| Abstract | The search for nitric oxide cheletropic traps (NOCTs) of the 7,7,8,8-tetraalkyl-o-quinodimethane type which would have properties appropriate for monitoring the formation of nitric oxide in cell cultures and in vivo by magnetic resonance techniques is described. In addition to the necessary condition that a NOCT reacts rapidly with NO to yield a persistent nitroxide radical, two additional properties were sought: (i) thermal stability at the temperature of interest (37°C) and (ii) water solubility. To these ends, a number of 1,1,3,3-tetraalkyl-2-indanones (and a related naphthalene derivative) were synthesized and subjected to UV photolysis in solution, a procedure which generally (though not in all cases) caused the elimination of carbon monoxide and formation of the corresponding o-quinodimethane. The thermal instability of many of these compounds is due to a 1,5-sigmatropic hydrogen atom transfer which, for example, converts 7,7,8,8-tetramethyl-o-quinodimethane (1) to o-isopropyl-α-methylstyrene (1P) with a half-life of only ca. 140 s at 37°C. Several o-quinodimethanes were discovered which were, for all practical purposes, completely stable at 37°C. The most suitable lipid-soluble NOCT discovered was 7-(2-indenyl)-7,8,8-trimethyl-o-quinodimethane (5), which is stable and reacts very rapidly with NO to form a persistent nitroxide. Various derivatives of 5 were also examined and found to be equally, or almost equally, effective NOCTs. Water solubility was explored by addition of water-solubilizing groups to the ring of 1. The carboxylic acid group, 13, was found to be particularly suitable, since the carboxylate anion 14 conferred excellent water solubility without interfering with either the nitric oxide trapping reaction or the necessary photoelimination of carbon monoxide from the starting indanone. Of even greater importance, the carboxylate group had no apparent effect on the rate of the thermal 1,5-sigmatropic rearrangement; i.e., the rates of decay of 14 and 1 were equal within experimental error. It is concluded that NOCTs of the o-quinodimethane class having long lifetimes and a high reactivity toward NO can now be prepared with appropriate lipophilic, hydrophilic, or amphiphilic properties. These NOCTs should prove suitable for exploratory use in biological systems. |
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