The substituent effect on the cycloheptatriene-norcaradiene equilibrium. Reaction of singlet oxygen with substituted cycloheptatrienes

2006 ARKIVOC  
Cycloaddition reaction of singlet oxygen and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) to various cycloheptatriene derivatives was investigated. The addition of PTAD to 3,8adihydroazulen-1(2H)-one gave exclusively a norcaradiene adduct whereas the addition to 3,4dihydroazulen-1(2H)-one resulted in the formation of a cycloheptatriene adduct. Photooxygenation of dihydroazulen-1(2H)-one afforded solely a [2+4] cycloaddition product derived from cycloheptatriene. Photooxygenation of the reduced
more » ... duct, 1,2,3,8atetrahydroazulen-1-yl acetate gave the all possible cycloaddition products. The product distribution was not affected upon reduction of the carbonyl group. On the other hand, photooxygenation of dimethyl cyclohepta-3,5,7-triene-1,3-dicarboxylate gave mainly addition products derived from the norcaradiene structure. The formation of the products was explained by a photochemically allowed 1,7-suprafacial hydrogen shift under the reaction conditions followed by singlet oxygen addition. Cycloaddition of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) to 3,8a-dihydroazulen-1(2H)one (5). To a stirred solution of bicyclic tirenone 5 6 1.0 g (6.85 mmol) in 50 mL of dichloromethane at room temperature was added in small portions, 1.3g (7.43 mmol) of the PTAD over a period of ca. 30 min. After stirring at room temperature for 1.5 h, the solvent was evaporated and the solid residue was recrystallized from dichloromethane/ethylacetate (2:1) to
doi:10.3998/ark.5550190.0008.814 fatcat:yzzn2fh5qngadp7uotk4pcndfa