Glucoside methyl 6-bromo-6-deoxy

Several other methods for the introduction of halo atoms at C-6 in hexose derivatives have been known since the late 1920s. One of the earlier methods involved the reaction of methyl 2,3,4-tri-0-acetyl-6-0-trityl-a-D-glucopyranoside with phosphorus pentachloride which resulted only in an 8% overall yield of methyl 6-chloro-6-deoxy-a-D-glucoside (57). In contrast, the reaction of methyl 2,3,4-tri-0-acetyl-6-0-trityl-a-D-altropyranoside with phosphorus tribromide and bromine afforded a 73% yield of the corresponding 6-bromo-6-deoxy derivative (91). 

Halogen atoms have been placed at C-6 of substituted 6-deoxy-hexoses by a variety of interesting ways, and the products provide potential routes to 6-deoxyhexoses. Helferich and coworkers 4 reported a low yield of methyl 6-chloro-6-deoxy-a-D-glucoside which they prepared by reaction of the (suitably protected) corresponding 6-trityl ether with phosphorus pentachloride. Sinclair 44 has provided a convenient procedure for the preparation of the 6-chloro derivative in yields of 30-35% by allowing methyl a-D-glucopyranoside to react with sulfur monochloride, SjClj, in N,/V-dimethylformamide. A much better yield (73%) of the 6-bromo derivative was reported1 in the reaction of methyl 2,3,4-tri-O-benzoyl-6-O-trityl-a-D-altroside with phosphorus tribromide and bromine. 

In another instance, when 2,4,5-tribromoimidazole is used, methyl 4,6-0-benzylidene-3-bromo-S-deoxy-a-D-allopyranoside 45 is selectively formed from the methyl glucoside 34 (O Scheme 27) [47]. This results from the greater nucleophilicity of C-3 hydroxyl, which can form alkoxyphosphonium ion easily. 

Among tetrahydropyran anhydrosugar derivatives the most common are 3,6-anhydro-pyra-noses. An example illustrating the synthesis of such types of compounds is the preparation of methyl 3,6-anhydro- -D-glucoside from methyl 2,3,4-tri-0-acetyl-6-bromo-6-deoxy-/3-D-gluco-pyranoside upon treatment with barium hydroxide. 3,6-Ahydro-D-galactose and 3,6-ahydro-D-mannose are prepared in a similar way (O Fig. 20). 

The hydrolyses of methyl 3-bromo- and 3-chloro-3-deoxy-a-D-glucoside and 2-bromo- and 2-chloro-2-deoxy-a-n-altroside have been followed, but the data obtained are insufflcient to permit accurate comparisons to be made. 

The synthesis of 6-deoxy-D-glucose (see D-Quinovose, p, 99) is carried out from 2,3,4-tri-0-acetyl-6-bromo-6-deoxy-a-D-glucosyl bromide. The dihalide is converted to the methyl 2,3,4-tri-0-acetyl-6-bromo-6-deoxy-i -D-glucoside by treatment with silver carbonate and methyl alcohol, and the bromine is exchanged for a hydrogen atom by reduction with zinc and acetic acid 227). 

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