Pyrylium-3-olates

Substituted derivatives of the pyrylium-3-olates (65) have been isolated. These brightly colored solids are transformed to pale yellow or colorless valence tautomers (66) by ultraviolet (UV) radiation.The process (65 66) is reversed by visible light and such photochromic behavior is of... 

The pyrylium-3-olates (65) form salts under acidic conditions. The tri-... 

The most important pyran betaines are the heteroaromatic pyrylium-3-olate (258) and 2-benzopyrylium-4-olate (259) systems. Numerous analogues of both are known, and it has been shown that these have a high propensity to participate in cycloadditions (72JOC3838)>... 

Pyridinium-3-olates 33 (Z = NR, Y = O) are converted photochemically into the bicycle 34 (Z = NR, Y = O) corresponding pyrylium-3-olates and especially isochromylium-4-olates isomerize more easily (cf. 5556). 

Pyridinium-3-olates and pyrylium-3-olates react with a variety of monoenes, dienes, and trienes. Each of these systems react readily with dienophiles to yield cycloadducts of type 433. For Z = NMe, an electron-withdrawing X group is required in the dienophile, but with Z = 0 or A-(2-pyridyl) even unactivated alkenes react. 1-Phenylpyridi-nium-3-olate and benzyne give 434 dienes give adducts of type 435. Fulvenes behave as trienes to give adducts across the 2,6-positions 436. 2-Benzothiopyrylium-4-olate 437 gives a thermal dimer across the 1,3-positions. 

A heavily used sequence has the formation of a pyrylium-3-olate produced in situ from oxidation of a furan and thence to a 2-acetoxypyran-4-one, which loses acetate, as illustrated by the example forming 438 <2001TL4947>. 

The pyrylium-3-olates (65) form salts under acidic conditions. The triphenyl compound (65 R = R = R" = Ph, R = H) is oxidized by air to the butenolide 73, which is also formed when a solution of the same precursor is photolyzed (A = 3660 A) in the presence of a benzophenone sensitizer. This oxidative rearrangement (65 R = R = R" = Ph, R = H - 73) has been rationalized in terms of the triketone intermediate 71 cyclizing to the zwitterion 72 followed by rearrangement. ... 

Pyrylium-3-olates, formally 3-hydroxy-pyryliums rendered overall neutral by loss of the phenolic proton, though this is not usually the method for their formation, undergo cycloadditions across the 2,6-positions and in so doing parallel the reactivity of pyridinium-3-olates (8.7). Even unactivated alkenes will cycloadd when tethered and thus the process is intramolecular. Usually, the pyrylium-3-olate is generated by elimination of acetic acid from a 6-acetoxy-2//-pyran-3(67/)-one (a pyranulose acetate ) (see 18.2). 

Diazocarbonyl compounds containing a suitable ester function such as (38) undergo a sequential Rh-catalysed extrusion of nitrogen and intramolecular capture of the resulting pyrylium-3-olate leading to cyclohepta[l,2-ft]pyrans <97JPC205>. 

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