Pyrones protonation

Protic solvents such as i-PrOH and t-BuOH favor the diastereoselectivity of the reaction of 3-hydroxy-2-pyrone with acrylates [49b]. Further examples of proton-promoted Diels-Alder reactions are reported in Section 4.8. 

Protonation of vinylogous 4-pyrones 41A yields a mixture of tautomers 4IB and 41C in a ratio that is influenced by the solvent polarity.13... 

Heterocyclic systems have played an important role in this historical development. In addition to pyridine and thiophene mentioned earlier, a third heterocyclic system with one heteroatom played a crucial part protonation and methylation of 4//-pyrone were found by J. N. Collie and T. Tickle in 1899 to occur at the exocyclic oxygen atom and not at the oxygen heteroatom, giving a first hint for the jr-electron sextet theory based on the these arguments.36 Therefore, F. Arndt, who proposed in 1924 a mesomeric structure for 4//-pyrone, should also be considered among the pioneers who contributed to the theory of the aromatic sextet.37 These ideas were later refined by Linus Pauling, whose valence bond theory (and the electronegativity, resonance and hybridization concepts) led to results similar to Hiickel s molecular orbital theory.38... 

Intermediates such as 224 resulting from the nudeophilic addition of C,H-acidic compounds to allenyl ketones such as 222 do not only yield simple addition products such as 225 by proton transfer (Scheme 7.34) [259]. If the C,H-acidic compound contains at least one carbonyl group, a ring dosure is also possible to give pyran derivatives such as 226. The reaction of a similar allenyl ketone with dimethyl mal-onate, methyl acetoacetate or methyl cyanoacetate leads to a-pyrones by an analogous route however, the yields are low (20-32%) [260], The formation of oxaphos-pholenes 229 from ketones 227 and trivalent phosphorus compounds 228 can similarly be explained by nucleophilic attack at the central carbon atom of the allene followed by a second attack of the oxygen atom of the ketone at the phosphorus atom [261, 262], Treatment of the allenic ester 230 with copper(I) chloride and tributyltin hydride in N-methylpyrrolidone (NMP) affords the cephalosporin derivative 232 [263], The authors postulated a Michael addition of copper(I) hydride to the electron-... 

It has been known since the turn of the century that amongst oxygen-containing compounds crystalline salts with protonic acids are most readily formed by 2,6-dimethyl-4-pyrone [190] (Collie and... 

Tickle, 1899). Which of the two oxygens was the site of proton addition became clear only when Hantzsch (1919) demonstrated the close optical analogy between the salts and methiodides of dimethyl-pyrone and pyridinium salts and argued that a benzene-like ring arises in the pyroxonium salts [191]. The discussion of the structure... 

Degani et al., 1968). A comparison of the basicity of these compounds with those of the corresponding pyridone analogues (2- and 4-quinolones and 9-acridones) is given in Table 4. The order of basicities for these compounds is NH > S > Se > O. They are all protonated on the carbonyl oxygen. Such is the resonance stabilization of the pyrone cations that they do not accept any further... 

Pyrones are also formed from D-fhreo-2,5-hexodiulose ( 5-keto-D-fructose ) (67) when it is heated in aqueous solution.89 Compounds 68 and 69 are formed in high yield, with 68 preponderating. Kojic acid (68) results from /3-eliminations in which the ring remains intact, but the formation of 5-hydroxymaltol (69) requires ring opening, because of the absence of a proton on C-2. 

Bromo-2-pyrones and 3-bromocoumarins give furan- and benzofuran-2-carboxylic acids by ring fission and subsequent closure, e.g. (228) — (229) (116) or (117) — (230). Pyrone rings are opened by aqueous acid in some cases, probably by successive protonation and attack of a water molecule, e.g. dehydroacetic acid (231) gives (233) which immediately forms (232) or (234) with HC1 or H2S04, respectively. 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

A dihydro-derivative (4) of strobiline (M+ 193) was also obtained. In the mass spectrometer, the pyrrolidine ring is opened to give fragments of m/z 165 and 164 (base peak), followed by a retro-Diels-Alder reaction to give fragments of m/z 137 and 136, respectively. The location of the two additional hydrogen atoms was deduced from the relative u.v. extinctions of (3) and (4), which is consistent with the dihydro-y-pyrone skeleton in the latter compound. This was confirmed by 3H n.m.r. spectra that showed no vinylic protons. 

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