Regiospecific annelations of substituted tropones with ethane-1,2-dithiol

Abstract

2-Butyl-, 2-phenyl-, and 3-chloro-tropone were found to react with ethane-1,2-dithiol in methanol in the presence of boron trifluoride–ether to give annelated products via addition of one sulphur atom to C-7 and replacement of the carbonyl oxygen by the other sulphur atom. The yields were low with the chloro-compound, but good in the other two cases. The same tropones were also found to react with boron trifluoride–ether alone in methanol to give, slowly, isolable equilibrium adducts of BF₃ addition to the carbonyl oxygen. These adducts reacted with ethane-1,2-dithiol to give the same annelated products mentioned above. A variety of other 2- and 3-substituted tropones, viz 2-hydroxy-, 2-methoxy-, 2-dimethylamino-, 3-methoxy-, and 3-dimethylamino-tropone, also gave isolable equilibrium adducts of BF₃ addition to the carbonyl oxygen which, however, were unreactive towards ethane-1,2-dithiol. Tropones like 2-tosyloxy-, 2-chloro-, and 2-methylthio-tropone are placed in a third category because they failed to react with either boron trifluoride–ether alone in methanol or with ethane-1,2-dithiolboron trifluoride-ether in methanol.