Reducing agents chiral

The enandoselecdve reducdon ofy-nitroketones andy-nitrodiketones by the chiral reducing agent f+ - or f- -diisopinocamphenylchloroboranerDIP-Cl gives nitro alcohols having from 33 to 86% ee and nitrodiols v/ith complete diastereoselecdvity and > 95% ee. 

The reaction of configurationally unstable triorganotin halides with the chiral reducing agent introduced by Vigneron and Jacquet45H-Al(0-3,5-Me2Ph)2(0-... 

This might explain why one equivalent of the chiral reducing agent of Vigneron and Jacquet45 (cf. 3.3.) transforms compound (3) into optically active (12) ([a] 5 + 7.4) with 80% yield whereas, with 8 equivalents of this reducing agent, the chemical yield is increased up to 93 % but (72) is obtained as a racemic mixture. 

Chiral reducing agents derived from aluminum or boron reducing agents that involve one or more chiral organic ligands. 

In summary, a number of effective chiral reducing agents have been developed based on the modification of LAH. Excellent results have been obtained with aryl alkyl ketones and a,p-acetylenic ketones. However, dialkyl ketones are reduced in much lower enantiomeric excess. This clearly indicates that steric effects alone do not control stereoselectivity in these reductions. Systematic studies have been carried out with the objective of designing improved reagents. A better understanding of the mechanisms and knowledge of the active species is required in order to provide more accurate models of the transition states of the key reduction steps. 

Nasipuri and co-workers have extensively investigated complex 111 as a chiral reducing agent (131-138). The results of these investigations are summarized... 

The 1-substituted 3,4-dihydroisoquinoline (87) was reduced with chiral reducing agent (62) to the corresponding alkaloids (88) in excellent optical yields (e.e. = 60 to 87 %) 94). 

In the second step, achiral 9-borabicyclo[3.3.1]nonane (9-BBN) adds to the less hindered diastereotopic face of a-pinene to yield the chiral reducing agent Alpine-Borane. Aldehydes are rapidly reduced to alcohols. The reaction with deuterio-Alpine-Borane, which yields (R)-a-d-henzy alcohol in 98% enantiomeric excess ( ) reveals a very high degree of selectivity of the enantiotopic faces of the aldehyde group in a crowded transition state ... 

FIGURE 27.6 Heavy metal-free chiral-reducing agents based on Ge and Si chemistry. 

Reduction of one carbonyl group of the prochiral dione (53) with chiral reducing agents has produced mixtures of a- (the OH groups at either C-14 or C-17 a-oriented as the molecule is usually depicted) and /3 -alcohols with disappointingly low discrimination.18... 

Overview and General Considerations. This article will cover the title reagent and other chiral reducing agents derived from lithium aluminum hydride and chiral additives, with initial... 

Many different chiral reducing agents have now been prepared for this purpose. One such reagent, formed by reacting borane (BH3) with a heterocycle called an oxazaborolidine, has one stereogenie center (and thus two enantiomers). 

CBS reagent (Section 20.6A) A chiral reducing agent formed by reacting an oxazaborolidine with BH3. CBS reagents predictably give one enantiomer as the major product of ketone reduction. 

Oxazaborolidine (Section 20.6A) A heterocycle possessing a boron, a nitrogen, and an oxygen. An oxazaborolidine can be used to form a chiral reducing agent. 

Modification of LAH with (-)-)V-methylephedrine (14) and )V-ethylaniline affords another chiral reducing agent (50). " A variety of aromatic ketones are reduced by (SO) to (S)-carbinols in high optical yields (Scheme 8). Acyclic a,3-unsaturated ketones are also reduced to (S)-carbinols with high selectivity (76-92% ee), but cyclic enones are reduced with only moderate selectivity. 

Lithium borohydride decomposed by /V-benzoylcysteine (61) or /V/v -dibenzoylcystine (62), a sulfur-containing modifier, is a highly efficient chiral reducing agent. A complex prepared from (61), t-butyl alcohol and LiBH4 affords carbinols in maximum 92% ee by the reduction of aryl alkyl ketones in THF at -78 °C (Scheme 13). A LiBH4 complex with (62) and t-butyl alcohol is useful for the reduction of -keto esters to give (R)-P-hydroxy esters in up to 91 % ee. In both cases the use of r-butyl alcohol is essential in order to achieve efficient enantiofacial differentiation. ... 

The ability of coenzyme NAD(P)+ (57, R = sugar) and its mimics to accept and deliver hydride ion stereoselectively via 1,4-dihydropyridine intermediates has spurred much interest. Much work involving NADIP)" " and its simpler derivatives (56 R = PhCH2, BNAH R = C12H25, DNAH) as chiral reducing agents has been reported. ... 

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