Pyrylium salts hydride acceptors

It is known that tropylium may be prepared from tropylidene via hydride abstraction by PhgC or MegC carbonium ions therefore, it is very likely that here too the dehydrogenation is a hydride transfer from the 1,5-dione to an acceptor. A similar dehydrogenation of chromanones to chromones, with triphenylmethyl perchlorate was reported. A study of the electrooxidation of 1,5-diones on a rotating platinum electrode showed that 1,5-diaryl-substituted diones afford pyrylium salts in these conditions and that the half-wave potentials correlate with yields in chemical dehydrogenations. 

Pyrans and thiins are also easily aromatized, e.g. (483) + S2Cl2 — 1-benzothiinium ion. 2H-Thiins are aromatized by hydride acceptors such as triphenylmethyl cations to give thiinium salts, and similar conversions produce pyrylium salts from pyrans. 

Although the value of this route has been improved by developments in the synthesis of 1,5-diketones, it is often easier to generate the diketone in situ. Provided that the reaction is carried out in the presence of a hydride acceptor, a direct synthesis of pyrylium salts is available from simple precursors. 

The use of ethyl acetoacetate in this route allows the formation of a 3-ethoxycarbonyl-pyrylium salt (68JOC1102). Some care is appropriate in this type of reaction in the absence of an added hydride acceptor, however, since the chalcone may undergo a Michael condensation with the intermediate 1,5-dione, leading to side products. 

The dehydrogenation of penta-2,4-dien-l-ones is a convenient route to pyrylium salts unsubstituted at C-4 (17CB1008) the usual variations in hydride acceptors are allowed. A minor variation probably involves ionization of the chlorodienone (670) to a carbocation and hence to the pyrylium salt (Scheme 266) (66AG448). 

When reacted with aryl methyl ketones in acetic anhydride in the presence of a hydride acceptor, e.g. FeCl3, chalcones 32 yield trisubstituted pyrylium salts 34 Dilthey synthesis) ... 

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