I'm studying the oxydation of guanine using singlet oxygen and one step of the reaction is not very clear to me.
My problem is with the first step: it is a 4+2 Diels Alder cycloaddition, and Fukui's rules suggest that this kind of reaction doesn't happen photochemically, only thermally.
However, since the reaction is between guanine and singlet oxygen, the oxygen has been photochemically excited, and I don't think it would happen thermally with oxygen at the ground state because otherwise we would have constant oxydation.
So what exactly is happening there?
Hope the question is clear enough, thanks in advance for your answers!