1,5,‐Electrocyclizations—An Important Principle of Heterocyclic Chemistry

Abstract

AbstractThe principle of orbital control of pericyclic reactions has deepened our understanding of reaction phenomena and provided an excellent classification of these one‐step processes. The electrocyclic reaction of the pentadienyl anion ⇌ cyclopentenyl anion type is relatively unimportant in all‐carbon systems and has not even been verified in the case of the parent compound. In the heterocyclic series, however, where up to five C‐atoms of the pentadienyl anion are replaced by heteroatoms, a multitude of ring closures and ring openings find their ordering principle in the mentioned electrocyclic reaction. The replacement of the carbon atoms by heteroatoms can take place isoinically, i. e., with retention of the anionic character, or isoelectronically. An isoelectronic replacement of CR2 in position 1 by NR2 OR, and or CR in position 3 by NR or O leads to a charge‐free resonance structure for the open‐chain species; the migration of the charge during the electrocyclization results in a correlation with a cyclic zwitterion. Conversely, isoelectronic exchange of CR in position 2 by NR or O produces a conjugated 1,3‐dipole, which cyclizes to a charge‐free unsaturated five‐membered ring. Twofold isoelectronic exchange allows the whole process to take place in a cation. Selected examples are to shed light on the classification and the thermodynamics of this electrocyclic reaction.