Heterocyclic compound

A pyridine bearing an amino substituent; its NH donors and ring-N acceptor permit complementary hydrogen bonding.

A fused benzene–pyrrole heteroaromatic ring system. Electrophilic substitution normally favours C3; when that position is occupied, C2 substitution can retain the pyrrolic nitrogen–hydrogen bond.

A compound containing two joined pyridine rings; 2,2′-bipyridine is a common bidentate nitrogen ligand.

The cation obtained by protonation or suitable N-alkylation of imidazole.

Assembly of a pyrrole by carbon-nucleophile addition of an isocyanoacetate to an activated unsaturated partner, ring closure at isocyanide carbon and aromatization.

Replacement of the ring oxygen of an activated pyrone by nitrogen through nucleophilic addition, ring opening and reclosure. Atom mapping distinguishes it from simple side-chain amidation.

A five-membered aromatic ring with one oxygen and two adjacent nitrogens. Cyclodehydration of a diacylhydrazine is a useful synthesis.

A five-membered aromatic ring with sulfur at position 1 and nitrogen at position 3.

A saturated five-membered ring containing adjacent oxygen and nitrogen atoms.

A fused bicyclic aromatic heterocycle with nitrogen at position 2.

A five-membered aromatic ring containing two adjacent nitrogen atoms.

A mesoionic five-membered heterocycle that can undergo cycloaddition and carbon-dioxide extrusion to form a pyrazole.

The conjugate base of a triazole NH group.

A saturated three-membered ring containing sulfur and two carbons. Its strained carbon–sulfur bonds can undergo nucleophilic opening.

Sequential ring opening, five-membered cyclic imidodithiocarbonate formation, cleavage and reclosure can exchange the thiirane sulfur with the thiocyanate sulfur. Backside substitution at both ring carbons reverses the thiirane face while leaving remote stereocentres intact.

A six-membered oxygen heterocycle containing an exocyclic carbonyl. A charge-separated contributor with carbonyl oxygen negative and a pyrylium-like ring clarifies why aromatic character is weaker and more polarized than in benzene.

1,3-Dioxane is a six-membered ring with oxygen at positions 1 and 3. It commonly adopts chair conformations. Axial substituent penalties differ sharply between C2 and C5 because the relevant 1,3 contacts involve different ring atoms.

A 2-Oxazoline has the five-membered ring $\mathrm{O}-\mathrm{C}=\mathrm{N}-\mathrm{C}{\mathrm{H}}_2-\mathrm{C}{\mathrm{H}}_2$. A substituent at its imine carbon can be introduced by aromatic capture of an activated cyclic carbamate.