Heterocyclic compound - Five- and six-membered rings with two or more heteroatoms | Britannica
Intramolecular five-membered ring compounds and their applications - ScienceDirect
Heterocyclic compound - Six-membered rings with one heteroatom | Britannica
Exhaustive exploration of the conformational landscape of mono- and disubstituted five-membered rings by DFT and MP2 calculations - RSC Advances (RSC Publishing) DOI:10.1039/C9RA03524A