Chiral non-racemic reagents

ORGANIC SYNTHESES PROCEDURES FEATURING CHIRAL, NON-RACEMIC REAGENT PREPARATION 7... 

Organic Syntheses Procedures Featuring Chiral, Non-Racemic Reagent Preparation, Volumes 68-78... 

When two enantiomers react with an achiral reagent, they react at the same rate, but when they react with a chiral, non-racemic reagent, they react at different rates. 

Two enantiomers have exactly the same chemical properties except for their reaction with chiral, non-racemic reagents. 

This process, that resolves enantiomers on the basis of their different reaction rates, is called kinetic resolution [28], Conceptually it is similar to the reactions of Fig. 4, as the enantiomerically pure reagent (in this context, the kinetic resolving agent) differentiates between the two enantiomerically related molecules of the racemic substrate. Obviously, in the reactions of Fig. 4 it is one molecule that features the enantiomerically related entities (e.g., the two carbonyl faces of benzaldehyde) which are differentiated by the chiral non-racemic reagent. 

The (5,5)-enantiomer of the platelet activating factor antagonist MK-0287 is 20-fold more potent than the (f ,f )-enantiomer. Several syntheses of this molecule have been reported [56], The one described in Fig. 15 is based on the use of a chiral non-racemic reagent. 

With this epoxidation procedure it is possible to convert the achiral starting material—i.e. the allylic alcohol—with the aim of a chiral reagent, into a chiral, non-racemic product in many cases an enantiomerically highly-enriched product is obtained. The desired enantiomer of the product epoxy alcohol can be obtained by using either the (-1-)- or (-)- enantiomer of diethyl tartrate as chiral auxiliary ... 

Review articles have featured new organometallic reagents using highly reactive metals90 and the synthetic potential of remarkably stable chiral, non-racemic 2-lithiopiperidines and 2-lithiopyrrolidines.91... 

Catalytic enantioselective addition reactions of alkyllithium or -zinc reagents to preformed imines have been described [34, 35]. Hoveyda, Snapper, and co-workers have described a direct three-component variant of this reaction [36]. Accordingly, several chiral, non-racemic aromatic as well as aliphatic amines were synthesized by the reaction of the corresponding aldehydes, o-anisidine 132, and alkylzincs using a Zr catalyst with a chiral peptide ligand 133 (Scheme 9.24). 

These reagents have been used for the conversion of Q symmetrical chains into chiral non-racemic products. Condensation of mcio-dialdehydes with Cp(/ ,/ )-Ti[All] or Cp(5,5)-Ti[All] reagents led (after reduction with NaBH4) to optically active polyketides (eq 12). ... 

In this approach the substrate is attached to a chiral, non-racemic unit that controls the formation of one or more new chiral groups. Reaction of the coupled unit with a reagent or prochiral substrate is designed to produce one diastereomeric product in excess. The auxil-... 

This article presents a review of recent advances in the catalytic addition of alkylmetal reagents to olefins, excluding reactions that involve unsaturated carbonyl compounds as substrates (conjugate addition) [2]. As described below, these reactions provide efficient and selective routes to the synthesis of a wide variety of chiral, non-racemic organic molecules that can be used in the fabrication of a number of highly functionalized molecules. 

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