PHTHALIMIDESULFENYL CHLORIDE .9. A SIMPLE ACCESS TO ALPHA,ALPHA'-DIOXOTHIONES, A NEW CLASS OF BIS-HETERODIENES - SYNTHESIS OF 1,4-OXATHIIN SYSTEMS

alpha,alpha'-Dioxothiophthalimides 4a and 4b react with pyridine to generate the alpha,alpha'-dioxothiones 5a and 5b which undergo chemo- and regiospecific inverse electron demand Diels-Alder reactions With electron-rich alkenes to give 1,4-oxathiin heterocyclic systems. Enol ethers, silyl enol ethers, vinyl sulfides, vinyl amides, substituted styrenes, and electron rich alkynes and allenes can be fruitfully employed as dienophiles. Among the vinyl ethers tested tri-O-benzylglucal was successfully used as a dienophile. Ab initio molecular orbital calculations performed on thiones 5a and 5b as well as on a selected enol ether are consistent with the experimental results. When thione 5a was generated in the presence of 2,3-dimethyl-2-butene (31) and -(beta)-pinene (33) a selective ''thiophlic ene reaction'' was observed leading to the formation of sulfides 32 and 34. Thione 5b reacted similarly with 33 to generate the sulfide 35. Additionally a competition between the dienic versus dienophilic behavior of alpha,alpha'-dioxothiones 5a and 5b was observed in reactions with 2,3-dimethoxy-1,3-butadiene (45) and 2-methoxyfuran (50). A preliminary screening of the reactivity of 1,4-oxathiins 7c and 8b, used as model substrates, is also outlined.