Kazlauskas rule

The enzymatic KR between racemic amines and nonactivated esters using a lipase as biocatalyst is shown in Scheme 7.15. In the same manner as in the transesterification of secondary alcohols, this process fits Kazlauskas rule [32], where normally if the large group (L) has larger priority than medium group (M), the (R)-amide is obtained. In general, major size differences between both groups result in better enantios-electivities ( ). 

Scheme 7.15 Schematic representation of Kazlauskas rule in the enzymatic KR of an amine.

In recent years, a great variety of primary chiral amines have been obtained in enantiomerically pure form through this methodology. A representative example is the KR of some 2-phenylcycloalkanamines that has been performed by means of aminolysis reactions catalyzed by lipases (Scheme 7.17) [34]. Kazlauskas rule has been followed in all cases. The size of the cycle and the stereochemistry of the chiral centers of the amines had a strong influence on both the enantiomeric ratio and the reaction rate of these aminolysis processes. CALB showed excellent enantioselec-tivities toward frans-2-phenylcyclohexanamine in a variety of reaction conditions ( >150), but the reaction was markedly slower and occurred with very poor enantioselectivity with the cis-isomer, whereas Candida antarctica lipase A (GALA) was the best catalyst for the acylation of cis-2-phenylcyclohexanamine ( = 34) and frans-2-phenylcyclopropanamine ( =7). Resolution of both cis- and frans-2-phenyl-cyclopentanamine was efficiently catalyzed by CALB obtaining all stereoisomers with high enantiomeric excess. 

This apparent swap of selectivity is a result of the predictable steric interactions of most commercially available lipases with primary and secondary alcohols and carboxylic acids. In fact, a simple predictive tool, known as the Kazlauskas rules , has been developed where attack is favoured towards substrates of configuration shown in Figure 1.9. " ° These rules are highly predictive for secondary alcohols and less reliable for primary alcohols and carboxylic acids. 

Figure 1.9 Kazlauskas rules preferential action of a lipase on alcohols and carboxylic acids (M and L indicate medium- and large-sized substituents respectively)...

R)-selectivity toward simple secondary alcohols carrying one small and one relatively larger substituent at the hydroxyl methane center, and the selectivity in general increases with an increase in the size diflference between two substituents. The size of the small substituent limits the reactivity of substrate toward lipase. If it exceeds a three-carbon unit, the substrate reacts very slowly or does not react at a synthetically useful rate. Accordingly, the Kazlauskas rule is useful as a guideline for predicting substrates that can be efficiently resolved by lipase as well as the stereochemistry of resolved substrates. 

Figure 4.4 Preferential acylation mode of lipases for secondary alcohols with large substituent (L) in large hydrophobic pocket and medium-sized substituent (M) in smaller pocket as described by Kazlauskas rule [72].

Naturally occurring Upases are (R)-selective for alcohols according to Kazlauskas rule [58, 59]. Thus, DKR of alcohols employing lipases can only be used to transform the racemic alcohol into the (R)-acetate. Serine proteases, a sub-class of hydrolases, are known to catalyze transesterifications similar to those catalyzed by lipases, but, interestingly, often with reversed enantioselectivity. Proteases are less thermostable enzymes, and for this reason only metal complexes that racemize secondary alcohols at ambient temperature can be employed for efficient (S)-selective DKR of sec-alcohols. Ruthenium complexes 2 and 3 have been combined with subtilisin Carlsberg, affording a method for the synthesis of... 

Figure 6 The fast reacting enantiomer (a) and the slow reacting one (b) in the active side model for lipases derived from Kazlauskas rule.

First attempts to predict the selectivity of enzymes are dated back to 1964 when Prelog described an empirically determined rule for the addition of hydrogen to ketones by the yeast Culvaria lunata [13]. In 1991 Kazlauskas published the hydrolysis of acetates of secondary alcohols by Pancreatic cholesterol esterase, Pseudomonas cepacia and Candida rugosa and formulated the widely applicable Kazlauskas rule according to which esters of secondary alcohols with a specific substitution pattern of large (L) and medium (M) substituents are cleaved faster than the corresponding enantiomer [14]. 

Kazlauskas rule preferred enantiomer sequence rule order of large > medium assumed R r2 = alkyl, aryl R = n-Pr or longer = center of (pro)chirality... 

Furthermore, the majority of lipases show the same stereochemical preference for esters of secondary alcohols (Scheme 2.49), which is known as the Kazlaus-kas mle [341]. Assuming that the Sequence Rule order of substituents R and R is large > medium, the preferably accepted enantiomer lipase-substrate of type III possesses the (R)-configuration at the alcoholic center. It should be noted that the Kazlauskas rule for secondary alcohols (Type III) has an accuracy of >90%, whereas the predictability for the corresponding a-chiral acids (Type IV) is less reliable. 

CALB is an exceptionally robust protein which is deactivated only at 50-60°C, and thus also shows increased resistance towards organic solvents. In contrast to many other lipases, the enzyme appears to be rather rigid and does not show a pronounced effect of interfacial activation [430], which makes it an intermediate between an esterase and a lipase. This latter property is probably the reason why its selectivity could be predicted through computer modeling to a fair extent [431], and for the majority of substrates the Kazlauskas rule (Scheme 2.49) can be applied. In line with these properties of CALB, selectivity-enhancement by addition of water-miscible organic cosolvents such as t-butanol or acetone is possible - a technique which is rather common for esterases. All of these properties make CALB the most widely used lipase both in the hydrolysis [432-437] and synthesis of esters (Sect. 3.1.1). 

Dynamic resolution of various sec-alcohols was achieved by coupling a Candida antarctica lipase-catalyzed acyl transfer to in-situ racemization based on a second-generation transition metal complex (Scheme 3.17) [237]. In accordance with the Kazlauskas rule (Scheme 2.49) (/ )-acetate esters were obtained in excellent optical purity and chemical yields were far beyond the 50% limit set for classical kinetic resolution. This strategy is highly flexible and is also applicable to mixtures of functional scc-alcohols [238-241] and rac- and mcso-diols [242, 243]. In order to access products of opposite configuration, the protease subtilisin, which shows opposite enantiopreference to that of lipases (Fig. 2.12), was employed in a dynamic transition-metal-protease combo-catalysis [244, 245]. 

Scheme 4.4 The Kazlauskas rule for predicting lipase-enantiorecognition.

Fig. 14 The conformation of the large (L) and medium (M) substituent and the hydroxyl group of the fast reacting enantiomer of secondary alcohols as predicted by Kazlauskas rule...

C. antarctica lipase B and BCL follow the Kazlauskas rule [8] for enantioselectivity in the transesterification of secondary alcohols (i) the enantiomer shown in Figure 5.1 reacts more rapidly than the other if two substituents at the hydroxymethine center of secondary alcohol are different in size and (ii) the enantioselectivity increases with increasing difference in size between two substituents. 

According to the Kazlauskas rule, CAL-B and BCL normally provide (R)-products in the DKR of simple secondary alcohols such as 1-phenyl-l-alkanols (Scheme 5.2a). On the other hand, SC is stereocomplementary [8] to CAL-B and BCL. It provides (S)-products in the DKR of simple secondary alcohols (Scheme 5.2b). CAL-A and PSL are somehow different from CAL-B and BCL in substrate specificity and stereospecificity. They can accept sterically more demanding secondary alcohols such as 1,2-diarylethanols, which are poorly reactive with CAL-B and BCL. Interestingly, they are stereocomplementary toward these substrates CAL-A accepts... 

The symmetry of both processes has been represented in Figure 9.3 according to the Kazlauskas rule for secondary alcohols [43], which established empirically that lipases favors the hydrolysis of (K)-esters to form (i )-alcohols, maintaining unaltered the (S)-esters. On the other hand the formation of (R)-esters and (S)-alcohols is preferred in acylation reachons. This rule is directly linked to the size and Cahn-Ingold-Prelog preferences of the substituents. 

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