Intramolecular Diels-Alder reactions of 2H-thiopyran dienes

A systematic survey of IMDA reactions of 4-[tris(2-methylethyl)silyl]oxy-2H-thiopyran derivatives with potential dienophiles tethered at the C-2, C-3, C-5, and C-6 positions is presented. Cycloaddition was not observed with a C-3 or C-4 tether and an unactivated terminal olefin as dienophile. IMDA adducts could be obtained when dienophiles activated by a carbomethoxy group were employed. Compounds having the activated dienophile attached via a C-3 tether to C-2 of the 2H-thiopyran gave adducts with high stereoselectivity. Substrates having the dienophile attached to C-3 with a C-3 or C-4 tether cyclized readily. With an (E)-enoate as the dienophile, the stereoselectivity was poor (endo:exo = 1.1-2.5:1) and essentially independent of reaction conditions (i.e., thermal vs. Lewis acid mediated). With the (Z)-enoate, a 7:1 mixture of endo:exo IMDA adducts was obtained under thermal conditions; with Lewis acid catalysis, isomerization of the dienophile was competitive with cycloaddition. Type II IMDA adducts were not observed with C-5 tethered substrates. Compounds having the dienophile attached to C-6 via a C-3 or a five-atom tether also failed to give IMDA adducts. No evidence for isomerization of the 2H-thiopyran dienes by [1,5] sigmatropic rearrangement was observed. The endo adducts from IMDA reactions of the C-3 tethered substrates can be desulfurized to obtain synthetically useful trans-fused hydrindans and decalins with angular methyl groups.