Oxidation of guanine with singlet oxygen

I'm studying the oxidation 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 oxidation.

So what exactly is happening there?

You're right that ground state $\ce{^3O2}$ doesn't undergo this reaction.
$$\ce{TPP ->[h\nu] ^1TPP^*}$$ $$\ce{^1TPP^* ->[ISC] ^3TPP^*}$$ $$\ce{^3TPP^* + ^3O2->[sens] TPP + ^1O2}$$