Dienone intermediates

The reaction is undoubtedly intramolecular. Evidence for this comes from the absence of crossed-over products when two different ethers were rearranged together121. Further, optically active ortho- 1,3-dimethylallyl phenol is formed from optically active 1,3-dimethylallyl phenyl ether122 and the presence of dienone intermediates has been demonstrated. Claisen and Tietze123 first proposed intermediate dienones such as LXXIX and LXXX. This was established124 when... 

Early kinetic work127 showed that the formation of both ortho and para products was a first-order process and that the rates of reaction were insensitive to added acid or base and to change of solvent. The activation parameters were of the same order of magnitude for both reactions and the suggestion was made that both had a similar rate-determining step. Schmid et a/.128 showed that the formation of a dienone intermediate in the para rearrangement was also reversible since the radioactivity from allyl 2,6-dimethyl-4-allyl-y-14C phenyl ether LXXXVII became uniformly distributed in the y carbon atoms of the O- and C-allyl groups... 

From the study with the synthesised dienone127 and the 14C work129 it was possible not only to evaluate the individual rate coefficients for the separate steps involving the dienone intermediates, viz. 

The dienone intermediate (53a), as well as enolising to the phenol (52a), is itself capable of undergoing a Cope rearrangement to yield a second dienone (cf. 56a), whose enol is the p-substituted phenol (c/ 57a). Enolisation normally predominates, but where (51) has o-substituents, i.e. (54a), o-enolisation cannot take place, and only the p-phenol (57a) is then obtained. That this product is indeed formed not by direct migration of the allyl group, but by two successive shifts, is suggested by the double inversion of the position of the, 4C label in the allyl group that is found to occur ... 

Further confirmation of the two-fold shift, and of the double inversion of the position of the 14C label, is provided by trapping (cf. p. 50) the first dienone intermediate (55a) with maleic anhydride in a Diels-Alder reaction. An exactly analogous rearrangement is found to occur in allyl ethers of aliphatic enols, e.g (58) ... 

Dienes, 11 addition to, 194-198 cisoid conformation, 197, 350 conjugated, 11 Cope rearrangement, 354 cycUsation, 346 cycloaddition to, 348 Diels-Alder reaction, 197, 349 excited state, 13 heat of hydrogenation, 16,194 isolated, 11 m.o.s of, 12 polymerisation, 323 Dienone intermediates, 356 Dienone/phenol rearrangement, 115 Dienophiles, 198, 350 Digonal hybridisation, 5 Dimedone, 202 Dimroth s Et parameter, 391 solvatochromic shifts, 391 solvent polarity, 391 Y and,392 Dinitrofluorobenzene proteins and, 172... 

The cyclopropyl enone 31 was photoisomerized to the cyclopropanone 32 at liquid nitrogen temperatures.34) The unstable 32 was characterized on the basis of ir spectra and thermal and photochemical reactions (see Section 4.1.5). When 31 was irradiated at room temperature in 45% acetic acid, the phenol 33 was produced, most probably through 32 and the spiro dienone intermediate 34 (Scheme 5).35>... 

Diynes 6, which consist of a trimethylsilyl alkyne tethered to a tertiary propargyl alcohol, undergo ruthenium catalyzed cycloisomerization in aqueous acetone to form the dienone intermediate 7. Concomitant electrocyclization affords 2-trimethylsilyl-2-//-pyrans in high yield (Scheme 3) <2004OL4235>. 

The most widely studied aspect of arene-oxide chemistry is the aromatization reaction to yield phenols. The acid-catalyzed, spontaneous, and thermal rearrangements of epoxides to ketones have a parallel in the isomerization of arene oxides to dienone intermediates with subsequent aromatization to phenols. Prior to their availability by chemical synthesis, arene oxides were postulated as initial inter-... 

On the basis of the evidence discussed, the rearrangement of arene oxides to phenols clearly involves dienone intermediates. However, from kinetic results it is evident that several carbonium ion intermediates must also be involved. The evidence for involvement of carbonium ion intermediates is ... 

Benzoyl peroxide introduces a benzoyl unit mainly ortho to an existing hydroxyl (equation 35), but para products can be formed by [3,3] migration of the acyloxy group around the ring periphery of the dienone intermediate . The benzoate esters can easily be hydrolysed to the corresponding phenols. 

The reaction of phenols with nitrous acid gives the ortho- and para-nitroso products, which are formed through a neutral dienone intermediate, the proton loss from the latter being the rate-limiting step" " . It has been shown that the nitrous acid can act as a catalyst for the formation of the nitro derivatives. Thus the conventional preparation of nitro compounds by the oxidation of nitroso compounds may be replaced by methods using an electron-transfer pathway in certain cases. In the latter method, the phenoxide reacts with nitrosonium ion to give the phenoxy radical and nitric oxide radical. The nitric oxide radical is in equilibrium with the nitronium radical by reaction with nitronium ion. The reaction of the phenoxy radical with the nitroninm radical resnlts in the formation of the ortho- and para-mixo prodncts" . Leis and coworkers carried ont kinetic stndies on the reaction of phenolate ions with alkyl nitrites and fonnd that the initially formed product is the 0-nitrite ester, which evolves by a complex mechanism to give the ortho-and the para-nitro products". ... 

The reversible conversion of phenols to dienone intermediates is an important transformation in the synthesis of natural products. This rearrangement occurs efficiently in superacid solutions The corresponding version for the halophenols to give halodienones has been reviewed in an earlier volume of this series. 4-Bromo-2,4,6-trialkylcyclohexa-2,5-dienones have recently been synthesized by electrophilic bromination of the corresponding phenols. 

In the presence of suitable hydride donors the dienone intermediates formed in these reactions can be further reduced to the corresponding ketones. 3-Hydroxytetralin in the presence of HF—SbFs and methylcyclopentane gives 3-oxodecaline (equation 92) . 

In practice a particular aporphine alkaloid may be derived a priori from a number of diphenolic benzylisoquinoline precursors either by direct coupling or through the genesis of a dienone intermediate as seen above. Thus in the study of the superficially simple alkaloid boldine (79), a number of benzylisoquinoline precursors had to be tested. Experiments were carried out with labelled samples of ( )-norprotosinomenine (83), ( )-nororientaline (84), ( )-4 -o-methyl-... 

In the light of established pathways to aporphine alkaloids (see p. 15) study of the biosynthesis of glaucine (6) and related bases in Dicentra eximia might have been expected to yield orthodox results. On the contrary, however, a novel pathway was unearthed which implicates dienone intermediates of quite un-... 

Aporphine Alkaloids.—Isothebaine (65) derives from orientaline (62) along a pathway which involves thedienone(63)and the dienol(64). The biosynthesis of the aporphine alkaloids of Dicentra eximia is quite different but dienone intermediates are also implicated. By comparison the biosynthesis of bulbocapnine (66) is simple, for the alkaloid arises directly from reticuline (67), in Corydalis cava, by ortho-ortho phenol oxidative coupling. 

Claisen rearrangement Thermal conversion of an allyl phenyl ether to an o-allyl phenol. The rearrangement proceeds via a cyelohexa-dienone intermediate. 

In the aromatic Claisen rearrangement (65), the ether-phenol transformation occurs at about 2(X)° S-C allyl shifts are more difficult to achieve (66). As the formation of the dienone intermediates is rate-determining, the analysis of perturbance around the heteroatoms (X) leading to the intermediates should provide clues to the relative rates, for changes are the same elsewhere. Thus, the crux of the problem involves only the net change of a -X to a Qpi -X bond. Such a change is favored with X = O rather than that with X = S because trigonal carbon is harder. 

Q 13. Suggest the possible mechanism for the conversion of 2,6-dimethyl phenyl allyl ether to 4-allyl-2,6-dimethyl phenol. How will you prove that it passes through a dienone intermediate ... 

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