Flavan derivatives. XXVI. 3(3')-Hydroxyisochromano(4',3':2,3)chromans. Mechanism of formation of these hemiketals from 1'-Oxo-isochromono-(4',3':2,3)chromones

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Abstract
Reduction of the lactones of flavonol-2?-carboxylic acids [1?- oxoisochromono-(4?,3?:2,3)chromones] to hemiacetals by lithium aluminium hydride is shown to proceed by fission of the lactone ring accompanied by direct reduction of the chromone carbonyl group without fission of the chromone ring. The intermediate chrom-2-en-4-ols undergo nucleophilic displacement of the allylic (and benzylic) hydroxyl group by hydride ion attack at the 2- and 4-positions. Reduction of 7- methoxyisochromeno(4?,3?:2,3)chromone with lithium aluminium deuteride led to 7-methoxyisochromeno(4?,3?:2,3)chromen with 95% incorporation of two deuterium atoms.

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