A 1,6-anhydro

Write the structural formulas for (a) 1,6 anhydro P-o-gl ucopyranose (b) 1,6 anhydro P-D-altrose. Compound (b) is many times more stable than compound (a). Explain this on stereochemical grounds. 

In the presence of such acid catalysts as monochloroacetic acid212 or a Lewis acid,201 a 1,6-anhydro sugar, such as levoglucosan (48), is undoubtedly protonated (or conjugated) on the 1,6-ether bridge222 (53). 

S. Ogawa and D. Aso, Chemical modification of the sugar moiety of methyl acarviosin synthesis and inhibitory activity of eight analogues containing a 1,6-anhydro bridge,... 

It should be realised that minor conformational isomers may be important reaction intermediates. For example, treatment of 6-0-tosyl-D-glucopyranose with base results in the formation of a 1,6-anhydro derivative. The starting material exists mainly in the 4Ci conformation. However, for reaction to occur the alternative 4 conformation has to be adopted (Scheme 1.2). The introduction of protecting groups may alter the preferred conformation of saccharides. 

Mono O-debenzylation can be achieved with Lewis-acid-mediated conditions such as with TiCLt of SnCU- Good yields are obtained when the substrate has three continuous cri-orientated metal-chelating groups (Scheme 2.3b). Thus, in case of a 1,6-anhydro-mannose derivative chelation takes place between the exocyclic anomeric oxygen and the 2-0-and 3-O-benzyl groups. On the other hand, 1,6-anhydrogIucose derivatives are chelated between endocyclic oxygen and the functionalities on... 

Deoxy-6-thio-D-a yfo-hexose forms a 1,6-anhydro compound analogous to 270. S-Acetyl-5-deoxy-l,2-0-isopropylidene-6-thio-a-D-xyZo-hexofuranose is obtained either by nucleophilic displacement on the corresponding 6-p-tolylsulfonyloxy compound with potassium thioacetate or by the photochemical addition of thioacetic acid to... 

Scheme 5. Examples of nucleophiles often present in glycosylation reactions that can compete with the nucleophilic hydroxyl group (A) 1,6-anhydro formation, (B) thio exchange, (C) leaving-group return, (D) promoter-derived components such as triflate anions, (E) additives such as added bases, (F) solvents.

Ogawa later published the synthesis of a 3-amino-3,4-dideoxyhexiu-onic acid in route to ezomycin [24]. In that account, a 1,6-anhydro epoxy sugar was reacted with sodium azide followed by antimony pentachloride to give methyl 3-azido-2-0-benzoyl-a-D-glucospyranoside. Oxidation of the primary alcohol was achieved with potassium permanganate, and reduction of the azide was accomplished with hydrogenation (Fig. 8). 

Anhydro rings of the 1,3,6, and 3,4 types have all been opened by acetolysis. The first class of linkage can be broken specifically in the presence of a disaccharide (1— )-linkage. - As mentioned previously (see p. 16), it is also possible to cleave a 1,6-anhydro ring in the presence of benzyl ether groups by using very mild conditions. ... 

An unusual reaction of some 1,6-anhydroaldohexopyranose derivatives was uncovered on treatment of the levoglucosenone derivative (i ) with sodium in liquid ammonia, which is indicated in Scheme 5 likewise compounds (5) (rlbo or arablno) gave the glycal (6) in good yield after acetylation. By contrast, a 1,6-anhydro-B-D-... 

Another route, completely stereospecific at C-2, to a P-D-gluco isomer 54 starts from a 1,6-anhydro compound 53 easily obtained [38] by iodocyclization of D-glucal 52 (Scheme 12). 

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