2,5-Cyclohexadienone structure

Hop "bitter" acids are isomeric mixtures of cyclohexadienone structures in both keto and enol forms, substituted at various positions on the ring by hydroxyl, acyl, and alkenyl groups. See Figure 2. 

It is reported in the literature (5-7) that m-bromo-phenolic compounds are more stable than their o- or p-brominated counterparts. An o- or p-Brominated phenol forms an unstable cyclohexadienone structure via keto-enol tautomerization. Upon heating, this unstable cyclohexadienone structure generates free radicals which, in turn, abstract a proton from a neighboring molecule to form HBr. 

Cyclohexadienone structures have also been assigned to the incompletely characterized products from the action of Cb on calixarenes in the solid state Lamartine, R. Perrin, R. Perrin, M. ... 

Frames 3a and 3b—are analogues of phenols structure, frame 3c and 3d—analogues cyclohexadienons structure. From calculation of frames 3a and 3b in program MM2 follows that energy of steric interaction (Hs) in 3a is equal 76.9 kJ.mol, in 3b > 1000 kJ.mol that, apparently, is a consequence influence of a steric factor on a direction of recombination of radicals from an antioxidant 2. 

Zimmerman, H.E. and Jones, G., II, The photochemistry of a cyclohexadienone structurally incapable of rearrangement. Exploratory and mechanistic organic photochemistry. XLVII,/. Am. Chem. Soc., 92, 2753, 1970. 

Certain structural features can make the keto-enol equilibrium more favorable by stabi hzmg the enol form Enolization of 2 4 cyclohexadienone is one such example... 

Generally the name of a compound should correspond to the most stable tautomer (76AHCS1, p. 5). This is often problematic when several tautomers have similar stabilities, but is a simple and reasonable rule whose violation could lead to naming phenol as cyclohexadienone. Different types of tautomerism use different types of nomenclature. For instance, in the case of annular tautomers both are named, e.g., 4(5)-methylimidazole, while for functional tautomerism, usually the name of an individual tautomer is used because to name all would be cumbersome. In the case of crystal structures, the name should reflect the tautomer actually found therefore, 3-nitropyrazole should be named as such (97JPOC637) and not as 3(5)-nitropyrazole. 

The basic principle of all diazotizations of aromatic amines with a hydroxy- or a sulfonamido group in the 4-position relative to the amino group involves a deprotonation of the OH or NH group, respectively, after diazotization of the amino group. There is also a case of a deprotonation of a CH group in the 4-position of an aniline derivative, namely in the diazotization of 4-aminophenylmalononitrile (2.41) which, by the sequence of steps shown in Scheme 2-23, yields 3-diazo-6-dicyanomethylene-1,4-cyclohexadienone (2.42), as found by Hartzler (1964). This product can also be represented by a zwitterionic carbanion-diazonium mesomeric structure. 

Structural analogues of the /]4-vinylketene E were isolated by Wulff, Rudler and Moser [15]. The enaminoketene complex 11 was obtained from an intramolecular reaction of the chromium pentacarbonyl carbene complex 10. The silyl vinylketene 13 was isolated from the reaction of the methoxy(phenyl)-carbene chromium complex 1 and a silyl-substituted phenylacetylene 12, and -in contrast to alkene carbene complex 7 - gave the benzannulation product 14 after heating to 165 °C in acetonitrile (Scheme 6). The last step of the benzannulation reaction is the tautomerisation of the /]4-cyclohexadienone F to afford the phenol product G. The existence of such an intermediate and its capacity to undergo a subsequent step was validated by Wulff, who synthesised an... 

Pyran-4-ones bear an obvious structural similarity to the all-carbon cyclohexadienones discussed above. However, the original studies of their photochemical behavior revealed only dimerization processes to produce a cage product resulting from two successive head-to-tail [2 + 2]-photocycloadditions (Scheme 29)54. Much later, small amounts of substituted furfural 121 were observed during the irradiation of 11955a. It was speculated that 121 could arise from bicyclic epoxide 120, an intermediate analogous to those formed in cyclohexadienone photochemistry. Subsequent reports noted that further irradiation of... 

An example of 2,4,6-triphenylpyrylium-3-olate (65 R = R = R = Ph, R = H) reacting as a 1,3-dipole was first provided by Suld and Price who obtained a maleic anhydride adduct (C25HigO5). Subsequently, an extensive study of the cycloadditions of this species has been published by Potts, Elliott, and Sorm. With acetylenic dipolarophiles, compound 65 (R = R = R = Ph, R = H) gives 1 1 adducts that have the general structure 74 and that isomerize to 6-benzoyl-2,4-cyclohexadienones (76) upon thermolysis. This thermal rearrangement (74 -> 76) has been interpreted in terms of an intermediate ketene 75. The 2,3-double bond of adduct 74 (R = Ph) is reduced by catalytic hydrogenation. Potential synthetic value of these cycloadducts (74) is demonstrated by the conversion of compound 74 (R = Ph) to l,2,3,4,6-pentaphenylcyclohepta-I,3,5-triene (79 R= Ph) via the alcohol 78 (Scheme 1). ... 

The ring fission of 1,3-cyclohexadienes observed in the vitamin D series (Sec. IVA) is a general reaction for 1,3-cyclohexadienes and 2,4-cyclohexadienones (Sec. IIB). Irradiation of 1,3-cyclohexadiene (Formula 330) gives 1,3,5-hexatriene (Formula 331) (142,143), and a-phellendrene (Formula 332) gives a triene which is probably Formula 333 (143). The evidence for Structure 333 is not compelling. Methyl dehydro-... 

Structure (XII) with the attachment of the lactone bridge reversed can be ruled out for the following reason. Gibberellic acid decomposes slowly in aqueous solution to give gibberellenic acid first described by Gerzon, Bird, and Woolf (16) who suggested the homoannular diene structure (XV). We consider that the ultraviolet absorption (Amax 253 m/ e 22,400) is more in accord with the hetero-annular structure (XIV) and this is confirmed (25) by ultraviolet absorption (Amax 309 mfi e 16,500) of the derived dienone (XVII) which is decisive [cf. the ultraviolet absorption (Araax 310 m/ e 3900) of model cyclohexadienone (XVIII)]. 

The hypervalent iodine reagents PIFA and PIDA have also been used in the synthesis of naturally occurring structures, primarily the amaryllidaceae alkaloids and related species. Work by White s group showed the feasibility of this method for the synthesis of 6a-epipretazettine and (-)-codeine [45, 46]. In the early 1990s, Rama Krishna and co-workers demonstrated that PIDA can promote the oxidative phenolic coupling of diaryl substrates 38a-e to deliver cyclohexadienones 39a-e, respectively, in consistent 30 % yields for all of the substrates examined (Scheme 10) [47]. 

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