A-Lithio selenoxides

Applications Based on a-Lithio-selenoxides. Full details have appeared on the uses of cf-lithio-selenoxides, generated by low-temperature deprotonation of selenoxides with LDA. They are probably less nucleophilic than the corresponding cf-lithio-selenides. Their reactions leading to olefins, allylic alcohols, and (less well explored) enones are summarized in Scheme 15, and they have also been applied in the synthesis of silyl enol ethers (Scheme 16). 

This reaction allows the synthesis of (i) p,p -dienols by oxidation of p-hydroxy-y-alkenyl sel-enides or more conveniently from a-lithioalkyl selenoxides and enones (Scheme 136 and 166) (ii) p,5-dienols from l-lithio-3-alkenyl phenyl selenoxides and carbonyl compounds (Scheme 177) and (iii) 2-(r-hydroxyalkyl)-1,3-butadienes from 1-methylselenocyclobutyllithium and carbonyl compounds (Scheme 178). a,p-Unsaturated alcohols bearing a methylselenoxy or a phenylselenoxy group at the a-position do not lead on thermolysis to propargyl alcohols however, those be ng a (trifluoro-methylphenyl)selenoxy moiety at the a-position are valuable precursors of such con unds (Scheme 179). ... 

Reich has now described the ready preparation of a-lithio-selenides and -selenoxides, which smoothly condense with aldehydes and ketones. The resultant /3-hydroxy-selenides and -selenoxides can be reductively eliminated under very mild conditions to give tetrasubstituted olefins, which are not readily available from the Wittig reaction. The /3-hydroxyselenides can be converted into olefins under milder conditions than their sulphur equivalents, using methane-sulphonyl chloride in triethylamine. 

Applications of Selenium-stabilized Carbanions. The previous sections show that many selenides are not effectively deprotonated, and so the corresponding cf-lithio-selenides must be prepared by other routes, such as cleavage of selenoketals (see Scheme 1) or of seleno-orthoesters. An alternative (but indirect) procedure is the low-temperature (usually at —78 °C) deprotonation (by LDA) of selenoxides that are generated in situ at temperatures below 0 A further approach... 

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