Prochiral dicarboxylates

Asymmetrization of Prochiral Dicarboxylates Single-Step Process... 

Asymmetrization of a prochiral dicarboxylic acid diester catalyzed by lipases, where the stereo center of the product is located on the acyl side, becomes a single-step process because the polar carboxylic acid and/or amide formed are not well accepted as substrates by the Upase. One example is the enantioselective hydrolysis or ammonolysis of diethyl 3-hydroxyglutarate, as shown in Scheme 7.4, a reaction which leads to the formation of a precursor for the important chiral side chain of atorvastatin, lipitor [40, 41]. The S-enantiomer was formed with high e.e. (98%), but unfortunately this is the undesired enantiomer for the production of the pharmaceutically important product. Only a-chymotrypsin gave a predominance of the... 

A limited number of acyclic and cyclic prochiral dicarboxylic acid diesters were found to be good substrates for hydrolysis catalyzed by lipases (Table 11.1-12). Notable examples which give a good illustration of the potential of hydrolases as well as of the trial and error approach one relies on to a certain extent are the dithio acetal derivative 9 and the fluoro alkyl malonates 1-8. The dithio monoester 9 is obtained with different lipases with high enantioselectivities and yields despite its remote chiral center. Candida cylindracea lipase is the enzyme of choice for the synthesis of fluoro alkyl malonates with small alkyl groups. An astonishing observation was... 

Mono cylDiols. Enzymatic synthesis of chiral monoacyl diols can be carried out either by direct enzymatic acylation of prochiral diols or by hydrolysis of chemically synthesized dicarboxylates. 

The solvent present in biphasic reactions can still have an effect on the enzyme even though the enzyme functions primarily in an aqueous microenvironment. A particularly dramatic example is the lipase AH (lipase from Burkholderia cepac/fl)-catalysed desym-metrization of prochiral 1,4-dihydropyridine dicarboxylic esters, where either enantiomer can be accessed in high enantioselectivity by using either water-saturated cyclohexane or diisopropyl ether (DIPE) respectively (Scheme 1.60). The acyl group used in acylation and deacylation can also have a dramatic effect on enantioselectivity. " ... 

Scheme 1.60 Resolution of a prochiral 1,4-dihydropyridine dicarboxylic ester with lipase AH in the presence of cyclohexane or DIPE...

The boron atom dominates the reactivity of the boracyclic compounds because of its inherent Lewis acidity. Consequently, there have been very few reports on the reactivity of substituents attached to the ring carbon atoms in the five-membered boronated cyclic systems. Singaram and co-workers developed a novel catalyst 31 based on dicarboxylic acid derivative of 1,3,2-dioxaborolane for the asymmetric reduction of prochiral ketones 32. This catalyst reduces a wide variety of ketones enantioselectively in the presence of a co-reductant such as LiBH4. The mechanism involves the coordination of ketone 32 with the chiral boronate 31 and the conjugation of borohydride with carboxylic acid to furnish the chiral borohydride complex 34. Subsequent transfer of hydride from the least hindered face of the ketone yields the corresponding alcohol 35 in high ee (Scheme 3) <20060PD949>. 

VI. Highly Selective Nonenzymatic Chiral Induction onto Prochiral r-Symmetric Dicarboxylic Acids... 

We developed a novel method for a highly stereoselective differentiation between two identical groups in prochiral ir-symmetric dicarboxylic acids based on a completely new idea employing (4/ )-MCTT(1). The dipole-dipole repulsion between the carbonyl and the thiocarbonyl groups in the (4/ )-MCTT amide system was utilized in order to regulate the free rotatory molecule in the transition state for chiral induction (see Fig. 2). 

Table 11.7-1. Pig liver esterase-catalyzed enantiotopos-differentiating hydrolysis of prochiral cyclic dicarboxylic acid diesters in aqueous solution.

Table 11.1-12. Lipase-catalyzed enantiotopos-differentiating hydrolysis of prochiral acyclic and cyclic dicarboxylic acid diesters in aqueous solution (CCL Candida cylindracea lipase, PPL pig pancreas lipase, PSL Pseudomonas sp. lipase, CVL Chromobacterium viscosum lipase,...

The second method of determining tacticities is essentially an extension of the first, the only difference being that the chiral centre is placed in the side chain(s) of a monomer that would otherwise be prochiral. The adjacent olefinic protons in the polymer may be equivalent and uncoupled (HH or TT), or non-equivalent and coupled (HT). The application of this method to the case of 5,6-dicarboxylic esters of norbomadienes is described in Section 13.4.1.4. It turns out that an all-/rans polymer is syndiotactic and an all-cw polymer is isotactic, which is the opposite way round to what was found for 5,5-dimethylnorbomene (see above). 

In kinetic resolutions (Scheme 3.2-3.5) it is often the case that one of the products is required, while the other is not and must be discarded or recycled (e.g. racemised). Such operations can be wasteful or expensive. On the other hand, the biotransformation of wcso-compounds or prochiral compounds allows for the possibility of preparing an optically pure compound in quantitative yield. In Scheme 3.7, two examples of the use of meso-compounds are described. The diester (11) is made up of a complex dicarboxylic acid unit derivatised as the dimethyl ester. Pig liver esterase catalyses the hydrolysis of one of the ester groups to give the acid (12) (95% e.e.) in 96% yield. This compound is an excellent precursor of the natural product neplanocin. Note that the acid (12) is not a substrate for pie, and thus the reaction stops at the half-way stage. The compound (13), like (11), possesses a plane of symmetry. Hydrolysis catalysed by porcine pancreatic lipase (ppl) affords the alcohol (14) (>98% e.e.) in quantitative yield. The latter compound has been used to make fluorocarbocyclic adenosine (C -adenosine), a stable analogue of the naturally occurring nucleoside adenosine. 

Prochiral and meso-compounds have been widely transformed into chiral products through transesterification reactions as shown with the acylation of diols (Fig. 14,1 and II). In transesterification, the prochiral or meso-substrate can also be a dicarboxylic acid diester. Common to such compounds is a plane of symmetry... 

Oiganocatalysts can also be used to prepare polycyclic systems. Professor Jorgensen has found (Chem. Commun. 2008, 3016) that condensation of 14 with acetone dicarboxylate 17, again using catalyst 3a, gave the bicyclic P-keto ester 18. Matthew J. Gaunt of the University of Cambridge observed (J. Am. Chem. Soc. 2008,130,404) that for the cycliza-tion of 19, catalyst 3b was superior to catalyst 3a. The power of desymmetrization of prochiral intermediates was illustrated by the report (J. Am. Chem. Soc. 2008,130, 6737) from Benjamin List of the Max-Planck-lnstitute, Mulheim of the cyclization of 21 to 23. 

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