Selective reactivities to some mono and disaccharides
Munavu, Raphael M
MetadataShow full item record
The rate of the base-catalyzed transesterification of sucrose and ethyl myristate was shown to vary somewhat as a function of solvent and base catalyst, but the most significant effect was the 2-4 fold acceleratin caused by the presence of crown ether. Nevertheless, the transesterification product consisted of three monoesters of sucrose (derived from the primary alcohol functions), and a mixture of five minor components. The major product of these transesterification reactions was the 6 ester obtained in ca. 66% yield. Similarly, both methyl a- and 6-glucopyranosides gave 46 and 37% yields of the 6-monoesters. The transesterification of ... several 4,6-benzal derivatives of hexopyranosides gave mixtures of the 2-0- and 3-0-esters and furthermore the experimental conditions also,.caused isomerization by the shift of acyl_ groups among the neighboring hydroxyl groups. -- Similar results were obtained by using acylimidazoles ,and other such acylating agents. 2. The reported faiJure of lactose to give base catalyzed transesterification was confirmed and prompted the examination of the methyl 6-lactoside in this reaction. The product consisted of two mono- and two diesters. The monoesters are believed to be the 3 , - and 6- derivatives of the galactose portion, while the diesters are thought to be the 6,6' – and 3`,6 - compounds. 3. Excellent seletive esterification at the 2-0- position was achieved by al1owing a series of mono- and disaccharides to react with dibutyltin oxide, and by treating the resulting dibutyistannylene with acid halides. It appeats the preferential formation and reactivity of the stannylene derivatives depends on the coordination of the anomeric oxygen with tin complexed by the 2,3-hydroxyl groups. This requirement explains that .the selective acylation was limited to a.-substituted saccharides with the R configuration at C-2. 4. A benzalation procedure that functions under non-acidic conditions was developed by using the dithioacetal derived from benzaldehyde and 1,2-ethanedithiol as well as its disulfonium salts. This reaction was applied to a series of diols and monosaccharides. The application of the procedure to sucrose apparently gave a poor yield of the 4,6-0-benzylidene deiivative isolated as the hexaacetate. 5. The NMR spectra of the benzal derivatives prepared in this work showed a variation of the chemical shift of the PhCH signal with ring size and the presence of substituents. An attempt was made to rationalize the observed chemical shift differences in terms of buttressing and other longrange effects.