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I'm studying the oxydation of guanine using singlet oxygen and one step of the reaction is not very clear to me.

Oxydation of guanine

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!

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You're right that ground state $\ce{^3O2}$ doesn't undergo this reaction.

There is no direct excitation of oxygen either.

The key in these processes is the electronic excitation of tetraphenyl porphyrine (TPP). The excited singlet state of the dye undergoes intersystem crossing to a triplet state. The latter interacts with ground state oxygen and generates singlet oxygen.

$$\ce{TPP ->[h\nu] ^1TPP^*}$$ $$\ce{^1TPP^* ->[ISC] ^3TPP^*}$$ $$\ce{^3TPP^* + ^3O2->[sens] TPP + ^1O2}$$

Singlet oxygen then acts like the typical dienophile in a Diels-Alder reaction.

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