Dewar pyrones

Woodward Katz Tetrahedron 1959, J, 70 Liu Schmidt Tetrahedron 1971, 27, 5289 Dewar Pyron, Ref. 891 Papadopoulos Jenner Tetrahedron Lett. 1982, 23, 1889 Houk Loncharich Blake Jorgensen J Am. Chem. Soc. 

The results described above have been criticized by Dewar and Pyron (145). These authors suggest that the above analysis requires that the effect of... 

Owing to Michael J. S. Dewar s brilliant experimental and theoretical contributions in the field of tropolone derivatives, it was believed that tropone and the pair of isomeric a- and y-pyrones had considerable aromaticity, with the zwitterionic resonance formula of the carbonyl playing the role of a Z-type atom. Soon, however, it was shown that the extent of aromaticity was very low, and it increased only after combining with electrophiles, for example on protonation or alkylation (such as the formation of methiodides) (69JA5286, 72T5299, 030BC3410). 

A Dewar isomer of unsubstituted pyridine was identified by Wilzbach, but it was very unstable.63 Other Dewar isomers of six-membered heterocycles, isolated and fairly stable, are substituted with perfluoroalkyl groups. However, 1,2-dihydroheterocyclic compounds such as a-pyrone or a-pyridone derivatives are easily photoisomerized to bicyclic Dewar isomers, which are useful for the synthesis of cyclobutadienes. The photochemistry of 1,2-dihydroheterocyclic compounds will be discussed, followed by those of fully aromatic compounds. 

The reverse reaction has also been observed Irradiation of the Dewar a-pyrone in an argon atmosphere affords cyclobutadiene which is dimerized to tricyclo-[4.2.0.02-5]octadiene upon heating or photolyzed to acetylene (127) 134 135). 

Irradiation of 4,6-dimethyl-a-pyrone immobilized in a guanidinium-sulfonate-calixarene gives rise to a 4,6-dimethyl-Dewar-p-lactone, a carboxyl zwitterion and 1,3-dimethylcyclobutadiene, both in the solid crystalline state and in aqueous solution. Diarylmethyl carbocations have been generated within the cavities of non-protic zeolites (LiY, NaY, KY, RbY, CsY, and NaX) via laser-mediated decarboxylation of diarylacetic acids. 

Dewar and Pyron analyzed the reaction in a similar way but with localization energies and 1,4-bislocalization energies given by the more accurate MO treatment mentioned earlier. Their plot is somewhat better than Fig. 5.43. 

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