Intramolecular ene reactions of thioaldehydes giving α-mercapto lactones

Abstract

The crystalline acids 3b, 4b and 5b and 6b, formally cycloadducts of anthracene, 9,10-dimethylanthracene and cyclopentadiene, respectively, with thioxoacetic acid (HO2CCHS), have been used to prepare esters with various allylic and homoallylic alcohols. These esters dissociated when heated, either in toluene under reflux or under the conditions of flash vacuum pyrolysis (FVP), to liberate transient thioaldehydes (thioxoacetate esters), which underwent intramolecular ‘ene’ reactions. Thus, the anthracene derivative 3c, in toluene at 111 °C gave, via prenyl thioxoacetate 2c, cis-2-mercapto-3-isopropenylbutyrolactone 13 in good yield. The corresponding cyclopentadiene derivatives 5c and 6c gave the same product, as a mixture of cis and trans isomers, essentially quantitatively when subjected to FVP. Similar experiments, involving Type [1, 1] intramolecular ene reactions, gave the α-mercapto-γ-lactones 20, 21 and 22, and the epimeric amercapto-δ-lactones 26. Further, the cyclopentadiene adduct 6h of 2-methylprop-2-enyl thioxoacetate 2h gave, under FVP conditions, the α-mercapto-δ-lactone 27 by a Type [2, 1] ene reaction. Evidence supporting a concerted mechanism for the ene reactions is discussed.Improved preparations of the cycloadducts 3b and 4b of thioxoacetic acid 2b with anthracene and dimethylanthracene, respectively, are recorded.