Oxidation structural motifs

The aminoxyl radicals lend themselves to synthetically interesting procedures of oxidation, both in the radical form itself and in the oxoammonium form (from TEMPO). Major advantages appear to be the mild operating conditions, the range of substrates susceptible to transformation and the selectivity in the oxidation of specific structural motifs. 

Studies of the effect of a-helix inducing solvents such as trifluoroethanol (TFE) on the product distribution of oxidative folding of endothelin-1 (6) (0.01 mM peptide concentration in 80% TFE, pH 9.5) clearly revealed that induction of the a-helical conformation favors the onset of the cystine-stabilized a-helix structural motif with concomitant enhancement of isomer 1 (88 12 vs 75 25) (Table 1)J491 A similar enhancement of the correctly folded 6 by the addition of TFE has also been observed under other conditions. [5°1... 

Early proposals suggested [72] that catalase contains a p-oxo-bis(p-carboxylato)-dimanganese core. The UV-Vis spectra of this structural motif present in model complexes exhibit 480-520 nm d-d absorptions [73] similar to the UV-Vis absorption bands of manganese catalases. The EPR studies of oxidized T. ther-mophilus catalase [74] also suggested a MnIIIMnI" p-oxo-bis(p-carboxy 1 ato) core as a possible structural motif for the active site. 

The question of structural motif aside, these complexes are of interest owing to the changes in physical properties that occur on partial oxidation. The report by Edelman1711 that Ni(dpg)2 could be oxidized by I2 or Br2 to afford Ni(dpg)2X (X = Br, I) appears to be the first in what is now an extensive group of reports on the oxidation of the bis dionedioximates) of the Ni triad as well as of planar metallomacrocycles in general. Edelman logically guessed that Ni(dpg)2X was a compound of Nim, a relatively unusual,... 

Many natural products display structural motifs biosynthetically derived from ortho-quinol precursors, and some even feature ortho-quinol moieties in their final structural arrangement [1, 6]. Asatone (7) and related neolignans can be put forward as classic examples of complex natural products derived from cyclodimerization of oxidatively activated simple phenol precursors (Figure 5) biomimetic syntheses of 7 have accordingly been accomplished by anodic oxidation (Section 15.2.1) and by Pelter oxidation (Section 15.2.2) of the naturally occurring phenol 9 [34, 36]. 

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