1.3- Oxazin-6-ones, photochemical isomerization

Other ring contractions that give azapentalene derivatives include the rearrangement of a fused oxazine to a pyrazolo[l,5-a]indol-4-one with acetic anhydride,165 and an interesting photochemical contraction of a cyclic lactone, which produces an isomeric fused indolone by a mechanism thought to involve the first meta photo-Fries rearrangement [Eq. (19)].166 The product was used as an intermediate for the synthesis of mitomycin antibiotics (Section VI,A). 

Irradiation of pyran-2-one gives the ketene (46) reversibly. Similar reactions are known for aza and diaza analogues. Thus, l,3-oxazin-6-ones isomerize photochemically to ketene imines (47), and flash vacuum pyrolysis converts the oxadiazinone (48) reversibly into (49). 

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