"Trading" a C[double bond, length as m-dash]C moiety for four C-O bonds and a peroxide: combining ozone and O-nucleophiles for peroxidative ring expansion of cycloalkenes into medium-sized 1,2-dioxacycloalkanes.

Despite the long and rich history of C[double bond, length as m-dash]C double bond ozonolysis, ozonolysis of cyclic alkenes has generally been limited to the formation of ring-opening products and fragmentary evidence of unstable ozonide formation. We present a non-orthodox cycloalkene ring expansion reaction involving ozone and O-nucleophiles, which results in the formation of medium-sized peroxycycles. The mechanism is governed by a delicate balance: the cyclic structure of the substrate dictates conformational restrictions in the generated carbonyl oxide intermediate that kinetically retard the rapid [3 + 2] cycloaddition between the carbonyl and carbonyl oxide groups, enabling competitive trapping of the carbonyl oxide intermediate by an external O-nucleophile. Some of the synthesized peroxides exhibit ring-chain tautomerism, existing in either a hydroperoxide or a cyclic peroxy hemiacetal form. The use of hydroperoxides as O-nucleophiles leads to compounds containing both cyclic and exocyclic peroxide fragments. Additionally, these unique bisperoxides demonstrate intriguing reactivity, undergoing a highly selective rearrangement under basic conditions, which affects only one of the peroxide fragments. The developed peroxidative ring expansion of cycloalkenes via ozonolysis broadens the synthetic potential of classical ozonolysis and provides new pathways for constructing structurally diverse peroxides and polyfunctionalized products of their transformation.