Pyrones photodimerization

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. 

The compound (198), typical of 1,4-photodimerization anti products, was obtained from 2-aminopyridine in aqueous hydrochloric acid.285 4,6-Dimethyl-2-pyrone gives a mixture of similar 1,4-adducts (syn and anti) and the unsymmetrical 1,2-dimerization product (199).286... 

When excited by ultraviolet radiation. Form I undergoes a stereo-specific photodimerization to produce a head-to-tail dimer across the pyrone double bond. However, an examination of the structure reveals that considerable molecular motion must take place before the photoreaction can take place. In fact, the reaction might not proceed at all if... 

Yates P, Jorgenson MJ, Singh P. Photodimeric cage compounds. IV. Transformations of the cyclobutenes formed on degradation of the photodimers of 2,6-dimethyl- and 2,6-diethyl-4-pyrone. J. Am. Chem. Soc. 1969 91 4739-4748. 

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