Synthesis of Polyol Glycoside Subunit

If the temperature is maintained at -20 °C, a rapid reaction takes place to afford a 1 3 mixture of glycosides favoring the undesired axial anomer 50a. Interestingly, warming of the reaction mixture to -3 °C induces an anomer equilibration to give 50/ in a 4 1 mixture with 50a. 

The undesired axial anomer 50a can be recovered and resubmitted to the reaction conditions to give a 4 1 mixture of glycosides favoring 50)5. 

Although trityl perchlorate is used to accomplish the glycosidation of the C-8 hydroxyl in 44 with acetoxy glycoside 49, control experiments have demonstrated that no reaction takes place in the presence of 4 A molecular sieves or 2,6-di-terf-butylpyridine. This observation suggests that the actual catalyst is not trityl perchlorate, but perchloric acid. Consistent with this conclusion is the observation that catalytic amounts of a strong Brpnsted acid such as triflic or perchloric acid can catalyze the glycosidation of 44 with 49 in the absence of trityl perchlorate. 

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