Pyrylium intermediate

Enynals 104 can undergo a palladium-catalyzed cyclization to form the pyrylium intermediate 105, which upon deprotonation affords the triene 106. In the presence of DM AD, a [2+2]-cycloaddition then occurs to form the tricyclic 4//-pyran intermediate 107, which undergoes expansion of the cyclobutene ring to afford the bicyclic 4//-pyrans 108 (Scheme 34) <2004S1409>. 

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. 

Pyrylium intermediates have been widely used as a four carbon atoms unit in various [4-1-2] cycloaddition reactions. The in situ gold(III)-catalyzed formation of pyrylinm intermediates from a 2-alkynylbenzaldehyde followed by the [4-1-2] cycloaddition with alkynes and bond shift afforded trisubstituted naphthalenes (Scheme 21.42) [49]. 

FORMAL [4 + 2] BENZANNULATION VIA PYRYLIUM INTERMEDIATES 15.3.3 With Carbonyl Compounds... 

Patil et al. found that the benzannulation of o-alkynylbenzaldehydes 11 with styrene oxides 54 proeeeded in the presence of h (Scheme 15.22) [35]. Probably, styrene oxides would undergo Meinwald rearrangement to form enols, which would participate in [4 -t- 2] cycloaddition with pyrylium intermediates. 

Kim et al. found that two cycloaddition pathways, [4 + 2] and [3 + 2], are possible in the reaction of 61, having a gem diester on the carbon tether, and they are dependent on the reaction conditions. The gold-catalyzed reaction of 61 proceeded at room temperature to give benzocycloheptenone derivative 68 exclusively. The reaction probably proceeded through intramolecular [3 -I- 2] cycloaddition of Au-pyrylium intermediate 69, followed by a sequential rearrangement, as shown in 70. On the other hand, a 1 1 mixture of 62 and 68 was obtained when the reaction was conducted at no °C (Scheme 15.28) [40],... 

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