Doxyl radicals

Prochiral carbon radicals have enantiotopic faces reaction with chiral nitroxides can result in two possible diastereomeric products (Scheme 9). Our laboratory has been investigating the ability of chiral nitroxides to differentiate between the two enantiotopic faces of a transient prochiral carbon radical. In many of the examples, the prochiral radical is generated by the lead dioxide oxidation of a secondary benzylic hydrazine. Early work utilized a camphor-derived nitroxide 27, which was coupled to a secondary benzylic prochiral carbon radical with low but reproducible stereoselectivity (Scheme 10) [26]. The stereoselectivity jumped dramatically upon moving to a conformationally rigid nitroxide in the form of the steroid doxyl radical... 

The formation of l-hydroxy-3-imidazolines proceeds under mild weakly alkaline or weakly acidic conditions, in contrast to the strongly acidic media usually employed for obtaining precursors of doxyl radicals (Keana, 1978). l-Hydroxy-3-imidazolines are easily oxidized to nitroxides by molecular oxygen, Pb02, Mn02, etc. 

Figure 7 Plot of the change in the product of the coupling and maximum saturation factors as a function of macromolecular structure. At lower pH values, the spin-labelled lipids are present as vesicles and vesicular aggregates, while at higher pH values, micelles are formed. The higher psmax values for the micelles imply greater water accessibility to the radical site. The solid circles represent 16-DS (16-doxyl stearic acid, spin-labelled at the end of the lipid tail) while the open circles represent 5-DS (5-doxyl stearic acid, spin-labelled near the polar head group). Reproduced with permission from Ref. [70].

Thus other chiral nitroxides were developed to probe the stereoselective coupling reaction. Two conformationally rigid doxyl nitroxides prepared from camphene, camphoxyl nitroxides 29, were synthesized and coupled to several prochiral carbon radicals. In this series, the results were disappointing very low diastereoselectivities were obtained (Scheme 11) [27]. 

The dynamic lipid stmcture of membranes was studied by EPR - spectroscopy by use of computerized radio-spectrometer Bruker200D (Germany) and spin - probes two nitroxyl radicals 5- and 16-doxyl-stearic acids (5-and 16-DSA) from Sigma (USA). The radicals prepared in concentration... 

The sensitized cascade reported in Ref [16] consisting of triplet ds-trans photoisomerization of the excited stilbene includes the triplet sensitizer (erythrosin B), the photochrome stilbene derivative probe (4-dimethylamino-4 -aminostilbene), and nitroxide radicals (5-doxyl stearic acid) quenching the excited triplet state of the sensitizer (Figure 10.11). 

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