Pyrylium salts benzene derivatives

This procedure illustrates a general method for converting substituted pyrylium salts to nitrobenzene derivatives. The reaction has been the subject of several reviews. - The yields are generally high, and under these conditions only a single product is formed, in contrast to the nitration of 1,3,5-triphenyl-benzene. The preparation of 2,4,6-triphenylnitrobenzene from the corresponding pyrylium salt eliminates isomer separation problems, which are encountered when the direct nitration procedure is used. Also, labeled compounds can readily be prepared by this method. ... 

Table 4. Benzene derivatives from pyrylium salts and enamines33)...

Phosphorus replaces the oxygen of pyrylium salts in reactions with phenyl-phosphine or phosphine itself. The former reaction gives products whose structure has not been completely clarified,314 but in the presence of mineral acids the latter gives unambiguously derivatives of phosphorin (phospha-benzene) 315... 

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 ... 

Russian workers have worked out another approach to benzo-[bjselenophens starting from selenophen derivatives. Acylation of (514) gave (515), which upon treatment with perchloric acid gave the pyrylium salts (516). Upon treatment with secondary amines, (516) gave amino-substituted benzo[b]selenophens, e.g. (517). From the pyrylium salt (518), only (519) was obtained. On treatment of (518) with aqueous hydroxide, (520) was obtained. Other benzo[b]selenophens hydroxylated in the benzene ring have been prepared similarly. Through the condensation of... 

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