DOXYL nitroxides

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]. 

Nitroxides.—A new synthesis of several doxyl nitroxides from oxazolines has been described. ... 

The localization of a nitroxide in a membrane [30] depends upon the relative polarity of the nitroxide moiety and other polar groups that may be present in the molecules. If the nitroxide group is the only polar group, then the molecule will orient itself in the membrane so that the nitroxide is located in the aqueous phase. The cholestane spin label is one such example. If groups more polar than the nitroxide are present, then the nitroxide will be found within the bilayer. For example, doxyl-labeled fatty acids are oriented such that the carboxyl group is in the aqueous layer and the nitroxide is down within the membrane. 

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... 

Fig. 2 Chemical structures of common nitroxide spin probes Fremy s salt (potassium nitrosodi-sulfonate) TEMPO and derivatives (2,2,6,6-tetramethylpiperidine-l-oxyl), DOXYL (4,4-dimethyl-oxazolidine-l-oxyl) PROXYL (2,2,5,5-tetramethylpyrrolidine-l-oxyl) Dehydro-PROXYL (2,2,5,5-tetramethylpyrroline-l-oxyl) 5-DSA (5-DOXYL stearic acid) 16-DSA (16-DOXYL stearic...

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). 

Fig. 8. Structural models of large aggregates in ionomer solutions, and suggested locations for doxyl spin probes based on ESR results , probe head groups , nitroxide groups O, ionomer head groups.

J. F. W. Keana, E. M. Bernard, and R. B. Roman. Selective reduction of doxyl and proxyl nitroxide carboxylic acids to the corresponding alcohols with borane methyl srilphide. Synth. Comm., 1977, 8, 169. 

Another interesting and potentially important study showed that by addition of the lanthanide YbCh it is possible to align phospholipid bilayers magnetically, either perpendicular or parallel to the field, using a static magnetic field of 3.4 T. This was clearly shown at 95 GHz with the nitroxide spin probe 3P-doxyl-5a-cholestane, where excellent sensitivity and resolution were demonstrated. ... 

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. 

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