Studies on Thioxo- and Selenoxo- Malonates

Thermally labile cycloadducts of diethyl thioxomalonate (46) were found to be most easily synthesised, in reasonable yields, by heating the corresponding ketone (48) with phosphorus pentasulphide in pyridine. The cyclopentadiene adduct (93) and, more usefully, the anthracene adduct (95) were found to dissociate on heating to regenerate diethyl thioxomalonate which then reacted regiospecifically in situ with other dienes (retro-Diels-Alder reaction) or with (-)-B-pinene ('ene' reaction). These labile cycloadducts (93) and (95) were oxidised with peracid to the corresponding S-oxides (173) and (172). These S-oxides also dissociated on heating, to give diethyl thioxomalonate S-oxide (148) which reacted in situ, regioselectively, with other dienes. Flash vacuum pyrolysis of the anthracene adduct (172) also liberated (148), free of anthracene, which enabled studies on the 'isolated' species (148) at lower temperatures. Less success was achieved in synthesising cycloadducts of diethyl selenoxomalonate (229). This species was generated in situ by treating either diethyl selenocyanatomalonate (244) or the selenosulphate (250) with triethylamine in the presence of a diene. However, only the cyclopentadiene adduct (230) and the 2,3-dimethyl-1,3-butadiene adduct (244) were succesfully synthesised, in poor to moderate yields with low reproducibility