Pyrones formation from furans

There is considerable evidence for the formation of the bicycHc oxyaUyl zwitterion prior to the epoxy-cyclopentenone. Pavlik and Barltrop showed that irradiation in trifluoroethanol (TFE) or methanol furnished solvent adducts as primary photoproducts, and Barltrop and co-workers excluded the possible formation of the TFE adduct 6e from epoxycyclopentenone 3e. Barltrop demonstrated that irradiation of 3,5-dimethyl-4-pyrone le in furan/trifluoroethanol mixtures furnished the [4-l-3]-cycloadduct 9e as one of the major photoproducts (Scheme 3). A similar result was reported by West and co-workers using tetramethylpyrone Id. Interestingly, in both cases, a single diastereomer was observed, corresponding to approach of furan from the face opposite the epoxide oxygen and reaction via an endo... 

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. 

Psoralens, also known as furocoumarins, are naturally occurring or synthetic tricyclic aromatic compounds and are derived from the condensation of a coumarin nucleus with a furan ring [1-10]. Several new furocoumarins have been isolated from natural sources [10, llj. In addition, the synthetic methods for production of these molecules have been described and reviewed [10]. The synthetic methods can be organized on the basis of the key step used for the formation of the two different oxygenated rings, hi this respect, there are three possibilities (i) formation of the furan ring onto the coumarin, (ii) formation of the pyrone ring onto the benzofuran and... 

Most of the synthetic applications of retro-Diels-Alder reactions involve cyclopentadiene or furan derivatives, but there are other dienes that have appropriate functionality for this process. One example is illustrated by heating pyrone 102 in a sealed tube at 200°C with bis(trimethylsilyl)acetylene to give 103, after initial formation of cycloadduct 104 and loss of carbon dioxide via a retro-Diels-Alder reaction.The retro-Diels-Alder usually requires higher temperatures than the Diels-AIder reaction, and the normal Diels-AIder product can be obtained without competition from the retro reaction. When the retro Diels-AIder reaction is desired, flash vacuum pyrolysis is a common technique used in synthesis. " Retro-Diels-Alder reactions can also be catalyzed by Lewis acids (sec. 11.6.A).An example that uses furan as a retro-Diels-Alder synthon is taken from work by Cannone et al., who used the retro-reaction as a synthetic route to 4-substituted... 

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