Thioketones can be used for the preparation of five- and six-membered sulfur heterocycles via (3+2)- and (4+2)-cycloadditions, respectively. Thioketones can show high reactivities towards 1,3-dipoles and dienes. Polarized donor-acceptor cyclopropanes (D-A cyclopropanes) can undergo (3+2)-cycloaddition reactions with a variety of dipolarophiles.

Grzegorz Mlostoń, University of Łodź, Poland, and colleagues have developed an approach to the diastereoselective (8+3)-cycloaddition of D-A cyclopropanes with tropothione, the thio analogue of 2,4,6-cycloheptatrien-1-one (general reaction pictured). The team reacted different dimethyl 2-aryl cyclopropane-1,1-dicarboxylates with tropothione in the presence of the Lewis acid Sc(OTf)₃, using CH₂Cl₂ as the solvent. The reactions were performed at room temperature.

The researchers found that tropothione smoothly undergoes (8+3)-cycloadditions with D-A cyclopropanes. The corresponding fused thiopyrans were obtained in moderate to good yields and in a diastereoselective manner. The products were formed as single diastereomers with cis-orientation of the H atoms at the C2 and C5 positions. Overall, the work provides a simple and selective procedure for the synthesis of fused thiopyrans with a troponoid unit, as well as insights into cycloaddition reactions involving lesser explored thiocarbonyl compounds.

• Diastereoselective (8+3)‐Cycloadditions of Donor‐Acceptor Cyclopropanes with Tropothione,
  Grzegorz Mloston, Mateusz Kowalczyk, Marcin Palusiak, Gwyndaf A. Oliver, Heinrich F. Koeller, Daniel B. Werz,
  Eur. J. Org. Chem. 2023.
  https://doi.org/10.1002/ejoc.202301182