Abstract
Sterically shielded nitroxides of the pyrrolidine series have shown the highest resistance to reduction. Here we report the synthesis of new pyrrolidine nitroxides from 5,5-dialkyl-1-pyrroline N-oxides via the introduction of a pent-4-enyl group to the nitrone carbon followed by an intramolecular 1,3-dipolar cycloaddition reaction and isoxazolidine ring opening. The kinetics of reduction of the new nitroxides with ascorbate were studied and compared to those of previously published (1S,2R,3′S,4′S,5′S,2″R)-dispiro[(2-hydroxymethyl)cyclopentan-1,2′-(3′,4′-di-tert-butoxy)pyrrolidine-5′,1″-(2″-hydroxymethyl)cyclopentane]-1′-oxyl (1).
Highlights
Shielded nitroxides are currently attracting much attention due to their high resistance to bioreduction [1,2]
Introduction of spirocyclic moieties has a smaller effect on the reduction rates of nitroxides than the introduction of linear alkyl substituents does; spirocyclic nitroxides may have much longer spin relaxation times at 70–125 K [3] and even at room temperature [4]
We recently reported the synthesis of sterically shielded pyrrolidine nitroxide 1 via a stereospecific consecutive assembly of two spiro-(2-hydroxymethyl)cyclopentane moieties
Summary
Shielded nitroxides are currently attracting much attention due to their high resistance to bioreduction [1,2]. We have previously reported that oxidation of secondary amines with a spiro(2-hydroxymethyl)cyclopentane moiety at the α-carbon with the H2O2/WO42− system is ineffective whereas conversion of these amines to the corresponding nitroxides can be performed using m-chloroperbenzoic acid (m-CPBA) [6,15]. Signal at 2.26 ppm in the 1H NMR spectrum to the methine proton on the C(9) carbon atom (see Supporting Information File 1).
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