Another Photochromism of a Spironaphtoxazine Due to the Geometric Isomerization of the Open-Form

The photodecoloration rate of colored spironaphtho[1,2-b]oxazine 2 was much slower than the corresponding [2,1-b]oxazine 1. Existence of another photochromic system between two of the open-form isomers of 2, in addition to the well-known one for 1 between the closed and the open forms, was proved.