Benzoates chirality

Fig. (2). Determination of the absolute stereostructure of broussonetine C (1) by the benzoate chirality method.

The absolute stereochemistry of broussonetine L (8) was determined by the combination of the benzoate chirality method and the Mosher s method [35-37]. A carbamate (8a) was prepared from broussonetine F (4) by reaction with phenyl chloroformate in tetrahydrofuran-H20 (7 3), and a diacetate (8b) was prepared from 8a with acetic anhydride in pyridine. Finally, a dibenzoate (8c) was obtained by benzoylation of 8b. The CD curve of 8c showed a negative Cotton effect (Ae237 -30.9) and a positive effect (Ae223 +15.9) to confirm a counter-clockwise chirality between two benzoyl groups, Fig. (3) [20]. 

H. Y. Aboul-Enein and M. R. Islam, Direct HPLC separation of thalidomide enantiomers using cellulose tris-4-methylphenyl benzoate chiral stationary phase, J. Liquid Chromatogr., 14 667... 

A GLC-MS approach to identifying the isomers of coumaroyl- and caffeoyl-D-quinic acid, and syntheses of 3,4,5-tri- and 1,3,4,5-tetra-galloylquinic acids involving a description of their chirotopical properties using the benzoate chirality rule, have been reported. 

Aboul-Enein HY, Ali I, Laguene M, Felix G (2002) Molecular modeling of enantiomeric resolution of methylphenidate on cellulose tris benzoate chiral stationary phase. J Liq Chromatogr Relat Technol 25 2739-2748... 

Triflates of phenols are carbonylated to form aromatic esters by using PhjP[328]. The reaction is 500 times faster if dppp is used[329]. This reaction is a good preparative method for benzoates from phenols and naphthoates (473) from naphthols. Carbonylation of the bis-triflate of axially chiral 1,1 -binaphthyl-2,2 -diol (474) using dppp was claimed to give the monocarboxy-late 475(330]. However, the optically pure dicarboxylate 476 is obtained under similar conditions[331]. The use of 4.4 equiv. of a hindered amine (ethyldiisopropylamine) is crucial for the dicarbonylation. The use of more or less than 4.4 equiv. of the amine gives the monoester 475. 

The potential for use of chiral natural materials such as cellulose for separation of enantiomers has long been recognized, but development of efficient materials occurred relatively recently. Several acylated derivatives of cellulose are effective chiral stationary phases. Benzoate esters and aryl carbamates are particularly useful. These materials are commercially available on a silica support and imder the trademark Chiralcel. Figure 2.4 shows the resolution of y-phenyl-y-butyrolactone with the use of acetylated cellulose as the adsorbent material. 

In this section, we will neglect the crystal structures of the mesogenic perfluorinated phenyl benzoates [23-27], benzoic acids [6, 28-31], cinnamic acids [7, 32, 33], dicarboxylic acids [34, 35], and cinnamate compounds [8, 36-40]. The single crystal X-ray analyses of chiral mesogenic carboxylates are described in Sect. 6. 

The three chiral centres in (45) mean that a mixture of isomers was formed. These could be separated as their crystalline benzoates. 

Carbanions derived from optically active sulfoxides react with esters, affording generally optically active )S-ketoesters ° . Kunieda and coworkers revealed that treatment of (-t-)-(R)-methyl p-tolyl sulfoxide 107 with n-butyllithium or dimethy-lamine afforded the corresponding carbanion, which upon further reaction with ethyl benzoate gave (-l-)-(R)-a-(p-tolylsulfinyl)acetophenone 108. They also found that the reaction between chiral esters of carboxylic acids (R COOR ) and a-lithio aryl methyl sulfoxides gave optically active 3-ketosulfoxides The stereoselectivity was found to be markedly influenced by the size of the R group of the esters and the optical purity reached to 70.3% when R was a t-butyl group. 

Figure 1-17. Exciton chirality of acyclic allylic benzoates and the sign of the predicted benzoate Cotton effects. The thick line denotes the electric transition moment of the benzoate group. Reprinted with permission by Am. Chem. Soc., Ref. 61. 

The synthesis of lycorane (13) by Mori and Shiba-saki121 is breathtaking for its use of three consecutive Pd catalyzed C-C bond forming reactions. Thus, Pd-catalyzed asymmetric allylic substitution of a benzoate in meso 7 in the presence of the chiral bisphos-phine 8 leads to the regioselective formation of 10 in 40 % ee It is easy to overlook this low level of enantioselectivity when we are faced with the subsequent elegant Pd-catalyzed reactions Pd-catalyzed intramolecular animation is followed by a Pd-catalyzed Heck coupling to afford 12, which is then readily converted to the target molecule... 

It was now important to again examine the biological activity of the enantiomers in this series. These were prepared using the chiral amino acids and the activity of these compounds particularly when compared to the benzoate analogs is striking. 

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