Organolithium reagents racemization

Table 2. Stereoselective Addition of Organolithium Reagents to Racemic Hydrazones l2 OR1...

In some cases the yields were poor due to competing deprotonation of the substrate by the organolithium reagent. Deprotonation was the predominant reaction with methyllithium or when (Z)-2-(l-alkenyl)-4,5-dihydrooxazoles were employed. The stereochemical outcome has been rationalized as occurring from a chelated transition state. The starting chiral amino alcohol auxiliary can also be recovered without racemization for reuse. 

Reactivity of oxiranes with organolithium reagents TABLE 9. Kinetic resolution of racemic oxiranes using HCLA 53... 

Generation of Enantioenriched, Configurationally Stable Organolithium Reagents. (1 S,2E)-1 -(N.Af-Diisopropyl-carbamoyloxy)-l-methyl-2-butenyllithium-(—)-sparteine is configurationally stable in solution and is obtained by kinetic resolution of the racemic 2-alkenyl carbamate by n-butyllithium-(—)-sparteine with >80% de (eq 4). The enantioenriched allylstan-nane, obtained on y-stannylation, was used as chiral homoenolate reagent. The methoxycarbonylation (a, inversion) yields enantioenriched 3-alkenoates. ... 

Chiral boronic esters react with organolithium reagents to form diorganylalkoxyboranes (borinic esters). Subsequent reaction with the anion of dichloromethyl methyl ether then yields chiral ketones by rearrangement of both of the groups on boron (Scheme 42). No racemization is observed in this sequence and alkyl-, aryl- or alkynyl-lithium reagents can be used. 

This rapid ligand exchange reaction was observed not only in simple aryl sulfoxides but also bulky sulfoxides with racemization of the recovered sulfoxide. The reaction is initiated by attack of organolithium reagent on the sulfinyl sul-... 

Final displacement to yield the required sulfoxides occurs in good yield only with organolithium reagents, but is accompanied by significant racemization. The use of Grignard reagents eliminates racemization but results in lower yields. 

Unsaturated a-Amino-acids.—Protection of the primary amino-group of ethyl glycinate as a stabase adduct, renders it stable to organolithium reagents hence the lithiated species (196) is readily prepared. Subsequent conversion into racemic 2-aminopent-4-ynoic acid (197) and vinyl glycine (198) is easily achieved (Scheme 90). ... 

The stereochemical course of the electrophilic substitution - retention, inversion, or partial racemization - of a given chiral benzylic organolithium by an electrophilic reagent still cannot be predicted on the basis of the present knowledge [Eq. (57)] it depends mainly on the substituents in the carbanion, the electrophile and the complexing ligands at the lithium cation (Sect. 3.3) [64,129, 130]. 

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