Hydroxyphenazines

Tautomerism questions arise with hydroxyphenazines and quinoxalines which contain hydroxyl groups in the aromatic ring. Does 5-hydroxyquinoxaline (62) show any tendency to exist as the tautomer (63) or 1-hydroxyphenazine (64) to exist as (65) Analogous tautomeric forms can also be written for 6-hydroxyquinoxaline and 2-hydroxyphenazine. 

Little information is available at the present time on these systems, but UV data 51JCS3204 indicate that the hydroxy forms rather than the vinylogous amide forms are favoured and these systems are true hydroxyquinoxalines and hydroxyphenazines . 

B. a-Hydroxyphenazine (demethylalion). A solution of 4.2 g. (0.02 mole) of a-methoxyphenazine, from A above, in 125 ml. of 55% hydrobromic acid (Note 7) is placed in a 250-ml. round-bottomed flask fitted with a reflux condenser. The flask is immersed in an oil bath, and the solution is heated to 110-120° for 5 hours the evolved gases are absorbed with water in a trap. The reaction mixture is cooled to room temperature, diluted with about 125 ml. of water, almost neutralized with sodium hydro.xide (Note 8), and extracted six times with 30- to 40-ml. portions of ether. The combined ether extracts arc extracted with 25-ml. portions of 10% sodium hydroxide solution (Note 9) until no more purple sodium salt is remox ed from the ether. The aqueous extracts are combined, made acid to litmus with dilute acetic acid, and re-extracted four times with 50-ml. portions of ether. The combined ether extracts are dried over anhydrous sodium sulfate, and the ether is removed by distillation on a steam bath. The residue is recrystallized as follows It is dissolved in the least possible amount of hot alcohol, water... 

C. Pyocyanine alkylation). A solution of 2 g. (0.011 mole) of a-hydroxyphenazine in 13.4 g. (10 ml., 0.1 mole) of methyl sulfate (Note 10) is placed in a 250-ml. Erlenmeyer flask fitted with a calcium chloride drying tube and heated at 100° (oil bath) for 10 minutes. The solution is allowed to cool to room temperature, and about 7.5 ml. of dry ether is added. The dark brown solid is filtered on a 7-cm. Eiichner funnel and washed with about ISO ml. of dry ether in several portions (Note 11). 

The residue, without recrystallization, is pure enough for the preparation of a-hydroxyphenazine. 

Hydroxyphenazine (Hemipyocyanine) [528-71-2] M 196.2, m 157-158", pK 1.61, pKj 8.33. Chromatographed on acidic alumina with benzene/ether. Crystd from benzene/heptane, and sublimed. 

Phenazines — The phenazines are biosynthesized by the shikimic acid pathway, through the intermediate chorismic acid. The process was studied using different strains of Pseudomonas species, the major producers of phenazines. The best-known phenazine, pyocyanine, seems to be produced from the intermediate phenazine-1-carboxylic acid (PCA). Although intensive biochemical studies were done, not all the details and the intermediates of conversion of chorismic acid to PCA are known. In the first step, PCA is N-methylated by a SAM-dependent methyltransferase. The second step is a hydroxylative decarboxylation catalyzed by a flavoprotein monooxygenase dependent on NADH. PCA is also the precursor of phenazine-1-carboxamide and 1-hydroxyphenazine from Pseudomonas species. - - ... 

Murakami E, U Deppenmeier, SW Ragsdale (2001) Characterization of the intramolecular electron transfer pathway from 2-hydroxyphenazine to the heterodisulfide reductase from Methanosarcina thermophila. J Biol Chem 276 2432-2439. 

The best-studied system producing antibiotics in the rhizosphere are fluorescent pseudomonads, producing up to seven different compounds, as summarized in Fig. 9 2-hydroxyphenazine-l-carboxylate, phenazine-l-carboxylate, 2-hydroxy-phenazine, pyrrolnitrin, pyocyanine, 2.4-diacetylphloroglucinol. and pyoluteorin (48). Nine genes have been identified in the synthesis of phenazine-1-carboxylic... 

Methanophenazine 239 functions as an electron carrier in the cytoplasmic membrane. The etherification of 2-hydroxyphenazine 237 with mesylate 238 was a key step in a total synthesis of 239 <00AC(E)2470>. 

The result of the retrosynthetic analysis of rac-lO is 2-hydroxyphenazine (9) and the terpenoid unit rac-23, which may be linked by ether formation [29]. The rac-23 component can be dissected into the alkyl halide rac-24 and the (E)-vinyl halide 25. A Pd(0)-catalyzed sp -sp coupling reaction is meant to ensure both the reaction of rac-24 and 25 and the ( )-geometry of the C-6, C-7 double bond. Following Negishi, 25 is accessible via carboalumination from alkyne 27, which might be traced back to (E,E)-farnesyl acetone (28). The idea was to produce 9 in accordance with one of the methods reported in the literature, and to obtain rac-24 in a few steps from symmetrical 3-methyl-pentane-1,5-diol (26) by selective functionalization of either of the two hydroxyl groups. 

The synthesis of 2-hydroxyphenazine (9) caused unexpected problems, though. The resultant yield of 9 was very low with both the condensation of 2-hydroxy-1,4-benzoquinone (33) [30], available from 1,2,4-trihydroxybenzene... 

To further elucidate the function of methanophenazine (10) as electron carrier its redox potential was determined [45]. As Scheme 2 reveals, the redox potential of lO/dihydro-10 must range somewhere between the values for F420H2/F420 ( 0 =-360 mV) [46] and H2/2H ( =-420 mV), and the redox potential of the heterodisulfide CoB-S-S-CoM (22). Whilst the values of F420H2/F420 and H2/2H" have long been known, the redox potential of 22 remained an unknown quantity. 2-Hydroxyphenazine (9) had proved to be a satisfactory model sub-... 

Table 2 Synthesis of phenazine ethers 10,44a-g by etherification of 2-hydroxyphenazine (9) with ROMs(-Br) and their redox potentials vs. SHE as determined by cyclic voltammetry in phosphate buffer at pH 7 using HMDE...

The same problem occurs with the synthesis of lavanducyanin (65) [85,87] and phenazinomycin (67) [88]. Here, Kitahara et al. used the same approach in that they first generated the phenazine skeleton and performed the AT-allylation as the last step. While the AT-methylation of the unprotected 1-hydroxyphenazine (109) proceeds without any major problems and provides pyocyanin (1) in acceptable yields [89], model experiments already indicate that the allylation of 110 could not be so readily accomplished. As expected, the yields were poor. The only way to bring about the synthesis of 65 is under high-pressure conditions by reaction of 110 with 111, and even so the yield of the natural product remains quite low. Similar problems are encountered with the synthesis of 67 by allylation of 110 with the terpenoid component 112. 

That the second option can also be successfully used has recently been revealed by our synthesis of 2-methoxyphenazine (117) [90]. The reduction of o-bromo-o -nitrodiphenylamine 132 accessible via intermolecular Pd-catalyzed JV-arylation provides the o-amino-o -bromodiphenylamine 133, which can then be cyclized to give 117 in a Pd-catalyzed intramolecular AT-arylation by employing Pd2(dba)3 as the Pd complex and 134 as the phosphine ligand. It should be noted that the outcome of both the intermolecular and the intramolecular AT-arylations heavily depends on the appropriate choice of the Pd complex as well as the phosphine. Ether cleavage leads to 2-hydroxyphenazine (9). 

Rapid and sensitive HPLC methods were developed for the detection of an antimicrobial growth promoter and its main metabolites containing quinoxaline-2-carboxylic acid <2005MI1495>. The major phenazine pigments of Pseudomonas aeruginosa such as 1-hydroxyphenazine and phenazine-l-carboxylic acid <1997JCH(A)(771)99>, and 2-aminophenazine as an impurity in a bactericide <1999MI632>, were also analyzed by HPLC methods. 

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