Pyrylium

Dyes pyrylium also used for thiapyrylium and benzothiapyrylium. 

The normal pattern of coupling constants for aromatic six-membered rings is found in the heterocyclic aza systems, except that the ortho coupling to a proton a to a heterocyclic nitrogen is reduced from 7-8 Hz to 4.5-6 Hz. The J2.3 of pyrylium salts is still lower... 

The mass spectrum of 2-pyrone shows an abundant molecular ion and a very prominent ion due to loss of CO and formation of the furan radical cation. Loss of CO from 4-pyrone, on the other hand, is almost negligible, and the retro-Diels-Alder fragmentation pathway dominates. In alkyl-substituted 2-pyrones loss of CO is followed by loss of a hydrogen atom from the alkyl substituent and ring expansion of the resultant cation to the very stable pyrylium cation. Similar trends are observed with the benzo analogues of the pyrones, although in some cases both modes of fragmentation are observed. Thus, coumarins. 

Finally, a novel synthetic route involves formation of the pyridine ring from a fused pyran intermediate, e.g. (139) - (140) (70CB1250, 80JOC1918, 73JCS(P1)823). If a pyrylium salt is used, a quaternary pyridopyrimidinium salt such as (141) is formed 77KGS14S4). 

Because of the increased importance of inductive electron withdrawal, nucleophilic attack on uncharged azole rings generally occurs under milder conditions than those required for analogous reactions with pyridines or pyridones. Azolium rings are very easily attacked by nucleophilic reagents reactions similar to those of pyridinium and pyrylium compounds are known azolium rings open particularly readily. 

Another interesting reaction of the pyrylium salt (396) has been reported (73TL2195). With nitrous acid in alcohol, (396) gave an intermediate (402) which on heating in acetic acid gave the diacylisoxazole (403). The structure of (402) was determined by X-ray crystallography. These ring interconversions are shown in Scheme 96. 

Diazoalkanes add to the carbon-carbon double bonds of 2,3-diphenylthiirene 1-oxide and 1,1-dioxide. The adducts lose SO or SO2 to give pyrazoles and related compounds (Scheme 103) (80CB1632). Mesoionic oxazolones (75CLH53), 4-methyl-5-phenyl-l,2-dithiolene-3-thione (80JOU395) and pyrylium betaines (72JOC3838) react similarly via intermediate adducts (Scheme 104). Enamines (Scheme 96) and ynamines add to the double bond of 2,3-diarylthiirene 1,1-dioxides to give acyclic and cyclic sulfones by a thermal. 

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

Benzofuro[3,2-c]pyrroles reduction, 4, 1079 Benzofuro[3,2-6]pyrylium salts synthesis, 4, 995 Benzofuro[2,3-c][l, 2]thiazine... 

H-l-Benzopyran-4-ones — see Chromones 4H-l-Benzopyran-4-ones, 2-phenyl — see Flavones Benzopyranopyrazoles synthesis, S, 317, 341 Benzopyrano[4,3-c]pytazol-4-one synthesis, 3, 712 Benzopyrano[4,3-6]pyridine synthesis, 3, 712 Benzopyrano[4,3-ii]pyrimidine synthesis, 3, 712 Benzopyrans nomenclature, 1, 23 pyrylium salt synthesis from, 3, 873 reactions... 

Wolff rearrangement, 7, 193 pyrylium salt synthesis from, 3, 871... 

Naphtho[l,2-h]pyrylium salts, 5,6-dihydro-2,4-diphenyl-synthesis, 3, 869 Naphtho[2,l-h]pyrylium salts synthesis, 3, 866 Naphthoquinolines, tetrahydro-synthesis, 2, 469 N aphtho[ 1,2-7]quinolines Skraup synthesis, 2, 467... 

K-pyran synthesis from, 3, 761-762 pyridazine synthesis from, 3, 54 pyrylium salt synthesis from, 3, 872 reduced, 2, 45... 

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