Binaphthol ethers

Binaphthol ethers 121 undergo diastereoselective rearrangement to give alcohols 122.88... 

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]. 

The principal variations on the normal crown synthesis methods were applied in preparing mixed crowns such as those shown in Eq. (3.55) and in forming isomers of the dibinaphthyl-22-crown-6 systems. The latter has been discussed in Sect. 3.5 (see Eq. 3.21) . The binaphthyl unit was prepared to receive a non-naphthyl unit as shown in Eq. (3.57). Binaphthol was allowed to react with the tetrahydropyranyl ether or 2-chloroethoxyethanol. Cleavage of the THP protecting group followed by tosyla-tion of the free hydroxyl afforded a two-armed binaphthyl unit which could serve as an electrophile in the cyclization with catechol. Obviously, the reaction could be accomplished in the opposite direction, beginning with catechol". ... 

A combination of diethylzinc with sulfonamides 18 or 19 offers another possibility for the enantioselective acetate aldol reaction39,41. The addition of silyl enol ethers to glyoxylates can be directed in a highly enantioselective manner when mediated by the binaphthol derived titanium complex 2040. 

Optical Resolution with 1,1 -Binaphthol Monoethyl Ether... 

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). 

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... 

Cram and co-workers have experimented extensively with chiral recognition in crown ethers derived from various 3-binaphthols (73). In nonpolar solvents, these chiral ethers complex salts of PEA and various chiral a-amino esters (with fast exchange), inducing nonequivalence in their NMR spectra. The senses of proton nonequivalence induced in these solutes have been used to support proposed structures of the diastereomeric solvates (74). 

Binaphthol- and biphenyl-derived ketones (9 and 10) were reported by Song and coworkers in 1997 to epoxidize unfunctionalized alkenes in up to 59% ee (Fig. 3, Table 1, entries 9, 10) [37, 38]. Ketones 9 and 10 were intended to have a rigid conformation and a stereogenic center close to the reacting carbonyl group. The reactivity of ketones 9 and 10 is lower than that of 8, presumably due to the weaker electron-withdrawing ability of the ether compared to the ester. In the same year, Adam and coworkers reported ketones 11 and 12 to be epoxidation catalysts for several trans- and trisubstituted alkenes (Table 1, entries 11,12). Up to 81% ee was obtained for phenylstilbene oxide (Table 1, entry 25) [39]. 

Diastereoselective rearrangement of binaphthol-derived ether 45 to 46 (equation 24) has been developed by Kiyooka and colleagues for the stereoselective functionalization of binaphthol . 

The incorporation of two nonidentical chiral residues, each supporting C2 symmetry, into a mactocyclic poly ether affords a chiral crown compound with C2 symmetry provided its structure is constitutionally symmetrical. Thus, base-promoted reaction of the half-crown diol prepared from (5)-birraphthol with the half-crown ditosylate d-72 synthesized tom diacetone-manrritol affords (144) the 20-crown-6 derivative (S)-d-113 with C2 symmetry. When d-72 is condensed in like fashion with (/ 5)-binaphthol, then the diastereoisomeric 20-crown-6 derivative (/ )-d-114 can be separated chromatogiaphically tom (S)-d-113. In this matmer, (/ 5)-binaphthol is resolved by the carbohydrate unit during the synthesis. 

The monobenzhydiyl derivative of (S)-binaphthol has played an important role, not only in the synthesis of chiral bisbinaphthyl crown ether derivatives, for example, (55)-124, containing two different bridges between the two binaphthyl units, but also in the provision of an entry into the constimtionally isomeric derivative (5S)-125. Rational stepwise syntheses of macrocyles containing three binaphthyl units have been devised and applied to the synthesis of (SSS)-126 and (RSS)-127. Cleariy, in all these procedures, the C2 symmetry of the chiral building block restricts the number of products (to one ) and defines the symmetries of the macrocycles formed. 

Figure 5 Crystal structures of (a) [18]crown-6 complex with two malonodinitrile molecules (b) binaphthol crown ether complex with water (reproduced with permission from 77CB2249 and 78AX(B)3387)...

Chiral enantiopure 2,2 -dihydroxybiaryls are important as chiral ligands and are also structural motifs occurring in some natural products. Hydrolase-catalyzed resolution by acylation in organic solvents of some dihydroxybiaryls has been successfully achieved. Thus, the racemic binaphthols 111-113 have been resolved by mono acylation with vinyl acetate in t-butylmethyl ether ( ME) at 45 °C catalyzed by Pseudomonas sp. lipase (Scheme 4.35) [106]. In a similar way the 2,2 -dihydroxybiphenyl 114 can be acylated with vinyl acetate catalyzed by PSL immobilized on celite in TBME at 45 °C (Scheme 4.35) [107]. Butanolysis of the racemic monobutyrate of binaphthol rac-115 catalyzed by CALB in toluene at 80 °C for 72 h gives (R)-binaphthol (93% ) at ca. 50% conversion [108]. 

Enantiotopos discrimination, 93, 128, 142, 234, 235, 331 Ene reactions asymmetric, 223 binaphthol, 222 chiral metal complexes, 222 intramolecular, 226 methyl glyoxylate, 290 Enol silyl ether substrates, 128, 228, 230 Enol substrates, 28 Enolates ... 

Asymmetric reduction of alkyl aryl ketones with trialkoxysilanes is promoted by a catalytic amount of chiral nucleophiles [39]. The reactive species is a transiently prepared hypervalent silicon hydride. 2, 4, 6 -Trimethylacetophenone was reduced with equimolecular amounts of trimethoxysilane in the presence of the monolithio salt of (R)-BINAPHTHOL (substrate Li=20 l) in a 30 1 ether-TMEDA mixed solvent at 0 °C to afford the R product in 90% ee (Scheme 21) [40]. The presence of TMEDA was crucial to achieve high yield and enantiose-lectivity. Reduction of less hindered ketonic substrates preferentially gave the... 

Enantioselective Claisen rearrangement of allyl (a-trimethylsilyl)vinyl ethers in the presence of aluminium binaphthol derivatives gives ft-chiral y, <5-unsaturated acyl silanes with good ee (Scheme 33)114. 

An aldol reaction of a trimethoxysilyl enol ether, catalysed by a lithium binaphtholate, shows anti diastereoselectivity and modest ees under dry conditions, but addition of water brings about syn adduct formation, with higher ee.131... 

Kagan and Schiffers carefully studied the effect of the lithium salts of BINOL (17) and related axially chiral binaphthols on the reduction of a variety of ketones with trialkoxysilanes [24]. They found that diethyl ether, with TMEDA as an additive, was the best solvent for asymmetric reduction of ketones. In the presence of 5 mol% of the monolithium salt of BINOL (17), acetophenone (1) could be reduced with trimethoxysilane in 80% yield and with 61% ee. Enantiomeric excesses > 90% were achieved under the same conditions with 2, 4, 6 -trimethyl-acetophenone (18) or a-tetralone (19) as substrates. Aliphatic ketones such as... 

Schuster et al. reported that the photochromism of a fulgide could change the helical pitch length of a cholesteric liquid crystal. 27 They added an indolylfulgide 3 (5.2 % w/w) to a cholesteric liquid crystal composed of 4-cyano-4 -pentylbiphenyl 26 (5CB) and 1.35 % (w/w) of a chiral cyclic ether (R)- 27, prepared from (R)-binaphthol. The cholesteric pitch length could be changed reversibly between 30 and 42 pm, after UV and visible light irradiation, respectively. 

In the synthesis of the swivel-cruciform binaphthyl-based oligomers 27 and 28, bromination of commercially available l,l -binaphthol followed by a Williamson reaction gives binaphthyl ethers with various alkoxy groups [43], The dibromi-... 

It is remarkable that a biphenyl unit in combination with a chiral amine 11 plays a role of an alternate binaphthol, inducing atropisomerism and giving ethylcyclohexanone in 96% ee [13]. Alexakis also succeeded in his search for efficient copper salts and solvents and reported the combination of CuTC (copper thiophene-2-carboxylate) and ether as a suitable choice. It is also noteworthy that trapping of the resulting zinc enolate with TMSOTf gave a TMS enolate 26, which is a useful intermediate for further synthetic manipulation [26]. Tandem... 

Purify the residue by flash chromatography on silica, eluting first with diethyl ether [110 mL to separate (S)-binaphthol 53] then diethyl ether THF (1 1) to yield (/ )-4-(5,5-dimethyl-2-oxo-2x5-[1,3,2]dioxaphosphinan-2-yl)-2,2,5,5-tetramethyl-3-thiazolidine, 57 (87 mg, 99%, 99% e.e.), as a solid. 

Trilluoroacetaldehyde (fluoral) undergoes facile highly stereoselective condensation with silyl enol ethers under titanium(ll) chloride binaphthol catalysis to give 2. ... 

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