Thioaldehyde Diels-Alder Reactions

Thioaldehydes containing virtually any a-substitutent can be generated by photofragmentation of phenacyl sulfides. Donor-substituted derivatives are reactive toward electron-rich dienes and give 2 + 4 cycloadducts with regiochemistry corresponding to advanced C-C bonding in the transition state. Acceptor-substituted thioaldehydes react in the opposite regiochemical sense with C-S bonding advanced. A number of unusual thioaldehydes have been trapped, including the parent HCH=S, Me₃SiCHS, Ph₂P(O)CH=S, PhSO₂CH=S, and CNCH=S, as well as more conventional alkyl- or acyl-substituted derivatives.