Pyranoid compounds, conformation

The pyranoid monosaccharides provide a wide range of asymmetric molecules for study by the c.d. spectroscopist. However, these compounds are not without their difficulties. In aqueous solution, these compounds exist in a complex equilibrium involving the two possible chair conformers of the pyranoses, the furanoses, a and p anomers, and the acyclic form, as well as septanoses for aldohexoses and higher sugars. 

The force field calculations of the enthalpies (AAiT) of the four hemiketals of 6-methylpyrano[2,3-/ ]dioxane-8a-ols clearly indicate that the /3-m-fused link is the most favored, followed by the a-cis- and /3-/ra r-isomers, while the a-/ra r-compound, due to the pyranoid ring being forced into 4 conformation with an axial methyl group, is the least likely to be formed <1991MI235>. 

In the case of aliphatic and aromatic amines as the starting material, where the carboxyl group is absent, the isolation of the corresponding glycosylamines has been reported (3). Structure analyses revealed the compound to be a /3-pyranoid having a C -(D) conformation. 

The special electronic structure of the anomeric center in pyranoid derivatives results in experimentally significant differences in molecular geometry (e.g., as reflected by bond lengths and valence angles) about the anomeric carbon atom between a- and 0-pyranoses and a- and /3-pyranosides. In the article by Jeffrey, a discussion of the structural properties of the anomeric center in pyranoses and pyranosides is given, and a comparison is made of crystallographic data of some carbohydrates with the results of theoretical calculations performed on model compounds. Paulsen and co-workers also present x-ray crystallographic data in their discussion of some aspects of the conformational analysis of pentopyranosyl acetates, benzoates, and halides, in comparison with extensive conformational data compiled by Durette and Horton for these compounds in solution. 

According to Stoddart (1), the unexpected preference for an electronegative substituent on C-l of a pyranoid ring to assume the axial orientation, first discussed by Edward (2) and later termed "the anomeric effect" by Lemieux (3), is now a generally recognized phenomenon in the conformational analysis of heterocyclic compounds. The preference for axial orientation is related to destabilization of the equatorial conformer in which a polar bond lies between two electron pairs on a vicinal oxygen atom. 

With paraldehyde and sulfuric acid at room temperature, methyl 4,6-0-ethylidene-a-D-mannopju anoside (32%) and methyl 2,3 4,6-di-0-ethyli-dene-a-D-mannopyranoside (55%) were isolated. The corresponding (8-d anomers have also been prepared. Assuming that the CA conformation is the most probable, the 4,6-acetals of methyl mannopyranosides are readily available model compounds for comparing the reactivities of equatorial (C-3) and axial (C-2) hydroxyl groups in the pyranoid ring. 

N.m.r. studies on pyranoid sugar derivatives containing an epoxide ring indicate that the compounds adopt a half-chair conformation similar to that observed for unsaturated, pyranoid sugars. From an analysis of the spin-coupling data for some methyl 2,3- and 3,4-anhydroaldopyranosides, it was found that the particular half-chair form adopted is determined by the anomeric effect and the tendency of the 5-(hydroxymethyl) or 5-methyl group to assume the equatorial... 

By use of the semi-empirical molecular orbital method AMI, the optimised geometries and energies of the pyranoid forms of D-fructose (14), L-sorbose (15), 5-deoxy-D-rhreo-hexulose (16), and carba-D-fmctose (17) have been determined. The findings that a low-energy conformation exists for each of the three sweet compounds (14), (16), and (17) [but not for the non-sweet analogue (15)] in which HO-2 is hydrogen-bonded to 0-3, is thought to provide an explanation for the relative sweetness of these compounds. ... 

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