Esterases, reaction with diesters

For desymmetrization of diesters 3 via their hydrolysis in water, pig Hver esterase [12], o -chymotrypsin [12, 13a], and Candida antarctica Hpase (CAL-B) [14] were successfully used. However, further studies showed that respective anhydrides 5 can be used as substrates for enzyme-catalyzed desymmetrization in organic solvents [15]. The desired monoesters 4 were obtained in high yield in this way, using immobilized enzymes Novozym 435 or Chirazyme L-2 (Scheme 5.3). After the reaction, enzymes were filtered off, organic solvents were evaporated, and the crude products were crystalHzed. This was a much simpler experimental procedure in which control of the reaction progress was not necessary, and aU problems associated with extraction of products from aqueous phase and their further purification were omitted [15]. 

For a new potential -3-receptor agonist a pig liver esterase-based enantioselec-tive synthesis was devised (Scheme 6.18). The substituted malonic acid diester was hydrolysed at pH 7.2 and yielded 86% of the (S)-monoester with an ee of 97% [62]. This reaction immediately demonstrates the great advantage of starting with a symmetric molecule. The enzyme very efficiently desymmetrizes the diester and excellent yields with high optical purities are obtained. No extra steps are necessary and no additional chemicals need to be added. 

Construction of the enantiomerically pure cyclopentane unit for 161 hinged on the ability of an enzyme to react preferentially with one of two enantiotopically related functional groups. To this end, the reaction of the meso diester 261 with commercially available pig liver esterase resulted in almost exclusive hydrolysis at the 1/ ester group to afford the mono ester 262 in 92% yield and >99% ee. Reduction to the alcohol was accomplished via the intermediate acid chloride, which was then cyclized to the lactone 263. Oxidative ring opening followed... 

The total synthesis of (-)-l-O-methylsweroside aglucone (486, R = Me) has been described by Ikeda and Hutchinson. We briefly mentioned (Vol. 4, p. 504) an earlier synthesis of the racemate of 486 (R = Me) by Hutchinson et al., and the key intermediate, 488, is the same, but now it has been prepared in optically active form. The starting material was (-)-489, made either by enzymatic in vitro oxidation of ( )-cyclohexene-4,5-bismethanol, or enantio-tropic hydrolysis of the corresponding diesters with pig-liver esterase. Two one-carbon homologations and a Pummerer reaction then converted this material... 

The chemistry of complex lipids is dominated by regioselective hydrolysis reactions of (1) the glyceryl fatty acid esters and (2) phosphate diesters. Both types of reactions are routinely performed with the corresponding esterases. A large variety of lipid active transferase enzymes is also commercially available. Phospholipases Aj, A, C, and D, for example, split any of the four ester bonds of a phospholipid regioselectively. The product without a fatty acid side chain at C2 of glycerol is called a lysophospholipid. Lecithin-cholesterol-acyltransferase transfers the fatty acid at C2, often linoleic acid, to the OH group at C3 of cho-... 

An enzymatic resolution of dimethyl (i , S)-malate with pig liver esterase (PLE) relies on selective hydrolysis of the (5)-diester to monoacid 14, leaving dimethyl (i )-malate (203) behind [10,22]. The reaction is performed at 0 °C in 20% aqueous methanol, and the desired (i )-203 is obtained in 42% yield with 93% ee (maximum theoretical yield 50%). 

Examples of enantiomorphogenic selectivity are provided by the pig liver esterase (PLE) catalyzed hydrolysis of diester 45 to give, by way of intermediate [46], aldehyde 47 (in preference to 48). A non-enzymatic example is provided by the reaction of 2-(2-bromoallyl)-l,3-cyclopentadiene (49) with 2-bromoacrolein (50) in the presence of 10 mol % of catalyst 51 to give 52 (99% ee, exo/endo 99 1), en route to key gibberellic acid precursor 53. ... 

Enantioselective Hydrolysis of Diethyl Methyl-(4-Methoxy-phenyO-Propanedioate. The (5 )-monoester (26) (Fig. 6B) is a key intermediate for the syntheses of 33-receptor agonists. The enantioselective enzymatic hydrolysis of diester (27) to the desired acid ester (26) by pig liver esterase (32) has been demonstrated. In various organic solvents, the reaction yields and e.e. of the monoester (25) were dependent upon the solvent used. High e.e. (>91%) were obtained with methanol, ethanol, and toluene as a cosolvent. Ethanol gave the highest reaction yield (96.7%) and e.e. (96%). 

In this two-step resolution, the enzyme catalyzes the hydrolysis of the diester to monoester, then the monoester to the diol. Each step contributes to the overall enantioselectivity. The reaction mixture is biphasic buffered water containing the cmde enzyme and an ether solution of the diester. The cmde enzyme is bovine pancreas acetone powder that contains many enz5mies, but the substrate rearts only with the cholesterol esterase so the other enzymes do not interfere, other than to make the work-up messy. The enz5mie cholesterol esterase requires a bile salt, taurocholate, for fiiU activity. This bile salt helps emulsify the two phases. Cholesterol esterase seems to behave more like a lipase than an esterase in this example since it works with the substrate in the organic phase. The dipentanoate ester is chosen to simplify the separation of starting diester and product diol (Figure 5.7). 

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