Selenenic acids synthesis

When selenenic acids are generated in the presence of alkenes, addition reactions similar to those of selenenyl halides occur. Oxidation and syn elimination of the adducts (42) provide a convenient synthesis of allylic alcohols (equation... 

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... 

Elimination reactions leading to olefins are usually performed on the corresponding selenoxides 3 9,1 12) (Scheme 2 a). These are often unstable and decompose at room temperature to olefins and selenenic acid (further oxidized to the more stable seleninic acid by excess of oxidant). Hydrogen peroxide in water-THF, ozone and further treatment with an amine or tert-butyl hydroperoxide without or with alumina proved to be the method of choice for such a synthesis of olefins. The reaction is reminiscent of the one already described with aminoxides or sulfoxides 22) and occurs via a syn elimination of the seleninyl moiety and the hydrogen attached to the P-carbon atom. However it takes place under smoother conditions. 

Since seleninic acids and their anhydrides are much more stable than the corresponding selenenic compounds, they can be employed as starting materials for the synthesis of selenenic acids and their anhydrides. Reduction of seleninic acids and their anhydrides with appropriate reducing reagents such as hydrazines [35], hypophosphorous acid (H3PO2) [36], and thiols [37] is a useful alternative method for the synfhesis of selenenic acids and their anhydrides. 

The synthesis of a stable selenenic acid was also achieved by utilizing a Bmt group and its reactivity was investigated in detail. In the catalytic cycle of GPx and many of the synthetic mimics of GPx, it is thought that a selenenic acid intermediate is formed by oxidation of a selenol with peroxides. However, no experimental evidence has been obtained for this important chemical transformation even trapping of an intermediary selenenic acid has not been reported. 

Fig.(5). An alternative synthesis of the decalone (50) from the Wieland-Miescher ketone (1) is described, its hydroxymethylene derivative on subjection to selenenation followed by oxidation afforded the unsaturated aldehyde (52) in excellent yield. The protection of the aldehyde moiety of (52) followed by the conversion of the resulting product to the adduct (54) led to the formation of the enol ether (55) which on hydroxylation yielded a mixture of hydroxy-aldehyde (56) and (57). This on acid hydrolysis afforded muzgadial (58) in 85% yield.

Important uses of selenenic and seleninic acids and anhydrides (see also the later sections) have been established, in the synthesis of )ff-silyloxy-a-(phenyl-seleno)aldehydes from allyl silyl ethers (Scheme 3) and in the high-yield syntheses of 2-acetoxyalkyl phenyl selenides [using PhSe(0)OH and AcOH j and... 

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