Oxocarbons and related compounds. 27. Synthesis of dihydrocyclobuta[a]naphthalene-1,2-diones and cyclobuta[a]naphthalene-1,2-diones via annulation of alkoxy-(1-alkenyl)benzenes with 3-chloro-3-cyclobutene-1,2-dione. Scope and limitations

Article abstract of DOI:10.1002/prac.199733901100

The reaction of alkoxy-(1-alkenyl)benzenes with semisquaric chloride (3) has been investigated systematically. 1,2-Dialkoxy- and 1-alkoxy′-2-alkoxy″-4-(1-alkenyl)benzenes (6a-j) and (11a-i) react with 3 to give the 3,4-dihydrocyclobuta[a]naphthalene-1,2-diones(8a-j) and (12a-i). Treatment of the dihydrocyclobuta[a]naphthalene-1,2-diones with 1.2 equiv. bromine effects dehydrogenation and affords cyclobuta[a]naphthalene-1,2-diones(9a-e) and (13b-f). Any efforts to extend this annulation reaction to dimethoxy-(1-alkenyl)benzenes with the methoxy groups in other than the 1,2-positions, e. g. 14a, b, 16a, b have been unsuccessful. The reaction of 1,2,3-trimethoxy-4-(1-propenyl) [and 4-(1-butenyl)]-benzenes (18a) and (18b) with semisquaric chloride (3) leads to the elimination of HCl and CH₃OH and gives 5,6-dimethoxy-3-methyl [and 3-ethyl]-cyclobuta[a]naphthalene-1,2-diones (20a) and (20b). The reaction pathway of this novel annulation reaction is discussed.