A Theoretical Perspective on the Stereochemistry of Benzoanellated Aroyl-X,N-Ketene Acetal Derivatives

Ketene 1,3-oxazoles and their derivatives present intriguing structures for the study of rotational barriers due to their pseudo–double-bond character stemming from resonance in the ketene moiety. A diverse range of compounds featuring this motif underwent quantum-chemical investigation to elucidate the nature of the stereochemical singularity observed in numerous cases. Because rotational barriers in most instances are too high to permit rapid interconversion, the findings are ascribed to thermodynamic rather than kinetic factors in the gas phase and within an implicit polar medium. The stabilities are attributed to internal hydrogen bonding where feasible. However, in cases where this is not possible, chalcogen bonding rather than steric effects governs the stereochemical preferences, particularly when S and Se comprise the heterocycle of these compounds. These findings hold promise for guiding the design of compounds whose properties hinge on stereochemistry and resonant structures, such as dyes.