Asymmetric reactions experimental compounds

Since the early times of stereochemistry, the phenomena related to chirality ( dis-symetrie moleculaire, as originally stated by Pasteur) have been treated or referred to as enantiomericaUy pure compounds. For a long time the measurement of specific rotations has been the only tool to evaluate the enantiomer distribution of an enantioimpure sample hence the expressions optical purity and optical antipodes. The usefulness of chiral assistance (natural products, circularly polarized light, etc.) for the preparation of optically active compounds, by either resolution or asymmetric synthesis, has been recognized by Pasteur, Le Bel, and van t Hoff. The first chiral auxiliaries selected for asymmetric synthesis were alkaloids such as quinine or some terpenes. Natural products with several asymmetric centers are usually enantiopure or close to 100% ee. With the necessity to devise new routes to enantiopure compounds, many simple or complex auxiliaries have been prepared from natural products or from resolved materials. Often the authors tried to get the highest enantiomeric excess values possible for the chiral auxiliaries before using them for asymmetric reactions. When a chiral reagent or catalyst could not be prepared enantiomericaUy pure, the enantiomeric excess (ee) of the product was assumed to be a minimum value or was corrected by the ee of the chiral auxiliary. The experimental data measured by polarimetry or spectroscopic methods are conveniently expressed by enantiomeric excess and enantiomeric... 

The facile arylation of aldehydes with arylboronic acid has prompted the exploration of asymmetric versions of this reaction. However, this field has been scarcely explored and only few examples have been reported in the literature, with moderate results. The first diastereoselective example was described by Ftirstner and coworkers. By reacting the Gamer aldehyde 15 with phenylboronic acid under their set of experimental conditions (i.e. RhClj-SH O, IPr HCl) (Scheme 7.4) [21], the secondary alcohol was obtained in higher selectivity than that observed in the addition of phenylmagnesium bromide reported by Joullie (de = 94% versus 66%), with the anti isomer as the major compound [29]. 

Both experimental and theoretical studies have been reported of fluoro-denitration and fluoro-dechlorination reactions using anhydrous tetrabutylammonium fluoride in DMSO. The absences of ion pairing and strong solvation are critical in contributing to the reactivity of the fluorinating agent24 Quaternary ammonium salts derived from cinchona alkaloids have been shown to be effective catalysts in an improved asymmetric substitution reaction of /1-dicarbonyl compounds with activated fluoroarenes. The products may be functionalized to yield spiro-oxindoles.25... 

Some of the most pertinent virtues of asymmetric epoxidations and dihydroxylations were already present in their classical versions. Both reactions are highly chemo-selective and can be carried out in the presence of many other functional groups. More important with respect to stereochemistry, each reaction is stereospecific in that the product faithfully reflects the E or Z configuration of the starting olefin (the nucleophilic epoxidation of a,P-unsaturated carbonyl compounds is an important exception). And one should not underestimate the importance of experimental simplicity in most cases, one can carry out these reactions by simply adding the often commercially available reagents to a substrate in solvent, without extravagant precautions to avoid moisture or air. 

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