Hydrolase-catalyzed acylation

Scheme 4.4 Irreversible hydrolase-catalyzed acylation with an activated acyl donor.

Whereas resolutions of secondary alcohols by hydrolase-catalyzed acylation in organic solvents often proceed with high E-values (see Section 4.2.1.2), primary alcohols are much more difficult to resolve. There are, however, many examples of successful resolutions of primary alcohols, and a selection of these will be described here. 

Some l-(2-furyl)-l-alkanols have also been resolved by hydrolase-catalyzed acylations (Scheme 4.20). Thus l-(2-furyl)-l-ethanol (46) is efficiently resolved by acylation with vinyl acetate catalyzed either by Lipozyme IM or PPL [77]. Resolution with a more complex acyl donor, ethoxyvinyl methyl fumarate, catalyzed by Lipase LIP (from Pseudomonas aeruginosa) has also been achieved [95]. The... 

Hydrolase-catalyzed acylation can be used to purify a diastereo- and enantiomerically enriched product. For example dimethylzinc addition to the racemic aldehyde 77 furnishes the racemic phenylsulfanylbutanol 78 (Scheme 4.29) in a 95/5 (2R, 3R )/(2R, 3S )-mtio. When this is treated with Chirazyme L2 (CALB) and vinyl acetate in heptane it is resolved with a high E-value (>400) [91]. However the diastereomeric ratio in the remaining substrate and produced ester is virtually unchanged. To circumvent the problematic contamination with the undesired diastereomers, enantiomerically enriched aldehyde 77 was reacted with dimethyl-zinc to furnish one major stereoisomer of 78 contaminated with a small amount of a mixture of the other three (Scheme 4.29). Because the two major contaminants had the opposite configuration at position 2 relative to the major product, these contaminants were efficiently removed from the major product and the trace byproduct by treatment with the 2R-selective Chirazyme L2 (CALB) and vinyl acetate in heptane to furnish virtually diastereo- and enantiomerically pure acetate (2R,3R)-79 or the alcohol (2S,3S)-78 (Scheme 4.29) [91]. 

As described above, the resolution of many types of secondary alcohols by hydrolase-catalyzed acylation in an organic solvent is usually possible after screening for a selective lipase and optimization of the reaction conditions. 

Several chiral racemic alkylamines have been successfully resolved using hydrolase-catalyzed acylation reactions with esters as acyl donors. A few examples are described here (Table 4.2). 

Hydrolase-catalyzed acylation of 1-arylethylamines has been studied extensively, and the reactions usually proceed with excellent E-values, providing easy access to both enantiomers of the amine after chemical hydrolysis. Some examples are listed in Table 4.3. 

Hydrolases, especially lipases are ideal to perform acylations such as direct esterification of free acids, acylation of alcohols, or amines in low-water organic media [29, 30, 60, 79, 81, 83]. Accordingly, the majority of the examples of hydrolase-catalyzed acylations in continuous-flow mode are enantiomer selective KRs (Figures 9.7-9.9 and Tables 9.5-9.6). Most of the earlier studies were aimed at the biocatalytic production of chiral pharmaceutical intermediates and were performed at a relatively large scale using immobilized lipases in PER [60, 99-102]. 

Three routes to enantiopure compounds using hydrolase-catalyzed reactions, (a) Kinetic resolution starts with racemic substrate and converts one enantiomer into product. This separation yields one enantiomer as the product alcohol and one as the starting acetate, both with a maximum yield of 50%. (b) Desymmetrization of a prochiral compound transforms one of prochiral groups to yield a chiral product with a maximum yield of 100%. (c) A dynamic kinetic resolution combines rapid racemization of racemic starting material with a hydrolase catalyzed acylation of one enantiomer. The maximum yield is 100%. 

Acyl donors for hydrolase-catalyzed acylations. Reactive acyl donors like (a) vinyl acetate and (b) succinic anhydride react effectively irreversibly with alcohols. The tautomerization of the vinyl alcohol to the keto form makes the acylation favorable. These acyl donors are not suitable for amines because they react spontaneously. Simple esters like (c) triacetin are less expensive acyl donors. All three acetyl groups from triacetin can react. For alcohols, these simple ester acyl donors are reversible so are not suitable when kinetically controlled selectivity is important. Simple esters are good, effectively irreversible, acyl donors for amines. 

Mechanism for hydrolase-catalyzed acylation reactions of different nucleophiles such as alcohols or amines. 

Use of Organic Solvents for Hydrolase-Catalyzed Acylation Reactions... 

Satisfyingly a broad number of hydrolase-catalyzed DKR processes have been reported, mainly using lipases as described in previous comprehensive bibliographic revisions [99-103]. Prior to the description of selected examples for the DKR resolution of racemic alcohols via hydrolase-catalyzed acylation, three issues must be highlighted and explained in detail. These are (i) the use of an adequate acyl donor, (ii) the role of the solvent in the reaction, and (iii) the nature of the racemization agent and its compatibility with the hydrolase. 

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