Optical purity functions

For the construction of oxygen-functionalized Diels-Alder products, Narasaka and coworkers employed the 3-borylpropenoic acid derivative in place of 3-(3-acet-oxypropenoyl)oxazolidinone, which is a poor dienophile in the chiral titanium-catalyzed reaction (Scheme 1.55, Table 1.24). 3-(3-Borylpropenoyl)oxazolidinones react smoothly with acyclic dienes to give the cycloadducts in high optical purity [43]. The boryl group was converted to an hydroxyl group stereospecifically by oxidation, and the alcohol obtained was used as the key intermediate in a total synthesis of (-i-)-paniculide A [44] (Scheme 1.56). 

Fig. 5-14. The optical purity (R/S) of the outflows as a function of the number of transfer units (NTU) in the apparatus at equal flow rates. Different lines are given for different a values.

Similarly to the P-CHj group, secondary phosphine-boranes react smoothly in the presence of a base (BuLi, NaH) under mild conditions to afford other kinds of functionalized phosphine-boranes in good to high yields, without racemi-zation. Yet the success of deprotonation/treatment with an electrophile process to afford substituted phosphine derivatives without any loss in optical purity may depend on the deprotonation agents employed. Use of butyllithium usually provides the products with high enantiomeric excess in good to high yields [73]. 

Aldol-type reactions of nitrones (303) occur with electron-deficient ketones, such as a-keto esters, a, 3-diketones, and trifluoromethyl ketones. These reactions are catalyzed by secondary amines. The use of chiral cyclic amines A1-A7 leads to a-(2-hydroxyalkyl)nitrones (304) in moderate yields and rather high optical purity (Scheme 2.120) (381). The mechanism of the nitrone-aldol reaction of iV-methyl-C-ethyl nitrone with dimethyl ketomalonate in the absence and presence of L- proline has been studied by using density functional theory (DFT) (544). 

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

In contrast to phenolic hydroxyl, benzylic hydroxyl is replaced by hydrogen very easily. In catalytic hydrogenation of aromatic aldehydes, ketones, acids and esters it is sometimes difficult to prevent the easy hydrogenolysis of the benzylic alcohols which result from the reduction of the above functions. A catalyst suitable for preventing hydrogenolysis of benzylic hydroxyl is platinized charcoal [28], Other catalysts, especially palladium on charcoal [619], palladium hydride [619], nickel [43], Raney nickel [619] and copper chromite [620], promote hydrogenolysis. In the case of chiral alcohols such as 2-phenyl-2-butanol hydrogenolysis took place with inversion over platinum and palladium, and with retention over Raney nickel (optical purities 59-66%) [619]. 

Functionalized organomagnesium compounds Synthesis and reactivity 545 afforded the benzotetrahydrofuran 134 without loss of optical purity (equation 67). 

Protection of peptide aldehydes as semicarbazones serves two purposes (1) it reduces or eliminates racemization during chromatographic separation, and (2) the semicarbazone protects the aldehyde functional group during subsequent peptide coupling reactions. Crude aldehydes can be immediately reacted with semicarbazide hydrochloride to give the corresponding semicarbazones. The semicarbazones are purified on a silica gel column and are subsequently deprotected with 37% formaldehyde/HCl. To determine the optical purity, the aldehydes are immediately reduced with sodium borohydride followed by determination of the optical rotation of the alcohol (Table 3).[5]... 

Hydrogenation of 2,2,2-trifluoroacetophenone and its derivatives with a mixture of trans-RuCl2[(S)-xylbinap][(S)-daipen] and (CH3)3COK in 2-propanol gives the S alcohols quantitatively with a high optical purity (Scheme 1,64) [258]. Unlike with many chiral borane reagents [264], the sense of enantioface discrimination is the same as in hydrogenation of acetophenone. The electronic effects of 4 -substituents on the enantioselectivity are small. These chiral fluorinated alcohols are useful as components of new functionalized materials [265]. 

Chiral separation by HPLC is a practically useful method not only for determining optical purity but also for obtaining optical isomers, and numerous CSPs are presently on the market. In order to achieve the efficient resolution of chiral compounds, we have to choose a suitable chiral column and eluent. The polysaccharide-based CSPs have a high chiral recognition ability and offer a high possibility for the successful resolution of racemates including aliphatic and aromatic compounds with or without functional groups under normal and reversed-phase conditions. 

Marchelli used the copper(II) complex of histamine-functionalized P-cy-clodextrin for chiral recognition and separation of amino acids [27]. The best results were obtained for aromatic amino acids (Trp). Enantioselective sensing of amino acids by copper(II) complexes of phenylalanine-based fluorescent P-cyclodextrin has been recently published by the same author [28, 29]. The host containing a metal-binding site and a dansyl fluorophore was shown to form copper(II) complexes with fluorescence quenching. Addition of d- or L-amino acids induced a switch on of the fluorescence, which was enantioselective for Pro, Phe, and Trp. This effect was used for the determination of the optical purity of proline. 

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