Nitroaromatic compounds, functional groups

Corbett MD and Corbett BR, Bioorganic chemistry of the arylhydroxylamine and nitrosoarene functional groups, in Biodegradation of Nitroaromatic Compounds, Spain JC, Ed., Plenum Press, New York, 1995, 151. 

Plant biotransformation parallels liver biotransformation and is conceptually divided into three phases. Phase I typically consist of oxidative transformations in which polar functional groups such as OH, NH2, or SH are introduced. However, reductive reactions have been observed for certain nitroaromatic compounds. Phase II involves conjugation reactions that result in the formation of water soluble compounds such as glucosides, glutathiones, amino acids, and malonyl conjugates or water-insoluble compounds that are later incorporated or bound into cell wall biopolymers. In animals, these water-soluble Phase H metabolites would typically be excreted. In Phase III, these substances are compartmentalized in the plant vacuoles or cell walls. For additional details, the reader is referred to reviews on the subject by Komossa and Sandermann (1995), Pflugmacher and Sandermann (1998), and Burken (2003). Enzymatic conversion rates typically follow Michaelis-Menten kinetics and are temperature-dependent (Larsen et al., 2005 Yu et al., 2004,2005, 2007). 

Reductive conditions favor the hydroxylamine side of what can be considered an equilibrium reaction between the hydroxylamine and nitroso functional groups trapped between the arylamine and nitroaromatic oxidative extremes. This is why all the standard methods for the partial reduction of nitroaromatic compounds inevitably yield the hydroxylamine product rather than the nitroso product. For example, a common technique to accomplish such partial reduction is the use of zinc powder and ammonium chloride in hydroxylic solvents (19). Stronger reducing conditions, such as zinc powder in acidified hydroxylic solvents, result in the complete reduction of the nitro group to the amine. This is because the acidic conditions provide a catalyst for the N-0 bond breaking reaction. 

Certain nitroaromatic groups are susceptible to photochemical reactivity (43). A well-known nitro group degradation reaction occurs for the API nifedipine (Fig. 60). Under ultraviolet as well as visible radiation, the nitro group of nifedipine is rapidly converted to a nitroso compound along with aroma-tization to a substituted pyridine ring (99). In the presence of molecular oxygen, the nitroso functionality is re-oxidized to the nitro derivative (100). 

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