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.