Phenanthridine N-oxides

Phenanthridine N-oxide forms 6-cyanophenanthridine N-oxide in fair yield when treated with aqueous potassium cyanide in the presence of potassium ferricyanide.331... 

The mechanism of the related photorearrangement of heterocyclic N-oxides is still a matter for debate. Evidence that rearrangement of the phenanthridine N-oxides (48) to the N-substituted phenanthridones (49) and the dibenzo[d,f]-1,3-oxazepines (50) proceeds by way of biradical intermediates rather... 

Phenanthridine 5-oxide cation Phenanthridin-N-oxid-Kation 44.002/40.620 H9/4... 

Albini, A., Fasani, E., and Frattini, V., Medium and substituent effect on the photochemistry of phenanthridine N-oxides. Is an intermediate of biradical character involved in the photorearrangement of heterocycHc N-oxides , /. Chem. Soc., Perkin Trans. 2, 235,1988. 

The first reduction wave for phenanthridine jV-oxide in dimethyl-formamide (at 25°) appears at — 1.774 (vs. s.c.e.) in good agreement with the HMO energy of the lowest vacant orbital277 a mechanism for the reduction has been proposed.278 No esr signal could be detected during controlled-potential electrolysis at the appropriate potential, perhaps because of the relative instability of the anion radical. This behavior, which is paralleled by isoquinoline, contrasts sharply with that of the oxides of other polynuclear N-heteroaromatic systems (e.g., quinoline and acridine).277... 

Quinoline 1-oxide undergoes nucleophilic attack by ozone to yield a hydroxamic acid (128), and 40% of the starting N -oxide is recovered (Scheme 74). When an excess of ozone is employed the aldehydes (129) and (130) are obtained. Formation of these products has been attributed to electrophilic attack by ozone rather than further oxidation of (128), because in a separate experiment (128) yielded carbostyril on treatment with ozone. Isoquinoline 2-oxide yields 2-hydroxyisoquinolin-l-one, and acridine 10-oxide gives 10-hydroxyacridone and acridone in a similar manner to the above. Likewise, phenanthridine 5-oxide affords mainly 5-hydroxyphenanthridone. Quinoline 1-oxide undergoes oxidation by lead tetraacetate as shown (Scheme 75). 

Photolysis of aromatic Schiffs bases in strongly acidic media results in cyclisation to phenanthridines (A. Padwa. Chem. Rev.. 1977, 77, 37). Irradiation of anils of polyfluoroaromatic ketones in trifluoroacetic acid also yields phenanthridines by oxidative cyclisation examples are quoted which involve cleavage of a C-F bond (N. I. Danilehko et al., Izvestia, 1980, 1606). 

As weak bases (cf. p. 428), acridine and phenanthridine are N-protonated by strong protic acids, N-alkylated by alkyl halides, and N-oxidized by peroxy acids. 

Another photocyclization to a benzo[c]phenanthridine was reported (127). Oppenauer oxidation of ( )-ophiocarpine (92) with potassium fm-butoxide and benzophenone in dioxane effected C-6—N bond cleavage to afford the hydroxyisoquinoline 219 via berberinephenolbetaine (121) (Scheme 39). Although photolysis of 219 gave only the oxepine 221, that of its methyl ether 220 furnished directly norchelerythrine (222) through electrocyclization followed by spontaneous elimination of methanol. 

Oxidative photocyclization of benzylideneaniline appears to proceed efficiently only in the presence of strong acid. The phenanthridine 31, however, has been prepared by irradiation of the imine 3228 few other examples of the photocyclization of arylimines have been reported.29 Strong acid is also required for successful photocyclization of azobenzenes to benzo-[c]cinnolines. Here, protonation is claimed to lower the reactive n, n excited state below the level of the unreactive n, it state. 2-Phenylazopyridine,... 

The photocyclization of enamides has been widely employed in the construction of heterocyclic systems the N-acryloyl-2-aminopyridines 37, for example, are converted on irradiation to the lactams 38.36 Numerous benzylisoquinoline alkaloids have been prepared using this approach, and in particular, the syntheses of benzo[c]phenanthridine alkaloids have been reviewed.37 Thus, irradiation of the [Z]-l-ethylidene-2-benzoyltetra-hydroisoquinoline 39 affords the corresponding 8-oxoberberine 4038 competing photoisomerization to the E-isomer is observed but cyclization occurs only via the Z-isomer. Examples of syntheses of Amaryllidaceae and indole alkaloids have also been reported. In this way, the precursor 41 of ( )-lycoran has been obtained by oxidative cyclization of the enamide 42.39... 

The formation of a phenanthridine lactone by the persulfate oxidation of 2 -cyanobiphenyl-2-carboxylic acid is interesting in that little is known of the addition of radicals to the C=N system. The mechanism proposed below (Scheme 1) also accounts for the occurrence of fluorenone as a by-product.64... 

The nitrogen conversions reported in Table II were measured with the chemiluminescent N0X analyzer. During the oxidation of the phenanthridine char at 873 K, no differences were seen in the instru-mentally measured concentration of NO and N0X. This indicates that NO and N are the principal nitrogen species present. Both NH3 and HCN and can be converted to NO by the stainless steel catalyst in the analyzer (7). If either of these two species were present, the measured N0X value would be higher than the NO value. 

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