Racemates s. under

Racemates s. Stereoisomers Racemization s. under Radical reactions Radical reactions... 

The racemate of the monomer was found to be iso-stractural with its enan-tiomorph, as it crystallizes in the same space group as a sohd solution, where the sec-butyl groups of opposite handedness are disordered. However, an accurate determination of the phase diagram between S(+)l and R(—)1, under equilibrium conditions, revealed the presence of an immiscibiUty gap in the range 60 40 to 40 60 [49]. Therefore, the crystallization of a large batch of racemic 1 under thermodynamically controlled conditions was associated with the precipitation of equal amounts of crystals of either handedness, with a constant internal composition, as defined by the boundaries of the eutectic. The presence of an immiscibihty gap imphes two different effects on the one hand it interferes with the requirements of an absolute asymmetric synthesis from racemic 1, while on the other hand it provides a most efficient way in which to amplify chirahty via the crystalhzation of nonracemic mixtures of compositions, which are outside the boundaries of the eutectic. Enantiopure oHgomers could be generated from mixtures of molecular composition R S of 60 40 [50]. 

Davies s group also extended this methodology toward the asymmetric syntheses of substituted cyclopentenes. Although chiral monocyclic VCPs isomerized to racemic cyclopentenes under the reaction conditions, the use of fused chiral VCPs led to the products in high yields with excellent enantioselectivity f Scheme 11.14T ... 

A wide diversity of products may be generated from the transformation of a racemic mixture under the action of an enantiopure reagent. An interesting situation is the preferential formation of two products, one deriving exclusively from the (R)-substrate and the other one deriving from the (S)-substrate (Scheme 2.4a). This is an enantiodivergent reaction - Pjj, Sg -> Pg. The actual situation is... 

Di(isopinocampheyl)borane Resolution of racemic ethylene derivatives with optically active boranes s. 19, 720 Lithium tetrahydridoborate s. under (C2H )gN,HCl Sodium tetrahydridoborate/boron fluoride Boranes from ethylene derivatives s. 16, 753... 

Finally, an elegant synthesis of brefeldin A started from 71 to afford (S)-72 with 99% e.e. [106], and a kinetic resolution by BY has been used for the synthesis of endo-brevicomin [107,108] (Fig. 21). Thus, reduction of racemic 75 under anaerobic conditions afforded via 76 a mixture of endo-11 (corresponds to a syn-selective reduction) and exo-78 [107]. 

Studies of reaction mechanisms ia O-enriched water show the foUowiag cleavage of dialkyl sulfates is primarily at the C—O bond under alkaline and acid conditions, and monoalkyl sulfates cleave at the C—O bond under alkaline conditions and at the S—O bond under acid conditions (45,54). An optically active half ester (j -butyl sulfate [3004-76-0]) hydroly2es at 100°C with iaversion under alkaline conditions and with retention plus some racemization under acid conditions (55). Effects of solvent and substituted stmcture have been studied, with moist dioxane giving marked rate enhancement (44,56,57). Hydrolysis of monophenyl sulfate [4074-56-0] has been similarly examined (58). 

Ibuprofen is an analgesic sold under various names including Advil, Motrin, and Nuprin. The material is sole as a racemic mixture, but only one enantiomer acts as ar analgesic. The other enantiomer is inactive. Assign R oi S forms to the two enantiomers of ibuprofen. 

The configuration of the amine was retained, except in the case of amino acid derivatives, which racemized at the stage of the pyridinium salt product. Control experiments showed that, while the starting amino acid was configurationally stable under the reaction conditions, the pyridinium salt readily underwent deuterium exchange at the rz-position in D2O. In another early example, optically active amino alcohol 73 and amino acetate 74 provided chiral 1,4-dihydronicotinamide precursors 75 and 76, respectively, upon reaction with Zincke salt 8 (Scheme 8.4.24). The 1,4-dihydro forms of 75 and 76 were used in studies on the asymmetric reduction of rz,>S-unsaturated iminium salts. 

A key transformation in Corey s prostaglandin synthesis is a Diels-Alder reaction between a 5-(alkoxymethyl)-l,3-cyclopenta-diene and a ketene equivalent such as 2-chloroacrylonitrile (16). As we have already witnessed in Scheme 3, it is possible to bring about a smooth [4+2] cycloaddition reaction between 5-substituted cyclopentadiene 15 and 2-chloroacrylonitrile (16) to give racemic 14 as a mixture of epimeric chloronitriles. Under these conditions, the diastereomeric chloronitriles are both produced in racemic form because one enantiotopic face of dienophile 16 will participate in a Diels-Alder reaction with the same facility as the other enantiotopic face. In subsequent work, Corey s group demonstrated that racemic hydroxy acid 11, derived in three steps from racemic 14 (see Scheme 3), could be resolved in a classical fashion with (+)-ephe-... 

The noteworthy successes of a relevant model study12 provided the foundation for Merck s thienamycin syntheses. In the first approach (see Schemes 2 and 3), the journey to the natural product commences from a readily available derivative of aspartic acid this route furnishes thienamycin in its naturally occurring enantiomeric form, and is noted for its convergency. During the course of this elegant synthesis, an equally impressive path to thienamycin was under parallel development (see Schemes 4 and 5). This operationally simple route is very efficient (>10% overall yield), and is well suited for the production of racemic thienamycin on a commercial scale.. x... 

Reaction of racemic a-alkyl-a-(l H-1,2,4-triazol-l-yl)acetophenones with Grignard reagents in boiling diethyl ether affords exclusively the (RS/S -enantiomeric pairs. On the other hand, reaction of racemic a-alkoxy-x-(17/-l,2,4-triazol-l-yl)acetophenonc with Grignard reagents leads, under chelation control, to the (R/ /5S)-enantiomeric pair82. 

For an efficient enzymatic DKR the following requirements must be fulfilled (i) the KR must be very selective ( > 20) (ii) the racemization must be fast (at least 10 times faster than the enzyme-catalyzed transformation of the slow reacting enantiomer, krac >10 kent-s) (hi) the racemization catalyst must not react with the product of the reaction (iv) the KR and the racemization must be compatible under the same reaction conditions. 

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