2-Pyrones, substituted, formation

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. 

As noted above, formation of a furan [4 + 3]-cycloadduct during irradiation of a 4-pyrone was advanced as evidence for the zwitterionic intermediate. This process can be moderately efficient (equation 4)68, and can be envisioned as an approach to substituted cyclooctanoids. Besides the formation of three new carbon-carbon bonds, an additional attractive feature is the complete diastereoselectivity, arising from a compact [4 + 3]-cycloaddition transition state with approach from the face opposite the epoxide. However, the generality of the intermolecular reaction is limited, as competing [2 + 21-photodimerization, solvent trapping and rearrangement often predominate58. 

When the initial vinylcarbene complex is substituted with a second me-thoxycarbonyl group (complex 169), a different reactivity pattern is observed. Addition of methyldiphenylphosphine or dimethylphenylphosphine to 169 results in formation of the expected vinylketene complex 170. However, the analogous reaction with triphenylphosphine yielded a complex mixture at room temperature, and upon heating the simple ligand-substituted product 171 is formed. When 169 is reacted with carbon monoxide, the pyrone complex 172 is formed. Finally, reaction of the vinylketene... 

The same product is obtained from 3-phenylchehdamic acid, the pyrone oxygen of which is already substituted by imino-nitrogen, only three molecules of biguanide being required for the formation of the final product (CXLVII). Similar reactions occur (464) with the parent chelidonic acid and with y-pyrone. 

When the pyrone 297 was exposed to base in dichloromethane, formation of the oxidopyrylium occurs and subsequent cycloaddition transpires. Lee found that when the allene tether was 3 (n = 0), cycloaddition occurs to form the bicy-clo[5.3.0]decadiene 298 exclusively, rather than the alternative bicyclo[5.2.0] nonadiene product 299. Increasing the tether length by one resulted in formation of the exo-substituted double bond (299) in 45% yield. Increasing the tether length by one carbon, to four, completely retarded the cycloaddition (Scheme 4.83). 

Recently, the reaction of masked ortho-benzoquinone [92] with C60 was tested [93]. The [4+2] cycloaddition reaction of such electron-deficient dienes with fullerenes resulted in the formation of highly functionalized bicyclo [2.2.2] octenone-fused fullerenes. The reactants were generated in situ by the oxidation of the readily available 2-methoxy phenols with hypervalent iodine agents. For the several different masked ortho-benzoquinones that were tested, it was found that the yield of the cycloadducts depends on the nature of the starting materials and the reaction conditions. Other Diels-Alder reactions of such electron-deficient dienes with electron-poor fullerenes involved tropones [94], 1,3-butadienes substituted with electron-withdrawing groups [95], and 2-pyrone [96]. 

If the diene fragment of the 2-pyrone and the acetylene dienophile are unsymmetrically substituted, the formation of two isomeric benzene products is possible. The model reaction between 4,5,6-triphenyl-2-pyrone (20) (XXIII) and phenylacetylene yields approximately equal amounts of 1, 2, 3, 4-(XXV) and 1,2,3,5-tetraphenylbenzene (XXVI). Therefore, this position isomerism would be expected to materialize during polymer formation when similar bispyrone monomers are polymerized with diethylnylbenzene. 

Various substituted cyclobutanes and cyclobutenes can be utilized in the synthesis of 2-pyrones . For example, 2-hydroxyl cyclobutylidenes 230 undergo base-promoted rearrangements to afford 2//-pyran-2-ones 231 via formation and ring closure of an intermediate 1,5-keto ester (Scheme 131) <2005TL8237>. 

Evidence for the preferential formation of the rraRS-substituted product of selenium dioxide allylic oxidation is seen in the synthesis of part of (13Z)-retinoic acid (equation 48). Reaction took place exclusively at the exocyclic double bond without rearrangement. Allylic oxidation of this nature has also been used in the synthesis of 6-conjugated 2-pyrones (equation 49). This intermediate was employed in the total synthesis of natural pyrones such as yangonin. 

Gogte synthesis. Formation of a-pyrone derivatives by rearrangement of acyl-substituted gluta-conic anhydrides. 

Reactions of substituted bis(3-aIkoxybenzoyl) peroxides in neat phenols afford mainly 8-alkoxy-677-dibenzo[fc, d]pyran-6-ones and orf/zo-benzoyloxylation products of the phenol. For example, bis(3,4-dimethoxybenzoyl) peroxide (561) in neat p-methylphenol was completely decomposed in 1 h at 60 °C with the formation of a dibenzo-a-pyrone derivative 562 (60%) together with an ortho-ortho coupled product 563 (21%) and benzoate 564 (5%). In contrast, dibenzoyl peroxides having no mefa-electron-releasing substituents gave mainly ort/zo-benzoyloxyphenols. For example, decomposition of bis(4-methoxybenzoyl)... 

Formation of N-substituted 4-amino-2-pyrones has been observed in reactions of ketene with some of its derivatives (64JOC2513). 

Pyrones also add Grignard nucleophiles at the carbonyl carbon, C-4 dehydration of the inunediate tertiary alcohol product with mineral acid provides an important route to 4-mono-substituted pyrylium salts." More vigorous conditions lead to the reaction of both 2- and 4-pyrones with two mole equivalents of organometallic reagent and the formation of 2,2-disubstituted-2H- and 4,4-disubstituted-4//-pyrans, respectively." Perhaps surprisingly, hydride (lithium aluminium hydride) addition to 4,6-dimethyl-2-pyrone takes place, in contrast, at C-b." ... 

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