Absolute configuration salt effect

All of the above examples are acetates of active alcohols. Here, we also mention the acetate of a phenol, namely the provitamin a-tocopheryl acetate, whose natural enantiomer of absolute configuration (2R,47 ,87 ) is shown as 8.73. a-Tocopheryl acetate is a substrate of cholesterol esterase (EC 3.1.1.13), and was hydrolyzed in rats faster than its (2S,47 ,87 )-epimer. In vitro experiments required a-tocopheryl acetate to be dispersed as a micellar pseudosolution, and the nature of the bile salt used to prepare micelles had a profound effect on the substrate stereoselectivity of the reaction [95] [96], Only when the micelle composition approximated that of the gastrointestinal tract did the in vitro substrate stereoselectivity resemble that seen in vivo. 

Imidazole-containing [10]metacyclophane 7 was efficiently synthesized from cyclododecanone and the racemic 7 obtained was effectively resolved to a pair of enantiomers by chiral HPLC (on a Daicel OD-H column) [22], The absolute configuration of the first-eluted (-)-isomer was determined to be Sj, by the X-ray crystal structural analysis of (+)-10-camphorsulfonic acid salt of 7, which displayed a negative Cotton effect at the imidazole band. Unfortunately, the CD intensities are not clear as the concentration of the cyclophane is not stated in the report (Fig. 1). 

The CD spectra of enantiopure pyridinophane 17 and bipyridine derivative 18 were reported. Both spectra were relatively complicated compared to those of substituted [2.2]paracyclophanes, apparently showing 6-8 positive/negative Cotton effect peaks. While the lAel value for the main band of compound 18 (20-30 M-1 cm-1) was reduced to half that of compound 17 (30-50 M 1 cm ), the spectrum of planar chiral bipyridine 18 was very sensitive to the added metal salts. The absolute configuration was determined by comparison with the theoretical spectrum at the DFT/SCI level [31]. 

Tertiary phosphines, in the absence of special effects 2 ), have relatively high barriers 8) ca. 30-35 kcal/mol) to pyramidal inversion, and may therefore be prepared in otically stable form. Methods for synthesis of optically active phosphines include cathodic reduction or base-catalyzed hydrolysis 3° 31) of optically active phosphonium salts, reduction of optically active phosphine oxides with silane hydrides 32), and kinetic 3 0 or direct 33) resolution. The ready availability of optically pure phosphine oxides of known absolute configuration by the Grignard method (see Sect. 2.1) led to a study 3 ) of a convenient, general, and stereospecific method for their reduction, thus providing a combined methodology for preparation of phosphines of known chirality and of high enantiomeric purity. 

The formal [3+3] cycloaddition of vinylogous amide 172 was effected with chiral amine salts to provide 173 enantioselectively (Scheme 49). These reactions were studied with respect to solvent, counterion, and temperature effects, and the probable mechanism and absolute configuration of the products were determined <05JOC4248>. 

The reaction of optically active allyl(difluoro)phenylsilanes with aryl triflates affords optically active allylarenes with high stereoselectivities, wherein the absolute configuration of the newly generated chiral carbon can be controlled by the choice of a fluoride salt and the solvent polarity (Scheme 29). Aryl and alkenyl triflates are effective coupling partners not only for aUylsilanes but also for alkenyl-, aryl-, alkyl-, and alkynylsilanes as we have been seen in some examples of the preceding sections. 

Resolution of l- -propo5y-3-methyl-3-phospholene 1-oxide (754) using TADDOL derivatives (755) or Ca salts of 0,0 -dibenzoyl-(2R,3R)- (756a) or 0,0 -di-p-toluoyl-(2R,3)-tartaric acid (756b) has been developed (Scheme 212). Both antipodes of l-n-propo qr-3-phos-pholene 1-oxide (754) have been obtained with ee >96% by exploiting the different antipode preference of the TADDOL derivatives (755). The absolute configuration of the 3-phospholene oxide (754) enantiomers has been determined by CD spectroscopy, assisted by TD-DFT quantum chemical calculations. Moreover, it has been found that the solvent used for resolution had a significant effect on the efficiency of the resolutions. ... 

In a similar way, the absolute configuration of a chiral primary amines can be deduced from the NMR complexation effect produced by the addition of a Ba + salt on a single MPA amide, prepared from either R) or (5)-MPA. ... 

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