The evolution of the chromano-chromanone unit present in rotenone and related compounds is of much interest. From a consideration of a number of structures occurring in nature, it is suggested that 2-hydroxy-methyl-2′-hydroxy isoflavanone or its equivalent is an intermediate and it undergoes ring closure by dehydration.
Based on this hypothesis, a synthesis of 7-hydroxy-chromeno-(3′∶4′; 2∶3)-chromone has now been achieved. 2-Methyl-7∶2′-diacetoxy isoflavone is converted into a 2-bromomethyl compound by treatment with N-bromo succinimide. On hydrolysis and treatment with potassium carbonate in acetone, it yields 7-hydroxy-chromeno-(3′∶4′∶2∶3)-chromone.
Though attempts to prepare the required diacetoxy isoflavone directly from 2∶4∶2′-trihydroxy phenyl benzyl ketone have not been successful, it could be obtained starting from 2∶4-dihydroxy-2′-methoxy phenyl benzyl ketone and treatment of the intermediate 2-methyl-7-acetoxy-2′-methoxy isoflavone with aluminium chloride in benzene solution, followed by acetylation.
Methylation of 2-methyl-7∶2′-dihydroxy isoflavone with excess of methyl iodide gives rise to the 7-methyl ether, indicating the existence of a considerable difference in the reactivities of the 7 and 2′-hydroxyl groups. The negative ferric reaction of these 2′-hydroxy compounds shows absence of chelation.