The higher order (8 + 2) cycloadditions (HOCs) of tropothione with selected cycloaliphatic and aromatic thioketones

Freshly prepared tropothione (1a) reacts with less hindered cycloaliphatic thioketones (adamantanethione (2a), 3-thioxo-2,2,4,4-tetramethylcyclobutanone (2b), and 2,2,4,4-tetramethylcyclobutan-1,3-dithione (2c)) in THF solution at ambient conditions via (8 + 2) cycloaddition (‘cross cycyloaddition’) yielding fused 1,3-dithiolane derivatives in a regioselective manner, in moderate yields. In contrast, cycloadditions of 1a with aromatic thioketones were successfully carried out only with the most reactive thiofluorenone (2d) and thiobenzophenone (2e). Whereas the (8 + 2) cycloaddition with 2d occurred in analogy to cycloaliphatic representatives in a regioselective manner yielding a single cycloadduct, thioketone 2e gave unexpectedly a mixture of regioisomeric products, identified as the isomeric 1,3-dithiolane 3d and 1,2-dithiolane 4b, respectively. In general, the (8 + 2) cycloadducts were isolated and identified by means of spectroscopic methods. The phosphorylated dithioester 5b reacted with 1a yielding the anticipated (8 + 2) cycloadduct as a ca. 3:1 mixture of two diastereoisomers. The structure of the crystalline (8 + 2) cycloadduct 3a, obtained in the reaction of 1a with 2a, was unambiguously confirmed by the single crystal X-ray diffraction analysis and the structure of this unique, 1,3-dithiolane is discussed.