Dienones nitro

An ipso attack on the fluorine carbon position of 4-fIuorophenol at -40 °C affords 4-fluoro-4-nitrocyclohexa-2 5-dienone in addtion to 2-nitrophenol The cyclodienone slowly isomenzes to the 2-nitrophenol Although ipso nitration on 4-fluorophenyl acetate furnishes the same cyclodienone the major by-product is 4 fluoro-2,6-dinitrophenol [25] Under similar conditions, 4-fluoroanisole pnmar ily yields the 2-nitro isomer and 6% of the cyclodienone The isolated 2 nitro isomer IS postulated to form by attack of the nitromum ion ipso to the fluorine with concomitant capture of the incipient carbocation by acetic acid Loss of the elements of methyl acetate follows The nitrodienone, being the keto tautomer of the nitrophenol, aromatizes to the isolated product [26] (equation 20) Intramolecular capture of the intermediate carbocation occurs in nitration of 2-(4-fluorophenoxy)-2-methyIpropanoic acid at low temperature to give the spiro products 3 3-di-methyl-8 fluoro 8 nitro-1,4 dioxaspiro[4 5]deca 6,9 dien 2 one and the 10-nitro isomer [2d] (equation 21)... 

Methylanisole. The competition between ortho and ipso attack [analogous to that depicted in (83)] applies to the simultaneous nitration and demethyla-tion of 4-methylanisole. The identification of 4-nitro-4-methylcyclohexa-2,5-dienone as the metastable intermediate in charge-transfer nitration (Kim et al., 1993) is particularly diagnostic of the ipso adduct (84) that is also apparent in the electrophilic nitration of 4-methylanisole (Sankararaman and Kochi, 1991). The common bifurcation of nitration pathways resulting from para (ortho) and ipso attack on the various aromatic donors, as noted above, indicates that the activation step leading to the Wheland intermediate and... 

The intramolecular nitrile oxide-alkene cycloaddition sequence has been used for the assembly of a great variety of natural products. A target that has received special attention is that of taxol (156), undoubtedly due to its unique structural features, its potent anticancer activity, and its hmited availability from natural sources (318,319). In 1984 Kozikowski et al. found that the treatment of nitro dienone 158 (obtained from the p-benzoquinone derivative 157) with p-chlorophe-nyl isocyanate under Mukaiyama conditions afforded the unexpected eight-mem-bered ring 159, which is related to ring B of taxol (156) (Scheme 6.79). 

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". ... 

Methyl-4-nitro-2,3,5,6-tetrabromocyclohexa-2,5-dienone, a regiospecific nitrating agent was obtained in 88% yieid by the addition at 100°C over 10 mins, of nitric acid (d 1.52) to 4-methyl-2,3,5,6-tetrabromophenol in acetic acid followed by reaction for 2 hours at 5 C (ref.28). 

The 2,4,6-triphenylpyrylium ion reacts with nitromethane to give the dienone 39. This is followed by an intramolecular nitro-aldol condensation forming 2,4,6-triphenylnitrobenzene 40. By way of the ring-opened phosphorane 41, action of phosphorylidene produces the 2-substituted 1,3,5-triphenyl-benzene 42 ... 

However, 4-hydroxy-1-nitro-1,2,3,5,6-pentamethylbenzenium ion is converted into a benzyl cation at a temperature at which the corresponding dienone undergoes no rearrangement into a benzyl derivative... 

The phenoxy radical, derived from an H-abstraction reaction by NO, couples with another -NO molecule to give 4-nitrocyclohexa-2,5-dienone, which readily rearranges, after a keto-enol tautomerism to 2,6-di- -butyl-4-nitrophenol. 2,4-di-t-butylphenol and 4-r-butylphenol react in the same way, whereas the presence of three substituents on the aromatic ring of 2,4,6-tri-r-butylphenol and 2,6-di- butyl-4-methylphenol does not allow keto-enol tautomerism in these cases 2,4,6-tri- -butyl-4-nitrocyclohexa-2,5-dienone and 2,6-di-r-butyl-4-methylnitrocyclohexa-2,5-dienone were the sole nitro compounds obtained. The reaction between NO and 2,6-di-t-butylphenol or 2,4-di- -butylphenol in methanol resulted in phenoxyl radical dimerisation together with nitration. As shown in (Scheme 5.100), phenoxyl radical dimerises to give an... 

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