THERMAL AND LEWIS ACID INDUCED CYCLOADDITION OF THIOALDEHYDE S-OXIDE (MONOSUBSTITUTED SULFINES) TO DIENES. III

The stereochemical course of the 1,4-cycloaddition of thioaldehyde S-oxides (monosubstituted sulfines) to 2,3-dimethylbuta-1,3-diene, buta-1,3-diene, and cis- and trans-penta-1,3-diene was investigated. Unexpectedly, the reactions of buta-1,3-diene and 2,3-dimethylbuta-1,3-diene with Z-monoaryl sulfines afforded cis/trans mixtures of the corresponding dihydrothiopyran S-oxides, in which the relative amounts of the two isomers depended upon the initial diene/sulfine ratio. A Z to E isomerization of the dienophiles during the cycloaddition was responsible. On the other hand, Z/E mixtures of aliphatic tert-butyl sulfine gave, with 2,3-dimethylbuta-1,3-diene, only the corresponding trans cycloadduct. Catalysis of the reaction by Lewis acids, heretofore largely unexplored, was also investigated