A DEER (double electron–electron resonance) spin–echo technique was applied to measure the electron–electron dipolar spectrum of a frozen toluene solution of the biradical, 2,6‐bis[(((2,2,5, 5‐tetramethyl‐1‐oxypyrrolin‐3‐yl)carbonyl)oxy)]‐anthracene. Modulation of the DEER spin–echo envelope was observed and identified as originating from the dipolar coupling between the two nitroxide spins of the biradical. Fourier transformation of the modulated components of the echo envelope yielded a dipolar spectrum from which a spin–pair separation of 19.73±0.14 A was calculated. Constraints on the relative orientation of the two nitroxide spin moieties were obtained by analysis of the effect of the microwave pulse orientational selectivity on the DEER modulation amplitudes. Molecular models of the studied compound exhibit structures that correspond well with the structural information deduced by DEER spectroscopy.

Double electron–electron resonance spin–echo modulation: Spectroscopic measurement of electron spin pair separations in orientationally disordered solids

A new antibiotic, CJ-17,665 (I) was isolated from the fermentation broth of Aspergillus ochraceus, CL41582. It inhibits growth of multi-drug resistant Staphylococcus aureus, Streptococcus pyogenes, and Enterococcus faecalis, with MICs of 12.5, 12.5 and 25 microg/ml, respectively. The structure contains a diketopiperazine and an indole N-oxide moiety that is unusual in natural products.

A new antibiotic CJ-17,665 from Aspergillus ochraceus.

Kinetics and mechanism for acid-catalyzed disproportionation of 2,2,6,6-tetramethylpiperidine-1-oxyl

3.02 – Imidazoles

Interactions between unpaired electrons may be examined via a variety of physical techniques. Of particular interest in biological systems are the interactions observable by electron paramagnetic resonance (EPR) between various pairs or clusters of paramagnetic transition metals and/or organic free radicals. These paramagnetic centers may be natural components of the biological system or they may be species added as probes or labels. Interactions between these centers may affect the EPR spectra enough to invalidate the interpretations that would be valid in the absence of interaction. For example, if a paramagnetic metal is present, a spin-labeled biomolecule may yield an EPR spectrum that is not validly interpreted in terms of changes in the motion of the label. If, however, the effect of the metal on the nitroxyl EPR spectrum were understood in detail, additional information could be obtained from the EPR spectrum. An assessment was made several years ago of the effect of the dipolar interaction between a metal and a nitroxyl radical on the EPR line shape of the nitroxyl in a rigid matrix (Leigh, 1970). This approach has been used by many investigators to obtain metrical information on spin-labeled biomolecules. Others have studied the metal-organic radical interactions in vitamin B12-dependent enzyme reactions also seeking metrical information (Pilbrow, 1982). Examples are considered in Section 10.

Resolved Electron-Electron Spin-Spin Splittings in EPR Spectra

Nitroxyl radicals were synthesized with a rigid system of conjugated bonds, containing both imidazoline and imidazoline oxide, as well as imidazoline and imidazoline oxide radical fragments. The EPR method was used to study the characteristic features of intramolecular electronic spin exchange in nitroxyl biradicals of the imidazoline series with a rigid system of conjugated multiple bonds. It was shown that the introduction of the N-oxide group into the imidazoline radical fragment leads to a considerable increase (tens of times) in the value of the exchange integral, characterizing the electronic spin exchange over the system of conjugated bonds.

Influence of the N-oxide group in biradicals of the imidazoline series on the intramolecular spin exchange over systems of conjugated bonds

1.

Reaction of 2-acyl-1-hydroxy-3-imidazoline 3-oxides with monosubstituted hydrazines leads to monosubstituted hydrazones, derivatives of 2-acyl-1-hydroxy-3-imidazoline 3-oxide, which on oxidation give stable nitroxyl radicals of the imidazoline series.




2.

In the reaction between 2-acyl-3-imidazoline 3-oxides and hydrazine, an intramolecular cyclization takes place with the formation of a new heterocyclic system, 2,3,6,8-tetraazabicyclo[3.2.1]oct-3-ene.




3.

The oxidation of 6,8-dihydroxy-4,5,7,7-tetraalkyl-1-R-2,3,6,8-tetraazabicyclo[3.2.1]oct-3-enes leads to bicyclic nitroxyl radicals, 8-hydroxy-4,5,7,7-tetraalkyl-1-R-2,3,6,8-tetraazabicyclo[3.2.1]oct-3-en-6 oxyls, which on oxidation give 1,2,4-triazine 4-oxides.

Formation of 2,3,6,8-tetraazabicyclo[3.2.1]oct-3-ene derivatives in reaction between 2-acyl-3-imidazoline 3-oxides and hydrazine and their oxidation to the corresponding nitroxyl radicals and 1,2,4-triazine 4-oxide

1.

The use of the Raman spectroscopy method makes it possible to unequivocally determine the position of the band of the stretching vibrations of the

group in the spectra of cyclic nitroxyl radicals.




2.

In the vibrational spectra of five-membered cyclic nitroxyl radicals the band of the stretching vibrations of the

group is found in the 1433–1443 cm−1 region. The position of the ΝN\(\mathop - \limits^ \bullet \)O band in the spectra of cyclic nitroxyl radicals depends on the size of the ring.

Use of Raman spectroscopy to identify nitroxyl group of cyclic nitroxyl radicals

The position of the tautomeric equilibrium in amino-, hydroxy, and mercaptonitrones was determined by the 13C NMR method for the case of the derivatives of 3-imidazoline-3-oxide. It was shown that the tautomeric equilibrium in the OH and SH derivatives is shifted toward the oxo and thioxo forms (∼95%). The chemical shifts of the carbon atoms of the nitrone group in the α-N-, α-O-, and α-S-substituted nitrones lie in the region of 137–150 ppm.

NMR spectra of cyclic nitrones. 5. 13C NMR spectra of the potential tautomeric systems of amino-, hydroxy-, and mercaptonitrones in the series of 3-imidazoline 3-oxide

G. I. Shchukin

Papers

Influence of the N-oxide group in biradicals of the imidazoline series on the intramolecular spin exchange over systems of conjugated bonds

Formation of 2,3,6,8-tetraazabicyclo[3.2.1]oct-3-ene derivatives in reaction between 2-acyl-3-imidazoline 3-oxides and hydrazine and their oxidation to the corresponding nitroxyl radicals and 1,2,4-triazine 4-oxide

Use of Raman spectroscopy to identify nitroxyl group of cyclic nitroxyl radicals

NMR spectra of cyclic nitrones. 5. 13C NMR spectra of the potential tautomeric systems of amino-, hydroxy-, and mercaptonitrones in the series of 3-imidazoline 3-oxide

Preparation of α-aminonitrones from 3-imidazoline-3-oxide α-methoxynitrones