Double Serial Inversion

During this epimerization process, it was found that the reactivity in the 4-position was however much higher than in the 2-position. At room temperature, the epimerizatiou reaction in the 4-position occurred instantaneously, completed withiu teu to tweuty minutes, whereas in the 2-position the epimerization reaction proceeded very slowly under these conditions. This result incited us to make use of the reactivity difference between the different positions to develop a new method, stepwise inversion of the hydroxyl groups amounting to a double serial inversion protocol, by which carbohydrate structures where one position is a hydroxyl group and the other positions were protected with ester groups could be obtained. 

Using the same initial step for the double serial inversion strategy, from methyl glucoside 37, the 2,4-tiiflate intermediates 50 could be produced via a Inflation process (Scheme 15). The 4-triflates of these intermediates were subsequently inversed to the corresponding 4-0-acetyl intermediates 51 by substitution with tetrabutylammonium acetate, followed by inversion of the 2-position by tetrabutylammonium nitrite, to yield a mixture of methyl... 

In addition, due to the fact that methyl glucoside 3 was produced in a lower yield (70%) than methyl galactoside 1 (90%), following the double serial inversion strategy, an... 

Scheme 17. Alternative double serial inversion strategy to intermediate 50...

Based on the efficient multiple carbohydrate esterifications and Lattrell-Dax carbohydrate epimerization, novel and convenient double parallel- and double serial inversion methods have been developed, by which methyl p-D-mannosides and methyl P-D-talosides have been efficiently synthesized in very high yields at very mild conditions in few steps. The results also indicate that an ester group can, either in parallel or serially, induce its two neighboring groups in the epimerization reacriom... 

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