Furan ring opening — isocoumarine ring closure: A recyclization reaction of 2‐carboxyaryldifurylmethanes

A general method for the synthesis of isocoumarine derivatives has been developed. Bis(5‐R‐2‐furyl)methylbenzoic acids (R = methyl, ethyl) underwent recyclization and subsequent cyclization into tetracyclic isochromene‐1‐one derivatives under treatment with hydrogen chloride in methanol. It has been shown that intermediate 4‐(5‐R‐furan‐2‐yl)‐3‐(3‐oxo‐3‐R‐propyl)‐isochromene‐1‐ones can be obtained selectively by varying a concentration of the hydrogen chloride and reaction times. In the case of R = tert ‐butyl only corresponding 4‐[5‐( tert ‐butyl)‐2‐furyl]‐3‐(4,4‐dimethyl‐3‐oxopentyl)‐1‐isochromenones were isolated regardless of the reaction conditions.