Heterocyclic compound

"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."

"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 …"