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Chiral oxazaborolidines

The most successful of the Lewis acid catalysts are oxazaborolidines prepared from chiral amino alcohols and boranes. These compounds lead to enantioselective reduction of acetophenone by an external reductant, usually diborane. The chiral environment established in the complex leads to facial selectivity. The most widely known example of these reagents is derived from the amino acid proline. Several other examples of this type of reagent have been developed, and these will be discussed more completely in Section 5.2 of part B. [Pg.110]

Recent advances in the asymmetric catalytic reduction of ketones using chiral oxazaborolidines as ligands 98MI64. [Pg.273]

Reduction of carbonyl compounds with chiral oxazaborolidine catalysts 98AG(E)1987. [Pg.273]

A series of chiral boron catalysts prepared from, e.g., N-sulfonyl a-amino acids has also been developed and used in a variety of cycloaddition reactions [18]. Corey et al. have applied the chiral (S)-tryptophan-derived oxazaborolidine-boron catalyst 11 and used it for the conversion of, e.g., benzaldehyde la to the cycloaddition product 3a by reaction with Danishefsky s diene 2a [18h]. This reaction la affords mainly the Mukaiyama aldol product 10, which, after isolation, was converted to 3a by treatment with TFA (Scheme 4.11). It was observed that no cycloaddition product was produced in the initial step, providing evidence for the two-step process. [Pg.160]

Although it is known that in some cases the lithium salts of chiral amino alcohols are even better catalysts than the chiral ligands themselves, the use of metals other than lithium has rarely been investigated. The oxazaborolidines A and B and the aluminum analog C have been used as catalysts for the enantioselective addition of diethylzinc to benzaldehyde35 (Table 32). [Pg.177]

Corey, E.J. Helal, C.J. (1998) Reduction of Carbonyl Compounds with Chiral Oxazaborolidine Catalysts A New Paradigm for Enantioselective Catalysis and a Powerful New Synthetic Method. Angewandte Chemie International Edition, 37, 1986-2012. [Pg.188]

In the same area, a (5)-tryptophan-derived oxazaborolidine including a p-tolylsulfonylamide function has been used by Corey et al. to catalyse the enantioselective Diels-Alder reaction between 2-bromoacrolein and cyclo-pentadiene to form the corresponding chiral product with an unprecedented high (> 99% ee) enantioselectivity (Scheme 5.27)." This highly efficient methodology was extended to various 2-substituted acroleins and dienes such as isoprene and furan. In addition, it was applied to develop a highly efficient total synthesis of the potent antiulcer substance, cassiol, as depicted in Scheme 5.21... [Pg.204]

Other S/N ligands have been investigated in the enantioselective catalytic reduction of ketones with borane. Thus, Mehler and Martens have reported the synthesis of sulfur-containing ligands based on the L-methionine skeleton and their subsequent application as new chiral catalysts for the borane reduction of ketones." The in situ formed chiral oxazaborolidine catalyst has been used in the reduction of aryl ketones, providing the corresponding alcohols in nearly quantitative yields and high enantioselectivities of up to 99% ee, as shown in Scheme 10.60. [Pg.338]

Catalytic Enantioselective Reduction of Ketones. An even more efficient approach to enantioselective reduction is to use a chiral catalyst. One of the most developed is the oxazaborolidine 18, which is derived from the amino acid proline.148 The enantiomer is also available. These catalysts are called the CBS-oxazaborolidines. [Pg.416]

Liao and Li enantioselectively synthesized and studied the antifungal activity of optically active miconazole and econazole. The key step was the enan-tioselective reduction of 2-chloro-l-(2,4-dichlorophenyl)ethanone catalyzed by chiral oxazaborolidine [10]. [Pg.8]

Boranes have opened the door to asymmetric reduction of carbonyl compounds. The first attempt at modifying borane with a chiral ligand was reported by Fiaud and Kagan,75 who used amphetamine borane and desoxyephedrine borane to reduce acetophenone. The ee of the 1-phenyl ethanol obtained was quite low (<5%). A more successful borane-derived reagent, oxazaborolidine, was introduced by Hirao et al.76 in 1981 and was further improved by Itsuno and Corey.77 Today, this system can provide high stereoselectivity in the asymmetric reduction of carbonyl compounds, including alkyl ketones. [Pg.367]

New chiral oxazaborolidines that have been prepared from both enantiomers of optically active inexpensive a-pinene have also given quite good results in the asymmetric borane reduction of prochiral ketones.92 Borane and aromatic ketone coordinate to this structurally rigid oxazaborolidine (+)- or (—)-94, forming a six-membered cyclic chair-like transition state (Scheme 6-41). Following the mechanism shown in Scheme 6-37, intramolecular hydride transfer occurs to yield the product with high enantioselectivity. With aliphatic ketones, poor ee is normally obtained (see Table 6-9). [Pg.370]

The reduction of dialkylketones and alkylaryl ketones is also conveniently accomplished using chiral oxazaborolidines, a methodology which emerged from relative obscurity in the late 1980s. The type of borane complex (based on (,V)-diphenyl prolinol)[39] responsible for the reductions is depicted below (10). Reduction of acetophenone with this complex gives (/ )-1 -phenylethanol in 90-95% yield (95-99% ee) [40]. Whilst previously used modified hydrides such as BiNAL-H (11), which were used in stoichiometric quantities, are generally unsatisfactory for the reduction of dialkylketones, oxazaborolidines... [Pg.13]

Chiral oxazaborolidines. Enantioselective reduction of ketones with a reagent prepared from BH, and the chiral vic-amino alcohol 1 (12,31) is now known to involve an oxazaborolidine. Thus BH3 and (S)-l, derived from valine, react rapidly in THF to form 2, m.p. 105-110°, which can serve as an efficient catalyst... [Pg.110]

Enantioselective reduction of ketones.1 The ability of diborane in combination with the vic-amino alcohol (S)-2-amino-3-methyl-l,l-diphenyl-l-butanol (12, 31) to effect enantioselective reduction of alkyl aryl ketones involves formation of an intermediate chiral oxazaborolidine, which can be isolated and used as a catalyst for enantioselective borane reductions (equation I). [Pg.239]

Surface modification of skeletal nickel with tartaric acid produced catalysts capable of enantiose-lective hydrogenation [85-89], The modification was carried out after the formation of the skeletal nickel catalyst and involved adsorption of tartaric acid on the surface of the nickel. Reaction conditions strongly influenced the enantioselectivity of the catalyst. Both Ni° and Ni2+ have been detected on the modified surface [89]. This technique has already been expanded to other modified skeletal catalysts for example, modification with oxazaborolidine compounds for reduction of ketones to chiral alcohols [90],... [Pg.147]

Corey, E. J. Shibata, T. Lee, T. W. Asymmetric Diels-Alder Reactions Catalyzed by a Triflic Acid Activated Chiral Oxazaborolidine. J. Am. Chem. Soc. 2002,124,3808-3809. [Pg.171]

Chiral 1,2,3-oxazaborolidines simply obtained from a-amino acid derived sulfonamides and borane were first applied in Diels-Alder reactions by Taliasu and Yamamoto228, and Helmchen and colleagues229. Yamamoto prepared catalysts from a-aminobutyric acid derived arylsulfonamides and found that the enantioselectivity of the reaction between... [Pg.411]

Cross-linked polymers bearing IV-sulfonyl amino acids as chiral ligands were converted to polymer bound oxazaborolidine catalysts by treatment with borane or bromoborane. In the cycloaddition of cyclopentadiene with methacrolein, these catalysts afforded the same enantioselectivities as their non-polymeric counterparts238. [Pg.416]


See other pages where Chiral oxazaborolidines is mentioned: [Pg.239]    [Pg.241]    [Pg.509]    [Pg.239]    [Pg.241]    [Pg.509]    [Pg.78]    [Pg.94]    [Pg.111]    [Pg.212]    [Pg.9]    [Pg.377]    [Pg.298]    [Pg.200]    [Pg.102]    [Pg.179]    [Pg.353]    [Pg.520]    [Pg.117]    [Pg.143]    [Pg.168]    [Pg.122]    [Pg.159]    [Pg.160]    [Pg.508]    [Pg.1003]    [Pg.111]   
See also in sourсe #XX -- [ Pg.239 , Pg.240 , Pg.241 ]




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Chiral oxazaborolidine Lewis acid

Chiral oxazaborolidine catalyst

N-protonated chiral oxazaborolidine

Oxazaborolidine chiral

Oxazaborolidine polymeric chiral

Oxazaborolidines

Oxazaborolidines, chiral reductions

Oxazaborolidins

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