Heterocyclic compound

A heterocyclic compound or ring structure is a cyclic compound that has atoms of at least two different elements as members of its ring(s). Heterocyclic chemistry is the branch of chemistry dealing with the synthesis, properties and applications of these heterocycles.

Quotes

• 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.
  • K. Peter C. Vollhardt, Neil E. Schore (2011) Organic chemistry : structure and function 6th ed. Ch. 25 : Heterocycles

• 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 …
  • George S. Zweifel and Michael H. Nantz Modern Organic Synthesis (2006), Ch. 2. Stereochemical Considerations in Planning Syntheses