Addition conjugated with

The racemization process involves removal of the a-hydrogen to form the enolate anion, which is favoured by both the enolate anion resonance plus additional conjugation with the aromatic ring. Since the a-protons in esters are not especially acidic, the additional conjugation is an important contributor to enolate anion formation. The proton may then be restored from either side of the planar system, giving a racemic product. 

Absorption at 240-250 nm by 4//-pyrans has been noted (61CB1784), although this has been disputed and in general 4//-pyrans are characterized by a weak shoulder at ca. 225 nm (69JOC3169). Introduction of an ethoxycarbonyl or acetyl group at C-3 causes a shift of the maximum to 270 and 284 nm, respectively, which are further shifted to 285 and 296 nm by a second of these substituents. These maxima are considerably different from those of 0-alkoxy a,/3-unsaturated esters and ketones indicative of additional conjugation with the second double bond of the pyran. 

A number of compounds substituted in this way were prepared. Thus Meister360 obtained 74 by the hydrolysis of dioxolan (73). Bulla-tenone (71), which is obtained from Myrtus buUata,361,362 is very similar, in spite of the additional conjugation with the phenyl ring, as is the compound 72, which is prepared by the hydration of a diynediol.363 The structure of a similar compound, prepared also from an acetyleneglycol,364 is not exactly known. 

Compound A is a cyclic, nonconjugated keto alkene whose carbonyl infrared absorption should occur at 1715 cm-. Compound B is an a,B-unsaturated, cyclic ketone additional conjugation with the phenyl ring should lower its IR absorption below 1685 cm-1. Because the actual IR absorption occurs at 1670 cm-1, B is the correct structure. 

The following is an example of an aldol reaction with a nonenolizable sink. The additional conjugation with the aromatic ring makes the follow-up elimination favorable. 

Cyclopentadiene contains conjugated double bonds and an active methylene group and can thus undergo a Diels-Alder diene addition reaction with almost any unsaturated compound, eg, olefins, acetylene, maleic anhydride, etc. The number of its derivatives is extensive only the reactions and derivatives considered most important are discussed. 

The additional parameter r is adjusted from reaction to reaction it reflects the extent of the additional resonance contribution. A large r corresponds to a reaction with a large resonance component, whereas when r goes to zero, the equation is identical to the original Hammett equation. When there is direct conjugation with an electron-rich reaction center, an equation analogous to Eq. (4.17) can be employed, but a is used instead of [Pg.210]

A significant modification in the stereochemistry is observed when the double bond is conjugated with a group that can stabilize a carbocation intermediate. Most of the specific cases involve an aryl substituent. Examples of alkenes that give primarily syn addition are Z- and -l-phenylpropene, Z- and - -<-butylstyrene, l-phenyl-4-/-butylcyclohex-ene, and indene. The mechanism proposed for these additions features an ion pair as the key intermediate. Because of the greater stability of the carbocations in these molecules, concerted attack by halide ion is not required for complete carbon-hydrogen bond formation. If the ion pair formed by alkene protonation collapses to product faster than reorientation takes place, the result will be syn addition, since the proton and halide ion are initially on the same side of the molecule. 

The addition of diazomethane to 17j -hydroxy-5a-androst-l-en-3-one (7) gives the A -pyrazoline (8) in which the C=N bond is conjugated with the 3-keto group. °°... 

When the enamine is in conjugation with a carbonyl function, as in a-aminomethylene aldehydes (528,529), ketones (530), or esters (531), a Michael addition is found in vinylogous analogy to the reactions of amides. An application to syntheses in the vitamin A series employed a vinyl lithium compound (532). 

Perhaps the most striking difference between conjugated and nonconjugated dienes is that conjugated dienes undergo an addition reaction with alkenes to yield substituted cyclohexene products. For example, 1,3-butadiene and 3-buten-2-one give 3-cycIohexenyl methyl ketone. 

Predominant formation of s-products on reaction of A -acyliminium ions, containing an additional conjugated double bond, with propylzinc iodide likewise arises from A(1 3) strain as discussed133. 

In contrast, a-alkoxycuprate 10, prepared from the corresponding stannane, gave the conjugate addition product with only 0-20% ee upon treatment with 2-cyclohexenone in the presence ofchlorotrimethylsilane67. However, the conjugate addition of 10 to ethyl propynoatc in the presence of chlorotrimethylsilane provided 11 in 85% yield with nearly 96% ee67. 

This reagent reacts with a,/3-unsaturated ketones to give kinetic products of exclusive 1,4-addition (2). With cyclic substrates, a strong preference for axial addition is observed, as is a susceptibility to steric hindrance. Transformation into the corresponding silylcuprate species permits conjugate addition to a wider variety of a,/3-unsaturated substrates (3,4). 

Michael addition is a 1,4-addition reaction of a nucleophile to an a, /1-unsaturated system in which the double bond is conjugated with a carbonyl group, enabling the formation of the corresponding enolate as an intermediate (equation 27). 

Data are also available with a-acetylenic aliphatic sulphones, which involve only two steps i.e., saturation of the triple bond without subsequent cleavage of the Caliphalic—S bond, since it is not reactive. However, the introduction of an aromatic ring to the S02 group does not lead, contrary to what is observed with enones, to a potential shift toward less reducing potential values. Thus, the aromatic moiety introduced apparently does not bring any additional conjugation effect but even seems to decrease the activation of the unsaturated bond, as shown by data in Tables 6 and 7 where most of the potentials refer to the same saturated calomel electrode under similar experimental conditions. 

For ortho, meta, and para diphenylbenzene we consider eight unexcited structures of the type indicated by the letter A in Fig. 5, thirty-six first-excited structures of type B, and thirty-six of type C, in which there is conjugation with the central ring. This includes all first-excited structures for m-diphenylbenzene. For ortho and para diphenylbenzene, however, there are in addition nine first-excited structures of type D, in which the two end rings are conjugated together. The secular equation for the ortho compound is... 

In some cases, conjugate addition has been performed on systems where a double bond is conjugated with a cyclopropyl ring. An example is ... 

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