Selenides carbonyl compound synthesis from

Another example of alkene synthesis by the pyrolysis of selenoxide is given in Scheme 4.14. The enolate derived from 4.18 reacts with either PhSeBr or PhSeSePh to form selenide 4.19. Oxidation of 4.19 gives selenoxide 4.20, which undergoes sy -elimination to give a,P Unsaturated carbonyl compound 4.21. 

For the synthesis of a-selenoalkyllithiums, the selenium-lithium exchange reaction is a good alternative to the almost impossible metallation of unactivated selenides.Thus it has been found that a large variety of selenoacetals, often readily available from carbonyl compounds and selenols, react with butyllithiums to provide a-selenoalkyllithiums - in very high yields (Scheme 2 see also Section 2.6.2.3). 

The combination of reactions described above (Sections 2.6.4.2 to 2.6.4.5) allows the selective synthesis of a large variety of alcohols, allyl alcohols, alkenes, epoxides and carbonyl compounds from p-hydroxyalkyl selenides. These products often can be obtained from two ca nyl compounds by activation of one of them as an a-selenoalkyllithium (Schemes 161-196). 

Selenoxide elimination is now widely used for the synthesis of a,p-unsaturated carbonyl compounds, allyl alcohols and terminal alkenes since it proceeds under milder conditions than those required for sulfoxide or any of the other eliminations discussed in this chapter. The selenoxides are usually generated by oxidation of the parent selenide using hydrogen peroxide, sodium periodide, a peroxy acid or ozone, and are not usually isolated, the selenoxide fragmenting in situ. The other product of the elimination, the selenenic acid, needs to be removed from the reaction mixture as efficiently as possible. It can disproportionate with any remaining selenoxide to form the conesponding selenide and seleninic acid, or undergo electrophilic addition to the alkene to form a -hydroxy selenide, as shown in... 

The stereoisomeric mixture of -hydroxyalkyl selenides resulting from the reaction of the a-selenoalkyllithium and the carbonyl compound has been often cleanly and easily separated into its constituents by liquid chromatography on silica gel (Schemes 124,133,134, and 170 172).200.206.222,226,229,258 59 jj,jg has, therefore, allowed the synthesis of each of the two stereoisomers of various di- and tri-substituted alkenes (Schemes 124,170 and 171 Scheme 172, a) and epoxides (Scheme 124 Scheme 172, b), which are otherwise obtained as intractable mixtures of stereoisomers through the conventional phosphorus or sulfur ylide methods. Last but not least, 2-lithio-2-methylselenopropane can be used as the precursor of various compounds bearing gem dimethyl substituted carbons, such as squalene, oxido-squalene, lanosterol and cholesterol. Use of commercially available perdeuterated or Ci or — 2 acetone allows the straightforward synthesis of the corresponding labelled compounds... 

Epoxides via methyl selenoacetals. Krief et al. have reported the synthesis of epoxides from two carbonyl compounds. The first step involves preparation of a dimethyl selenoacetal (1), followed by conversion to an a-methyl seleno-carbanion (a). These highly reactive carbanions react with even hindered carbonyl compounds to give 0-hydroxy methyl selenides (2), which are converted into selenonium salts by reaction with methyl iodide or dimethyl sulfate. 

The reaction of dithiocarbamates and diselenocarbamates with a-halogenated carbonyl compounds has proved to be a useful first step in the synthesis of l,3-dithiole-2-thiones and their analogues. Thus 2-dimethylamino-l,3-dithiolium salts (38) > and diselenolium salts (39) " are readily obtainable by this route, and these salts are converted into thiones (40 X = Y = S or Se, Z = S) or selones (40 X = Y = S or Se, Z = Se) with hydrogen sulphide or hydrogen selenide. The dihydroxybenzo-l,3-dithiole-2-thione (41) has been prepared from p-benzoquinone and dithiocarbamic acids. ... 

To overcome this limitation, the vinyl sulfide and selenide were used as the substrates instead of mono- and 1,2-disubstituted olefins [129]. With these substrates, the ene products are formed with almost complete enantioselectivity. The carbonyl-ene adducts have been applied to the synthesis of natural compounds such as (R)-(—)-ipsdienol, an insect aggregation pheromone [130] (Scheme 14.49). For trisubstituted olefin substrates, a more Lewis acidic catalyst prepared from 6,6 -Br2BINOL gave better results [128b, c]. 

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