Pyrylium betaines

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. 

A more complex cycloaddition type is observed when diphenyl cyclopropenone and its thio analogue are reacted with the pyrylium betaine 451276 and the products obtained were assigned structures 450 and 452, respectively. 

The structure of the pyran-2-one, 165, is closely related to that of the pyrylium salt (Equation 13). One of the resonance structures of pyran-2-ones is the pyrylium betaine, 166. As might be expected, pyran-2-one shows some aromaticity, which is manifested in some of its reactions however, it also displays many of the reactivity patterns associated with 1,3-dienes and lactones, depending on the conditions it is subjected to. 

Pyrylium betaines. A soln. of 2-(a-acetopropyl)-3, 4-dimethoxy-4, 5-diacetoxy-benzophenone in glacial acetic acid heated 20 hrs. at 240° in a sealed tube 1,6-anhydro-l- (3-methoxy-4-hydroxyphenyl) -3-methyl -4-ethyl -6,7-dihydroxyiso-benzopyranol. Y 54%. A. Muller and M. Lempert-Sr ter, M. 96, 369 (1965). 

Cycloadditions and Rearrangements. The addition of 2-oxyallyl cations to furan provides a route to oxabicyclo[3,2,l]octanes complementary to the cycloaddition of cyclopropanones to furan. Careful experimental studies have led to yields of preparative importance both with furan and with cyclopentadiene. Following the route to azabicyclo-octanes, dipolar addition to the pyrylium betaine (99) affords oxa-analogues (Scheme 23). Also reported are the addition of fiiran to 1-cyanonaphthalene, the formation of various cycloadducts of tropone and tropolone (Scheme 24), and the phototransformations of (100) (Scheme 25) and (101) (Scheme 26). Thermal addition gives (102) from (103) and similarly other 8-oxabicyclo-octanes are prepared from acyclic precursors. ... 

The Dimroth-Reichardt betaines are synthesized by reactions of p-aminophenols with pyrylium salts bearing aryl substituents in ortho position. 

Pyrylium 3-oxides 54 (Scheme 25) should be considered as heteroaromatic six-membered mesomeric betaines. A diketo-oxepine 44 (Scheme 26) has two equivalent identically polarized limiting structures... 

The betaines 297 are easily prepared from a pyrylium salt and the appropriate 4-aminophenol followed by deprotonation. Typically, 2,4,6-triphenyl-pyridinium iodide and 4-aminophenol give the blue-black betaine (297 R = R = Ph, R = H) which also occurs as a red hexahydrate. With methyl iodide this compound gives 7V-(p-methoxyphenyl)-2,4,6-triphenyl-pyridinium iodide. 10,221... 

These compounds are usually written in the uncharged form (788, 789 Z = NH, NR, O, S), but canonical forms of types (790) or (791) are of comparable importance, i.e. the compounds can also be considered as betaines derived from pyridinium, pyrylium and thiinium cations. They possess considerable stability and aromaticity in that in many of their reactions they revert to type . 

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)>... 

The photostationary state concentration of 15 depends on the wavelength used for excitation, but a sufficient concentration could not be obtained to allow isolation of 15 (62JA1315). Besides, the formation of betaines 15 is also complicated by their interaction with the initial epoxyinda-nones 14 (Section III,E,2). The high reactivity of benzo[c]pyrylium-4-oxides as dipolarophiles (Section III,E,2) initiated the search for new approaches to their synthesis, one of which is the intramolecular carbenecarbonyl reaction which yields the unstable l-methoxy-4-oxide 16 (81BCJ240). 

Examples are known for deprotonation of hydroxyl groups attached either to the 2-benzopyrylium cation itself or to its subsituents. In both cases, the stability of the betaine thus formed is determined by the position of the deprotonated group. Thus, unstable betaine 15, obtained as described in Section II, A, may be formally considered as products of deprotonation of the nonisolated 4-hydroxybenzo[c]pyrylium salts (84CC702 86NKK1622). A valence-isomeric structure 14 is possible, and photochemical processes were discussed in Section II,A. 

Deprotonation of 3-hydroxy-substituted benzo[c]pyrylium salts 239 leads to betaines 240a having an ori/io-quinonoid structure 240b as a resonance form (84TL3659). 

Calculations of the structure of the mesoionic thiopyrylium-3-olate 74 suggest that the C-S bond lengths are similar to those in the pyrylium cation at ca. 168 pm, perhaps supporting the fully charge-separated betaine structure. However, the charge at oxygen is closer to 0.5 than the 1.0 expected for such a structure. Furthermore, the nucleus-independent chemical shift value is appreciably lower than that for the thiopyrylium cation. These data point toward an ylidic structure with an acceptor moiety rather than an aromatic cation and an exocyclic oxyanion <2002IJQ(90)1055>. 

H-Pyran-2-ones show characteristic IR-absorption in the region of 1730 cm . When this value is compared to the C=0 absorption of other six-membered lactones (3-5), especially to those influenced by C=C double bonds adjacent to the lactone moiety (4/5), the conclusion seems to be justified that a pyrylium-olate betaine structure 2 does not play a significant role in the electron distribution of the 2H-pyran-2-one system. 

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