Synthesis, C-H bond functionalisation and cycloadditions of 6-styryl-1,2-oxathiine 2,2-dioxides

A series of 6-styryl-1,2-oxathiine 2,2-dioxides have been efficiently obtained by a two-step protocol from readily available (1 E ,4 E )-1-(dimethylamino)-5-arylpenta-1,4-dien-3-ones involving a regioselective sulfene addition and subsequent Cope elimination. Pd-Mediated direct C-H bond functionalisation of the 6-styryl-1,2-oxathiine 2,2-dioxides and a wider selection of 5,6-diaryl substituted 1,2-oxathiine 2,2-dioxides proceeded smoothly to afford C-3 (hetero)aryl substituted analogues and the results are contrasted with those of a complementary bromination - Suzuki cross-coupling sequence. Whilst the cycloaddition of benzyne, derived from in situ fluoride initiated decomposition of 2-(trimethylsilyl)phenyl trifluoromethanesulfonate, to the substituted 1,2-oxathiine 2,2-dioxides resulted in low yields of substituted naphthalenes, the addition of 4-phenyl-1,2,4-triazoline-3,5-dione to the 6-styryl-1,2-oxathiine 2,2-dioxides afforded novel 5,9-dihydro-1 H -[1,2]oxathiino[5,6- c ][1,2,4]triazolo[1,2- a ]pyridazine-1,3(2 H )-dione 8,8-dioxides through a silica-mediated isomerisation of the initial [4 + 2] adducts. 1,2-Oxathiine 2,2-dioxides readily undergo C-H activated coupling at C-3. The 6-styryl derivatives participate in cycloadditions with PTAD to afford 1 H -[1,2]oxathiino[5,6- c ][1,2,4]triazolo[1,2- a ]pyridazine-1,3(2 H )-dione 8,8-dioxides.