Enantiotopos selectivity

A related approach is the enantiotopos-selective ester hydrolysis with PLE or other lipases of mejo-diesters 16-19 and of mesodiacetates 20-24. 

This transformation has been extended recently by the development of methods for the enantiotopos selective lithiation of a prochiral 2-butenylcarbamate and the subsequent formation of chiral 2-butenyltitanium reagents from these species (Scheme 10-94) [180]. The reaction of -2-butenylcarbamate 279 with a solution of 5ec -butyllithium and (-)-sparteine generates a crystalline lithium complex 280-sparteine. Upon addition of 4 equivalents of Ti(Oi-Pr)4, a homogeneous titanium complex 283 (see below) is formed. The titanium reagent is then allowed to react with aldehydes to produce diastereomerically pure homoallylic alcohols 282 with good to high enantioselectivity. The high enantioselectivity observed in the... 

Enzymatic Baeyer-ViUiger oxidations have been shown to be highly enantioselec-tive in the desymmetrization of cychc ketones to produce lactones with considerable synthetic potential [144]. So far, it was an open question as to how much the inherent enantiotopos selectivity of the monooxygenases employed would be transferred to corresponding regioselectivities if nonsymmetrical enantiomerically pure ketones are the substrates (Scheme 3.90). 

Hashmi ASK, Hamzic M, Rominger F, Bats JW (2009) Gold catalysis enantiotopos selection. Chem Eur J 15 13318-13322... 

By enantiotopos-differentiating deprotonation the lithiated complex is formed in a reagent-controlled reaction with excellent selectivity. The lithiated center of the complex is assumed to have the S configuration, as follows from the carboxylation, to give an (7 )-lactic acid derivative based on the reasonable assumption of metalloretentive electrophilic attack. Trapping with chlorotrimethylstannane gave the corresponding chiral (.S -SjS-dimethyl-l-trimethylstannyl-alkyl-l-oxa-4-azaspiro[4.5]decane-4-carboxylates. Enantioselectivity of the overall transformation is excellent. 

Selectivity in enantiotopos-differentiating acylation and phosphorylation of meso-diols can rival that of enzymes. The organocatalysts employed include chiral phosphines, chiral diamines, chiral DMAP derivatives and peptides identified from combinatorial libraries. The highest selectivity in meso diol desymmetrization has been achieved with a planar-chiral Fu catalyst. It seems the substrate scope of this process is not yet broadly explored. Because of their sequential variability it is to be... 

The face selectivity observed during the enantioselective epoxidation of 9led several groups46-55 to apply the reaction to l,4-pentadien-3-ol (10) which contains a prosterogenic center. This example has already been cited in Section A.l as an enantiotopos- and diastereoface-differentiat-ing reaction. As expected, the epoxidation of 10 proceeds with both high enantio- and diastereoselection to provide mixtures of 11 which are > 95 % one diastereomer. The enantiomeric products 11 are valuable intermediates for the synthesis of numerous chiral natural products. 

In the formation of carboxylic esters in an anhydrous organic solvent, its hydrophobicity and the water activity have a major influence on the reaction [30, 36, 134l Hence, the organic solvent used can significantly influence the selectivity of a lipase-catalyzed enantiotopos- or enantiomer-differentiating reaction. Furthermore, the acyl donor may influence reactivity and selectivity. 

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