Reactions via Pyrylium Intermediates

SCHEME 21.42 Au-catalyzed [4+2] cycloaddition of 2-alkynylbenzaldehyde with alkynes. 

Interestingly, the reactions of the 2-alkynylbenzaldehyde with alkynes were conducted in the presence of Cu(OTf)2 (5 mol %) and a Brpnsted acid (1 equiv) afforded decarbonylated naphthalenes 

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In this subchapter, I summarized the intermolecular two-component cycloaddition reactions, which are able to construct benzenoid aromatic rings. Four types of transition metal-catalyzed [4-r2] cycloaddition reactions (Diels-Alder reactions, reactions of enynes with alkynes, reactions via pyrylium intermediates, and reactions via acylmetallacycles) are described. 

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