Pyrylium cations/ions/salts aromaticity

The substitution of a CH unit in benzene by 0+ (the oxonia group) gives rise to the pyrylium cation (3). Since this ring still possesses 6 ir-electrons, it may be expected to exhibit aromatic properties. As the oxygen is primarily tricovalent, the pyrylium ring may be formally regarded as a cyclic oxonium ion. However its enhanced stability relative to aliphatic and alicyclic oxonium salts is doubtless due to its aromatic nature. 

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. 

Pyrylium salts with a free - or -position react in a similar way without ring fission, flavylium adds dimethylaniline and the product aromatizes to give cation 312 xanthylium ions form adducts at the 9-position with -diketones, -keto esters, and malonic esters (e.g., 313). 

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