2-Methyl-5,6-naphtho- -pyrylium

The reaction of methoxybutenone with 2-naphthole (FeCF/HCl) involves the addition of the latter to the double bond (adduct 285), cyclocondensation to give hemiacetal 286, and the formation of 2-methyl-5,6-naphtho-(l,2)-pyrylium fer-rochlorate (287) (60MI1). 

The chlorovinyl immonium salts (654), obtained through Vilsmeier formylation of methyl ketones, behave as 1,3-dicarbonyl compounds. In the presence of strong base they react with methyl ketones to form dienones (655). On treatment with a mixture of perchloric and acetic acids the pyrylium salt results (Scheme 259) (71JPR1110). In a similar reaction 2-naphthol is converted into the naphtho[2,1 -b]pyrylium salt (656) (79S241). 

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