Anhydrohexofuranoses

Individual 1,6-anhydrohexofuranoses (for a review, see Ref. 3) are formed, together with 1,6-anhydrohexopyranoses, as minor products in proportions of l-5%, by (a) pyrolysis of hexoses, for example, 

D-galactose,116 196 294,295 792 D-glucose,196,272,296,299 D-mannose,113,196 and of some D-glucopyranosides,291 oligosaccharides,196,291 starch,190,196,251 cellulose,196,208,214,232,250,258 and dextran,190 and (b) treatment of hexoses with acids.99,109,113,114-116,119,121,790 An alternative route consists in isomerization of triacetates of 1,6-anhydrohexo-pyranoses or -furanoses, or both, in the presence of trifluoromethanesulfonic acid in nitro-methane.421,422,453,791 

The longest known compound in this series is 1,6-anhydro-a-D-galactofuranose (261). It is relatively easily obtainable, and is usually prepared by pyrolysis of D-galactose116,294,295,792 it can also be obtained by pyrolysis of lactose, melibiose, and raffinose.196,291 An alternative route consists in the reaction of 0.2 M hydrochloric acid, or 0.1 M sulfuric acid, with D-galactose.114,115 The anhydride 261 was originally mistaken for l,3-anhydro-/3-D-galactopyranose, because of 

It follows from the H-n.m.r. spectra of the 1,6-anhydrohexofuranose peracetates (see Table IV) that their 1,3-dioxane ring exists in the alCa4 conformation99,113,119,121,793 and their oxolane ring favors the E0 conformation,119 although, for the D-g alacto triacetate (271), the E4 conformation of the oxolane ring was assumed.119,783 For l,6-anhydro-/3- 

D-mannofuranose (262), the E0 arrangement has been verified by X-ray crystallographic analysis.794 

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Two representatives of a remarkable new class of glycosans containing the < 1,4 > < 1,6 > (1,6-anhydrohexofuranose) ring system recently have been isolated as additional products of the pyrolytic vacuum distillation of carbohydrates. From starch, Dimler, Davis and Hilbert7 obtained... 

IV. Effect of the 1,6-Anhydro Ring on the Rate of Acid Hydrolysis of the Furanoside Structure of 1,6-Anhydrohexofuranoses... 

Deulofeu, Venancio, The Acylated Nitriles of Aldonic Acids and Their Degradation, 4, 119-151 Dimler, R. J., 1,6-Anhydrohexofuranoses, A New Class of Hexosans, 7, 37-52 Doudoroff, M. See Hassid, W. Z. Dubach, P. See Mehta, N. C. 

Throughout the text, such shortened names as 1,6-anhydrohexopyranoses, 1,6-an-hydrohexoses, or, sometimes, hexosans will be used for 1,6-anhydroaldohexopyranoses, and 1,6-anhydrohexofuranoses or 1,6-anhydrofuranoses for 1,6-anhydroaldohexofuranoses, respectively. 

Aldohexoses cyclize in aqueous, mineral acids to give mainly 1,6-anhydrohexopyranoses 1,6-anhydrohexofuranoses are simultaneously formed, but to a far lesser degree (see Sect. IX,1). An equilibrium is established for the 1,6-anhydrohexoses, whose composition can be calculated from thermodynamic data according to Angyal and Dawes99 ... 

Some analogs of 1,6-anhydrohexofuranoses have also been described. Derivatives 257 and 258 of l,6-anhydro-l(6)-thio-/3-D-gluco-furanose were respectively prepared from 255 and 256 by the action of acids.788,788 Similarly, l,6-anhydro-2,3-di-0-methyl-l(6),4-dithio-/3-D-glucofuranose (259) was obtained from the corresponding methyl glycopyranos ide. 789... 

A simple relationship exists between the structure and the optical rotation of 1,6-anhydrohexofuranoses,119,121,791 as shown in Scheme 15. [M]d values can be calculated for the 1,6-anhydrohexofuranoses121 and, presumably, also for their amino, fluoro, and deoxy derivatives by using the partial molar rotational contribution of the hydroxyl groups and of the fundamental skeleton. O.r.d. measurements in the 200-nm region seem also to be potentially valid for structural estimation.797... 

Mass spectroscopy is another potentially useful tool for the identification of 1,6-anhydrohexofuranoses,471 but the gas-liquid chromatographic technique is the method of choice for their determination for example, in pyrolytic tar from polysaccharides, or in mixtures with other 1,6-anhydrohexoses.190,196... 

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