Phosphorus ylides 0-oxido

Schlosser, M., Coffinet, D. a-SubstItutlon plus carbonyl olefinatlon via P-oxido phosphorus ylides (SCOOPY) reactions. Stereoselectivity of allyl alcohol synthesis via betaine ylides. Synthesis 1971, 380-381. 

Schlosser, M., Christmann, K. F., Piskala, A. Olefinatlon reactions with phosphorus ylides. II. P-Oxido phosphorus ylides in the presence and absence of soluble alkaline metal salts. Chem. Ber. 1970, 103, 2814-2820. 

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... 

In this context, one may also pay attention to the so-called "betaine-ylides" that act as key intermediates in stereocontrolled Wittig olefination reactions. They are generated from the ordinary adducts obtained by the combination of a phosphine ylide and an aldehyde in the presence of lithium bromide (or another soluble lithium salt). When the P-betaines are a-deprotonated with phenyllithium, the stereocenter at the phosphorus-adjacent carbon atom becomes configurationally mobile. In this way, erythro/threo mixtures can spontaneously convert into virtually pure /Areo-betaine ylides (p-lithiooxy ylides, P-oxido ylides). Reprotonation and subsequent elimination of triphenylphosphine oxide affords trans olefins, whereas a-substitution by electrophiles other than acids leads to branched alkenes exhibiting a well-defined stereochemistry "("three-dimensional"" Wittig reaction or SCOOPY method). ... 

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