Hetero Oxa-Claisen Rearrangement: Perspective on New Synthetic Pathways and Structures

The substrate of the classical Claisen rearrangement is ether containing double bonds in α and β positions. Both double bonds can belong to the aromatic system. There are about half a dozen known rearrangements in which one or more carbon atoms of the starting allyl vinyl ether were replaced with hetero-atoms such as sulfur, nitrogen or phosphorus. Such processes can be considered versions of the Claisen rearrangement. It seemed worthwhile to determine which of the possible versions can offer valuable products or useful alternatives to existing synthetic methods. Surprisingly, the literature data are very scarce. The article looks into plausible rearrangements of 3-oxa-1,5-hexadienes in which one or more carbon atoms were replaced with atoms different than carbon. We call these reactions heterooxa-Claisen-rearrangements (HOCR). Particularly interesting are processes forming: • A new bond between an aromatic ring and a hetero-atom, • A sequence containing several (various) hetero-atoms, • Aliphatic α-amino, α-thio and α-hydroxy-carbonyl compounds. It is worth noting that (like in the classical Claisen rearrangement) the driving force for many of the described processes is the formation of the thermodynamically stable carbonyl (or P=O) group. Additionally, transition state optimizations for selected example reactions show similar gas phase activation energies to known Claisen rearrangements, indicating their plausibility in the laboratory. Keywords: Claisen rearrangement, variants, hetero-atoms, optimization, activation energy.