Hetero-Diels-Alder Reactions of α-Nitrosoalkenes with Ferrocenyl, Hetaryl and Cycloaliphatic Thioketones

The in situ generated α-nitrosoalkenes react with ferrocenyl, hetaryl and cycloaliphatic thioketones yielding 4H-1,5,2-oxa- thiazines as products of the hetero-Diels-Alder reaction. These products are formed in a perfect regioselective manner. A similar reactivity is displayed by a thiochalcone (1,3-diphenylprop-2-ene-1-thione), and in that case, the [4 + 2]-cyclo-addition occurs also chemoselectively and regioselectively along the C=S bond acting as a heterodienophile. The stability of the 4H-1,5,2-oxathiazines depends on the type of substituents, and in the case of ferrocenyl thioketones, the diferrocenyl representative is the reagent of choice. The replacement of one ferrocenyl group by an ethyl group leads to a dramatic decrease of the product stability. Sterically crowded cycloaliphatic thioketones derived from 2,2,4,4-tetramethylcyclobutane-1,3-dione are exceptional dienophiles in reactions with trifluoromethyl-substituted nitrosoalkenes, and in these cases, a remarkably stable 3-trifluoromethyl-4H-1,5,2-oxathiazine derivative was obtained. For the first time, perfectly stable and non-odorous 2,2,4,4- tetramethylcyclobutane-1,3-dithione was explored as an active heterodienophile for a two-fold hetero-Diels-Alder reaction