From pyrylium salts

Azulene, 4,b,8-trimethyl-from pyrylium salts, 3, 660 Azulenes... 

Second, there exists a number of reactions starting from pyrylium salts and involving substitution at the pyrylium ring, or modification of existing substituents. These reactions will be described here briefly they will be discussed in more detail in the forthcoming second part of this review. 

Since 5-cyanodion-l-ones are difficultly accessible other than starting from pyrylium salts, this method has a theoretical rather than practical interest. 

Indolizines, aromatic heterocycles with 12-jr-elec-tron system salts, isomeric with indole and isoindole, are prepared in good yields from pyrylium salts with active a-methylene groups.215 216 Aromatics derived from pyrylium salts by substitution of one or two fi-CW group(s) with a heteroatom are also possible. 

Bipyrans can be prepared by quantitative cathodic reductions of the corresponding pyrylium and benzopyrylium salts. According to the nature of the substituents, dimerizations obtained from pyrylium salts take place at the C-2 or C-4 positions (Scheme 156) [274]. 

K. Dimrolh and K. H. Wolf, Aromatic Compounds from Pyrylium Salts, in Newer Methods of Preparative Organic Chemistry, voL 3,... 

The reactions in Methods A, B and C, which all start from pyrylium salts are analogous to the well-known conversions of 2.4.6-substituted pyrylium salts 27 with ammonia, primary amines, hydrogen sulfide or the anions of CH activated compounds to the corresponding heterocyclic or isocyclic aromatic systems The first step involves addition of the basic phosphine at C-2 (or C-6) to form 2S. Ring-opening, ring-closure and elimination of water are likely steps in the formation of the product 2. 

Nucleophilic addition at the 2-position of pyrylium salts (223) occurs readily under mild conditions and when ammonia or primary amines are used the subsequent ring-opening/ring-closure sequences give pyridines (224) and pyridinium salts (222), respectively (Section 3.2.1.6.4.iii). The process is most useful for the synthesis of 2,4,6-trisubstituted pyridine derivatives. Thiinium salts (226) are conveniently prepared from pyrylium salts (225) by treatment with sodium sulfide (Section 3.2.1.6.5), Thiinium salts (226) react with ammonia and amines similarly to their pyrylium analogues. 

The formation of furans from pyrylium salts under oxidative conditions has been mentioned in Section 2.23.2.2. Ring opening and recyclization of 4-chloromethyl-2,6-dimethyI-pyrylium perchlorate (83) by aqueous alkali and DMF leads to 2-methylfuran-4-ylacetone (84) (77CHE918). When 2,3,4,5-tetraphenylpyrylium perbromide is hydrolyzed to the dienone and then subjected to bromination-dehydrobromination, 2-benzoyl-3,4,5-triphenylfuran (85) is formed. 

Further details of the formation of chromenes from pyrylium salts can be found in Chapter 2.23. 

The facile nucleophilic displacement of a 4-methoxy group from pyrylium salts provides syntheses of 4-substituted pyrans. In the presence of triethylamine, 4-methoxypyrylium salts react with aromatic nitro compounds to give 4-benzylidene-4//-pyrans (73JOC2834). 

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. 

H-1,2- Diazepines acylation, 7, 602 nucleophilic reactions, 7, 603 protonation, 7, 601 from pyrylium salts, 3, 660 4-subs tituted synthesis, 7, 604 synthesis, 7, 598, 599 thermal reactions, 7, 600... 

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