Chiral auxiliary binaphthol

Another method for ketone reduction, BINAL-H asymmetric reduction, can also be used in co-side chain synthesis. An example of applying BINAL-H asymmetric reduction in PG synthesis is illustrated in Scheme 7-27. This has been a general method for generating the alcohol with (15. -configuration. The binaphthol chiral auxiliary can easily be recovered and reused. As shown in Scheme 7-27, when the chiral halo enone 91 is reduced by (S -BINAL-H at — 100°C, product (15S)-92 can be obtained with high enantioselectivity. 

Bidentate chiral auxiliaries have since been examined. While camphane-2,3-diol and (5-binaphthol gave disappointing results, tartrate-derived (TADDOL) ligands were found to be very promising as chiral inductors [44]. Particularly interesting results were obtained by using complex 21, readily available from natural (P,J )-(+)-tartaric acid (Scheme 13.21). 

Recently, Schaumann et al. 153,154 an(j Bienz et tf/.155,156 have developed dependable routes for the resolution of racemic functionalized organosilanes with Si-centered chirality using chiral auxiliaries, such as binaphthol (BINOL), 2-aminobutanol, and phenylethane-l,2-diol (Scheme 2). For instance, the successive reaction of BINOL with butyllithium and the chiral triorganochlorosilanes RPhMeSiCl (R = /-Pr, -Bu, /-Bu) affords the BINOL monosilyl ethers 9-11, which can be resolved into the pure enantiomers (A)-9-ll and (7 )-9-11, respectively. Reduction with LiAlFF produces the enantiomerically pure triorgano-H-silanes (A)- and (R)-RPhMeSiH (12, R = /-Pr 13, -Bu 14, /-Bu), respectively (Scheme 2). Tamao et al. have used chiral amines to prepare optically active organosilanes.157... 

With a twist on the Sharpless asymmetric epoxidation protocol, Yamamoto and co-workers <99JOC338> have developed a chiral hydroxamic acid (17) derived from binaphthol, which serves as a coordinative chiral auxiliary when combined with VO(acac)j or VO(i-PrO)j in the epoxidation of allylic alcohols. In this protocol, triphenylmethyl hydroperoxide (TiOOH) provides markedly increased enantiomeric excess, compared to the more traditional t-butyl hydroperoxide. Thus, the epoxidation of E-2,3-diphenyl-2-propenol (18) with 7.5 mol% VO(i-PiO)3 and 15 mol% of 17 in toluene (-20 °C 24 h) provided the 2S,3S epoxide 19 in 83% ee. 

Optically pure l,l -binaphthol and its derivatives have been evaluated as versatile chiral auxiliaries and ligands in asymmetric transformations. Research in this area has provided many efficient and useful methods for the preparation of key chiral building blocks, some of which have been used for the construction of complex natural products. The wide ranging and important applications of such compounds in organic synthesis have stimulated great interest in developing efficient methods... 

The chiral boron complex prepared in situ from chiral binaphthol and B(OPh)3 is utilized for the asymmetric aza-Diels-Alder reaction of Danishefsky s diene and imines [67] (Eq. 8A.43). Although the asymmetric reaction of prochiral imine affords products with up to 90% ee, the double asymmetric induction with chiral imine by using oc-benzylamine as a chiral auxiliary has achieved almost complete diastereoselectivity for both aliphatic and aromatic aldimines. This method has been successfully applied to the efficient asymmetric synthesis of anabasine and coniine of piperidine alkaloides. 

The classical Ullmann reaction has also attracted renewed interest in recent years. Miyano has prepared optically enriched binaphthyl derivatives by intramolecular biaryl coupling, as shown in equation (9). " Diastereoselectivities of up to 70% have been obtained with a binaphthol chiral auxiliary in the linking chain. Intermolecular cross-coupling is best accomplished with preformed arylcopper compounds. "... 

Enantiomerically pure binaphthol is used as a chiral auxiliary. For example, it has been used to prepare chiral aluminum hydride reducing agents, chiral Lewis acids catalysts,8 and chiral crown ethers.9... 

JR)- and (S)-binaphthols 1.44 [230-235] have sometimes been used as chiral auxiliaries. Examples include reduction of y-ketoester 1.45 [236], nucleophilic substitution of binaphthol ethers (G = binaphthol) 1.46, by organomagnesium reagents, organocuprate additions to binaphthol monodnnamates [237], and alkylations of arylacetic or crotonic esters [238,239]. 

Axial-chiral enantiomerically highly enriched binaphthols 4, which are highly useful chiral auxiliaries, are accessible either through acylation of the racemic diol with vinyl acetate or deacylation of the racemic diacetate with butanol (Table 11.1-22), both catalyzed by Pseudomonas cepacia lipase. 

Chloral. Yamamoto has found that the organoaluminum reagent prepared from MeaAl and (/ )-(+)-3,3 -bis(triphenylsilyl)binaphthol catalyzes the ene reaction of chloral and pentafluorobenzaldehyde with 1,1-disubstituted alkenes at -78 °C, giving the expected ene adducts in 40-90% yield and 60-90% enantiomeric excess. Use of the sterically hindered chiral auxiliary is necessary. Low yields of racemic products are obtained with 3,3 -diphenylbinaphthol. 

Apart from enantio- and diastereoselective synthesis, binaphthols as the widely used chiral auxiliaries have been obtained in optically pure form by several method... 

Uemura et al. [103] developed an efficient catalytic and enantioselective oxidation of prochiral sulfides. As in the Kagan system, a titanium(IV) complex is produced in situ from a titanium alkoxide and two / -(-F)-binaphthols (Scheme 1.12) as chiral auxiliaries (in place of diethyl tartrate), in the presence of a large amount of water (more than 1 equivalent). The oxidation of methyl p-tolyl sulfide gives the corresponding sulfoxide in 45 h in 90% yield and 73% ee. 

An alternative pathway via a single-electron transfer mechanism has also been invoked in some cases (Eq. 14.10). Chiral auxiliaries such as binaphthols can make Grignard and related reactions asymmetric ... 

Chiral 1,1 -binaphthol derivatives are well established as readily available chiral catalysts and auxiliaries for the production of various useful optically active compounds. Tanabe et al. investigated [11] a crystalline-liquid resolution of (1R) -// . v - c h rysanthemic acid utilizing l,l -binaphthyl monoethyl ether (25) (Scheme 3). 

Thus, chiral monophosphine auxiliaries have a real potential for various catalytic processes and certainly deserve further attention. In addition, their synthesis allows a modular approach to a variety of structures. Due to the importance of the binaphthol framework for efficient enantioselection, the latter has been the most frequently used for the construction of monophosphine monodentate auxiliaries. 

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