Turanose orthoesters

These conclusions do not comply with the explanation given by Haworth, Hirst and Samuels and by Pacsu for the reaction mechanism of the acid-catalyzed hydrolysis of the rhamnose and turanose orthoesters, respectively. It seems, however, that the experimental data of these authors can be re-interpreted without difficulty, to fit into the picture given for the reaction mechanism of the maltose orthoester. The first and very rapid reaction, which Haworth and coworkers associated with the removal of a methoxyl residue, would, as in the maltose orthoester, involve the rupture of the bond between the central carbon atom and that oxygen atom which is linked to carbon atom 1. This process liberates the 3-form of L-rhamnose substituted at position 2 by a methyl hydrogen orthoacetate residue. However, since this intermediate is... 

On removal of the six acetyl groups from hexaacetylturanose methyl 1,2-orthoacetate, either by alcoholic ammonia or by a trace of sodium methoxide according to the method of Zempl n and Pacsu, crystalline turanose methyl 1,2-orthoacetate (Vlllb) was obtained. It crystallized from ethanol in the form of tabular crystals with m. p. 137 and [o]d + 114.6 in aqueous solution. Hydrolysis with alkali did not eliminate the remaining acetyl group until the methyl glycosidic group with which it was linked in the orthoester formation had been removed. The instability of the turanose methyl 1,2-orthoacetate in aqueous solution was illustrated by a gradual decrease in rotation in water from [aln +113.3 to a constant value of [a]n +72.7 in sixty-four hours. 

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