2,5-Di-tert-butyl-2,5-diethylpyrrolidine-1-oxyls: Where Is a Reasonable Limit of Sterical Loading for Higher Resistance to Reduction?

The pyrrolidine nitroxides with four bulky alkyl substituents adjacent to the N-O∙ group demonstrate very high resistance to reduction with biogenic antioxidants and enzymatic systems. This makes them valuable molecular tools for studying the structure and functions of biomolecules directly in a living cell and for functional EPR and NMR tomography in vivo. The first example of highly strained pyrrolidine nitroxides with both ethyl and tert -butyl groups at each of the α-carbon atoms of the nitroxide moiety with cis -configuration of the tert -butyl groups was prepared using a three-component domino reaction of tert -leucine and 2,2-dimethylpentan-3-one with dimethyl fumarate with subsequent conversion of the resulting strained pyrrolidine into 1-pyrroline-1-oxide and addition of EtLi. The nitroxide has demonstrated unexpectedly fast reduction with ascorbate, the rate constant k 2 = (2.0 ± 0.1) × 10 -3 M -1 s -1 . This effect was explained by destabilization of the planar nitroxide moiety due to repulsion with the two neighboring tert -butyl groups cis to each other.