Sulfur Oxidation Increases the Rate of HIRE-Type [1.4]Thiazepinone Ring Expansion and Influences the Conformation of a Medium-Sized Heterocyclic Scaffold

The hydrated imidazoline ring expansion (HIRE-type) reaction was investigated for a series of di(hetero)arene-fused [1.4]thiazepinones in comparison with their sulfone counterparts. The sulfones were found to undergo ring expansion at a much higher rate compared to the thioethers, much in line with the current mechanistic understanding of the process. Moreover, the amide bond cis- and trans-isomers of the ring-expanded products were found, in the case of sulfones, to be stabilized through an intramolecular hydrogen bond. The latter phenomenon was studied in detail by NMR experiments and corroborated by X-ray crystallographic information.

Automated summary

The mechanism of a thiazepinone ring expansion reaction was investigated, revealing that sulfone derivatives exhibit a higher rate of ring expansion compared to thioethers. Additionally, in certain cases, the cis- and trans-isomers of the resulting products were found to be stabilized through intramolecular hydrogen bonds.