Hydrogen Bromide → Robustaflavone

Participants

Reaction Steps

Deprotection of 10 was initially attempted using standard mineral acid conditions, such as HBr and HI. In all cases, the use of mineral acids resulted in Wessely-Moser rearrangement21, and amentoflavone was the major product isolated. Similar results were obtained using aqueous (HBr and HI) or anhydrous (HBr in HOAc) conditions. Complete demethylation of 10 was achieved by treatment with between about 8 and about 15 equiv BBr3, preferably 12 equivalents BBr3, in a suitable solvent, e.g., CHCl3 at reflux, at a temperature ranging between about 25° C. and about 80° C., preferably about 60-62° C. If desired, BCl3 may be used in place of BBr3. Attempts to deprotect using amounts of BBr3 less than 8 equivalents or at temperatures lower than 50° C., did not achieve complete demethylation, and the resulting products contained significant quantities of partially demethylated materials. Following demethylation, crude robustaflavone was obtained in 88.9% yield. Column chromatography through silica gel (toluene/pyridine/formic acid, 20:10:1) afforded robustaflavone in 30% yield.

Provenance & Evidence