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Cross-conjugation is a special type of conjugation in a molecule, when in a set of three Pi bonds only two pi-bonds interact with each other by conjugation, the third one is excluded from interaction [1]. In classical terms it means that the strict alternation of single and double bonds --CH=CH–CH=CH–CH-- (i.e., conjugated) is interrupted by two consecutive single bonds at each cross-conjugated point in the cross-conjugated pathway: --CH=CH–C(=CH)–CH=CH--. Examples of cross-conjugation can be found in molecules such as benzophenone, divinylether, dendralenes and fullerene. The type of conjugation has an impact on reactivity and molecular electronic transitions.
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Ethyl methacrylate is an example of crossed conjugation.
CH2=C(-Me)-C(=O)-Oet
The ethylenic bond does not enter into resonance with COOEt group. The contribution to resonance is only the path that leads to greater resonance energy, which in this case is the carbethoxy group.
The structure of p-benzoquinone is known as crossed conjugated system; it contains three( or more) conjugated double bonds which are not arranged in a continuous chain. It behaves as an alpha-beta unsaturated ketone, most of its addition reactions being typical 1,4-additions, followed by aromatisation. The driving force of these additions is largely due to the fact that the resonance energy of quinone is 20.9kJ per mole, and when it converts into the aromatic compound, the product has gained an increased stabilization energy of atleast 125.5kJ per mole.
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Cross conjugation is a phenomenon where it decreases the efficiency of resonance in a molecule.
CH3-O-
- CH3 , Here the lone pair on the oxygen atom participates in the resonance effectively but
CH3-O-
-O- CH3, here the other lone pair on the oxygen atom decreses the effectiveness by which the resonance takes place because the lone pair on the oxygen atom decreases the magnitude of 
+ charge on C atom.