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