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We wish to report (1) that photoisomerization of pyridine to a Dewar pyridine, 2-azabicyclo[2.2.0]hexa-2,5-diene (I), occurs in the liquid phase at 2537 Á; (2) that photoreduction of pyridine occurs in aqueous sodium borohydride, yielding 2-azabicyclo[2.2.0]hex-5ene (II), and (3) that I is an intermediate in the formation of II, as well as in the photohydration2 of pyridine to 5-amino-2,4-pentadienal (III). The Dewar pyridine, which is the first valence isomer of pyridine or its derivatives to be found,3 reverts completely to pyridine within 15 min at room temperature, but fortunately has a high activation energy, 16 kcal mol-1, for rearomatization. The picolines and several lutidines also form thermally unstable photoisomers which are reduced by borohydride and hydrolyzed by water.

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