Synthesis, stereochemistry, and chiroptical spectra of cyclopropyl lactones and thionolactones

Several optically active substituted 3-oxabicyclo[3.1.0]hexan-2-ones and their thiocarbonyl analogues were synthesized, and their circular dichroism spectra are reported. It was found that the n-π∗ Cotton effect sign is determined by the helicity of an inherently chiral chromophore formed by the lactone or thiolactone group and the cyclopropyl moiety. The π-π∗ Cotton effect of thiocarbonyl compounds shows opposite sign to that observed for the lowest energy transition. The crystal structures of two compounds were solved to establish their molecular geometries.