Heterocycles saturated, anomeric effects

In a previous section we have discussed the importance of n—a conjugative interactions in dictating the axial-equatorial preference of an electronegative substituent at C—1 of a pyranose ring, i. e. the anomeric effect . Here, we shall extend the discussion of n—a conjugative interactions to other saturated heterocyclic molecules. 

An anomeric effect is observed in 3,4-dihydro-2H- pyrans. For example, a 2-alkoxy group preferentially occupies an axial position (71DOKd96)367>. Indeed, a study of the NMR spectra of some 2-alkoxy-3,4-dihydro-2//-pyrans and their 4-methyl derivatives established that the anomeric effect was more important in the unsaturated heterocycles than in the corresponding tetrahydropyrans (72BSF1077). The axial preference of an alkoxy group is even more accentuated when the double bond is associated with a fused benzenoid ring, as in the 2-alkoxychromans. It is also of interest to note that the role of the polarity of the solvent on the conformational equilibrium is less important than for the saturated analogues. 

Some substituents of saturated heterocycles prefer to be axial the anomeric effect... 

X-ray diffraction studies have suggested that the anomeric effect operates in 2-phosphinoyl derivatives of the saturated heterocycles, where the 2-substituent has no lone electron pairs. For example, in 2-diphenylphosphinoyl-l,3-dithiane, the substituent occupies an axial position, in contrast to the 1,3-dioxane and 1,3-oxathiane derivatives, where the same substituent occupies an equatorial position <84JOC3026,88JOC3609,88TL6801>. 

The anomeric effect is an empirical, thermodynamic stereoelectronic effect that states the preference for heteroatoms to occupy the axial position in saturated six-membered heterocyclic ring systems, notably carbohydrates (Lemieux and Koto, 1974 Szarek and Horton, 1979). Features of the anomeric effect had been remarked upon many years before its advent in... 

Anomeric Effect in Saturated Heterocyclic Ring Systems... 

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