[SinkingTako]

I was reading something about pericyclic reactions, and it's stated that for [4+2] cycloadditions of cations and anions, it can only happen if the cation formed (Something like (A) and (B)) is stabilised by substituent that carries a lone pair. I don't get how the stabilisation works, or the resonance structures. I tried to bring the lone pair down, then move the double bond to the side, but this will lead to a structure with much greater charge (like (C)). So how does it work? Thanks!

[TheUnassuming]

In this case I believe the resonance stabilization comes from the allyl itself.  So instead of moving electrons from the heteroatom, shift electrons from the neighboring pi system of the alkene to get the resonance structure. The heteroatom with electrons to donate might stabilize this resonance structure through induction than actually forming a separate resonance form of the molecule.

[SinkingTako]

Okay, thank you! So I supposed it's just the normal resonance between the 3 bottom carbons, then the heteroatoms can act in a way like hyperconjugation so the positive charge is spread over all the atoms ? (though hyperconjugation is probably the wrong term here)

[spirochete]

Iminium ions, oxocarbenium ions and acyllium ions (google these terms if you don't know what I'm referring to) are all quite stable cations because they have resonance donation from 2nd row heteroatom lone pairs. But the cations you drew look to be essentially cross conjugated with the heteroatom lone pairs. I've never heard of ions like that forming. It's probable that they are quite unstable, because the cationic carbon is primarily experiencing inductive withdrawal from the electronegative element as opposed to resonance donation.

Where did you find pictures of those ions? Did you just make them up yourself? I'm not sure how valid what you're saying is: relatively unstable reactive intermediates can still do pericyclic reactions.