Enantioselective enzymatic protection

The stereogenic center at C20 is introduced by enantioselective enzymatic hydrolysis of MOM-protected malonic acid dimethyl ester derivative 60 (Scheme 10) with pig liver esterase (PLE). The asymmetric compound 61 is obtained in 90 % yield and 98 % ee. Amide formation with Mu-... 

Considerable attention has also been given to enantioselective enzymatic hydrolysis of esters of a-amino acids. This is of particular importance as a means of preparing enantiopure samples of unusual (non-proteinaceous) a-amino acids. The readily available proteases a-chymotrypsin (from bovine pancreas) and subtilisin (from Bacillus lichenformis) selectively hydrolyze the L-esters, leaving D-esters unreacted. These enzymatic hydrolysis reactions can be applied to V-protected amino acid esters, such as those containing r-Boc and Cbz protecting groups. 

In an alternate process for the preparation of the C-13 paclitaxel side chain, the enantioselective enzymatic hydrolysis of racemic acetate ci5 -3-(acetyloxy)-4-phenyl-2-azetidinone 38 (Eignre 16.10B), to the corresponding (S)-alcohol 39 and the nnreacted desired (l )-acetate 38 was demonstrated [63] nsing lipase PS-30 from Pseudomonas cepacia (Amano International Enzyme Company) and BMS lipase (extracellnlar lipase derived from the fermentation of Pseudomonas sp. SC 13856). Reaction yields of more than 48% (theoretical maximnm yield 50%) with EEs greater than 99.5% were obtained for the (R)-38. BMS lipase and lipase PS-30 were immobilized on Accnrel polypropylene (PP), and the immobilized lipases were reused (10 cycles) without loss of enzyme activity, productivity, or the EE of the product (R)-38. The enzymatic process was scaled up to 250 L (2.5 kg substrate input) using immobilized BMS lipase and lipase PS-30. Prom each reaction batch, R-acetate 38 was isolated in 45 mol% yield (theoretical maximum yield 50%) and 99.5% EE. The (R)-acetate was chemically converted to (R)-alcohol 39. The C-13 paclitaxel side-chain synthon (2R,3S-37 or R-39) produced by either the reductive or resolution process could be coupled to bacattin III 34 after protection and deprotection to prepare paclitaxel by a semisynthetic process [64]. 

For resolution of the racemate 12 two different procedures can be applied 124 the en-antioselective enzymatic deacylation of chloroacetyl-DL-a-aminosuberic acid at pH 7.2 with Taka-acylase or the enantioselective salt precipitation of Z-dl-Asu-OH with D-tyrosine hydrazide according to the method of Vogler et alJ25 Complete enzymatic digestion is achieved in about ten days at 37 °C, and the optically pure L-enantiomer is obtained in 80% yield but the overall efficiency is lower than that of the chemical method. Fractional crystallization affords in good yields the Z-l-Asu-OH derivative 13 which is then used directly as a suitably protected intermediate in subsequent derivatization steps (see Scheme 6). Moreover, the recovery of the D-enantiomer from the mother liquors is also easy in this case. 

Reduction of l-(chloro or bromo) -3-butyn-2-one (27e,f) with recLBADH affords enantiopure R-alcohols 28e,f, resulting in an interesting switch of the enantioselectivity of the enzymatic reduction. As the enantiomers (S) -28e,f can be obtained by recLBADH-catalyzed reduction of 27b-27d and subsequent removal of the si-lyl-protecting group, this enzyme offers unique access to a pair of enantiomers via the same oxidoreductase. Due to the high volatility of the substrates (27e,f) these transformations were only performed on an analytical scale. 

Electroenzymatic reactions are not only important in the development of ampero-metric biosensors. They can also be very valuable for organic synthesis. The enantio- and diasteroselectivity of the redox enzymes can be used effectively for the synthesis of enantiomerically pure compounds, as, for example, in the enantioselective reduction of prochiral carbonyl compounds, or in the enantio-selective, distereoselective, or enantiomer differentiating oxidation of chiral, achiral, or mes< -polyols. The introduction of hydroxy groups into aliphatic and aromatic compounds can be just as interesting. In addition, the regioselectivity of the oxidation of a certain hydroxy function in a polyol by an enzymatic oxidation can be extremely valuable, thus avoiding a sometimes complicated protection-deprotection strategy. 

Starting from achiral materials, two stereoisomeric phosphonylated dihydroxy pyrrolidines (275) and (276), containing four stereogenic centers, have been synthesized enantioselectively, employing a combination of enzymatic and transition-metal-mediated methods. Both compounds contain features of the transition state of the enzyme-catalysed fucosyl transfer reaction and represent building blocks of potential inhibitors against this class of enzymes. The synthesis of new sugar-derived phosphonic acids e.g. (277) from protected... 

This review covers the catalytic literature on condensation reactions to form ketones, by various routes. The focus is on newer developments from the past 15 years, although some older references are included to put the new work in context. Decarboxylative condensations of carboxylic acids and aldehydes, multistep aldol transformations, and condensations based on other functional groups such as boronic acids are considered. The composition of successful catalysts and the important process considerations are discussed. The treatment excludes enantioselective aldehyde and ketone additions requiring stoichiometric amounts of enol silyl ethers (Mukaiyama reaction) or other silyl enolates, and aldol condensations catalyzed by enzymes (aldolases) or catalytic antibodies with aldolase activity. It also excludes condensations catalyzed at ambient conditions or below by aqueous base. Recent reviews on these topics are those of Machajewski and Wong, Shibasaki and Sasai, and Lawrence. " The enzymatic condensations produce mainly polyhydroxyketones. The Mukaiyama and similar reactions require a Lewis acid or Lewis base as catalyst, and the protecting silyl ether or other group must be subsequently removed. However, in some recent work the silane concentrations have been reduced to catalytic amounts (or even zero) this work is discussed. 

DSM uses an enzymatic procedrue for the preparation of this product (capacity > 2,000 tonnes). Z-protected (L)-aspartic acid is reacted enantioselectively with racemic phenylalanine methyl ester, using thermolysin from Bacillus proteolyticus. Unreacted (D)-phenylalanine methyl ester is separated off, racemised and recycled. The remaining step is cleavage of the Z-protecting group to get to the desired product... 

The preparation of glycerol-based Ca-synthons and intermediates represents an interesting example where enzymatic transesterification is widely exploited. Alcoholysis of tributyrin with PPL was already shown to produce a chiral diglyceride (Fig. 3). Alcoholysis of trityl-protected dibutyrin with methanol in organic solvents shows some essential features about regio- and enantioselectivity in bifunctional compounds (Fig. 18) (25). Most lipases, such as lipases PS and AK and Novozym 435, first regioselectively produce the primary alcohol from the diester. With lipase AK, the reaction leads to effective kinetic resolution (A), whereas Novozym 435-catalyzed reaction (B) is not enantioselective. However,... 

Over the last decade in vitro enzymatic catalysis has established itself as an indispensable tool in the synthesis of small molecules both at the academic and industrial level. Examples can be found in the production of pharmaceutical intermediates where biotechnology is generating significant turnover and reducing environmental impact [1]. The success of enzyme catalysis in these reactions is based on the selectivity and efficiency of enzymes by promoting reactions that are not easily accessible by conventional techniques. Examples are the replacement of tedious protection/deportation chemistry (chemo- and regioselectivity) and asymmetric synthesis of chiral compounds (enantioselectivity). 

The alkenes used were either commercially available (65-67) or could be accessed by one or two steps (63 and 64 Scheme 13). As shown, using enzymatic resolution of racemic l-octen-3-ol (68), ° we were able to synthesize alkenes 69 and 70 with high enantioselectivity. When alkenes 69 or 70 were reacted with bicyclic core 58, two diastereomers were produced that were separated by column chromatography to get protected analogs 63 and 64 with high enantiopurities. 

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