76 Pyryliums, Benzopyryliums, Pyrones and Benzopyrones
produces 2-pyrone and bromination gives 3,5-dibromo-2-pyrone. (Note the easy ipso replacement of carbon dioxide
by bromine: the intermediate is highly activated for decarboxylation.)
Ring synthesis of 1-benzopyryliums, coumarins and chromones
All practical syntheses of benzene ring-fused-pyryliums and benzopyrones start from phenols. For example, resor-
cinol reacts with 1,3-diketones in acidic solution to give 1-benzopyrylium salts. Changing the oxidation level
of one of the carbonyl groups to that of an ester, that is reaction of resorcinol with a -keto-ester, allows the
synthesis of coumarins. The initiating step in these syntheses is electrophilic attack on the aromatic ring by the
O-protonated 1,3-dicarbonyl compound, the presence of an activating hydroxyl para to the point of attack facili-
tating the sequence.
Salicylaldehyde reacts with aryl methyl ketones in acid to produce fl avylium salts (2-aryl-1-benzopyrylium salts): aldol
condensation is followed by ring closure and then dehydration. Once again, changing the oxidation level of the second
component – condensing a salicylaldehyde with acetic anhydride – produces a coumarin (the Perkin reaction - see
page 191).
ortho-Hydroxyaryl methyl ketones can be simply O-acylated giving phenolic esters. Carrying out a base-induced
rearrangement on these (arrows) leads to a 1,3-diketone phenolate anion that closes to a chromone when acidifi ed.
2-Arylchromones are generally known as ‘fl avones’.