Oxane rings

The chair conformation is the preferred conformer for the oxane ring and substituted derivatives. In the case of poly-substitution (e.g., 1,3-diaxial groups), twist conformers can also participate in the equilibrium. Substituents can adopt the axial and equatorial positions ring interconversion between the chair conformers is fast on the NMR timescale at ambient temperature but becomes slow at low temperature (AG = 10.3 kcal mol- ) (73JA4634). 

The conformations of spiro-compounds containing the oxane ring have been reported (Scheme 5) (81CJC1132). They exist only in conformation 9a at room temperature at lower temperature, no indication of a second conformer was found. The methyl-substituted derivatives 10a and similar tricyclic analogs occur in the same conformation 11a. 

The structure of the thromboxane A2. Thromboxanes differ from prostaglandins in having a cyclic ether (oxane) ring structure. 

Heating the trifluoromethanesulfonate 89 in the presence of 2,6-di-tert-butyl-4-methylpyridine gave methyl 3,5-anhydro-2,6-dideoxy-/J-D-xylo-hexofuranoside 91. This rearrangement involves oxane-ring contraction 90 followed by internal sulfonate displacement.353... 

One recognizes an oxane ring (tetrahydropyran) substituted by three secondary alcohol functions in an equatorial orientation, a side chain carrying a primary alcohol function and finally a hemiacetal hydroxyl carried by carbon 1. This intramolecular hemiacetal is derived from the addition of the oxygen carried by C-5 to an aldehyde function. 

The most frequently used measure of the anomeric effect is based on the comparison of the stabihty of 2-substituted oxane (tetrahydropyran THP) and cyclohexane. In general, conformational properties of the oxane ring are similar to those of cyclohexane, with dominance of a chair conformation. It is further presumed that steric interactions in oxane are the same as in cyclohexane, with preference for equatorial positions of bulky substituents. The Gibbs energy of the anomeric effect, AG(AEl), can be expressed as the... 

In summary, experimental data on the isomeric abundances at anomeric equilibrium reveal that the preference for the axial position depends on several, interconnected factors which were clarified in surveys on carbohydrate stereochemistry, and these provided a background for ensuing theoretical studies. The elucidation of this relationship in complex carbohydrates is greatly facilitated by measurements on the simple derivatives of oxane, and qualitative trends have already been established. Table II illustrates sever possibilities of the quantification of the energetic aspect of the anomeric effect. The procedure most frequently used, based on Eq. 7, suffers from the ambiguity of the A values for the oxane ring and by their presumed variation with solvent. 

Usually, acidic water with a pH ranging from 3.0 to 4.0 is used to catalyze the hydrolysis reaction. However, a glycidoxy-functional (epoxy-fimctional) silane readily opens its oxane ring in this pH range. For this silane, pH 5-6 may be needed to prevent the ring opening reactions. These neutral silanes may need to be hydrolyzed for tens of minutes at room temperature for complete hydrolysis, provided that the concentration of the silane is relatively low. 

Ye, T. and Pattenden, G. (1998) Synthetic studies towards phorboxazole A. A concise stereoselective synthesis of the C20-C26 pentasubs-tituted oxane ring unit. Tetrahedron Lett., 39, 319-322. 

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