Cycloadditions of substituted benzopyran-4-ones to electron-rich dienes: a new route to xanthone derivatives

Article abstract of DOI:10.1016/S0040-4020(01)86849-6

The benzopyranones 1 and 3 reacted with 2,3-dimethyl-1,3-butadiene in the presence of titanium (IV) chloride to give the corresponding (4 + 2) cycloadducts 8 and 11, the former undergoing facile deformylation to give 9 and 10. Compounds 1, 3, and 4 underwent efficient uncatalysed cycloaddition to 1- methoxy-3-(trimethylsilyloxy)-l,3-butadiene 12 to give the respective adducts 13,14, and 18 as mixtures of C-l stereoisomers. Heating the 3-arylsulphinylchromone 5 with the diene 12 afforded 3-hydroxyxanthone 23 in 50% yield, the presumed cycloaddition - elimination sequence constituting a new route to xanthone systems. Desilylation of 13,14, and 18 in acidic media provided 25,26, and 27 respectively.