Studies of Thioaldehydes and Dienophilic Dithioesters

Studies have been carried out on the base-mediated ehmination reactions of N-phthaloyl sulfenamides under various conditions. In particular, ethyl phthalimidosulfanylacetate was treated with triethylamine alone to yield the thioaldehyde, ethyl thioxoacetate, and with 4-dimethylaminopyridine (DMAP) (with or without triethylamine) to yield the dithioester, diethyl 3-thia-2-thioxopentanedioate. The transient, dienophilic products were trapped by Diels-Alder reactions with various dienes. The ratio of thioaldehyde and dithioester adducts were dependent on the amount of DMAP used and also on the reactivity of the diene. In the absence of any diene, elimination of the phthaloyl sulfenamide led to the polymerisation, catalysed by base, of the transient dienophile. Different types of polymer were formed according to the base used. Triethylamine generated a thioaldehyde polymer while DMAP and combinations of it with other bases led to the dithioester polymer. Depolymerisation was accomplished by the addition of base to the polymers and again the dienophilic intermediates that were generated were captured by dienes to form the corresponding cycloadducts. The labile dithioester was isolated after immediate addition of acid to the reaction mixture to prevent polymerisation being catalysed by the DMAP. It was isolated in a pure state, after chromatography, as a purple oil. Diels-Alder reactions of this purified dithioester gave the corresponding cycloadducts free from other products. Elimination reactions were also studied with other ?-oxo phthaloyl sulfenamides. The cycloadduct of ethyl thioxoacetate and anthracene was converted into the corresponding S-ethyl sulfonium salt. This reacted with base to form a pair of isomers, which have been assigned tentative structures. Similarly, the S-ethyl sulfonium salt of the cycloadduct of the dithioester and dimethylbutadiene was treated with diazabicyclononane (DBN) to give a cyclopropanecarboxylate identical to that formed from the thioaldehyde cycloadduct. A known racemic, chiral auxiliary, trans-2,5-bis(methoxymethyl)pyrrolidine was prepared and N-acylated with chloroacetyl chloride. This chloroacetyl derivative was converted into the corresponding Bunte salt. Base-mediated elimination yielded a chiral thioaldehyde, which was trapped with both cyclopentadiene and cyclohexadiene to yield 4 diastereomers in each case. The cyclopentadiene endo adducts were formed in 91% d.e. and the cyclohexadiene endo adducts in 75% d.e. In both cases the major endo diastereomer was separated from the others by chromatography. The X-ray crystal structure obtained for the major, endo cyclohexadiene adduct provided the relative configuration of the chiral centres; the stereoselective formation of this cycloadduct has been rationalised