THE CHEMISTRY OF COUMARIN DERIVATIVES .3. SYNTHESIS OF 3-ALKYL-4-HYDROXYCOUMARINS BY REDUCTIVE FRAGMENTATION OF 3,3'-ALKYLIDENE-4,4'-DIHYDROXYBIS[COUMARINS]

Treatment of 3,3'-alkylidene-4,4'-dihydroxybis[coumarins] 1 with NaBH3CN 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).