Stereoisomerization of β-Hydroxy-α-sulfenyl-γ-butyrolactones Controlled by Two Concomitant 1,4-Type Nonbonded Sulfur−Oxygen Interactions As Analyzed by X-ray Crystallography

We have synthesized nine β-hydroxy-α-sulfenyl-γ-butyrolactones having different substituents. The syn-anti or syn-syn lactones (or any mixture of both) invariably isomerized into syn-anti/syn-syn lactones in a ratio of ∼6/4. The other two possible isomeric lactones (anti-syn or anti-anti) were never observed. The crystal structures of syn-syn lactones have been determined. We found sulfur−oxygen distances to be less than the sum of the van der Waals radii (3.3 Å), with the angle formed among the hydroxylic oxygen, sulfur, and quaternary aromatic carbon being close to 180°. In addition, carbonylic oxygen−sulfur is directed <40° from the perpendicular to the C−S−C. Theoretical calculations were performed which emphasize the directionality of the sulfur−oxygen interaction. The X-ray and theoretical studies demonstrate that two concomitant, attractive 1,4 intramolecular interactions of divalent sulfur with both carbonyl and hydroxyl oxygens are the driving force for the aforementioned stereochemical preference. Then nonbonded sulfur−oxygen interactions would control the stereoselectivity of the reaction. The same features are observed in the X-ray structure of a β-hydroxy-α-sulfinyl-γ-butyrolactone.