Vicinal chiral

In an analogous manner, a -substituted phenyl ketones have been used to afford Michael adducts containing two vicinal chiral tertiary centers in both high diastereo- and enantioselectivity (eq 10, Table 2),... 

For synthetic purposes, the most usefiil types are those lyases that depend on pyruvate or dihydroxyacetone phosphate (DHAP) as the aldol donor component [38,42]. The latter enzymes catalyze the addition of DHAP to the carbonyl group of an aldehyde to form 3,4-dihydroxyketose 1-phosphates which contain two new vicinal chiral centers at the termini of the C-C bond created. Because the aldolization is doubly chirogenic this calls for four corresponding enzymes with stereoselectivities matching those of the four possible diastereomeric products. 

The stereochemistry of the olefin is crucial for the enantioselectivity of the carbolithiation. Whereas the asymetric carbolithiation of ( )-cinnamyl alcohol gives the (S)-alkylated product 93, the reaction of the (Z)-isomer, under the same experimental conditions, leads to 92 Li, the enantiomer of 92Li. After acidic hydrolysis, 92 Li was converted to 93 with an (/ ) absolute configuration in 70% ee. Addition of diphenyl disulfide to 92 Li leads to the fonnation of two vicinal chiral centers with a diastereoselectivity of 98 2 and an enantioselection of 70% [128], as described in Scheme 7-109. However, a racemic product is obtained when the allylic alcohol is not substituted, as is the case with prop-2-en-l-ol. 

In this section, we will focus on linear cascades combining m-TAs with hydrolases or lyases. For example, the synthesis of (2S,3S)-2-aminopentane-l,3-diol from propanal and p-hydroxypyruvate was facilitated [50] by coupHng a mutated transketolase from E. coli (TK D469T) [51] and an m-TA from C. violaceum [31] (Scheme 4.14). Such vicinal chiral amino alcohols represent an important class of building blocks and pharmaceutical intermediates. 

Quinine decorated with a fluorous pony-tail (326) has been developed as a catalyst for the Michael addition of a-fluoro-/ -ketoesters ArC0CHFC02Et to A-substituted maleimides. The resulting products with two vicinal chiral centres were obtained with <87% ee and >20 1 dr, and the catalyst was recycled by fluorous solid-phase extraction. ... 

Syntheses and use of vicinal diamines with one or two N atoms included in heterocycle as chiral auxiliaries 98AG(E)2580. 

In most cases of diastereoselective nucleophilic addition reactions where achiral organometallic reagents are added to chiral carbonyl compounds, the chirulity inducing asymmetric center is in close vicinity to the newly created center and cannot be removed without the loss of chirality of either the inducing center or the newly formed center. This type of reaction is very useful in propagating chirality in a molecule from one center to an adjacent one, or in immolative processes. 

D Chiral or achiral. v/H-carbanions with an additional, configurationally stable stereo-genic center. Very often the additional stereogenic center is in close vicinity (a- or -position) to the anionic center. 

Even in Ugi reactions of chiral 4,5-dihydro-3//-pyrrole derivatives with aryloxy substituents vicinal to the cWo-cyclic imino group, a low stereoselectivity was found with either chiral or achiral isocyanides and benzoic acid leading to substituted 2-aminoearbonyl-3-aryl-oxy-1 -benzoylpyrrolid ine derivatives82. 

Sulfoxides (R1—SO—R2), which are tricoordinate sulfur compounds, are chiral when R1 and R2 are different, and a-sulfmyl carbanions derived from optically active sulfoxides are known to retain the chirality. Therefore, these chiral carbanions usually give products which are rich in one diastereomer upon treatment with some prochiral reagents. Thus, optically active sulfoxides have been used as versatile reagents for asymmetric syntheses of many naturally occurring products116, since optically active a-sulfinyl carbanions can cause asymmetric induction in the C—C bond formation due to their close vicinity. In the following four subsections various reactions of a-sulfinyl carbanions are described (A) alkylation and acylation, (B) addition to unsaturated bonds such as C=0, C=N or C= N, (C) nucleophilic addition to a, /5-unsaturated sulfoxides, and (D) reactions of allylic sulfoxides. 

Chiral heterocyclic compounds containing vicinal oxygen and nitrogen atoms were achieved by an asymmetric Diels-Alder reaction [111] of chiral acylnitroso dienophiles 111. The latter were prepared in situ from alcohols 110, both antipodes of which are available from camphor, and trapped with dienes (Scheme 2.46). Both the yield (65-94 %i) and diastereoisomeric excess (91-96%) were high. 

The cationic pathway allows the conversion of carboxylic acids into ethers, acetals or amides. From a-aminoacids versatile chiral building blocks are accessible. The eliminative decarboxylation of vicinal diacids or P-silyl carboxylic acids, combined with cycloaddition reactions, allows the efficient construction of cyclobutenes or cyclohexadienes. The induction of cationic rearrangements or fragmentations is a potent way to specifically substituted cyclopentanoids and ring extensions by one-or four carbons. In view of these favorable qualities of Kolbe electrolysis, numerous useful applications of this old reaction can be expected in the future. 

Chiral epoxides and their corresponding vicinal diols are very important intermediates in asymmetric synthesis [163]. Chiral nonracemic epoxides can be obtained through asymmetric epoxidation using either chemical catalysts [164] or enzymes [165-167]. Biocatalytic epoxidations require sophisticated techniques and have thus far found limited application. An alternative approach is the asymmetric hydrolysis of racemic or meso-epoxides using transition-metal catalysts [168] or biocatalysts [169-174]. Epoxide hydrolases (EHs) (EC 3.3.2.3) catalyze the conversion of epoxides to their corresponding vicinal diols. EHs are cofactor-independent enzymes that are almost ubiquitous in nature. They are usually employed as whole cells or crude... 

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