Through-Space “α-Effect” between the Bridging Oxygen Atoms in Diepoxybenzo[de]isothiochromene Derivatives

In this manuscript, we report the synthesis and X-ray characterization of a series of diepoxybenzo[de]isothiochromenes obtained by the tandem [4+2] cycloaddition between dialkylacetylenedicarboxylate dienophiles and bis(furan-2-ylmethyl)sulfane (bis-diene). For the synthesis, we have used four different dienophiles (RO2C–C≡C–CO2R, R = Me, Et, iPr, tBu) and kinetic conditions, so the “pincer”-adduct is exclusively formed in good/moderate yield even for the bulky CO2tBu-group. The polycyclic nature of the resulting products and the close distance between the O-bridge atoms provoke an interesting through space lone pair···lone pair (LP···LP) orbital interaction that is conceptually similar to the “alpha-effect”, which has been analyzed theoretically. Remarkably, this through-space “alpha-effect” enhances the ability of the O-atoms to participate in hydrogen bonding interactions, as demonstrated by DFT calculations. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim