Alcohol enantioselective

Much effort has been put into finding optically active reducing agents that will produce one enantiomer of the alcohol enantioselectively, and considerable success... 

Enantioselective addition of diethylzinc to benzaldehyde has been promoted by indole-containing chiral oxazolidines (which are able to use both O and N atoms to effect metal coordination in the transition state), and by chiral o-hydroxyphenyl diaza-phospholidine oxide,and by chiral aziridino alcohols.Enantioselective addition of dialkylzinc to prostereogenic ketones has been promoted using chiral camphorsul-fonamide derivatives. 

Reactions where NLE have been discovered include Sharpless asymmetric epoxi-dation of allylic alcohols, enantioselective oxidation of sulfides to sulfoxides, Diels-Alder and hetero-Diels-Alder reactions, carbonyl-ene reactions, addition of MesSiCN or organometallics on aldehydes, conjugated additions of organometal-lics on enones, enantioselective hydrogenations, copolymerization, and the Henry reaction. Because of the diversity of the reactions, it is more convenient to classify the examples according to the types of catalyst involved. 

Allyl bromide 1-Propene, 3-bromo- (106-95-6), 76, 60, 221 Allyl chloride 1-Propene, 3-chloro- (107-05-1), 77, 254 l ALLYLGLYCINE 4-PENTENOIC ACID, 2-AMINO-, (R)- (54594-06-8), 76, 57 Allylic alcohols, enantioselective cyclopropanation, 76, 97 ALLYLINDATION, 77, 107... 

Finally, a titanium(IV) pillared clay (Ti-PILC) catalyst has been prepared.71 In the presence of tartaric acid esters as chiral ligands Ti-PILC is an effective, heterogeneous catalyst for the asymmetric epoxidation of allylic alcohols. Enantioselectivities were comparable to those observed in the homogeneous system - and reactions could be carried out at concentrations up to 2M with a simple work-up via filtration of the catalyst. 

Me3SiN3, cat Ti(0-i-Pr)4, cat chiral diols or amino alcohols (enantioselective) Me3SiN3, cat TiQ2(0-i-Pr)2-dialkyl tartrate (enantioselective)... 

RCH =CHBCy2, R2Zn, chiral amino alcohol (enantioselective)... 

R2Zn, several chiral amino alcohols (enantioselective) 1L 25 2823 (1984) (FhCHO only) 28 6163 (1987) (ArCHO only) 30 6427 (1989)... 

Chiralallenylboronic esters.1 These reagents can react with aldehydes to provide /(-acetylenic alcohols enantioselectivity (60-95% ee). Thus, allenylboronic acid 1, prepared as shown, reacts with aldehydes in the presence of ( + )-DET or ( + )-DlPT to give (S)-alcohols (2), whereas (R)-alcohols (2) are obtained in the presence of (-)-DF.T or ( —) DIPT, both in optical yields of 85-95%. The selectivities are lower in reaction with aryl and z,/3-unsaturated aldehydes. 

The groups of Pete and Rau also employed chiral amino alcohols for the enantioselective protonation of simple enols 23a-c that were photochemically generated from 2-/-butyl indanones and tetralones 22a-c by a Norrish type II photoelimination (Scheme 9) [41,42]. Best enantioselectivities were obtained at — 40°C in acetonitrile with 0.1 equivalent of the chiral amino alcohol. In the case of indanone 22a, the selectivity reached 49% ee with (— )-ephedrine ent-18) and could not be further enhanced by the camphor derived inductor 20. With this amino alcohol, enantioselectivities over 80% ee were induced in the case of tetralone 22b. A benzyl substituent in place of the methyl group led to substantial decrease of the selectivity to 47% ee. Linear ketones gave low yields and enantioselectivities around 9% ee. 

There are many synthetic applications of epoxide formation. Titanium alkoxides in the presence of diethyltartrate as a chiral ligand catalyze the epoxidation of allylic alcohols enantioselectively (Sharpless reaction). In the presence of singlet... 

Recently, the preparation of the chiral biphenyl (6) and its use as a modifying agent with LAH has been reported." A complex of LAH-(6)-EtOH (1 1 1) at —78°C gives the best enantiose-lectivities in the reduction of prochiral ketones. Similar to Noyori s reagent, use of the LAH complex with (S)-(6) leads to the (S)-alcohol. Enantioselectivity is usually high for aromatic ketones (acetophenone 97% ee, 93% yield). This reagent reduces 2-octanone in higher enantioselectivity (76% ee) than 3-heptanone (36% ee). 

Silyl enol ethers react with aldehydes in the presence of chiral boranes or other additives " to give aldols with good asymmetric induction (see the Mukaiyama aldol reaction in 16-35). Chiral boron enolates have been used. Since both new stereogenic centers are formed enantioselectively, this kind of process is called double asymmetric synthesis Where both the enolate derivative and substrate were achiral, carrying out the reaction in the presence of an optically active boron compound ° or a diamine coordinated with a tin compound ° gives the aldol product with excellent enantioselectivity for one stereoisomer. Formation of the magnesium enolate anion of a chiral amide, adds to aldehydes to give the alcohol enantioselectively. 

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