The chemistry of coumarin derivatives. Part 3. Synthesis of 3‐alkyl‐4‐hydroxycoumarins by reductive fragmentation of 3,3′‐alkyiidene‐4,4′‐dihydroxybis[coumarins]

Treatment of 3,3′‐alkylidene‐4,4′‐dihydroxybis[coumarins] 1 with NaBH 3 CN in refluxing MeOH affords 3‐alkyl‐4‐hydroxycoumarins 2 and 4‐hydroxycoumarin ( 3 ; Scheme 1 ). The reaction might take place via hydride trapping of alkylidenechromandiones C formed from 1 in a retro‐Michael reaction. Such a retro‐Michael reaction of 1 might be biologically relevant. The presence of C during the reductive fragmentation 1 → 2 is suggested by Diels‐Alder and nucleophilic trapping of the alkylidenechromandiones C as well as from cross‐over experiments with coumarins other than 3 ( see Scheme 2 ). The reductive fragmentation of 1 allows the chemo‐ and regioselective synthesis of a variety of 3‐alkyl‐4‐hydroxycoumarins 2 (see Table ).