Thioaldehydes in Synthesis

Thioaldehydes, ZCHS, where Z is an electron-withdrawing group, have been prepared by base-mediated, 1,2-elimination reactions of sulfenyl derivatives, ZCH2SX, where X is a heteroatomic leaving group, e.g. Cl, N-phthaloyl, SO2Tol and SO3Na. The transient thioaldehydes were trapped in situ with conjugated dienes to give Diels-Alder cycloadducts. The cycloadducts of anthracene, 9,10-dimethylanthracene and cyclopentadiene dissociate in toluene at 111°C and thereby serve as synthetically useful, auxiliary precursors of the labile thials. α-Alkylation of anthracene adducts provides precursors of thioketones and S-oxidation precursors of sulfines. Anthracene and cyclopentadiene adducts of unsaturated esters of thioxoacetic acid, RO2C. CHS, have been used to study intramolecular ene cyclisations of thioaldehydes. The Diels-Alder adducts of thioaldehydes have been used in the synthesis of new opiate analgesics, derived from thebaine, and of 6-thiashikimic acid, a sulfur analogue of the key intermediate in the biosynthesis of aromatic amino acids.