Tetrahydropyran Pyranose Sugars

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

Barnes, J J, Davidson, A H, Hughes, L R, Procter, G, The synthesis of optically active tetrahydropyrans by the addition of a stabilized Wittig reagent to pyranose sugars, J. Chem. Soc., Chem. Commun., 1292—1294, 1985. 

Aiming at the pyranose form of sugars, normal type hetero-Diels-Alder reactions were extensively used for the synthesis of functionally substituted dihydropyran and tetrahydropyran systems (5-10) (see routes A - D in the general Scheme 1) which are also important targets in the "Chiron approach" to natural product syntheses (2.) Hetero-Diels-Alder reactions with inverse electron demand such as a, p-unsaturated carbonyl compounds (l-oxa-1,3-dienes) as heterodienes and enol ethers as hetero-dienophiles, are an attractive route for the synthesis of 3,4-dihydro-2H-pyrans (11). 

This structural characteristic, called anomeric effect, plays an important part in the chemistry of the carbohydrates, since the pyranose form of sugars corresponds structurally to the 2-substituted tetrahydropyrans as shown above. 

To assist in the interpretation of carbohydrate infrared spectra, one approach is to make use of model compounds which are of a simple nature but have similarities with the structure of the carbohydrate being examined. For example, tetrahydropyran is such a model compound since it contains the pyranose ring which is found in sugars. A knowledge of its spectrum can assist in the study of, and interpretation of, carbohydrate spectra. 

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