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Two ketone carbonyls separated by one carbon. The intervening C–H bonds are relatively acidic, and the enol can be stabilized by pi conjugation and an intramolecular hydrogen bond.

An alkene bearing two thioalkoxy groups on one carbon, ${\mathrm{R}}_2\mathrm{C}=\mathrm{C}(SR\prime {)}_2$. pi conjugation to a carbonyl can allow addition–elimination at that beta carbon with thiolate departure.

For a monocyclic, approximately planar, continuously conjugated pi system, Hückel's rule predicts aromatic closed-shell filling for $4n+2$ pi electrons, with $n=0,1,2,\dots$. Every atom on the circuit must have an appropriately oriented orbital; counting double bonds without checking pi conjugation is insufficient.

The two outer benzene rings are clearly aromatic; the central pyrone ring has a weaker, polarized six-pi aromatic contribution. Each outer ring has the familiar six-electron circuit. For the central ring, a useful charge-separated contributor moves the exocyclic carbonyl pi pair onto oxygen and donates one endocyclic oxygen lone pair into the ring. Its four fused carbon p centres contribute four electrons and the ring-oxygen lone pair supplies two: a six-pi, pyrylium-like circuit, with positive charge on the ring oxygen and negative charge on the carbonyl oxygen. This does not make three independent benzene rings of equal aromatic stabilization; localized carbonyl bonding and the fused benzene circuits remain important contributors.

The three oxygens play different roles. The endocyclic ether oxygen donates a p-type lone pair to pi conjugation. The carbonyl oxygen accepts pi density in the charge-separated contributor. The phenolic oxygen donates into the outer ring as a substituent. Its in-plane lone pair is not an extra pair to add to every ring's electron count. In particular, neither the external C=O pair nor both lone pairs on a ring oxygen should be counted as independent extra ring electrons. Structural studies of xanthone support a largely planar conjugated framework, while the division into strongly aromatic outer rings and a more polarized central ring is an electronic-structure interpretation.^[ref]