D-Mannose methyl 1,2-orthoacetate

Levene and Sobotka repeated Dale s experiments using crystalline acetylmannosyl bromide as the starting material. Their product was identical with the y-form of Dale s new methyl mannoside acetates. Deacetylation by alkali led to a dry, hygroscopic mass, which gave the correct analysis for D-mannose methyl 1,2-orthoacetate. From the crystalline acetylmannosyl bromide, on treatment with ethyl alcohol and silver carbonate, they prepared the analogous ethyl orthoacetate (Illb) with m. p. 81-82° and C ]d —28°. On saponification, it behaved in a manner analogous to the methyl derivative prepared from the same source. For the deacetylation of the triacetyl-n-mannose methyl 1,2-orthoacetate, Bott, Haworth and Hirst employed 0.5 N sodium hydroxide in ethanol at 0°. 

When a chloroform solution of hexaacetyl-4- 8-D-glucosyl-D-mannose methyl 1,2-orthoacetate was treated with hydrogen chloride, a rapid change in rotation occurred in that the initial specific rotation of —12.7 became constant in six minutes, [ajo -f29.8 . This change in rotation was similar to that observed with alcoholic solutions. On evaporation, the chloroform solution gave a dextro-rotatory heptaacetyl-4- 8-D-glu-cosyl-D-mannosyl chloride with the structure of the common type of acetylglycosyl halides. 

The first observation of the instability of carbohydrate orthoesters toward alkali came from Haworth, Hirst and Miller in connection with their experiments on the simultaneous deacetylation and methylation of L-rhamnose methyl 1,2-orthoacetate. These authors noticed that methylation by methyl iodide and silver oxide in the presence of solid sodium hydroxide resulted in the formation of crystalline methyl tri-methyl-/3-L-rhamnopyranoside. A similar result was obtained by Bott, Haworth and Hirst on the simultaneous deacetylation and methylation of triacetyl-D-mannose methyl 1,2-orthoacetate by the use of excessive quantities of dimethyl sulfate and alkali. The reaction produced a mixture of a. and /3 forms of methyl tetramethyl-D-mannopyranoside but the yield was only 40%. When the acetylated orthoester was submitted to methylation with silver oxide and methyl iodide in the presence of sodium hydroxide, the product was mainly trimethyl-rhamnose methyl 1,2-orthoacetate. This result indicates that for the alkaline hydrolysis of orthoesters, hydroxyl ions are necessary. Such ions are present in the dimethyl sulfate-alkali process, but are absent in the methyl iodide treatment except when the reaction mixture contains a little water either by accident or from the decomposition of the sugar molecule. Haworth, Hirst and Samuels examined the behavior of dimethyl-L-rhamnose methyl 1,2-orthoacetate in alkaline solution. When the substance was heated under various conditions with 0.1 A alkali at 70 there was no appreciable hydrolysis at the end of ninety minutes, whereas at 80 for... 

Copolymerization of l,4-anhydro-2,3-di-0-(to7-butyldimethylsilyl)-a-D-xylopy-ranose (39) with l,4-anhydro-2,3-di-0-benzyl-a-i>xylopyranose (27), followed by desilylation with tetrabutylammonium fluoride gave a partially benzylated sterero-regular (l -->5)-a-D-xylofuranan. [37] A branched polymer (40) was obtained when it was glycosylated with 3,4,6-tri-0-acetyl-P-D-mannose l,2-(methyl orthoacetate). Debenzylation of the polymer having D-mannosyl branches with sodium in liquid ammonia yielded (1 — 5)-a-i>xylofuranans having 2- or 3-O-a-D-mannopyranosyl branches. 

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