Pyranoses hexopyranoses

The study of Lemieux and co-workers (38, 39) which first delineated the stereospecific dependences of the P.M.R. parameters of pyranose derivatives has since been confirmed by many workers (24, 40, 44). In the following section we shall discuss some of the more recently recognized stereospecific dependences, dealing first with the dependences of coupling constants and then with those of chemical shifts. Reference will be made to the P.M.R. parameters of the hexopyranose derivatives which were obtained in experiments outlined in the previous section, together with the parameters of the a-(4) and, / -(5) anomers of 1, 3, 4, 6-tetra-0-acetyl-2-deoxy-D-arafoino-hexopyranose. 

One of the most interesting stereospecific dependences which followed from the original study (38,39) of O-acetylated hexopyranose and pento-pyranose derivatives was that of the anomeric proton shifts. In general it was found that the axially oriented anomeric protons gave resonances to higher field than the equatorially oriented protons. Many subsequent studies of pyranose derivatives (24,40) and of inositol derivatives (43)... 

There are two forms of carba-sugar carba-pyranoses and -furanoses. The former, especially the carba-hexopyranoses, have been extensively studied during the past two decades, ever since their derivatives were found in Nature as components of important antibiotics. However, very little is known about carba-furanoses, except for 4a-carba-)3-L-arabinofuranose ... 

Additional n.m.r.-spectral data have been become available since about mid-1986. Further data for hexopyranosyl fluorides,as well as for 2-deoxy-2-fluoro-hexopyranoses and -hexopyranosides, " " have been reported by several groups, whereas only a few articles dealt with pento-pyranose or hexofor pento)furanose analogs " of these compounds. 

Aus 1,6-Anhydro-4-desoxy-2,3-di-0-p-toluolsulfonyl- p-D- vy/o-hexo-pyranose entsteht durch Einwirkung von CHaONa l,6-2,3-Dianhydro-4-desoxy-p-D-n o-hexopyranose, die weiter mit waBrigem Kaliumhydroxid selektiv zu l,6-Anhydro-4-desoxy-p-D-xy/o-hexopyranose geoffnet wird 33,34) ... 

The rule can be extended to pyranoses which form six-membered tetrahydropyran rings. The chemical shifts of the anomeric hydrogens in some hexopyranose pentaacetates are as follows28. 

In analogy to the a-hydroxy effects in cyclohexanols (vide supra), the chemical shifts of anomeric atoms in pyranoses are often stereochemistry dependent, as seen for the C-l shifts of the following hexopyranoses in deuterium oxide41-45. 

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). 

More broadly speaking, we can anticipate the following four orientations around the cyclic oxygen of a D-pyranose I and III for trans derivatives, and II and IV for cis (Fig. 2.9). With the exception of idose (and perhaps altrose), all of the trans derivatives, in this case the monocyclic a-D-hexopyranoses and their derivatives, exist under the only observable conformation, d- Ci, which corresponds to the local conformation I, doubly stabilized by the anomeric and coplanar effects. 

In fact, the conformation of pyranoses is dominated by two effects, not present in the cyclohexane, which appear at positions 2 and 6 of the oxane. One of them is characteristic of hexopyranoses and I propose that we call this the coplanar effect in order not to imply a particularly restrictive structure by using the name of an effect already present in methoxyethane. The other effect, present in all pyranoses, is referred to as anomeric. This name, taken from the nomenclature of sugars because it was first recognized in this family, in fact disguises its general nature since it is also present in methyl chloromethyl ether. The consequences of these effects can be modulated by cyclohexane-type interactions, but not to the point where more than a qualitative discussion is necessary. 

The calculated free-energies given in Table XV are relative to an imaginary hexopyranose that has no non-bonded interactions. The work provides a basis for predicting the stabilities of aldohexo-pyranoses in conformations that may apply to certain reaction intermediates, as well as to sugars in solution. 

Practical access to l,6-anhydro-/i-D-hexopyranoses by a solid-supported solvent-free procedure has been demonstrated by Bailliez et al. [106]. Microwave irradiation of 6-O-tosyl or 2,6-di-O-tosyl peracetylated hexopyranoses absorbed on basic alumina furnished the corresponding 1,6-anhydro-yS-D-hexopyranoses. Direct access to l,6 3,4-dianhydro-yS-D-altro-pyranose from D-glucose is also described (Scheme 8.38). 

Starting with the assumption that all pyranoses exist in a chair form, a hexopyranose will try to adopt a conformation, which allows an equatorial rather than an axial position for the hydroxymethyl (primary hydroxyl) group. Thus in the D-series, a Ci conformation will be preferred, except idopyranose in which an equatorial position for the hydroxymethyl group, Cl, must be at the expense of three axial hydroxyl groups. 

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