Lewis acid-promoted hetero Diels–Alder reaction of α,β-unsaturated thioketones

摘要: α,β-Unsaturated thioketones 1 were regenerated from their precursors, the dithiine-type dimers 3, and readily underwent Lewis acid-promoted hetero Diels–Alder reactions with a variety of carbonyl-activated dienophiles such as methyl acrylate 4, vinyl ketone 6, acrolein 7, N,N-dimethylacryl-amide 8, methacrylate 9, dimethyl fumarate 10, crotonate 11 N-p-tolylmaleimide 12, to give [4 + 2]cycloadducts 5 13–19, respectively. Among acids investigated, AlCl3 EtAlCl2, when used in diethyl ether dichloromethane, respectively, found remarkably accelerate reaction even at lower temperatures (25–35 °C); is very slow absence catalyst. A moderate enhancement stereoselectivity (endo/exo) was observed compared that thermal 80 °C benzene. The structure determination cycloadducts by 1H NMR spectroscopic studies are described configurations relative conformations also discussed.