Pyrylium perchlorate, synthesis from

Synthesis from Pyrylium and Nitrllium Salts. Russian workers31,59 have found that acetonylthiophenes yield pyrylium salts when allowed to react with anhydrides in the presence of 70% perchloric acid, and that these salts are converted into thienopyridines by treatment with ethanolic ammonia. For example 4-acetonyl-2-methylthiophene and acetic anhydride gave 2,5,7-trimethylthieno[2,3-c)pyrylium perchlorate (33), from which 2,5,7-trimethylthieno[2,3-c]pyridine (34) was obtained. 

Glutaconaldehyde anion serves as an interesting intermediate for the synthesis of heterocyclic compounds. The parent pyrylium perchlorate has been prepared from glutaconaldehyde and 70% perchloric acid in ether at -55°.° The reaction of glutaconaldehyde anion with alkyl and aryl isothiocyanates and... 

Enamines are also potential ketones and accordingly have been used as the ketonic component in this type of pyrylium synthesis (63AG(E)394). The initial adducts, which form pyrylium salts on treatment with perchloric acid, are similar to those derived from secondary amines and pyrylium salts. The significant feature of this variant is that it gives access to pyrylium compounds which are unsubstituted at the 4- and 6-positions but which carry substituents at C-2 and C-5. More conventional enamines, such as 1-piperidinocyclohexene, afford bicyclic pyrylium salts (652), whilst polycyclic salts (653) result if cyclic chlorovinyl ketones are used. 

In view of the development of the synthesis of various aromatic and heterocyclic compounds from pyrylium salts (80T679), the preparation of the unsubstituted pyrylium salt from pyridine is noteworthy (Scheme 269) (53CB1327). The pyridine-sulfur trioxide complex undergoes ring opening to the sodium salt of pent-2-ene-1,5-dial. This salt cyclizes in perchloric acid via the red oxonium salt. 

Like the 2-amino-2//-pyrans, analogous 2-azido-2//-pyrans have been observed to be unstable intermediates in the known synthesis of oxa-zine derivatives from pyrylium salts and NaN3 see Section V.G.3. Only some sterically hindered pyrylium cations can generate the intermediates, and attempts have been made to rationalize these observations by quantum-chemical approaches (84T3539 84T3549). Exceptionally, 4-azido-4//-pyran 124 was isolated after the reaction of 4-methyl-2,3,5,6-tetraphenylpyrylium perchlorate with NaN3 (84T3539). 

Thiochromans and Thiochromenes. - Cyclization of /3-(phenylmercapto)-propyl phenyl ketones (228) with perchloric acid gave a mixture of products, the pyrylium salt (229) and the thiochroman (230 R = Me or Ph, R = Ph or Me) predominating. An improved procedure has been developed for the synthesis of 1-thiaphenalene (232) from the acid chloride (231). ... 

By 1916 Dilthey had synthesized 2,4,6-triphenylpyrylium ferrichlo-ride (tetrachloroferrate) from acetophenone and benzaldehyde in acetic anhydride in the presence of ferric chloride. He discovered other syntheses of pyrylium salts having aryl and alkyl substituents, as aryl groups enhanced the stability of pyrylium salts. Most often, the anions were perchlorate and tetrachloroferrate, but tetrafluoroborate began to be appreciated. A simple synthesis of alkyl-substituted pyrylium salts had to wait till the treatment of mesityl oxide with acetic anhydride was described by Schneider and Sack using sulfoacetic acid, and by Diels and Alder using perchloric acid. Then a lack of interest followed till the late 1950s, when Karl Dimroth and Klaus Hafner in Germany described new reactions of pyrylium salts, and new synthetic approaches were developed in Bucharest and in London. 

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