Anhydro isomerization

Treatment of l,4 3,6-dianhydro-D-glucitol with boron trichloride gives l,6-dichloro-l,6-dideoxy-D-glucitol (20). Although methyl 6-chloro-6-deoxy-a-D-glucopyranoside (isolated as the tribenzoate) could be isolated from the reaction of methyl 3,6-anhydro-a-D-glucopyranoside with boron trichloride (21), the application to the isomeric furanoside derivative led to complex results. 

S)-1,5-Anhydro-3,4,6-tri-0-benzyl-1 -C,2-0-(ophenylenemethylene)-D-mannitol Note. The isomeric chromene would be named as a 2-0,1 -C-substituted system. 

C-Glycosyl derivatives may be prepared by utilizing glycosyl fluorides. Ishido and coworkers reported that the reaction of 2,3,5-tri-O-benzyl-a-(36a) or - -D-ribofuranosyl fluoride (36fi) with isopropenyl trimethylsilyl ether under BF3 catalysis (0.1 -0.05 mol. equiv. for the fluoride, in ether or acetonitrile) gave a mixture of 4,7-anhydro-5,6,8-tri-C)-benzyl-l,3-dideoxy-D-altro- (139, major) and -D-a//o-2-octulose (140) 139 was stated to isomerized to 140 (should be vice versa) under Lewis acid catalysis. Similar... 

It is to be noticed that although the anhydro ring of levoglucosan is stable to alkali (Tanret s preparative method consisted in the heating of aromatic jS-D-glucosides with aqueous alkali), it readily undergoes scission in the presence of acid. An isomeric glucose anhydride prepared in 1912 by E. Fischer showed, however, very different properties. 

Let us now return to the pioneering work of Fischer and Tiemann.4 Chitonic acid is produced from D-glucosamine by deamination and oxidation, in that order. If the order is reversed, however, i. e., if D-glucos-amine is first oxidized to D-glucosaminic acid (2-amino-D-gIuconic acid) and the latter substance subsequently deaminated, chitonic acid is not the product. Instead an isomeric 2,5-anhydro hexonic acid (chitaric acid) is obtained. These facts may be summarized thus ... 

Another example279 is the differentiation of the two isomeric anhydro derivatives of cyanobac-terin 3. For the (15Z)-isomer a signal enhancement of 11 % could be identified for H-16, when H-13 was irradiated. On the other hand, irradiation of H-13 of the (15 )-isomer led to a 3% NOE effect at H-18/H-22. 

To accommodate these facts, the earliest mechanisms proposed for degradation of D-fructose assumed that it was present in the furanose form, and that the ring remained intact. It was assumed that the initial reaction was the elimination of water, to form the 1,2-enolic form of 2,5-anhydro-D-mannose, and that further dehydration resulted in 2-furaldehyde. The necessity for D-glucose to isomerize to D-fructose was assumed to account for the much lower reaction-rate of D-glucose. This mechanism does not account for the observation that 2,5-anhydro-D-mannose is less reactive than D-fructose, nor is there any evidence that 2,5-anhydro-D-mannose is present in reacting D-fructose solutions. Nevertheless, similar mechanisms have since been proposed.13-16 Because of the ease of mutarotation of D-fructose... 

For the conversion of l,6 2,3-dianhydro-4-deoxy- and l,6 3,4-dian-hydro-2-deoxy-/3-DL-(i/ ro-hexopyranoses (261 and 262) lithium diethylamide was successfully employed.151 However, attempts at isomerization of methyl 2,3-anhydro-4-deoxy- or -4,6-dideoxy-DL-hex-opyranosides (263 R1 = CH2OH, CH3, R2 = Me) with butyllithium failed, because of predominance of secondary reactions (for example, opening of the oxirane ring with BuLi).14(i The yields of desired products were negligible or nil. 

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