Optical yields modifying reagent

The reduction of phenyl mesityl ketone was studied with LAH modified with amino alcohols 65 to 72 in ether (the ratio LAH alcohol ketone = 1.1 1.1 1) (83). Optical yields were modest, with the highest 39%, obtained with 65 as the chiral auxiliary reagent. It was observed that there is a relationship between the preferred enantiomeric product and the structure and absolute configuration of the carbons carrying the hydroxy and amino groups. Thus the threo... 

Correlation between the Rate, Optical Yield, and Absorbed Amount of Modifying Reagent on the Catalyst"... 

Modifying reagent Rate of hydrogenation (mmol/hr) Optical yield (%) Adsorbed amount of modifying reagent (mmol/g cat.)... 

Chiral sulfoxides (12, 92). Kagan et al.3 have reviewed the asymmetric oxidation of sulfides by a water-modified Sharpless reagent. Optical yields are generally highest in the oxidation of aryl methyl sulfides (—75-90%). 

In 1951 Bothner-By first attempted asymmetric reductions based on the conversion of lithium aluminum hydride (LAH) into a chiral alkoxy derivative by reaction with (+)-camphor. Since this pioneering work, the use of chirally modified LAH reagents has been the focus of much attention. In 1979, the first virtually complete enantiofacial recognition of prochiral carbonyl compounds was accomplished by using LAH modified with optically pure 2,2 -dihydroxy-1,1 -binaphthyl and a simple alcohol (BINAL-H). Asymmetric reduction with chiral 2,5-dimethylborolane also gave alcohols in high optical yields." Recently, excellent results have been obtained using a chirally modified sodium borohydride... 

Very recently it has been shown that electrode surfaces can be chemically modified.13 Although no useful reactions have come from this work, it has been shown that organic molecules can be covalently attached to electrode surfaces and that these modified surfaces impart selectivity to electrochemical reactions which is not otherwise available. Attempts have also been made to increase the selectivity of electrochemical reactions by adsorbing material on the electrode surface. In particular if chiral alkaloids are adsorbed on mercury, it Is then possible to perform the asymmetric reduction of prochiral ketones tc chiral alcohols. An optical yield of 54% has, for example, been reported for the reduction of 4-acetyl pyridine in aqueous-ethanol using strychnine as the catalytic, chiral reagent.11 ... 

Several new chiral modifications of lithium aluminium hydride have been reported, including those formed by reaction with chiral secondary benzylamines (14), with diols such as (15) derived from D-mannitol, or with terpenic glycols such as (16). These complexes reduce phenyl alkyl ketones to optically active phenyl carbinols, and enantiomeric excesses of up to 50% have been observed in the case of reagents derived from (14). However, in the diol complexes, believed to have structures of the type shown in (17), lower chiral selectivity is observed, e.g. up to ca. 12% in the case of (15), or up to an optical yield of 30% with an ethanol-modified complex of (16). Better results have been reported with the chiral diamine complex (18), derived originally from L-proline, which reduces acetophenone in 92% optical yield. Asymmetric induction with reagents in this class (i.e. derivatives of lithium aluminium hydride) is usually low in the reduction of aliphatic ketones, but a complex of UAIH4 and the amino-alcohol (19) has been shown to reduce... 

Aldehydes and ketones are readily transformed into the corresponding cyanohydrin trimethylsilyl ethers when treated with cyanotrimethylsilane in the presence of Lewis acids (eq 1), triethylamine, or solid bases such as Cap2 or hydroxyapatite. The products can be readily hydrolyzed to the corresponding cyanohydrins. The cyanosilylation of aromatic aldehydes can be achieved with high enantioselectivity in the presence of catalytic amounts of a modified Sharpless catalyst consisting of titanium tetraisopropoxide and L-(+)-diisopropyl tartrate (eq 2). Catalysis with chiral titanium reagents yields aliphatic and aromatic cyanohydrins in high chemical and optical yields... 

As a consequence of the wide choice of hydride reagents the classical methods such as reduction with sodium in ethanol almost fell into oblivion [579, 520]. Nevertheless some old reductions were resuscitated. Sodium di-thionite was found to be an effective reducing agent [262], and the reduction by alcohols [309] was modified to cut down on the temperature [755] or the time required [527], or to furnish chiral alcohols ( in good yields and excellent optical purity ) by using optically active pentyl alcohol and its aluminum salt [522]. Formation of chiral alcohols by reduction of pro-chiral ketones is... 

Various benzophenones and aryl alkyl ketones substituted with a fluorine atom on the ortho position were effectively converted into the corresponding alcohols with high to excellent enantioselectivities in the presence of the optically active ketoim- inatocobalt(II) complexes (14). The combination of o-F substituent and a modified lithium borohydride reagent contributed to the high yield and high enantioselectivity (88-96% ee).316... 

Enantioface-differentiating conjugate additions have also been performed using modified hydrolytic enzymes14. Thus, the addition of diethylamine to ethyl ( )-4,4,4-trifluoro-2-butenoatc in an organic solvent is catalyzed by a lipase from Candida cylindracea, a-chy-motrypsin or pig liver esterase to afford ethyl (-)-3-diethylamino-4,4,4-trifluorobutanoate in moderate yield (36-47%) and low optical purity (8-37% ee, as determined by l9F NMR using a chiral shift reagent)14. 

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