Chiral source

Syntheses of both enantiomers using the same chiral source, (a) Kobayashi, S. Ishitani, H. J. Am. Chem. Soe. 1994,... 

With chiral auxiliaries1,41 a remote chiral moiety is temporarily introduced into the substrate in order to direct the nucleophilic addition diastereoselectively. The chiral auxiliary can be removed from the initial addition product with complete conservation of the chirality of the desired product and also of the chiral auxiliary. The recovered chiral auxiliary can then be reused in further reactions. Therefore, chiral auxiliaries are used to chiralize an a priori achiral carbonyl substrate by the introduction of a covalently bound, but nevertheless easily removable, chiral source. 

Catalytic asymmetric aza-Diels-Alder reactions using a chiral lanthanide Lewis acid. Enantioselective synthesis of tetrahydroquinoline derivatives using a catalytic amount of a chiral source [98]... 

Since chiral amines are much more available than chiral isocyanates, ureas have often been obtained from reaction of an amine used as the chiral source with a stoichiometric amount of a non-chiral isocyanate [41]. Similarly, thioureas are obtained by reacting isothiocyanates with amines. The corresponding ureas and thioureas (examples in the following sections) are... 

Synthesis of optically pure compounds via transition metal mediated chiral catalysis is very useful from an industrial point of view. We can produce large amounts of chiral compounds with the use of very small quantities of a chiral source. The advantage of transition metal catalysed asymmetric transformation is that there is a possibility of improving the catalyst by modification of the ligands. Recently, olefinic compounds have been transformed into the corresponding optically active alcohols (ee 94-97%) by a catalytic hydroxylation-oxidation procedure. 

Since carbohydrates constitute an inexpensive and highly modular chiral source for preparing chiral ligands," Claver et al. have reported the use of a series of thioether-phosphite" and thioether-phosphinite furanoside ligands" in the test palladium-catalysed allylic substitution reaction. In the first type of ligand, a systematic variation of the donor group attached to the carbon atom C5 indicated that the presence of a bulky phosphite functionality had a positive effect on the enantioselectivity. Indeed, the enantioselectivity was controlled mainly by the phosphite moiety. This was confirmed by the use of a ligand... 

Control of Enantioselective Photoreactions in the Absence of a Chiral Source... 

The above results are valuable in that an optically active compound is produced in bulk from achiral material. Only a few successful examples of photochemical conversion of achiral into chiral material in the absence of a chiral source have been reported hitherto 49, and in these cases the conversion was carried out on a fragment of a chiral crystal. In our case, chiral crystals are available in bulk, and mass production of the chiral compound is possible. 

Tanaka et al.28 have synthesised a series of (S)-chiral Schiff bases as the highly active (yield 69-99%) and enatioselective (ee 50-96%) catalysts in the reaction of addition of dialkylzinc to aldehydes. The stereochemistry of the asymmetric addition was suggested. In a transition state when S-chiral Schiff base was used as chiral source, the alkyl nucleophile attacked Re face of the activated aldehyde and formed the R-configuration alkylated product [13]. 

More recently a Ru-catalyzed hydrogen transfer of acetophenone under microwave conditions with monotosylated (S,S)-diphenylethylenamine as a chiral source, has... 

Optically active sulfoxides are well known and several synthetic methods have been developed, with numerous papers published on asymmetric synthesis utilizing optically active sulfoxides as chiral sources.1,6... 

The chirality source in the synthesis of optically active nitrones (71) and (72) are known to be enantiopure chiral benzyl type hydroxylamines, (f )-a-methyl-benzylhydroxylamine (70a) and (f )-a-(hydroxymethyl)-benzylhydroxylamine (70b) (Scheme 2.25) (221). 

Intermolecular addition of activated methylenes to unsaturated systems has been investigated with silver,36 silver/ gold, and palladium catalysts. Thus, C-H addition of 2,4-pentandione to 1,3-cyclohexadiene occurs in THF at 0°C with 5mol% of palladium(ll) catalyst without base. Josiphos ligand 20 is used as a chirality source to induce... 

In the presence of a catalytic amount of chiral lanthanide triflate 63, the reaction of 3-acyl-l,3-oxazolidin-2-ones with cyclopentadiene produces Diels-Alder adducts in high yields and high ee. The chiral lanthanide triflate 63 can be prepared from ytterbium triflate, (R)-( I )-binaphthol, and a tertiary amine. Both enantiomers of the cycloaddition product can be prepared via this chiral lanthanide (III) complex-catalyzed reaction using the same chiral source [(R)-(+)-binaphthol] and an appropriately selected achiral ligand. This achiral ligand serves as an additive to stabilize the catalyst in the sense of preventing the catalyst from aging. Asymmetric catalytic aza Diels-Alder reactions can also be carried out successfully under these conditions (Scheme 5-21).19... 

Hydrogen atom transfer implies the transfer of hydrogen atoms from the chain carrier, which is the stereo-determining step in enantioselective hydrogen atom transfer reactions. These reactions are often employed as a functional group interconversion step in the synthesis of many natural products wherein an alkyl iodide or alkyl bromide is converted into an alkane, which, in simple terms, is defined as reduction [ 19,20 ]. Most of these reactions can be classified as diastereoselective in that the selectivity arises from the substrate. Enantioselective H-atom transfer reactions can be performed in two distinct ways (1) by H-atom transfer from an achiral reductant to a radical complexed to a chiral source or alternatively (2) by H-atom transfer from a chiral reductant to a radical. 

A review article reports information regarding the preparation, handling, and storage of this important 3-carbon chiral source.9 Our experience with the compound demonstrates that it tends to polymerize and readily adds water to form hydrate 1 in aqueous solution, from which it is extracted with only difficultly. Both hydrated and polymerized aldehyde can contaminate samples and result in lowered optical rotation values, even though no racemization has occurred. The present procedure provides... 

Enantiomerically pure sulfoxides play an important role in asymmetric synthesis either as chiral building blocks or stereodirecting groups [156]. In the last years, metal- and enzyme-catalyzed asymmetric sulfoxidations have been developed for the preparation of optically active sulfoxides. Among the metal-catalyzed processes, the Kagan sulfoxidation [157] is the most efficient, in which the sulfide is enantioselectively oxidized by Ti(OzPr)4/tBuOOH in the presence of tartrate as chirality source. However, only alkyl aryl sulfides may be oxidized by this system in high enantiomeric excesses, and poor enantioselectivities were observed for dialkyl sulfides. 

Sugars are often used as chiral precursors for the synthesis of optically active compounds, because they are readily available in large quantities and they are relatively inexpensive. The major restriction is that only the D-se-ries of sugars is usually available. An exception is arabinose, which is an attractive chiral source since both enantiomers are commercially available. 

A symmetric activation is also observed in the combination of (/f)-BINOL and Zr(0 Bu)4, which promotes enantioselective synthesis of homoallylic alcohols (Scheme 8.13). A 2 1 ratio of (/ )-BINOL and Zr(0 Bu)4 without any other chiral source affords the homoallylic alcohol product in 27% ee and 44% yield. Addition of (7 )-(+)-a-methyl-2-naphthalenemethanol ((/ )-MNM) leads to higher enantiomeric excess (53% ee) than those using only (7 )-BINOL. Therefore, (7 )-MNM can act as a chiral activator a higher ee can be achieved via activation of the allylation of benzaldehyde by addition of (7 )-MNM as a product-like activator. 

The above azomethine ylide cycloadditions have been extended to an enantioselective version involving amino alcohols both as chiral ligands and amine bases. Thus, reactions of the N-metalated azomethine yhdes derived from achiral methyl 2-(arylmethyleneamino)acetates, cobalt(II) chloride [or manganese(II) bromide], and chiral amino alcohols, 1 and 2 equiv each, with methyl acrylate as solvent have been performed to provide the enantiomer-enriched pyrrolidine-2,4-dicarboxylates with the enantioselectivities of up to 96% enantiomeric excess (ee) (128,129). However, a large excess of the metal ions and the chiral source (ligand and base) have to be employed. 

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