Furans pyrylium salts

Organic chemistry is based on carbon, but nitrogen is fundamental to heterocyclic chemistry. Although there are many important aromatic heterocycles without nitrogen atoms (thiophene, furan, pyrylium salts, etc.), it is clear that the majority of heterocyclic systems contain nitrogen atoms. Thus, NMR spectroscopy ( " N NMR yields the same chemical shifts... 

A general method for the attachment of a pyridine ring to a furan nucleus consists of the reaction of appropriate pyrylium salts (e.g. 55) with ammonia (Scheme 11) (73CHE416). Pyrylium and hence pyridine derivatives of benzofuran may also be prepared in this way. 

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. 

Pyrylium salts are useful precursors to other heterocyclic compounds such as furanes and pyridines. The perchlorates usually used are explosive, and tetrafluoro-borates are formed only in low yield. Sulfoacetates are nonexplosive if necessary for further synthetic uses, they can be converted into perchlorates.1... 

Bcnzo[/)]furan and derivatives, recent advances in chemistry of, Part 1, occurrence and synthesis, 18, 337 Benzo[c]furans, 26, 135 recent advances in the chemistry of, and related compounds, 73, 1 Benzofuroxans, 10, 1 29, 251 2//-l-Benzopyrans (chrom-3-enes), 18, 159 Bcnzofc] pyrylium salts syntheses, reactions, and physical properties, 50, 157 1,2-and 2,1-Benzothiazines and related compounds, 28, 73... 

Pyrylium salts are also suitable for the preparation of furans.7Z,7S Oxidation of these salts by aqueous hydrogen peroxide in the presence of perchloric acid leads to ring contraction, with the formation of furan derivatives. Nenitzescu and Balaban described the following example72 ... 

Oxidation of 5-hydroxymethylfuraldehyde 38 with hydrogen peroxide catalyzed by chloroper-oxidase, a hemeperoxidase from Caldariomyces fumago, proceeds with good selectivity to furnish 306 [238] (O Scheme 61). The 6-aminodeoxyglycal derivative 307 is similarly converted into the furan 308 [239]. The dithiane 309 gives the oxacyclohexadi-ene 310, on acid treatment [240]. Treatment of 311 with TMSOTf produces the pyrylium salt 312 [241]. 

Applications of di- and tri-phenylpyrylium salts continue to be exploited in the synthesis of pyridines, thiocarbonates, thiocyanates, " nitriles, and NN -diaryl-carbodi-imides. Conversion of pyrylium salts into 2-acyl-furans and 2-pyrones by sulphuric acid is dependent on the concentration of the acid. ... 

The oxidation of pyrylium salts with hydrogen peroxide gives furans (Equation (44)) <690PP63>. 

Condensation of benzo[fr]furan-3(2H)-one with 1 in acetic acid in the presence of triethylamine afforded 2-(3-benzo[fr]furyl)dimedone 481, which upon acylation with acid anhydrides in the presence of 70% perchloric acid led to the corresponding tetrahydrobenzo[t>]furo[2,3-cJ-pyrylium perchlorate 482 (94CHE283). Similarly, benzo[fr]thieno[2,3-c]-pyrylium perchlorate 484 (94CHE283) and benzo[t>]seleno[2,3-cJpyrylium salt 485 were prepared (94CHE283). 

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