2.3- anhydrohexopyranoses

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. 

A. Klemer and M. Kohla, One-step stereoselective syntheses of C-branched a-deoxycyclitols from 1,6-anhydrohexopyranoses, Liebigs Arm. Chem. p. 1662 (1984). 

A further stereochemical factor (discussed next) that has bearing on the regiochemistry of some photobrominaiions is the relative difficulty with which equatorial hydrogen atoms on pyranoid rings may be abstracted. In the cases of the tri-0-acylated 1,6-anhydrohexopyranoses, reaction is diverted to 06, because the anhydro bridge holds the pyranoid ring in the iC4 conformation, in which H-l and H-5 are both equatorial. 

All stereocenters in 1,6-anhydrohexopyranoses are of inverted orientation compared to those in the parent 4Ci(d) or 1C4(l) conformations of the corresponding hexopyranoses for example, see 21, 23, and l,6-anhydro-/J-D-glucopyranose (22). In chemical properties, these compounds resemble to a certain degree the methyl /f-D-hexopyranosides. They are relatively stable in alkaline media, but are readily hydrolyzed by acids. In aqueous acid solution, an equilibrium is established between the 1,6-anhydrohexo-pyranose and the corresponding aldohexose, whose composition correlates with expectations from conformational analysis and calculations from thermodynamic data.121 Extreme values, 0.2 and 86%, are observed respectively with 1,6-anhydro-/f-D-glucopyranose (22) and l,6-anhydro-/f-D-idopyranose (the latter has all hydroxyl groups in equatorial disposition). 

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

The basic skeleton of these anhydrides, l,6-dioxabicyclo[3.2.1]octane, is identical to that of 1,6-anhydrohexopyranoses (see Section II.4). For the 32 diastereoisomeric aldoheptoses, three different types of anhydrides may be formed in acid solution 1,6-anhydropyranoses, 1,6-anhydrofuranoses,... 

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. 

All cofigurational isomers of 2,3- and 3,4-anhydrohexopyranoses have been described. Generally, they adopt more or less flexible half-chair conformations (H) dependent on the absence or presence of an additional fused ring. Examples of the latter compounds are methyl 2,3-anhydro-4,6-O-benzylidene-a- and /1-hexopyranosides of the alio 70,272,273 manno 71,273 275 gui0 216,211 an(j 276 c0nfigurati0nS. 

Anhydroaldohexopyranoses show a tendency for conversion into 1,6-anhydrohexopyranoses if a free hydroxymethyl group is present at C-5 and the oxirane ring adopts the appropriate trans orientation, see Section II.l. 

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. 

E. Wu and Q. Wu, Preparation of hexopyranose 2-tosylates and their facile conversion into 1,2-anhydrohexopyranoses by internal displacement, Carbohydr. Res., 250 (1993) 327-333. 

J. M. Macleod, L. R. Schroeder, and P. A. Seib, Selective esterification of 1,6-anhydrohexopyranoses the possible role of intramolecular hydrogen-bonding, Carbohydr. Res., 30 (1973) 337-347. 

With the 1,6-anhydrohexopyranoses, condensation occurs at vicinal ds-diols or, in the case of 1,6-anhydro-/3-D-glucopyranose, at the diaxial 2,4-sites,61 despite the observation that this anhydride does not readily complex with the acid (see Section V,2). 

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