To expand the presently extremely limited repertoire of known nitronyl nitroxide α‐amino acids, we report here the synthesis of a novel, conformationally constrained, quaternary, chiral residue, Aic(CN), and its subsequent conversion into a blue‐colored, imidazolinyl nitronyl nitroxide‐bearing α‐amino acid, Aic(NN). Deprotection and peptide coupling reactions were examined. The configurational assignment of Aic(NN) was achieved by X‐ray crystallographic analysis of an appropriate tripeptide. This latter investigation, accompanied by an IR absorption conformational analysis, strongly supports the view that the Aic(NN) residue is an efficient peptide turn former. Numerous spectroscopic and magnetic properties of its derivatives and the tripeptide are also described. Of particular relevance for future applications are its UV/Vis absorption, NMR, and EPR signatures.

A set of azetidinium ions substituted at the nitrogen atom either by a benzyl group or a benzhydryl group were synthesized to delineate the scope of their ring expansion into 2‐phenyl‐ or 2,2‐diaryl‐pyrrolidines through a Stevens rearrangement. Whereas the regioselectivity of the rearrangement is very high, the diastereoselectivity is low when 2,3‐disubstituted pyrrolidines are produced, except in one case. The major undesirable side‐reaction is a Hofmann elimination leading to ring cleavage.

The intrinsically blue-colored Ullman imidazolinyl nitronyl nitroxide (NN) mono-radicals have found various applications, in particular as spin probes and organic magnetic materials. Here, we present the solution-phase synthesis, extensive characterization, and conformational analysis of the first peptidomimetics with two pendant, chiral nitronyl nitroxide free radical units. Two (R)-Aic(NN) residues, where Aic(NN) is 2-amino-5-nitronylnitroxide-indan-2-carboxylic acid, have been inserted at positions i and i+3 of the pentapeptide Boc-(R)-Aic(NN)-(Ala)-(R)-Aic(NN)-Ala-OMe and the hexapeptide Boc-[Ala-(R)-Aic(NN)-Ala]-OMe as well. The two compounds were obtained in good yields and high purities. Thanks to a combination of several spectroscopic techniques (IR absorption, NMR, VCD, and EPR) we gained clear evidence that both compounds adopt a right-handed 3-helical conformation with both nitronyl nitroxide pendants positioned on the same side of the helix. This peptidomimetic/free radical system is a potentially excellent template for the preparation of a set of appropriate analogs with exciting applications in the area of host-guest organic chemistry, or to spectroscopically evaluate in-depth the intramolecular exchange interactions in this type of probe.

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