The constitution of oroxylin - A: Part II. Attempted synthesis of oroxylin-A and the synthesis of wogonin

Abstract

The presence of baicalein in the root bark of Oroxylum indicum, Vent, has been confirmed, and sitosterol and galactose have also been isolated. In an attempt to synthesise the previously isolated oroxylin - A (5 : 7 - dihydroxy - 6- methoxyflavone), it was found that benzoylation of 2 ; 4 - dihydroxy-3 : 6- dimethoxyacetophenone proceeded normally (with ring closure) to give 7- hydroxy-5 : 8 - dimethoxyflavone, which was demethylated by aluminium chloride to 5 : 7 : 8-trihydroxyflavone, obtained, it is believed pure for the first time, and to wogonin (5 : 7-dihydroxy -8- methoxyflavone). This is the first synthesis of this naturally occurring flavone, The action of hydriodic acid on 7-hydroxy-5 : 8-dimethoxyflavone and on wogonin under the experimental conditions described gave baicalein (5 : 6 : 7-trihydroxyflavone), and it is suggested that this is due to the opening of the pyrone ring with subsequent rearrangement.