Enantioselectivity alcohol formation, chiral secondary

The formation of chiral secondary homoallylic alcohols via the enantioselective addition of allylic nucleophiles to aldehydes is an important tool in organic synthesis. An efficient way to achieve this transformation is to use allylic organometallic reagents in the presence of chiral Lewis acid catalysts. The most widely studied catalysts in the area... 

Capillary gas chromatographic determination of optical purities, investigation of the conversion of potential precursors, and characterization of enzymes catalyzing these reactions were applied to study the biogenesis of chiral volatiles in plants and microorganisms. Major pineapple constituents are present as mixtures of enantiomers. Reductions, chain elongation, and hydration were shown to be involved in the biosynthesis of hydroxy acid esters and lactones. Reduction of methyl ketones and subsequent enantioselective metabolization by Penicillium citrinum were studied as model reactions to rationalize ratios of enantiomers of secondary alcohols in natural systems. The formation of optically pure enantiomers of aliphatic secondary alcohols and hydroxy acid esters using oxidoreductases from baker s yeast was demonstrated. 

Chiral organophosphorus compounds have been found to act as catalysts in numerous enan-tioselective reactions. This review highlights recent developments in this area and more precisely in the kinetic resolution of secondary alcohols, the enantioselective borane reduction of ketones, and in the asymmetric C-C bond formation with the introduction of asymmetric two-center catalysts. 

While the polymerization of optically inactive AA-BB and AB monomers under DKR conditions leads to chiral polyesters, these approaches always result in limited molecular weights since a condensation product is formed that needs to be effectively removed. A solution for this would be to use the eROP of lactones, where no condensation products are formed during polymerization. In principle, the eROP of lactones can lead to very high MW polyesters (>80kgmol 1) [57]. Addition of a methyl substituent at the ro-position of the lactone introduces a chiral center. Peeters et al. conducted a systematic study of substituted e-caprolactones which revealed that monomers with a methyl at the 3-, 4-, or 5-position could be polymerized enantioselectively while a methyl at the 6-position (a-methyl-e-caprolactone, 6-MeCL) could not [58]. The lack of reactivity of the latter monomer in a Novozym 435-catalyzed polymerization reaction was attributed to the formation of S-secondary alcohol end-groups. These cannot act as a nucleophile in the propagation reaction since the lipase-catalyzed transesterification is highly R-selective for secondary alcohols. 

---