The conformation of spirocyclohexyl nitroxides is an important consideration: it has been proposed that such scaffolds exist as an equilibrium between open and closed conformations, where the former is preferred in solution (Figure ). Closed conformations are, however, expected to have higher stability in reducing environments due to more effective steric shielding of the nitroxide. Previous investigations have demonstrated that disubstituted fivemembered pyrrolidinyl nitroxides (e.g. 1, Figure ) adopt semiopen conformations, which causes a moderate improvement in reduction stability. Moreover, six-membered spirocyclic nitroxides can be forced to adopt semi-open conformations by introducing substituents on the cyclohexane rings (e.g. 2). However, spirocyclohexyl nitroxides have so far not been successfully induced to adopt fully closed conformations.