It is observed that whereas singlet oxygen is required for an ene reaction with an alkene to give an allyic hydroperoxide, triplet oxygen will react similarly with an enol to give an alpha-hydroperoxy aldehyde. (Example: see Chen HJ et al., Tetrahedron 69 (2013), 1112-1114.)
The drawing below shows two steps in the final sequence of reactions leading to the antimalaria drug artemisinin, as described in the cited article. The enol is formed by opening of the hemiacetal, and before it can tautomerize to the aldehyde, it reacts with atmospheric oxygen (in the dark, so presumably triplet oxygen) to give the alpha-hydroperoxy aldehyde shown. That in turn spontaneously folds up to give artemisinin (not shown). A concerted reaction of the enol with triplet oxygen, if it is a variant of the ene reaction, would be spin-forbidden.