Chiral carboxylates, solvent-dependent

Solvent Dependent CD of Pentaamminecobalt(lll) Complexes of Chiral Carboxylates... 

Some studies on Meyer s route to chiral carboxylic acids (1, 1 2, 10) have revealed that stereoselectivity in the deprotonation of chiral oxazolines is very dependent on the particular base-solvent combination used. A further route to chiral acids is by alkylation (with primary halides) and subsequent hydrolysis of the bis-anionic species (1) derived from acid anhydrides and /-ephedrine. Chemical yields are in the range 50—70%, while optical yields are around 75%. A full report has appeared on the general approach to chiral acids by the Michael addition of Grignard reagents to oxazepinedione derivatives (2), also derived from /-ephedrine. Optical yields of 80—99% have been claimed (2,11 3, 27). 

Stereoselective Alkylation of Prochiral Enolates. A limited amount of work has demonstrated the potential use of chiral amines in inducing stereoselectivity in the alkyla-tion/carboxylation of prochiral enolates. The selectivity of these reactions, like those described above, is highly dependent on solvent and temperature conditions. The use of ether at — 196°C provides optimal results in a particular system (eq 10). ... 

Inter molecular Additions to Cyclohexenones and Related Systems. Calculations have been reported that deal with a transition state analysis of the regioselectivity encountered in triplet-state (2 + 2)-cycloaddition reactions of cyclohex-2-enone. Diastereoselective (2 + 2)-cycloaddition of ethene to cyclohexenone carboxylates in the presence of chiral auxiliaries has been described. Yields of bicyclo[4.2.0]octanone derivatives can be obtained with de as high as 81%. The enone (16) can be tethered to a polymer substrate via the tether group R. Irradiation of this material in toluene with ethene at —78°C gives a 68% yield of the adduct (17). The de of the product is reasonable at 72%. Addition of 1,1-diethoxyethene to the cyclohexenone (18, R = Ph) results in the formation of the two stereoisomeric head-to-tail regioisomers (19) and (20). The outcome of the reaction is dependent on the rate of formation of 1,4-biradical intermediates. This can be seen in the dependence of the cisjtrans ratio on the solvent and on the temperature at which the reaction is carried out. Thus with enone (18, R = Ph) in acetone at 3°C, a cjt ratio of 4.4 is obtained, and this changes to 2.1 at —40°C. In acetonitrile the cjt ratio is only 1.9 at the same temperature. With the other derivative of (18), the cjt ratio is 1.3 in acetone and 0.8 in acetonitrile. Photoaddition of ethene to the enone carboxylate [21, R = (-)-8-(2-naphthyl)menthyl] results in the formation of the diastereoisomers (22) and (23) with a de of 56% at —78°C. The diastereo-selectivity can be enhanced by the addition of naphthalene derivatives to the solution. Thus with naphthalene, a de of 71% is obtained, and this can be increased to 83% with 1-phenylnaphthalene. ... 

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