Synthesis of 10-methyl-Δ4-octalins by Diels-Alder reactions of 2H-thiopyran surrogates for 1-ethenyl-2-methyleyclohexene

Article abstract of DOI:10.1139/v97-082

Diels-Alder reactions of 1-ethenyl-2-methylcyclohexene derivatives could be a versatile route to a variety of natural product skeletons that possess a 10-methyldecalin substructure with additional substitution at C-8 and C-9. These dienes are unreactive due (in part) to the presence of the vinyl methyl group, which destabilizes the necessary &-cis conformation. The use of 2H-thiopyran diene surrogates for 1-ethenyl-2-methylcyclohexene is investigated. The desired Diels-Alder adducts were not obtained by reaction of 6,7,8,8a-tetrahydro-5,5-dimethyl-3-tris(1-methylethyl)silyloxy-1H-2- benzothiopyran (7c) or 1,5,7,8-tetrahydrospiro[6H-2-benzothiopyran-6,2′-[1,3]dioxolane] (17) with N-phenylmaleimide. Reactions of 3-[2-(2-methyl-1,3-dioxolan-2-yl)ethyl]-4-[(tris(1-methylethyl)silyl)oxy]-2H- thiopyran (19) with maleimide, N-methylmaleimide, and N-phenylmaleimide under thermal conditions and with N-methylmaleimide, N-phenylmaleimide, 3-(propenoyl)-2-oxazolidinone, and methyl acrylate under Lewis acid mediated conditions (EtAlCl₂, TiCl₄) gave Diels-Alder adducts in moderate to good yields. In each case, those adducts were readily converted into products that are synthetically equivalent to Diels-Alder adducts of 17 with the same dienophiles. Desulfurization of those products gives 10-methyl-Δ⁴-octalins (2,3,4,4a,5,6,7,8-octahydro-4a-methylnapthalenes) that are synthetically equivalent to hypothetical Diels-Alder adducts of 1-ethenyl-2-methylcyclohexene-5-one.