0 anomer

Barrows S E, J W Storer, C J Cramer, A D French and D G Truhlar 1998. Factors Controlli Relative Stability of Anomers and Hydroxymethyl Conformers of Glucopyranose. Journal Computational Chemistry 19 1111-1129. 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

The following are the more stable anomers of the pyranose forms of D glucose D mannose and D galactose... 

Trimethylsilyl triflate—silver perchlorate (1 1) also cataly2es the highly stereoselective glycosylation of P-naphthol to provide predominantiy the P-anomer. 

Fmctose is sweeter than sucrose at low temperatures (- S C) at higher temperatures, the reverse is tme. At 40°C, they have equal sweetness, the result of a temperature-induced shift in the percentages of a- and P-fmctose anomers. The taste of sucrose is synergistic with high intensity sweeteners (eg, sucralose and aspartame) and can be enhanced or prolonged by substances like glycerol monostearate, lecithin, and maltol (19). 

The syn-anti conformational problem of a- and /3-pyrazofurins (756 one of the rare naturally occurring pyrazole compounds, see Section 4.04.4.4.3), which involves a rotation around a pyrazolic sp carbon atom and a sugar sp carbon atom, has been studied theoretically using the PCILO method (81MI40403). In agreement with the experimental observations, the /3 anomer is energetically more favourable than the a anomer, the preferred conformations being anti and syn, respectively. 

LiBF4, CH3CN, 70°, 3-8 h, 81-90% yield. This system of reagents also cleaves benzylidene acetals. Conventional reagents failed to cleave these glycosides. It is interesting to note that the /3-anomers are cleaved more rapidly than the a-anomers and that the furanoside derivatives are not cleaved. 

The Qt-anomer gives better selectivity for the 2-OH than does the /3-anomer (3 2). Note that the conditions used to remove the 4,6-O-benzylidene group are sufficiently mild to retain the sulfate. ... 

The rules previously mentioned for assignment of a- and /3-configurations can be readily applied to Haworth projection formulas. For the D-sugars, the anomeric hydroxyl group is below the ring in the a-anomer and above the ring in the /3-anomer. For L-sugars, the opposite relationship holds. 

The Mitsunobu reaction is usually used to introduce an ester with inversion of configuration. The use of this methodology on an anomeric hydroxyl was found to give only the /3-benzoate, whereas other methods gave mixtures of anomers. Improved yields are obtained in the Mitsunobu esterification when p-nitrobenzoic acid is used as the nucleophile/ Bis(dimethylamino) azodicarboxylate as an activating agent was... 

This synthesis appears to be quite general for the preparation of 1-substituted nucleosides and was used with small modifications for the synthesis of l-ribofuranosyl-6-azathymine and 2 -deoxyribo-furanosyl-6-azauracil and -6-azathymine. In the case of 2 -deoxy-ribofuranosyl a mixture of a- and )8-anomers is produced, their ratio depending on the reaction conditions. In the preparation of 2 -deoxy-ribofuranosyl-6-azathymine only one anomer was obtained having probably the )8-configuration, ... 

Chiral l,2,5-thiadiazolidin-3-one 5,5-dioxides 250 were synthesized by way of conventional methods, with the intention of introducing them in new pseudonucleosides as aglicone (96T(52)993). The synthetic methodology, in accord with previous reports, gave the l,2,5-thiadiazolidin-3-ones in good overall yield (35-55%). The glycosilation was performed on the protected heterocycles to prevent the condensation involving the N-2 atom, the most acidic reactive site. As expected only one anomer was obtained that is the p-one. 

Reaction of 3-amino-1-propanol and 5-bromo-5-deoxy-D-furanoxylose (25) in D2O was monitored by NMR (Scheme 4). The a-anomer of trihydroxypyridoPd-f l-LbSloxazine 26 formed 20 times faster, but the /3-anomer 27 was more stable (A / 7.3). The faster formation of the Q -anomer is a consequence of a kinetic anomeric effect that destabilizes the transition state for equatorial A -alkylation and formation of the /3-anomer 27 (OOJOC889). 

NMR investigations of A (l)-methyl 117 and A (5)-methyl 118 derivatives of 112 (R = H) revealed that they adopted a similar conformation as the parent 112 (R = H) (99T6759). Both A -methyl derivatives contain the methyl group in an axial position. All of them have a trans-ring junction. Whereas, cA-2/7,4//,9n//-dimethyl derivative 112 (R = Me) exists in a single conformation, its cis-2H,AH-trans-9aH diastereomer 119 is conformation-ally complex, consisting of al, 0 2, and fi anomers in equilbrium in D2O (OOJOC889). 

A iolyt, m. (Elec.) anolyte. anomal, anomalisch, a. anomalous. 

Several features of the rearrangement have been elucidated. Although in the treatment of the ester 26 with acetic acid the products were isolated in only 64% yield, evidence was obtained (22) that finally no 1,2-unsaturated compounds remained, since the noncrystalline portion on hydrogenation and deacetylation afforded only 3-deoxy-D-ribo and -d-arabino-hexose and no 1,5-anhydrohexitols. That the components of the final mixture were in equilibrium was indicated by the observation that the main component 27 underwent reaction in boiling acetic acid to give a solution with the same optical activity as that of the original reaction mixture. Thus the 2,3-unsaturated compounds are more stable than the hydroxyglycal derivatives and the a isomer 27 is more stable than its anomer 28. 

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