Heterocyclic compound

"The steric interaction between any axial substituent and a β-situated heteroatom is counted as zero. Thus, in 3-substituted tetrahydropyrans, the destabilization due to an axial R substituent is computed as half of the AR. … The anomeric effect refers to the tendency of a group X at C(1) of a pyranose ring to assume the axial rather than the equatorial orientation. This phenomenon is important in carbohydrate chemistry since it influences the composition of isomeric mixtures and hence their reactivities. … The anomeric effect is solvent and substituent dependent …"

"The electronic structure of the three heterocycles pyrrole, furan, and thiophene is similar to that of the aromatic cyclopentadienyl anion. The cyclopentadienyl anion may be viewed as a butadiene bridged by a negatively charged carbon whose electron pair is delocalized over the other four carbons. The heterocyclic analogs contain a neutral atom in that place, again bearing lone electron pairs. One of these pairs is similarly delocalized, furnishing the two electrons needed to satisfy the 4n + 2 rule."