Oxidation juglone

The importance of both electronic and steric effects is clear in cycloadditions as in cross-oxidations. One example is a heterocycHc modification leading to the thermodynamically less stable natural form of juglone derivatives such as ventiloquinones JT [124917-64-2] (84) and I [124917-65-3] (85) (83). The yields are 97% (84) from 6-chloro-2,3-dimethoxy-l,4-ben2oquinone [30839-34-0] and 100% (85) upon hydrolysis. 

Derivatives of the natural product juglone [77189-69-6] eg (109), have been obtained ia 90% yield ia a single reaction involving halogenation and oxidation by A/-bromosucciniinide (115). 

In 1885, from a detailed study of juglone (52) it was proposed that its stmcture was 5-hydroxy-l,4-naphthoquinone (9). This stmcture was confirmed by oxidizing 1,5-dihydroxynaphthalene with potassium dichromate in sulfuric acid (53). Juglone occurs in walnuts as a glycoside of its reduced form, 1,4,5-trihydroxynaphthalene (54). Later it was deterrnined that the sugar is in the 4-position (10) (55). 

Juglone is most readily synthesized by Bemthsen s method. However, this method is too drastic and results in low yields (56). Somewhat better yields are obtained by using Fremy s salt (potassium nitroso disulfonate) as the oxidant (57). By using thallium trinitrate to oxidize 1,5-dihydroxynaphthalene, yields as high as 70% of juglone have been reported (58). 

The toxic influence exerted by Juglans nigra (black walnut) on other plants has been attributed to 5-hydroxy-1,4-naphthoquinone (juglone) (11,31). Hydrojuglone is present in the root bark, leaves, and fruit husks. This compound is not considered toxic, but it is oxidized to the toxic juglone upon exposure to air (51). Rainfall... 

Many allelochemicals are decomposed in soil, either abiotically (37) or by microorganisms (95-100). Obviously, the attainment of active concentrations of allelochemicals in soil depends on the relative rates of addition and inactivation. It is important to understand also that microbial decomposition of allelochemicals does not necessarily result in a decrease in allelopathic activity. In fact, the reverse may be true. Hydrojuglone is oxidized in soil to juglone, a quinone that is inhibitory to some species at a 10 ° M concentration (101). Isoflavonoids produced by red clover are decomposed to even more toxic phenolic compounds (95) and to repeat, amygdalin from peach roots is changed to hydrogen cyanide and benzaldehyde which cause the peach replant problem (88), and phlorizin from apple roots is decomposed to several phenolic compounds that appear to be responsible for the apple replant problem (100). 

Quinone monoacetals. Wheeler et aI.2 have obtained the highest yields reported to date for oxidation of 1,5-naphthalenediol (la) to juglone (2a) by use of TTN in methanol. The yield is improved somewhat by use of TTN supported by Celite for... 

Oxidation of naphthols. 1,5-Dihydroxynaphthoquinone (1) is oxidized by HI04 in aqueous DMF to juglone (2) in 55% yield. Under the same conditions p-naphthol is oxidized to 1,2-naphthoquinone in 47% yield.1... 

The boundary between all oxidized forms and all reduced forms of a substance can be drawn from Equation (18) by expanding Q (Equation (17)) to include acid/base speciation. Figure 16.1 shows this for five substances that exhibit moderately complex, but well characterized, speciation as a function of pH (uncomplexed Fe(II)/Fe(III), iron porphyrin, juglone, lawsone, and anthraquinone disulfonate). The resulting Eh-pH diagram shows, for example, that the hydroquinone of lawsone is a reductant relative to anthraquinone disulfonate, below pH 7.5, but the relationship is inverted at higher pH. A similar crossing... 

When [bis(acyloxy)iodo]arenes have electron-withdrawing groups either in the phenyl ring or in the acyloxy moiety, their oxidizing power increases generally. In a comparative study of the conversion of 1,5-dihydroxy-naphthalene to juglone, the following yields were obtained, under similar conditions [62] ... 

Guingant and Barreto [53] published the pioneering paper describing the synthesis of ochromycinone (35) by a Diels-Alder reaction. The dienone (46) was prepared from 3-ethoxy-5-methyl-cyclohex-2-enone (45) in two steps (alternatively, a diene with SPh instead of OMe could be used). The Diels-Alder reaction with juglone (47) was catalyzed with boron triacetate to overcome the somewhat poor reactivity of the electron-deficient diene 46. The primary adduct 48 could not be isolated but directly eliminated and oxidized to ochromycinone (35) (Scheme 13). 

In related investigations Valderrama et al. [47] studied the reaction of naphthoquinone (40) and juglone (47) with the ketene acetal 51. Using this oxidation state, the oxygen substituent in the product is always preserved at the terminal position of dienes (compare anthracycline chemistry [55]). Thus, the phenol ether 52 a (61 %) and smaller amounts of the phenol 52b were smoothly formed after silica gel-promoted elimination and air oxidation of the primary Diels-Alder adducts. The authors also observed base-catalyzed air oxidation to the ketone 53, analogous to similar oxidations of benzylic carbanions (compare with [56]). 

I he strongly chelated pcr/-hydroxyl group of juglone can be methylated with methyl iodide and silver oxide in chloroform, but not in dimethyl sulfoxide, ... 

Oxidation of phenols. Barton et al. carried out some studies on the oxidation of phenols with hydrogen peroxide and samples of old cerium(IV) oxide. It was later found that the oxidations reported require activation if freshly prepared pure dioxide is used. It is dissolved in hot H2SO4, precipitated at pH 12 with sodium hydroxide, and then heated at ca. 900° for 24 hr. This material in combination with 30% H2O2 oxidizes phenols such as (1) to hydroperoxy-cyclohexadienones (2) in good yield. Other reported reactions are the oxidation of (4) to the oxide (5) and of (6) to juglone (7). This oxidation system probably... 

In nature, the oxidative dimerization of phenols is controlled by enzymes, as is demonstrated by the axial chirality of the 6,8 -coupled juglone derivative isodiospyrin. In synthesis, however, phenol oxidation only proceeds in high yields when the enzymatic reaction control is replaced by substituent control, that is, if all but one of the positions with high spin density in the radical (ortho-and para positions) are blocked. 

Oxidation of 1,5-dihydroxynaphthalene. Juglone (2) has usually been prepared by chromic acid oxidation of 1,5-dihydroxynaphthalene (1), but the yield by this route is rather low. A new method involves oxidation of 1 with peracetic acid in acetic acid to give a mixture of 2 and 3, which is readily separable. The unstable... 

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