Polar double bonds, asymmetric

ASYMMETRIC PROTONATION OF CARBANIONS AND POLAR DOUBLE BONDS APPLICATION TO TOTAL SYNTHESES... 

However, despite the considerable efforts devoted to address the fundamental issues toward the development of asymmetric protonation, its applications to natural or bioactive synthesis remain sporadic. Herein, two main strategies, namely the enantioselective protonation of metal enolates, especially silicon enolates and the protonation of polar double bonds, i.e., Michael acceptors, were depicted trough the most relevant synthetic applications. These two strategies led to the synthesis of fragrance, natural products, ° bioactive compoundsand... 

Asymmetric Protonation of Carhanions and Polar Double Bonds Application to Total Syntheses... 

Depolarization data on trimethylphosphine oxide are now available and the relationship between the symmetric and asymmetric POP vibrations has been equated for diphosphates, and some halogen and metal salt derivatives. The polarization of a carbonyl group produced by its conjugation with an ylide causes a large decrease in vco- This shift to lower frequency is increased further when a double bond is interposed, thus increasing the extent of conjugation. -... 

Enoate reductase reduces a,/3-unsaturated carboxylate ions in an NADPH-dependent reaction to saturated carboxylated anions. Useful chiral synthons can be conveniently prepared by the asymmetric reduction of a triply substituted C—C bond by the action of enoate reductase, when the double bond is activated with strongly polarizing groups [22]. Enoate reductases are not commercially available as isolated enzymes therefore, microorganisms such as baker s yeast or Clostridium sp. containing enoate reductase are used to carry out the reduction reaction. 

The polarized carbon-nitrogen double bond can also be reduced enantiospecifically using chiral hydrides (7lJCS(C)2560). Although only modest asymmetric inductions were observed, this method holds promise for the synthesis of chiral piperidines (equation 54). 

Optical yields are always low in asymmetric reduction of simple olefins where no polar groups are close to the double bond 41a,51). 

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