1,2-Anhydrohexopyranose derivative

The reaction can be applied with other 1,6-anhydrohexopyranose derivatives [30], but clearly it is restricted by the strongly basic conditions used and by the specific natures of the substrates. 

The conformational analysis of certain 1,2-anhydrohexopyranose derivatives, and the synthesis of branched-chain sugars from epoxide precursors are covered in Chapters 20 and 13 respectively. 

Various procedures have been used to obtain 1,6-anhydrohexopyranoses and their derivatives including acyl, alkyl, deoxy, azido, halo, and others ... 

Only two 1,7-anhydrohexopyranoses have been described thus far l,7-anhydro-D-g/ycero-/J-D-gw/0-heptopyranose (58a)247,250 and the corresponding D-ido isomer (58b).247 They appear in the reaction mixture with 1,6-anhydro derivatives after acid treatment of solutions of the parent aldoheptoses. 

Acetalation of l,6-anhydro-/l-D-mannopyranose (39) and -/1-D-galacto-pyranose with trichloroacetaldehyde in the presence of A,A -dicyclohexylcar-bodiimide affords endo-H diastereomers of 3,4- and 2,3-trichloroethylidene derivatives of l,6-anhydro-/l-D-altro- and -/1-D-gulopyranose, respectively.528 For isomerization of various 1,6-anhydrohexopyranoses acetates, see Section II.4. 

Neither the 1,6-anhydrohexopyranoses nor the corresponding fura-noses occur naturally as they have been isolated from natural sources, however, they must have been formed as secondary products from hexoses or their derivatives during the isolation process. 

Anhydrohexopyranoses are relatively stable in aqueous alkalis, even on heating,56,57 and this property can be utilized in their separation from free sugars they do not reduce Fehling solution. They are, however, readily hydrolyzed by mineral acids to reducing hexoses, especially when heated, and an equilibrium is established between the hexose and its 1,6-anhydro derivative (see Table I). 

The coupling constants J1S, J2.3, J3,4, and J4,3 reflect the conformation of the ring protons and, consequently, the configuration of a 1,6-anhydrohexopyranose (see Scheme 5). The antiperiplanar, a-a orientation of hydrogen atoms exists only for the derivatives having the... 

Separation of 1,6-anhydrohexopyranoses and their derivatives from each other, and from mixtures with other sugars, can be achieved by using various types of chromatography, for example, on paper,55,96,108, 112,138,140,301,314,322 layers 51-53>113 columns 82>" >38,140,297,301,310,453 and... 

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