Promoter chiral

Conjugate addition. In general, the use of chiral auxiliaries able to promote chirality transfer with a predictable stereochemistry on newly generated stereocenters is recognized as indispensable in asymmetric synthesis.79... 

A basic understanding of the structure and behavior of liquid-crystalline cellulosics has yet to evolve. From a conceptual point of view, the chirality of the cellulosic chain is most sensitively expressed in the super-molecular structure of the cholesteric phase, which may be described by the twisting power or the pitch. At present, no information is available about domains or domain sizes (correlation lengths) of supermo-lecular structures. The chirality in the columnar phases has not been addressed at all. The principal problem, i.e., how does chirality on a molecular or conformational level promote chirality on the supermolecular level, has not been solved. If this correlation were known, it would enable the determination of the conformation of cellulosic chains in the mesomorphic phase and the development of models for the polymer-solvent interactions for lyotropic systems. On the other hand, direct probing of this interaction would provide a big leap towards an understanding of lyotropic phases. 

The twist induced by different molecular species can be qualitatively related to the molecular structure of the chiral species [40]. The twist induced in a nonchiral liquid crystal solvent by a chiral dopant also depends on the nature of the solvent, and it has been proposed that chiral dopants can preferentially promote chiral conformations of the solvent molecules [41]. This effect has also been observed in isotropic solutions, where an enhanced optical rotation in solutions of a chiral biaryl in a cyanobiph-enyl solvent was attributed [42] to an induction of chirality via preferential interactions... 

Related results of promotion (catalysis) and inliibition of stereonuitation by vibrational excitation have also been obtained for the much larger molecule, aniline-NHD (CgH NHD), which shows short-time chirality and stereonuitation [104. 105]. This kind of study opens the way to a new look at kinetics, which shows coherent and mode-selective dynamics, even in the absence of coherent external fields. The possibility of enforcing coherent dynamics by fields ( coherent control ) is discussed in chapter A3.13. 

Nonpolar organic mobile phases, such as hexane with ethanol or 2-propanol as typical polar modifiers, are most commonly used with these types of phases. Under these conditions, retention seems to foUow normal phase-type behavior (eg, increased mobile phase polarity produces decreased retention). The normal mobile-phase components only weakly interact with the stationary phase and are easily displaced by the chiral analytes thereby promoting enantiospecific interactions. Some of the Pirkle-types of phases have also been used, to a lesser extent, in the reversed phase mode. 

Asymmetric aldol reaction promoted by chiral oxazaborolidinone 97YGK313. 

Chiral aluminum catalyst 2, prepared from Et2AlCl and a Vaulted biaryl ligand, is reported to be an effective Lewis acid catalyst of the Diels-AIder reaction between methacrolein and cyclopentadiene, affording the adduct in 97.7% ee [4] (Scheme 1.2). Although the Diels-AIder reaction with other a,/ -unsaturated aldehydes has not been described, that only 0.5 mol% loading is sufficient to promote the reaction is a great advantage of this catalyst. 

To overcome these problems with the first generation Brmsted acid-assisted chiral Lewis acid 7, Yamamoto and coworkers developed in 1996 a second-generation catalyst 8 containing the 3,5-bis-(trifluoromethyl)phenylboronic acid moiety [10b,d] (Scheme 1.15, 1.16, Table 1.4, 1.5). The catalyst was prepared from a chiral triol containing a chiral binaphthol moiety and 3,5-bis-(trifluoromethyl)phenylboronic acid, with removal of water. This is a practical Diels-Alder catalyst, effective in catalyzing the reaction not only of a-substituted a,/ -unsaturated aldehydes, but also of a-unsubstituted a,/ -unsaturated aldehydes. In each reaction, the adducts were formed in high yields and with excellent enantioselectivity. It also promotes the reaction with less reactive dienophiles such as crotonaldehyde. Less reactive dienes such as isoprene and cyclohexadiene can, moreover, also be successfully employed in reactions with bromoacrolein, methacrolein, and acrolein dienophiles. The chiral ligand was readily recovered (>90%). 

Yamamoto et al. have reported a chiral helical titanium catalyst, 10, prepared from a binaphthol-derived chiral tetraol and titanium tetraisopropoxide with azeotropic removal of 2-propanol [16] (Scheme 1.22, 1.23, Table 1.9). This is one of the few catalysts which promote the Diels-Alder reaction of a-unsubstituted aldehydes such as acrolein with high enantioselectivity. Acrolein reacts not only with cyclo-pentadiene but also 1,3-cyclohexadiene and l-methoxy-l,3-cyclohexadiene to afford cycloadducts in 96, 81, and 98% ee, respectively. Another noteworthy feature of the titanium catalyst 10 is that the enantioselectivity is not greatly influenced by reaction temperature (96% ee at... 

Another chiral titanium reagent, 11, was developed by Corey et al. [17] (Scheme 1.24). The catalyst was prepared from chiral ris-N-sulfonyl-2-amino-l-indanol and titanium tetraisopropoxide with removal of 2-propanol, followed by treatment with one equivalent of SiCl4, to give the catalytically-active yellow solid. This catalyst is thought not to be a simple monomer, but rather an aggregated species, as suggested by NMR study. Catalyst 11 promotes the Diels-Alder reaction of a-bro-moacrolein with cyclopentadiene or isoprene. 

With few exceptions chiral Lewis acids are usually moisture-sensitive and require anhydrous conditions, but bench-stable aquo complexes such as [Cu(S,S)-t-Bu-box)(H20)2](SbF6)2 were found to promote the Diels-Alder reaction as effectively as the anhydrous copper reagent. 

Although furan is usually a poor diene in the Diels-Alder reaction, the chiral copper reagent 24b promotes its asymmetric addition to acryloyloxazolidinone to afford the 7-oxabicyclo[2.2.1]hept-2-ene derivative in high optical purity (Scheme 1.40). Because a retro-Diels-Alder reaction occurs above -20 °C, the reaction must be performed at low temperature (-78 °C) to obtain a high optical yield. The bicy-... 

In all the reactions described so far a chiral Lewis acid has been employed to promote the Diels-Alder reaction, but recently a completely different methodology for the asymmetric Diels-Alder reaction has been published. MacMillan and coworkers reported that the chiral secondary amine 40 catalyzes the Diels-Alder reaction between a,/ -unsaturated aldehydes and a variety of dienes [59]. The reaction mechanism is shown in Scheme 1.73. An a,/ -unsaturated aldehyde reacts with the chiral amine 40 to give an iminium ion that is sufficiently activated to engage a diene reaction partner. Diels-Alder reaction leads to a new iminium ion, which upon hydrolysis af-... 

The complexation procedure included addition of an equimolar amount of R,R-DBFOX/Ph to a suspension of a metal salt in dichloromethane. A clear solution resulted after stirring for a few hours at room temperature, indicating that formation of the complex was complete. The resulting solution containing the catalyst complex was used to promote asymmetric Diels-Alder reactions between cyclopen-tadiene and 3-acryloyl-2-oxazolidinone. Both the catalytic activity of the catalysts and levels of chirality induction were evaluated on the basis of the enantio-selectivities observed for the endo cycloadduct. 

The synthesis of f-i-i-crotanecine is accomplished in 10 steps in a 10.2% overall yield, as shown in Scheme 8.42. The key step in the asymmetric synthesis is a Lewis acid-promoted, tandem inter [4-i-2 /intra [3-i-2 cycbaddidon between a ffumaroyloxyxiitroalkene and chiral fi-silylvinyl ether, in which the snbsdtuted silanes are used as hydroxy synthons. ... 

Corey used a chiral bromoborane 75 (1.1 equiv.) to promote the addition of tert-butyl bromoacetate (76) to aromatic, aliphatic, and a,P-unsaturated aldehydes to give the halo alcohols 77 with high enantio- and diastereoselectivities (Table 1.10) [35]. 

Jacobsen subsequently reported a practical and efficient method for promoting the highly enantioselective addition of TMSN3 to meso-epoxides (Scheme 7.3) [4]. The chiral (salen)Cl-Cl catalyst 2 is available commercially and is bench-stable. Other practical advantages of the system include the mild reaction conditions, tolerance of some Lewis basic functional groups, catalyst recyclability (up to 10 times at 1 mol% with no loss in activity or enantioselectivity), and amenability to use under solvent-free conditions. Song later demonstrated that the reaction could be performed in room temperature ionic liquids, such as l-butyl-3-methylimidazo-lium salts. Extraction of the product mixture with hexane allowed catalyst recycling and product isolation without recourse to distillation (Scheme 7.4) [5]. 

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