Enantiomer isolation enantiomeric enrichment

In 1996, Hamann and Hoft were able to obtain 1,2,3,4-tetrahydronaphthyl hydroperoxide (THPO, 16b) in enantiomerically enriched form starting from the racemic mixture by selective decomposition of one enantiomer in the presence of Jacobsen s catalyst 21 . Besides the enantiomerically enriched hydroperoxide (5)-16b, also the corresponding alcohol (/f)-l-tetralol (19b) was isolated in enantiomerically enriched form (opposite... 

In this enzymatic transformation, three optically active compounds were prepared in one step. Besides the enantiomerically enriched hydroperoxide (5)-16/17a, also the opposite enantiomer of the corresponding alcohol (/f)-19/18a and enantiomerically enriched (S)-sulfoxide 23 could be isolated (equation 13). 

The intramolecular PKR was used by Fox and Pallerla (150) to obtain a key intermediate in the enantioselective synthesis of (—)-pentalenene 176, the unnatural enantiomer of the angular triquinane natural product. Using Co2(CO)8 (60) mol%) as catalyst in tolnene and 1 atm of CO, enantiomerically enriched (91% ee) cyclopropene 174 gave the PK addnct 175 (4 1 dr) in 64% isolated yield. The use of the known PKR promoter tetramethylthiourea (TMTU) was found to be critical in obtaining good yield of the desired tricyclic product (Scheme 80). 

After filtration, the alcohol enriched in one enantiomer was obtained from the filtrate, the other one was isolated from the solid complex. Thermal methods such as distillation or sublimation were used for liberation of the optically active alcohols from the complexes. [38] Time-scaled experiments showed continous increasing of the enantiomeric excess (ee) in the crystalline diastereoisomeric complexes of 26 and 8 for 14 days, then these ee values practically did not change within the next 16 days (Figure 1). 

Even nowadays, particularly in industrial processes, the separation of enantiomers of racemic acids and bases is based on this molecular chiral recognition. The less soluble, i.e. the more stable of these diastereomer salts crystallizes even if the chiral agent in the better soluble salt is replaced by an achiral reagent of similar chemical character, or eventually eliminated, or substituated by a solvent. In this case, a mixture enriched with the more stable diastereomer can be isolated by filtration from the solution of the achiral salt of the enantiomeric mixture or the free enantiomers [2,3]... 

Although initially prepared and evaluated as a racemate, the NMDA antagonist activity was likely to reside primarily in a single enantiomer. The stereoselective nature of the NDMA receptor is well established, albeit not completely understood. Consequently, several attempts have been undertaken to develop synthetic protocols that would allow preparation of optically active compounds. Early reports of preparation of optically active co-amino-o-carboxyalkylphosphonic acids describe the preparation of (.S )-A P-3 from an optically active amino nitrile prepared by reaction of diethyl 1-formylphosphonate with hydrogen cyanide and (5)-(-)-a-methylbenzylamine. Acid hydrolysis, enrichment of the diastereomers by fractional recrystaUization, and debenzylation lead to the isolation of (.S )-A P-3 in 86% enantiomeric excess. " Recently reported procedures, which use chemoenzymatic processes, offer a more convenient and mild approach for the production of optically pure aminophosphonic acids. Enzymatic hydrolysis of amides using penicillinacylase (EC... 

Similar results were observed for the rearrangement of enantio-enriched secondary allyl ester (31) to give ketones (32) and (33). Both enantiomers reacted with retention of configuration in the presence of chiral ligands (30a, c-h), however in the case of i -(31) the product of the Carrol rearrangement was isolated with the same enantiomeric purity with both the chiral imine (30h) and the achiral imine (30b). The explanation of this phenomenon is however not clear (Scheme 11). 

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