Cross-conjugated

Another synthesis of a bridged hydrocarbon takes advantage of high elearon release from the /wra-position of phenolate anions, which may be used to transform the phenol moiety into a substituted cross-conjugated cyciohexadienone system (S. Masamune, 1961, 1964). 

Poor correlations are obtained for cross-conjugated polyene systems such as... 

The cross-conjugated system of two a,P-unsaturated carbonyl groups of both 1,2- and 1,4-quinones occurs in many polynuclear hydrocarbons, eg, 1,2-naphthoquinone [524-42-5] (8) and 1,4-naphthalenedione [130-15-4] (1,4-naphthoquinone) (9) (see Fig. 1). The carbonyl groups may be located in different rings, but occupy positions corresponding to the 1,2- or 1,4-orientation of monocyclic quinones, eg, in naphthalenes such as 2,6-naphthoquinone... 

Interesting structures can be formed by combinations of ring and side-chain substituents in special relative orientations. As indicated above, structures (28) contain the elements of azomethine or carbonyl ylides, which are 1,3-dipoles. Charge-separated species formed by attachment of an anionic group to an azonia-nitrogen also are 1,3-dipoles pyridine 1-oxide (32) is perhaps the simplest example of these the ylide (33) is another. More complex combinations lead to 1,4-dipoles , for instance the pyrimidine derivative (34), and the cross-conjugated ylide (35). Compounds of this type have been reviewed by Ramsden (80AHCl26)l). 

Clearly, in the case of (66) two amide tautomers (72) and (73) are possible, but if both hydroxyl protons tautomerize to the nitrogen atoms one amide bond then becomes formally cross-conjugated and its normal resonance stabilization is not developed (c/. 74). Indeed, part of the driving force for the reactions may come from this feature, since once the cycloaddition (of 72 or 73) has occurred the double bond shift results in an intermediate imidic acid which should rapidly tautomerize. In addition, literature precedent suggests that betaines such as (74) may also be present and clearly this opens avenues for alternative mechanistic pathways. 

This and other aromatic cross conjugated ketones give deeply colored... 

Metal-ammonia solutions reduce conjugated enones to saturated ketones and reductively cleave a-acetoxy ketones i.e. ketol acetates) to the unsubstituted ketones. In both cases the actual reduction product is the enolate salt of a saturated ketone this salt resists further reduction. If an alcohol is present in the reaction mixture, the enolate salt protonates and the resulting ketone is reduced further to a saturated alcohol. Linearly or cross-conjugated dienones are reduced to enones in the absence of a proton donor other than ammonia. The Birch reduction of unsaturated ketones to saturated alcohols was first reported by Wilds and Nelson using lithium as the reducing agent. This metal has been used almost exclusively by subsequent workers for the reduction of both unsaturated and saturated ketones. Calcium has been preferred for the reductive cleavage of ketol acetates. 

A variety of conjugated dienones are reduced by lithium-ammonia, presumably via dienyl carbanions analogous to the allyl carbanions encountered in enone reductions. Cross-conjugated l,4-dien-3-ones afford 4-en-3-ones as the major reduction products, indicating that the cyclohexadienyl carbanion (55) protonates largely at C-1. Some protonation at C-5 does occur as shown by examination of the NMR spectrum of the crude reduction product derived from the 17-ethylene ketal of androsta-l,4-diene-3,17-dione. The 17-ethylene ketal of androst-4-ene-3,17-dione is formed in 75%... 

Cross-conjugated dienones are quite inert to nucleophilic reactions at C-3, and the susceptibility of these systems to dienone-phenol rearrangement precludes the use of strong acid conditions. In spite of previous statements, A " -3-ketones do not form ketals, thioketals or enamines, and therefore no convenient protecting groups are available for this chromophore. Enol ethers are not formed by the orthoformate procedure, but preparation of A -trienol ethers from A -3-ketones has been claimed. Another route to A -trien-3-ol ethers involves conjugate addition of alcohol, enol etherification and then alcohol removal from la-alkoxy compounds. 

A"" -3-Ketones are more reactive than cross-conjugated A ""-3-ketones. A"" -3,3-CycIoethylenedioxy compounds can be easily prepared by acid-catalyzed reaction with ethylene glycol or by exchange dioxolanation. 3,3-Cycloethylenedioxy-A -dienes can be prepared from 3,3-cycloethy-lenedioxy-A -enes by allylic bromination and dehydrobromination. Acid hydrolysis yields A"" -3-ketosteroids. ... 

The investigation of the photochemistry of steroidal cross-conjugated dienones was initiated by both Barton and Jeger and was pursued most extensively by the latter group. Despite the frequently complex product patterns which result from prolonged dienone irradiations, some prepara-tively convenient and useful methods for the synthesis of novel systems have been worked out in this field. 

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). ... 

Photochemical cyclohex-2-enoneH> bicyclo (3.1.0) hexan-2-one rearrangement irradiation of testosterone acetate, 322 Photochemical rearrangements of cross-conjugated cyclohexadienones and their photoisomers... 

Photochemical oxacarbene formation, 307 Photochemical rearrangements of cross-conjugated cyclohexadienones, 330 Photochemical rearrangements of enol esters and enol lactones, 339... 

Conjugated, cross-conjugated, and homoconjugated fluoroalkenes react with halogens to yield predominantly 1,4-adducts Results from the reactions of a series of conjugated fluoroalkenes with elemental halogens are summarized in Table 4 In nearly all cases, the fraMs-l,4-addition products are formed exclusively [Il ... 

A cross-conjugated perfluorotnene is unreactive toward bromine, however, photochemical chlorination yields the 1,4 addition product [I2 (equation 5)... 

Dienamine 56a has been reported to undergo a 1,4 cycloaddition with acrylonitrile to form bicycloaminonitrile 57 in a 74% yield (61). A recent report has indicated that both possible 1,4-cycloaddition adducts are obtained from the reaction of acrylonitrile with a 1 1 equilibrium mixture of the linear- and cross-conjugated isomers of dienamine 56b (61a). 

While the usual eonsequence of hydration of enamines is eleavage to a secondary amine and an aldehyde or ketone, numerous cases of stable carbinolamines are known (102), particularly in examples derived from cyclic enamines. The selective terminal hydration (505) of a cross-conjugated dienamine-vinylogous amide is an interesting example which gives an indication of the increased stabilization of the vinylogous amide as compared to simple enamines, which is also seen in the decreased nucleophilicity of the conjugated amino olefin-carbonyl system. 

In contrast to the saturated azlactones, the Friedel-Crafts reaction of 2-substituted-4-arylidene-5-oxazolones is quite complex and may follow several different courses, often concurrently, depending on both reaction conditions and structural variations in the arylidene ring. This behavior is readily interpreted in terms of the a,)S-unsaturated carbonyl moiety and the cross-conjugated system containing nitrogen, both of which provide potential reaction sites in addition to the lactone carbonyl group. The reaction has been investigated " ... 

Tlie name fulvaleiie was first mentioned by R. Brown (49TFS296), who expanded the class of fulvenes for those compounds containing two cyclic poly-enic systems with a central double bond. Tlius, depending on the ring size, cy-clopropylidenecyclopropene 1, fulvaleiie 2, heptafulvalene 3 and the unsymmetrical hybrid molecules triapentafulvalene (calicene) 4, triaheptaful-valene 5, and pentaheptafulvalene (sesquifulvalene) 6 are members of this class of cyclic cross-conjugated systems (Scheme 1). 

Tliis chapter covers nitrogen-containing fulvalenes that can be obtained by replacement of CH=CH and/or CH, for example, types 1-3 starting from compounds 1-6. Compounds in which nitrogen atoms are arranged on the periphery of the cross-conjugated system as in 15 or 16, as well as derivatives in which the central double bond contains heteroatoms as in 17, are not included. For azoniafulvalenes of type 17 and related heterocyclic betaines see (94AFIC197). 

Concerning nomenclature, fulvalene 2 and its related systems 1 and 3-6 are the parent structures of this class of heterocyclic cross-conjugated compounds. Both ring systems are numbered as shown in formula 9 (1,4,5,8-tetraazafulva-lene) beginning at the heteroatoms. Alternatively, as in the case of heptafulva-lene 10 (3,3 -diazaheptafulvalene), the numbers 1-7 and l -7 can be used.Tlie use of the name of the parent heterocycle connected by an olefinic double bond is often favored for the nomenclature of electron-rich olefines, for example, bis[3-(2,6-diisopropylphenyl)-4,5-dimethylthiazol-2-ylidene] for compound 51a (97LAR365). Similarly, azafulvalenes of type 11 and 12 can be re-... 

An intensely colored by-product of the photolysis reaction of methyl-2-azidobenzoate has been identified as the first known derivative of 3,3 -diazaheptafulvalene 70 (94LA1165). Its molecular mass was established by elemental analysis and mass spectroscopy as that of a formal nitrene dimer, whereas and NMR studies demonstrated the twofold symmetry as well as the existence of a cross-conjugated 14 7r-electron system in 70. Involving l-azido-2,3-dimethoxy-5,6-dimethoxycarbonylbenzene in thermal decomposition reactions, the azaheptafulvalene 71 could be isolated and characterized spectroscopically and by means of X-ray diffraction. Tliis unusual fulvalene can be regarded as a vinylogous derivative of azafulvalenes (96JHC1333) (Scheme 28). 

Among the azafulvalenes, those possessing three nitrogen atoms in the cross-conjugated system are the smallest group to date.Tliere are a number... 

Although azafulvalenes as yet comprise a small class of cyclic cross-conjugated compounds, they have been the subject of a variety of theoretical investigations spanning a range from the Hiickel (HMO) theory to the... 

D. Pseudo-cross-conjugated Heterocyclic Mesomeric Betaines. 131... 

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