Percent enantioselectivity

Enantioselectivity (percent enantioselectivity, % es) In a reaction or reaction sequence in which one enantiomer (Ei) is produced in excess, the enantioselectivity is the mole fraction formed of the major enantiomer, expressed as a percent ... 

Sunoj also predicted the enantioselectivity offered by a series of bicyclic proline analogs in the aldol reaction of acetone with p-nitrobenzaldehyde. The two best performing catalysts are 59 and 60 both are predicted to give an ee greater than 90 percent, which exceeds the selectivity afforded by proline itself. The TS geometries all display the Houk-List characteristics. 

In a follow-up paper, Jacobsen showed that the chiral guanidinium 80 provides catalytic power and enantioselectivity in the Claisen rearrangement systems such as Reaction 6.36. Here the reaction proceeded in 81 percent yield with an ee of 84 percent. The reaction is also diasteroselective for Reaction 6.37, the diastere-oselectivity increases from 11 1 without catalyst to 16 1 with 80. 

A notable feature of this catalytic system is that asymmetric induction is lower at the early stages of the reaction. A subsequent study revealed that in the reaction of methyl acrylate and cyclopentadiene, the cycloadduct interacts with the catalyst in a fashion such that the enantioselectivity is intimately tied to the percent conversion (Scheme 7). 

The final definition concerns formation of enantiomers, and the terms enantioselective and enantiospecific are used. If a reaction produces an unequal mixture of enantiomers it is enantioselective. If it generates only one enantiomer of two possibilities, it is enantiospecific. The baker s yeast reduction (see sec. 4.10.F) of 134 gave 135 with >99% ee (S). (Here % ee means percent of enantiomeric excess.) A 0% ee means a 50 50 mixture (racemic mixture), 50% ee means a 75 25 mixture and 90% ee means a 95 5 mixture. The predominance of the (S) enantiomer makes this reaction highly enantioselective. 

Catalyst Performance. The enantioselectivity expressed as enantiomeric excess (ee, in percent) of a catalyst should be more than 99% for pharmaceuticals... 

To understand the literature in this field, several terms need to be defined. An important practical property of these systems is their stereoselectivity or enantioselectivity, which is their ability to produce one enantiomer in preference to the other. This property is commonly given as the percent enantiomeric excess, ee, determined from an analysis of the products. If R and S are the percentages of the major and minor... 

Evans and coworkers reported Ce(IV)-PyBOX catalyzed highly enantioselective nitrone cycloaddition by employing a,P-unsaturated 2-acyl imidazoles. Five mole percent of Ce(OTf)4/Ph-PyBOX promoted the reaction under mild conditions (0°C, EtOAc), giving products in up to 99% yield, 97% ee, and endojcxo =... 

The basic requirement for an enantioselective synthesis is a significant energy difference between the transition states leading to the possible products. The greater the energy difference, of course, the greater the preference for one of the products. The extent of enantioselectivity is expressed in terms of the percent enantiomeric excess. 

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