Chiral auxiliaries nucleophile containing

New strategies for preparing either 1,2-cis- or 1,2-trans-glycosides stereoselectively have been reported.61 A chiral auxiliary containing a nucleophile is used to control the anomeric selectivity of the glycosylation. A sample reaction is shown in Scheme 8. 

An interesting example of stereoselective coupling of an aromatic radical with a nucleophile is found in the reaction of 1-iodonaphthalene with the imide anion 12, containing a chiral auxiliary. In this reaction the diasteromeric isomers of the substitution compound are formed (Sch. 17) [87]. This reaction is highly dependent on the metal counter ion used. All the ions studied [Li, Na, K, Cs, Ti(IV)] are selective, but the highest stereoselection is reached with Li at low temperature (—78°) and with Ti(IV) (ca. 99% de) [87]. 

The use of C2-symmetric aziridines as chiral auxiliaries has been reviewed by Tanner <94AG(E)599>. Chiral A-acylaziridines such as (143) have been used in enolate chemistry (see Section 1.01.8.2). Aziridine-containing ligands such as (144) have been used for a variety of metal-catalyzed reactions such as the asymmetric dihydroxylation of alkenes, the palladium-catalyzed allylation of nucleophiles, asymmetric cyclopropanation, and aziridination <94AG(E)599,94TL4631). 

Use of ( )-l,4-dibromobut-2-ene in the SIRC reaction gives rise to vinylcyclopropanes (Table 7). The cyclopropanation is induced by bases such as sodium ethoxide, sodium hydride or sodium amide or can be carried out under phase-transfer conditions. The reaction is non-stereoselective however, a diastereomeric excess of 39-80% was obtained with nucleophiles containing a chiral auxiliary. When cyclic ketones are used as nucleophiles, spiro-cyclo-propanes, which serve as precursors for (Z)-jasmonate or methyl jasmonate (Table 7, entries 5-7), are formed. 

These boron enolates can be considered as chiral nucleophiles wherein chirality observed in the products of the aldol reactions arises from the chiral auxiliary mandelic acid. An alternative approach to the diastereo- and enantioselective carbon-carbon bond forming reaction is to react an achiral anion precursor with an electrophilic equivalent containing a chiral auxiliary derived from mandelic acid. 

Extensive studies have been carried out on nucleophilic substitution of a-halocarboxylic acid derivatives containing a chiral auxiliary in the carboxylic moiety. Racemisation of the labile chiral centre in the a-position to the carbonyl—induced by additives such as polar solvents, bases or halide salts—allows a high asymmetric induction through a DKR process to be obtained. This methodology has been recently recognised as a powerful synthetic method for asymmetric syntheses of a-heteroatom-substituted carboxylic acid derivatives. 

An interesting example of stereoselective coupling of an aromatic radical with a nucleophile is found in the reaction of 1-iodonaphthalene with an imide ion containing a chiral auxiliary. In this reaction, the diastereomeric isomers of the substitution compound are formed - ... 

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