ChemInform Abstract: SYNTHESIS OF 4‐ETHYNYL‐SUBSTITUTED PYRYLIUM SALTS

Alkynylcarbenium ions and related unsaturated cations

“…170 Later, other lithium acetylenide derivatives have also been involved in this reaction. 171 Three possible structures, 343 ± 345, have been proposed for the reaction intermediate; 170 based on the data of IR spectroscopy, the allene species 344 was preferred. The IR spectrum of salts 346 contains an absorption band at 2210 cm 71 typical of acetylene derivatives.…”

“…Later, this difference between the routes of nucleophilic addition to the pyrylium salt 342 was explained 173 by different conditions of metallation of phenylacetylene. In fact, Doney et al 172 used an equimolar amount of a standard solution of nbutyllithium, whereas other investigators 170,171 employed an excess of butyllithium, which then remained in the reaction mixture and acted as a base during the isomerisation of the compound 343a to 344a. 173 Study of dyes containing carbon ± carbon triple bonds have led to the synthesis of acetylenic analogues of triarylmethane dyes, which are able to acquire an alleno-quinoid structure and, hence, they exhibit interesting photochemical properties.…”

“…In the two latter cases [Eqns ( 5), ( 6)], the appearance of a carbenium centre results as well in the formation of a C:C triple bond. The oxidative dehydrogenation of ethynylpyran systems 171,181 [Eqn (7)] also belongs to this type of reactions.…”