Vinyl enzyme catalyzed acylation with

Recently Lin and coworkers have developed a selective synthesis of N-acyl and 0-acyl propanolol vinyl derivatives by enzyme-catalyzed acylation of propanolol using divinyl dicarboxylates with different carbon chain lengths (Scheme 7.10) [24]. Lipase AY30 in diisopropyl ether demonstrated high chemoselectivity toward the amino... 

DKR of secondary alcohol is achieved by coupling enzyme-catalyzed resolution with metal-catalyzed racemization. For efficient DKR, these catalyhc reactions must be compatible with each other. In the case of DKR of secondary alcohol with the lipase-ruthenium combinahon, the use of a proper acyl donor (required for enzymatic reaction) is parhcularly crucial because metal catalyst can react with the acyl donor or its deacylated form. Popular vinyl acetate is incompatible with all the ruthenium complexes, while isopropenyl acetate can be used with most monomeric ruthenium complexes. p-Chlorophenyl acetate (PCPA) is the best acyl donor for use with dimeric ruthenium complex 1. On the other hand, reaction temperature is another crucial factor. Many enzymes lose their activities at elevated temperatures. Thus, the racemizahon catalyst should show good catalytic efficiency at room temperature to be combined with these enzymes. One representative example is subtilisin. This enzyme rapidly loses catalytic activities at elevated temperatures and gradually even at ambient temperature. It therefore is compatible with the racemization catalysts 6-9, showing good activities at ambient temperature. In case the racemization catalyst requires an elevated temperature, CALB is the best counterpart. 

The effect of reaction temperature on starch acylation Studies were performed to assess how reaction temperature effects Novozym 435 catalyzed acylation with vinyl stearate of AOT-coated starch nanospheres. The results of this work are compiled in Table 2. By increasing the reaction temperature from 25 to 50 C but keeping all other reaction variables constant, the efficiency of the reaction increased from 6.7 to 40.0% and the D.S. increased from 0.2 to 1.2. Further increasing the reaction temperature to 50 and 60 °C resulted in a decrease in the reaction efficiency. Since late in the study 50 C was identified as the preferred temperature, 40 C was used for subsequent experiments described below. Acylation of starch did not occur for reactions performed at 40 and 60°C but without enzyme. Hence, we conclude that the acylation is catalyzed by Novozym 435 (CAL-B). 

Kinetic resolutions by means of the selective formation or hydrolysis of an ester group in enzyme-catalyzed reactions proved to be a successful strategy in the enantioseparation of 1,3-oxazine derivatives. Hydrolysis of the racemic laurate ester 275 in the presence of lipase QL resulted in formation of the enantiomerically pure alcohol derivative 276 besides the (23, 3R)-enantiomer of the unreacted ester 275 (Equation 25) <1996TA1241 >. The porcine pancreatic lipase-catalyzed acylation of 3-(tu-hydroxyalkyl)-4-substituted-3,4-dihydro-2/7-l,3-oxazines with vinyl acetate in tetrahydrofuran (THF) took place in an enantioselective fashion, despite the considerable distance of the acylated hydroxy group and the asymmetric center of the molecule <2001PAC167, 2003IJB1958>. 

Many such activated acyl derivatives have been developed, and the field has been reviewed [7-9]. The most commonly used irreversible acyl donors are various types of vinyl esters. During the acylation of the enzyme, vinyl alcohols are liberated, which rapidly tautomerize to non-nucleophilic carbonyl compounds (Scheme 4.5). The acyl-enzyme then reacts with the racemic nucleophile (e.g., an alcohol or amine). Many vinyl esters and isopropenyl acetate are commercially available, and others can be made from vinyl and isopropenyl acetate by Lewis acid- or palladium-catalyzed reactions with acids [10-12] or from transition metal-catalyzed additions to acetylenes [13-15]. If ethoxyacetylene is used in such reactions, R1 in the resulting acyl donor will be OEt (Scheme 4.5), and hence the end product from the acyl donor leaving group will be the innocuous ethyl acetate [16]. Other frequently used acylation agents that act as more or less irreversible acyl donors are the easily prepared 2,2,2-trifluoro- and 2,2,2-trichloro-ethyl esters [17-23]. Less frequently used are oxime esters and cyanomethyl ester [7]. S-ethyl thioesters such as the thiooctanoate has also been used, and here the ethanethiol formed is allowed to evaporate to displace the equilibrium [24, 25]. Some anhydrides can also serve as irreversible acyl donors. 

Synthesis of (-I-) calanolide A (Scheme 8-11) was achieved by enzyme catalyzed resolution of the aldol products ( )-53. Compound 7 with acetaldehyde by aldol reaction in the presence of LDA/TiCU stereoselectively produced a mixmre of ( )-53 and ( )-54 (94% yield), the ratio of which was 96 4. ( )-53 was then resolved by lipase AK-catalyzed acylation reaction in the presence of tert-butyl methyl ether and vinyl acetate at 40 °C to obtain 41% yield of (+)-55 and 54% yield of the acetate (—)-56. Mitsunobu cyclization of (+)-55 in the presence of tri-phenylphosphine and dielthyl azodicarboxylate afforded 63% yield of (-l-)-43 with 94% ee as determined by chiral HPLC. Luche reaction on (+)-43 with CeCla 7H2O and triphenyl phosphine oxide and NaBH4 in the presence of ethanol at 30 °C gave the crude product. It was purified by column chromatography on silica gel to give 78% yield of a mixture containing 90% of (+)-calanolide A and 10% (+)-calanohde B, which were further separated by HPLC. 

The racemic alcohol was prepared by lithium aluminum hydride reduction of (R,S)-4. An abbreviated library of commercially available enzymes (entries 1-11 in Tab. 1) was used to screen the vinyl acetate acylation. Enzymes in entries 2, 3, 4, 8, and 11 catalyzed the acylation with a relatively fast rate (>20% conversion in... 

Typically the reaction was carried out as follows to a mixture of lipase in the IL were added this racemic alcohol and vinyl acetate as the acyl donor. The resulting mixture was stirred at 35°C and the reaction course was monitored by GC analysis. After the reaction, ether was added to the reaction mixture to form a biphasic layer, and product acetate and unreacted alcohol were extracted with ether quantitatively. The enzyme remained in the IL phase as expected (Fig. 2). Two months later, Kim and co-workers reported similar results and Lozano and Ibora " reported other examples of lipase-catalyzed reaction in June. Further Park and Kazlauskas reported full details of lipase-catalyzed reaction in an IL solvent... 

We succeeded in showing that recycling of the enzyme was indeed possible in our IL solvent system, though the reaction rate gradually dropped with repetition of the reaction process. Since vinyl acetate was used as acyl donor, acetaldehyde was produced hy the hpase-catalyzed transesterification. It is well known that acetaldehyde acts as an inhibitor of enzymes because it forms a Schiff base with amino residue in the enzyme. However, due to the very volatile nature of acetaldehyde, it easily escapes from the reaction mixture and therefore has no inhibitory action on the lipase. However, this drop in reactivity was assumed to be caused by the inhibitory action of acetaldehyde oligomer which had accumulated in the [bmim][PFg] solvent system. In fact, it was confirmed that the reaction was inhibited by addition of acetaldehyde trimer. =... 

The rate of the reaction in [BDMIM]BF4 was superior to that in [BMIM]BF4. It is significant that no drop in the reaction rate was observed in the [BDMIM]BF4 system after 10 repeated uses of the enzyme, whereas the reaction rate was significantly reduced when the reaction was conducted in [BMIM]BF4 or in [BMIM]PF6 (282). [BDMIM]BF4 was found to be the best solvent for the recycled use of the enzyme under normal pressure conditions when vinyl acetate was used as the acyl donor. No reaction took place when [BDMIM]PF6 was used as the solvent in the lipase-catalyzed reaction. (The [BDMIM]PF6 was purified with particular care to rule out the possibility of contamination.) The replacement of the C2 proton of [BMIM]PF6 by a methyl group was therefore concluded to have a large influence on its biocatalytic compatibility. 

Once bearing some substituents, the decrease of polarity of the sucrose derivatives makes them soluble in less-polar solvents, such as acetone or tert-butanol, in which some lipases are able to catalyze esterifications. Unlike proteases, which necessitate most often the use of an activated acyl donor (such as vinyl or trifluoroethyl esters), lipases are active with simple esters and even the parent carboxylic acids in the presence of a water scavenger. The selectivity of the lipase-catalyzed second esterification is specific for OH-6 allowing the synthesis of mixed T,6 -diesters.123,124 For some lipases, a chain-length dependence on the regiochemistry was observed.125 Selectively substituted monoesters were thus prepared and studied for their solution and thermotropic behavior.126,127 Combinations of enzyme-mediated and purely chemical esterifications led to a series of specifically substituted sucrose fatty acid diesters with variations in the chain length, the level of saturation, and the position on the sugar backbone. This allowed the impact of structural variations on thermotropic properties to be demonstrated (compare Section III.l).128... 

Some of these catalyze the smooth racemization of chiral secondary alcohols at room temperature. However, a major problem which needed to be solved in order to design an effective combination of ruthenium catalyst and lipase in a DKR of secondary alcohols was the incompatibility of many of the ruthenium catalysts and additives, such as inorganic bases, with the enzyme and the acyl donor. For example, the ruthenium catalyst may be susceptible to deactivation by the acetic acid generated from the acyl donor when it is vinyl acetate. Alternatively, any added base in the racemization system can catalyze a competing selective transesterification of the alcohol, resulting in a decrease in enantioselectivity. Consequently, considerable optimization of reaction protocols and conditions was necessary in order to achieve an effective DKR of secondary alcohols. 

The acyl group is a common protecting group for alcohols that can be incorporated using several methods[138]. The most common acylation method involves the reaction of an alcohol with acetic anhydride in the presence of pyridine. But for some acid- or base-sensitive alcohols, the common procedures do not work very well. Transesterification using vinyl acetate offers an alternate route which is very mild and can be catalyzed by Cp2 Sm THF, enzymes, and acids. 

In the reaction with 2-octanol catalyzed by C. antarctica lipase B, Ohmer et al. compare four acyl donors differing only in (heir leaving groups [7] (Table 2). In the case of octanoic acid, its low compared with the other acyl donors makes it beneficial at high conversions since it competes well with the product ester for the free enzyme (Scheme 5). However, the initial rate is lower than for other acyl donors, resulting in a slightly longer reaction time than for the thio ester and vinyl ester. 

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