdianhydrosugar 9 was obtained. Previously, Brimacombe and colleagues observed the formation of similar dianhydrosugar when the 3,6-anhydro-D-glucal 7 was left in anhydrous 

Sodium iodide is believed to further enhance the alpha selective addition of nucleophiles under these conditions [32], Addition of sodium iodide to the CAN/acetonitrile/thiophenol reaction mixture with the anhydroglycal 7 gave a diminished amount of glycoside 10a,b (35%, a p, 2 1) and increased amounts of Perrier product (65%). Only the a-Ferrier product 11a was isolated. 

4 PREPARATION OF 6-O-TOSYL-d-GALACTAL AND REDUCTION WITH LrrmuM aluminum hydride 

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