Pyryliums, Benzopyryliums, Pyrones and Benzopyrones 75that abstracts hydride from the intermediate 4H-pyran is the triphenylmethyl carbonium ion (Ph 3 C), and acetic
anhydride facilitates formation of the 4H-pyran.
A suitable 1,5-diketone can be prepared and utilised in situ via the addition of an enol or enolate to a chalcone (a ‘chal-
cone’ is an ,-unsaturated ketone formed by aldol condensation of an aryl-aldehyde with an aryl methyl ketone). The
excess chalcone is the oxidant of the intermediate 4H-pyran.
The oxidative step is unnecessary if a 1,5-dicarbonyl compound already containing a C-C double bond is used as start-
ing material. A synthesis of pyrylium perchlorate itself (CAUTION: potentially explosive) illustrates this – the sodium
salt of glutaconaldehyde (pent-2-ene-1,5-dial), available via hydrolysis of the pyridine–sulfur trioxide complex, is ring
closed using perchloric acid.
Ring synthesis of 4-pyrones from 1,3,5-triketones (1,2-bond made)
A synthesis of 4-pyrone also involves formation of a 1,2-bond. The 1,3,5-tricarbonyl compound, which is easily pre-
pared by a double Claisen condensation of acetone with diethyl oxalate, closes to a 4-pyrone di-acid on mineral acid
treatment, the presence of the central carbonyl group of the starting material being refl ected by its presence in the ring
closed heterocycle.
Ring synthesis of 2-pyrones from 1,3-keto-aldehydes
(1,2- and 4,5-bonds made)
The classical synthesis of 2-pyrone involves the decarbonylation of malic acid (the tart component of apples and
other fruits, fi rst isolated from apple juice in 1785 by Scheele) and then the double condensation of two molecules of
the resulting formylacetic acid to give coumalic acid (2-pyrone-5-carboxylic acid). Decarboxylation of coumalic acid