Pyranose pento

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)... 

The structures of the a-D-aldo-pento and -hexo-pyranose monosaccharides are shown in Pig. 1. In all cases, these sugars will be studied as the six-membered-ring tautomer, as shown. 

One of the methods by which replacement of secondary hydroxyl groups by fluorine in pentofuranoses and pento- and hexo-pyranoses has been achieved is by cleavage of epoxide rings with fluoride ion. The epoxide rings are always cleaved trans-diaxially, according to the Fiirst-Plattner rule. 

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. 

Application of these principles to the structural analysis permitted establishment of the configurations of the ring-carbon atoms and the orientations of the protons thereon, and the stereochemistry of the phosphorus atom in these pento- and hexo-pyranoses. 

They are stable in basic solution, but are readily hydrolyzed with acids 79 this is particularly true for l,4-anhydro-/f-D-galactopyranose, which decomposes on silica gel. l,4-Anhydro-2,3-dideoxy-a -D-g7ycero-pento-pyranose (16a) represents the basic skeleton of 1,4-anhydroaldoses in the... 

Hphe term anomeric effect was introduced by Lemieux in 1958 as a result of a detailed study of the anomerization of acetylated pento-and hexo-pyranoses. The effect is well known to carbohydrate chemists, and refers to the tendency of an electronegative substituent at C-l of a pyranoid ring to assume the axial rather than equatorial orientation, in contrast to predictions based solely on steric grounds. However, the phenomenon is not restricted to carbohydrate systems, but is displayed in many types of heterocyclic compounds. Thus, the investigation of the anomeric effect has been of considerable interest to a variety of chemists, namely, theoreticians, structural chemists, physical organic chemists, and synthetic chemists. Chemists from all of these areas participated in the Symposium on The Origin and Consequences of the Anomeric Effect. This symposium was the first symposium devoted exclusively to a discussion of the anomeric effect, and it provided a mechanism for interactions between the diverse types of chemists. The chapters in this volume are not merely the texts as presented at the symposium, but they also incorporate some new interpretations by the authors resulting from these interactions. 

Infra-red Spectra of Carbohydrates. Part II, Anomeric Configuration of Some Hexo-and Pento-pyranoses, S. A. Barker, E. J. Bourne, R. Stephens, and D. H. Whiffen, /. Chem. Soc., 3468-3473 (1954). 

Ytterbium(III) triflate in methanol is an efficient catalyst for cleaving methoxy-acetates (e.g., 23 - 24). The methoxyacetate protecting group was introduced 30 years ago (C.B. Reese and J.C.M. Stewart, Tetrahedron Lett., 1968,4723) but has found little application so far, due to difficulties associated with its removal. Efficient, selective anomeric deacetylation of peracetylated reducing disaccharides has been achieved with hydrazine hydrate (1.2 equivalent) in acetonitrile. It has been observed that the BF3.Et20-catalysed glycosidation of several pento- and hexo-pyranose peracetates with simple alcohols is accompanied by selective deacetylation at the 2-position. ... 

On the basis of 2D H-n.m.r. spectral data, Cj conformations have been proposed for both the unhydrated and hydrated pyranose forms (23) and (25), respectively, of o-//ireo-pentos-2-ulose, whereas the unhydrated erythro-isomtx (24) appears to prefer the C4 conformation and its hydrated form (26) exists as an equiUbrium mixture of both conformers. Barriers of 46 and 32 kJ/moI, respectively, have been calculated for chair boat inversion in 1,6-anhydro- -o-gluco-pyranose and its triacetate the role of intramolecular H-bonding in determining the conformational energy of the former compound is discussed. ... 

The introduction of an additional asymmetric carbon atom leads to the existence of two isomers, (a and j8) of each type of ring, which are described as anomers, and C-i is now referred to as the anomeric carbon atom (C-2 in ketoses). In the D-series of sugars the a-hydroxyl is written on the right of the sugar chain in the cyclic form of the Fischer formulae and trans to the CHjOH in the Haworth formulae. It is necessary to relate the pento-pyranoses to the hexoses to determine whether the anomeric hydroxyl... 

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