2'-Hydroxychalcones
and α-alkoxy-2'-hydroxychalcones are converted
by sodium borohydride in isopropanol into flav-3-enes and
3-alkoxyflav-3-enes in the convenient new synthesis
which makes these flavenes readily available. Catalytic reduction of the flavenes
gives the corresponding flavans or 3-alkoxyflavans in
high yield, and the latter are obtained mainly in the 2,s-cis-form. The flavenes immediately give flavs
lium cations in the cold when treated with acids in
air, and oxidation of 5,7,3',4'-tetramethoxyflav-3-ene with benzoquinone in an acidic
medium gave the flavylium salt, isolated as the ferrichloride. Reduction of 5,7,3',4'-tetramethoxy-flavylium
chloride with lithium aluminium hydride gave 5,7,3',4'-tetramethoxy-flav-2-ene
identical with the flavene obtained from (-)-epicatechin tetramethyl ether, and confirms an earlier
investigation by Gramshaw, Johnson, and King. In its
N.M.R. spectrum the heterocyclic-ring protons of this flav-2-ene
give an ABX multiplet which is easily distinguished
from the ABX multiplet at much lower field
characteristic of flav-3-enes.
Geological field characteristic of the Kroh Formation in the Upper Perak shales, western Peninsula Malaysia
