2-Pyridone photochemical dimerisation

Of a comparatively small range of photochemical reactions described for quinolines and isoquinolines, perhaps the most intriguing are some hetero-ring rearrangements of quaternary derivatives, which can be illustrated by the ring expansions of their A-oxides to 3,1-benzoxazepines. As with 2-pyridones, 2-quinolones undergo 2h-2 photo-dimerisation involving the C-3-C-4 double bond. ... 

Photoreactions of Pyridones - Irradiation of 1-benzyl-1,4-dihydronicotina-mide (161) with the malonate derivative (162) affords a variety of products resulting from debromination and dimerisation. The dihydropyridine derivatives (163) are photochemically reactive in the solid phase. The formation of the products by irradiation has been shown to be a two step process affording the (2+2)-cycloaddition product (164) in the first step. Secondary irradiation of (164) then gives the cage compounds (165) in yields greater than 90%. 

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