Dienone-benzene rearrangement

Oxidation of 4-methyl-2-tert-butylphenol (225, Scheme 57) by 2,3-di-chloro-5,6-dicyano-l,4-benzoquinone in methanol affords the dibenzofuran 228. The intermediate oxepinobenzofuran 226, as its valence isomer 227, undergoes nucleophilic attack by methanol and subsequent dienone-benzene rearrangement with the migration of the methoxy group. 

Phosphorus pentachloride Dienone-benzene rearrangement Chloranenes from 2,5-cyclohexadienones... 

Phenyltrichloromethylmercury Arylcarboxylic acids by dienone-benzene rearrangement... 

Analysis of NMR data on futoquinol and the derived mixture of epimeric alcohols (58) led to structural proposal 7.10a which justifies the dienone-phenol and dienone-benzene rearrangements formulated in Chart 21 139). 

In view of the similarity between the reaction mechanisms proposed for the dienone-phenol and the dienol-benzene rearrangements it is hardly surprising that the two groups of reactions exhibit similar trends. The dienol 29) [171] and derivatives with 2-methoxy [X73], 3-methyl [174,175], or ii-hydroxyl substituents [ 176] follow path A leading mainly to 4-methyl derivatives (31). 

Barton and Cohen (48) were the first to suggest the seminal idea that dienones could be intermediates in the biosynthesis of aporphine alkaloids from benzylisoquinolines, since such dienones could rearrange to the aporphine nucleus either through a dienone-phenol or a dienol-benzene rearrangement as shown in the scheme on the following page. 

The dienol-benzene rearrangement has been used to advantage to explain the structures of several abnormal aporphine alkaloids (49) from a biogenetic point of view, and has also allowed the first s3uithesis of the aporphine alkaloid isothebaine (L) (50). (+ )-Orientaline (XLVII) was oxidized by alkaline ferricyanide to give a mixture of two dienones (XLVIII) in 4% yield. One of the dienones was reduced with sodium borohydride to a mixture of two dienols XLIX which, without... 

Two studies have appeared dealing with the phenolic oxidative coupling of the dihydric benzylisoquinoline LI. In the first study, oxidation of LI with potassium ferricyanide yielded two dienones (LII), one of which was obtained crystalline. The crystalline material was reduced with sodium borohydride to two noncrystalline dienols (LIII) which underwent dienol-benzene rearrangement in anhydrous methan-olic hydrogen chloride to ( )-corydine (XXVI). Rearrangement of the... 

The dienone-phenol rearrangement is widely employed for the synthesis of many polycyclic structures, the formation of which demands an expansion of one of the cycles. Thus, the aporphine-type plant alkaloids 307-310 can be obtained via the dienone-phenol rearrangement of orientalinone 305 and dienol-benzene rearrangement of orientaUnol 306 (equation 148). 

Analogous rearrangements were reported for similar systems such as proaporphines , the alkaloids of Croton sparsiflorusMorong and cannabinoids . The dienone-phenol and dienol-benzene rearrangements were studied in the eupodienone-1 series (a constituent of Eupomatia laurina R. Br.). These compounds 311 were transformed under a variety of acidic conditions into dibenzocyclooctene derivatives (equation 149). 

The reduction with borohydride and the acid-catalyzed dienone-phenol/dienol-benzene rearrangements leave the center of chirality at C-6a intact. Consequently, the absolute configuration of this center can be intercorrelated with that of the benzylisoquinoline, proaporphine, and aporphine alkaloids (Schemes 1, 2, and 20). 

The biosynthesis of aporphine alkaloids takes the course—laudanoso-line, norlaudanosoline, reticuline, or orientaline—by means of phenolic oxidation via the cyclohexadienone proaporphine (27, 28, 39, 40, 47, 49, 65, 66, 68, 69,147,148,150, 229, 231, 240, 250, 275, 349, 445, 451, 512) after dienone-phenol or dienol-benzene rearrangement, as described in more detail in the preceding section. The foregoing results, considered as a whole (39,49), provide good evidence that isothebaine is biosynthesized in P. orientale along the following pathway ... 

Dehydrotestosterone acetate (174) in nonprotic solvents (dioxane, benzene) undergoes a rearrangement to the isomer (175). This product is photolabile and isomerizes readily to new cross-conjugated dienones. Thus, ultraviolet irradiation of (174), its 1-, 2- and 4-methyl homologs, and its lOa-stereoisomer (188) in dioxane solution causes, in each case, a series of rearrangements as summarized on page 331 for (174) and (188). ... 

The photorearrangement of a dienone was noted<4) as early as 1830 in a study of the sesquiterpene a-santonin (1). However, the structure and stereochemistry of the various photoproducts were not conclusively established until 1965.(6) Upon irradiation in neutral media, a-santonin (1) undergoes rapid rearrangement to the cyclopropyl ketone, lumisantonin (2). However, if the irradiation is not terminated after a short period of time the lumisantonin itself rearranges into a linearly conjugated dienone (3). The dienone (3) can be isolated from the photolysis of either (1) or (2) in benzene or ether. In nucleophilic solvents (alcohol or water) the dienone (3) is also photo-chemically active and is further converted into an ester or an acid (photo-santonic acid) (4). 

The thermal ring closure reaction of a 1,3,5-triene to a 1,3-cyclohexadiene occurs by a concerted disrotatory electrocyclic mechanism. An example of the latter is the oxepin-benzene oxide equilibrium (7) which favors the oxepin tautomer at higher temperatures (Section 5.17.1.2). Oxepin (7) was found to rearrange to phenol during attempted distillation at normal pressure (67AG(E)385>. This aromatization reaction may be considered as a spontaneous rearrangement of the oxirane ring to the dienone isomer followed by enolization (equation 7). 

Among the very early reports (1911) of photochemical rearrangements of cross-conjugated dienones is the report that the diphenylketene-quinone adduct (Formula 70) on irradiation in benzene solution gives... 

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