Conjugated compounds, reaction with

Acetylenic Grignard reagents are effective for reaction with conjugated compounds such as -ionone, which is an important starting material for the preparation of carotenoids that readily undergoes enolation under basic conditions. Some examples of synthetic processes are given [Eqs. (30-33) 46-49]. 

Ordinary Grignard reagents react with a, -unsaturated carbonyl compounds and afford both 1,2-adduct and 1,4-adduct. However, methylsulfonyhnethylmagnesium bromide or p-tolylsulfonylmethylmagnesium bromide gave only 1,2-adducts in the reaction with conjugated carbonyl compounds such as crotonaldehyde, cinnamaldehyde, trans-4-phenyl-3-buten-2-one, benzalacetophenone and l,5-diphenyl-2,4-pentadien-l-one. 

Rees and co-workers in their study of the reactions of trithiazyl trichloride in the preparations of heterocyclic compounds have noted that the isothiazolo[5,4-, isothiazole compound 140 was produced in low yield on reaction with conjugated dienes, along with the other heterocyclic systems 142-145 in much higher yields (Equation 28). Since it is known that trithiazyl trichloride is in thermal equilibrium with its monomer NSCl (Equation 29), the authors propose the so-called criss-cross cycloaddition reaction (Equation 30) which has been reported for azabu-tadienes, but this represents the first example of such a criss-cross cycloaddition to an all-carbon diene <1998CC1207>. 

Alkenyl trityl ketones, prepared by reaction of 1 with a ketone followed by dehydration, undergo exclusive conjugate addition on reaction with organolithium compounds. The adducts are cleaved to primary alcohols on reduction with LiB(C2H5)3H. 

Two particular reactions are characteristic of the (3-functional alkyltin compounds. First, conjugative electron attraction by the functional group makes C-C=X a good leaving group, and the Sn-C bond has an enhanced reactivity in substitution (e.g. hydrolysis) and addition where C=X is a nitrile, ketone, ester, or amino group, the compounds add across the N=C bond in isocyanates, and undergo Reformatsky-type reactions with carbonyl compounds.91-92... 

With their extended system of conjugated double bonds, the carotenoids contain a reactive electron-rich system that is susceptible to reactions with electrophilic compounds. This structure is responsible for high sensitivity of carotenoids to oxygen and light. The central chain of conjugated double bonds can be oxidatively cleaved at various points, giving rise to the family of apocarotenoids. 

Except in the case of N-acetyl-cysteine, the chemical problems encountered In synthesis of the conjugates summarized under this heading are similar due to the amphoteric character of the nucleophiles. In general Che reactions with thiol compounds must be carried out In an inert atmosphere in order to minimize oxidation. The starting material, for synthesis of some of Che xenobiotic conjugates below, require multistep syntheses, but the procedures for their preparations will not be discussed here. 

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