SELECTIVE 1,4-ADDITION OF ARENES TO 3-CHLORO-3-CYCLOBUTENE-1,2-DIONE UNDER FRIEDEL-CRAFTS CONDITIONS - SYNTHESIS AND REACTIVITY OF 4-ARYL-3-CHLORO-2-HYDROXY-2-CYCLOBUTEN-1-ONES

The reaction of semisquaric chloride (7) with arenes 2 has been invest igated. In the presence of 1.1 equiv of AlCl3 and in the temperature r ange of -15 degrees C to rt the arenes 2a-q afford the 4-aryl-3-chloro -2-hydroxy-2-cyclobuten-1-ones (chloroenols) 8a-q in good yield. By co ntrast, 7 reacts with 1,4-dimethoxybenzene (21) in boiling CH2Cl2 to g ive a mixture of (2,5-dimethoxyphenyl)cyclobutenedione (9a) (27% yield ) and bis(2,5-dimethoxyphenyl)cyclobutenedione (10a) (8% yield). With 1,2,4-trimethoxybenzene (2r) in the presence of trifluoroacetic acid i s generated (2,4,5-trimethoxyphenyl)cyclobutenedione (9b) in 21% yield . The chloroenols 8 allow a series of valuable transformation reaction s: with diazomethane the chloroenol methyl ethers 11 are generated, wi th chlorine the 3-aryl-4-chlorocyclobutenediones 12, and with bromine in MeOH the 3-aryl-4-methoxycyclobutenediones 13. In DMSO or in aceton e/H2O the chloroenols 8 eliminate HCl, furnishing the arylcyclobutened iones 14. In a mixture of acetone -d(6)/D2O/DCl are obtained 4-aryl-cy clobutenediones-3-d 15. For the latter two processes the corresponding 3-aryl-4-chlorocyclobutane-1,2-diones 16 are postulated as intermedia tes. Thermolysis of the chloroenols 8 and the chloroenol methyl ethers 11 in refluxing m-xylene afforded the 3-chloro-1,2-dihydroxynaphthale nes 17 and the 3-chlorol-hydroxy-2-methoxynaphthalenes 18, respectivel y.