Phenols, from pyryliums

The clarification of the recyclization mechanism of 2-benzopyrylium salts into naphthols under the action of alkali permits some generalizations (low temperature at the beginning of reaction followed by heating, and the use of cosolvents that are unable to form stable adducts). This leads to an increase in the yield of this reaction, considered earlier (70MI1) as lacking preparative interest, similar to the formation of phenols from monocyclic pyrylium salts. Consider the example shown in Scheme 116. 

Benzene derivatives can also be obtained from pyrylium salts 43 which are substituted by R-CH2 in the 2- or 6-position. This is effected by C-2 addition of hydroxide ions, leading to a base-catalysed intramolecular aldol condensation. The reaction produces highly substituted phenols 44, which incorporate the R-CH2 carbon in the benzene ring ... 

Alkylcarboxonium (Alkoxycarbenium) Cations. The ionic structure of pyrylium salts was clearly stated by Hantzsch592 as early as 1922. In pyrylium salts 297, there is a contribution from carbocation structures, a fact apparent in the behavior toward strong nucleophiles leading to phenols. 

Table 1. Phenols 3 and 6 and nitrobenzenes 4 and 7 from 2,4,6-arylated pyrylium salts and nitromethyl-arenes... 

Table 2. Phenols and oxidation products from the reaction of pyrylium salts and nitromethane with 1 mole NR3 in unpolar solvents... 

In pyrylium salts there is contribution from carbocation structures, a fact apparent in the behavior toward strong nucleophiles leading to phenols. 

Pyrylium-3-olates, formally 3-hydroxy-pyryliums rendered overall neutral by loss of the phenolic proton, though this is not usually the method for their formation, undergo cycloadditions across the 2,6-positions and in so doing parallel the reactivity of pyridinium-3-olates (8.7). Even unactivated alkenes will cycloadd when tethered and thus the process is intramolecular. Usually, the pyrylium-3-olate is generated by elimination of acetic acid from a 6-acetoxy-2//-pyran-3(67/)-one (a pyranulose acetate ) (see 18.2). 

An interesting formation of highly substituted trihydroxybenzenes (36) and (37) together with the simpler phenol (38) involves the transfer of a methyl group from O to induced by a pyridinium halide, and the similarly congested 2,4,6-tris-(4-t-butylphenyl)-phenol (39) results from the pyrylium salt (40) (Scheme 6) formed by the reaction of 2 moles of the aldehyde Bu C6H4CHO with 1 of the ketone Bu CeH4COMe. 

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