Isomerization anhydro-ring

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. 

Potts KT, Choudhury DR (1978) Mesoionic compounds. 43. Ring annulation utilizing the isomeric anhydro-2- and 3-hydroxythiazolo[2,3-b]benzothiazolium hydroxide mesoionic... 

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. 

In the case of 1,6 2,3- (136) and l,6 3,4-dianhydro-/I-D-talopyranoses, the diaxial opening of the oxirane ring prevails, but a trend to diequatorial opening496 498 is apparent with l,6 2,3-dianhydro-4-deoxy- and 1,6 3,4-dianhydro-2-deoxy-/l-D-/yxo-hexopyranoses.169,432,493 Using 1,6 2,3- and l,6 3,4-dianhydrohexopyranoses as starting compounds allowed the preparation of a complete series of 12 isomeric 1,6-anhydro-monodeoxy-/Fn-hexopyranoses and 6 corresponding 1,6-anhydro-dideoxyhexoses,499 mainly by catalytic or complex hydride reductions.462,500... 

The aziridine ring is more stable than the oxirane ring in alkaline solution, as demonstrated by the low reactivity in attempts to accomplish isomerization of the hydroxyepimines to amino epoxides in alkaline media at room temperature, which contrasts with the rapid epoxide migration (see Sect. V,2). Isomerization of hydroxyepimines occurs only at high temperatures, and leads finally to the formation of amino derivatives of 1,6-anhydrohexoses.379,740 For example, when 166 is heated in 5% potassium hydroxide, 2-amino-l,6-anhydro-2-deoxy-/3-D-mannopyranose (168) is formed as the main product this can be explained by transient formation of 2-amino-l,6 3,4-dianhydro-2-deoxy-/3-D-altropyranose (167), and its subsequent, diaxial hydrolysis.379 Compound 167 is probably in equilibrium with epimine 166. Acid hydrolysis of the aziridine ring in 153 also follows a diaxial mechanism, without scission of the 1,6-anhydride bond, to give 4-amino-l,6-anhydro-4-deoxy-)8-D-mannopyranose756 (177). 

Several physical studies have been carried out on cyclodextrin derivatives. Thus the X-ray structure of per-(6-bromo-6-deoxy-2,3-di-0-methyl)P-cyclodextrin has been examined by single crystal X-ray diffraction analysis. The conformation deviates significantly from C symmetry. Likewise the X-ray structure of heptakis-2,3,6-tri-C>-methyl-P-cyclodextrin shows that one of the seven rings is inverted. Conformational analysis of an extensive set of per-O-substituted p-cyclodextrins reveals that several show conformational isomerism. The relevance of this to the use of cyclodextrins in molecular device design is examined. Related work undertook n.m.r. and molecular modeling studies on a-cyclodextrin in vriiich one of the units is in the 3,6-anhydro from. The work included the examination of two model disaccharide imits. ... 

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