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When a methylene chloride solution of acetophenone (2 equiv.) in the presence of a modified organoaluminum compound, methylaluminum bis(2,6-di-tert-butyl-4-methylphenoxide) (abbreviated as MAD), was irradiated in a quartz test tube for 5h with a high-pressure mercury lamp, the aldol reaction of acetophenone proceeded exclusively and gave 3-hydroxy-1,3-diphenylbutan-1-one (1) in 50% yield.　In the dark reaction of acetophenone in the presence of MAD, aldol product 1 was obtained in 10% yield.　This photo-induced aldol reaction of acetophenone using other organoaluminum compounds 3 ~ 6 or ordinary Lewis acids proceeded unsuccessfully.　A mechanism for this photo-induced aldol reaction of acetophenone with MAD is proposed as follows: 1) the acetophenone-MAD complex was formed immediately; 2) the aluminum enolate intermediate 2 may be formed by the irradiation of this MAD complex; 3) the reaction of the intermediate 2 with free acetophenone gave aldol product 1.　Similarly, the photoreaction of various acetophenone derivatives, such as 4-chloroacetophenone, 4-methoxyacetophenone, 2-methylacetophenone, 2’-acetonaphthone, and propiophenone, with MAD gave corresponding aldol products 7 ~ 11 in 18 ~ 30% yields.