Ketones nonconjugated alkenic

Rearrangements of nonconjugated alkenes have also been observed. Thus, a triterpenoid (partial structure 38) afforded the alkene (39 32% yield) upon reduction (Barton modification), a result attributed to initial base-induced isomerization of (38) to the corresponding a,3-unsaturated ketone followed by rearrangement during reduction. ... 

Nonconjugated alkenes may be assembled by using a siloxide elimination, but the nucleophile is usually made in a different way since bases are unable to remove a proton alpha to a silicon without conjugative stabilization. Organolithium reagents will add to triphenylvinylsilane and may then be used with an aldehyde or ketone as exemplified by the synthesis of the alkene precursor of the sex pheromone of the gypsy moth (Eq. 7.30) [52]. 

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. 

An important cyclization procedure involves acid-catalyzed addition of diene-ketones such as 58, where one conjugated alkene adds to the other conjugated alkene to form cyclopentenones (59). This is called the Nazarov cyclization Cyclization can also give the nonconjugated five-membered ring. ... 

This method provides a convenient synthesis of alkenes with the double bond in a relatively unstable position. Thus reduction of the p-toluenesulfonylhydrazones of a,(3-unsaturated aryl ketones and conjugated dienones gives rise to nonconjugated olefins. Unsaturated ketones with endocyclic double bonds produce olefins with double bonds in the exocyclic position. The reduction of p-toluenesulfonylhydrazones of conjugated alkynones furnishes a simple synthesis of 1,3-disubstituted allenes. ... 

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 regioselectivity of Friedel-Crafts acylations of unsymmetrical alkenes can often be predicted simply by consideration of the alternative carbenium ions formed in an initial electrophilic attack. Pathways via tertiary carbocations are generally preferred over those involving secondary ions. It is the subsequent fate of the initially generated ion that determines the products formed. As has been indicated already, elimination of a proton completes a substitution, although there is a predominance of nonconjugated unsaturated ketone formed, and treatment with base is required to form the conjugated product."... 

The acylation of alkenes gives rise to unsaturated ketones, which themselves may be further acylated under the same reaction conditions. This is particularly the case with the nonconjugated products that often are formed preferentially. The diacyl derivatives readily cyclize to form pyrylium salts when this can be accommodated, and the sequence represents one of the best strategies for the formation of pyrylium salts symmetrically substituted at the 2- and 6-positions. Friedel-Crafts acylation as a route to pyrylium salts has been reviewed, and compared with other synthetic strategies. ... 

Yields of pyrylium salts are often higher in diacylations of alkenes than in the acylation of an equilibrium mixture of unsaturated ketones. This may be attributed to the preferential formation of the nonconjugated isomer in the first acylation, whereas isomerization of the (usually) thermodynamically more stable isomer is required before an a,p-unsaturated ketone may acylate. 

Disubstituted alkenes react with 1-chloro- and l-bromo-2-trifluoroacetylacetylenes even at 20°C. However, the 1,2-substituted alkenes need heating at 70-80°C. Tarring does not occur and the reaction leads to a mixture of cyclobutene (A) and a product of ene reaction (B) (Scheme 4.8 and Table 4.1). The ene-adducts readily add bromine to the nonconjugated double bond, while cyclobutenes react slowly even at 0°C. The bromination allows the cyclobutenes to be separated by distillation. So, this approach made accessible a class of fluorine-containing cyclobutenyl ketones for use as intermediates in organic synthesis. 

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