Inversion of enantioselectivity

In all the reported examples, the enzyme selectivity was affected by the solvent used, but the stereochemical preference remained the same. However, in some specific cases it was found that it was also possible to invert the hydrolases enantioselectivity. The first report was again from iQibanov s group, which described the transesterification of the model compound (13) with n-propanol. As shown in Table 1.6, the enantiopreference of an Aspergillus oryzae protease shifted from the (l)- to the (D)-enantiomer by moving from acetonitrile to CCI4 [30]. Similar observations on the inversion of enantioselectivity by switching from one solvent to another were later reported by other authors [31]. 

In many cases, the racemization of a substrate required for DKR is difficult As an example, the production of optically pure cc-amino acids, which are used as intermediates for pharmaceuticals, cosmetics, and as chiral synfhons in organic chemistry [31], may be discussed. One of the important methods of the synthesis of amino acids is the hydrolysis of the appropriate hydantoins. Racemic 5-substituted hydantoins 15 are easily available from aldehydes using a commonly known synthetic procedure (Scheme 5.10) [32]. In the next step, they are enantioselectively hydrolyzed by d- or L-specific hydantoinase and the resulting N-carbamoyl amino acids 16 are hydrolyzed to optically pure a-amino acid 17 by other enzymes, namely, L- or D-specific carbamoylase. This process was introduced in the 1970s for the production of L-amino acids 17 [33]. For many substrates, the racemization process is too slow and in order to increase its rate enzymes called racemases are used. In processes the three enzymes, racemase, hydantoinase, and carbamoylase, can be used simultaneously this enables the production of a-amino acids without isolation of intermediates and increases the yield and productivity. Unfortunately, the commercial application of this process is limited because it is based on L-selective hydantoin-hydrolyzing enzymes [34, 35]. For production of D-amino acid the enzymes of opposite stereoselectivity are required. A recent study indicates that the inversion of enantioselectivity of hydantoinase, the key enzyme in the... 

Inversion of enantioselectivity based on the reaction mechanism and homology... 

The tertiary structure of glutamate racemase has already been resolved and it has also been shown that a substrate analog glutamine binds between two cysteine residues. These data enabled us to predict that the new proton-donating amino acid residue should be introduced at position 74 instead of Gly for the inversion of enantioselectivity of the decarboxylation reaction. 

As mentioned above, changing Cys 188 to a weak proton donor, Ser, and introducing Cys instead of Gly74 brought about the inversion of enantioselectivity of... 

An inversion of enantioselectivity was observed experimentally for the hydrosilylation of a series of para-substituted styrenes as shown in Table 1. We intend to examine the nature of the enatioselectivity by studying the catalytic cycle for styrene which reacts to give predominately the S form of the product (64% S ee) and for 4-(dimethylamino)styrene, which gives predominately the stereochemical opposite product (64% R ee). Although we have already examined the hydrosilylation of styrene in Section 3 and 4, in this section we focus on enantioselectivity of the catalytic process for comparison to the hydrosilylation with 4-(dimethylamino)styrene. 

As an inversion of enantioselectivity was observed experimentally for 4-(dimethylamino)styrene, (64% R ee) as compared to styrene (64% S ee), we have recalculated the relative thermodynamic stabilities of endo and exo isomers for each step of the catalytic cycle using this second substrate. These calculations allow us to verify the quality of our findings by checking if an inversion in the relative stabilities of the endo and the exo-ri3-silyl-allyl intermediates (with the endo being more stable than the exo) is observed with 4-(dimethylamino)styrene. Using 4-(dimethylamino)styrene as the substrate, the calculated relative stabilities of the intermediates in the Chalk-Harrod mechanism are shown as parenthetic values in Figure 15. 

Miyazawa, T., Kurita, S., Ueji, S., Yamada, T. and Shigeru, K., Resolution of mandelic acids by lipase-catalyzed transesterifications in organic media inversion of enantioselectivity mediated by the acyl donor. J. Chem. Soc. Perkin Trans. 1, 1992, 18, 2253-2255. 

Hydantoinases belong to the E.C.3.5.2 group of cyclic amidases, enzymes that catalyze the hydrolysis of hydantoins 7-11,147). Because synthetic hydantoins are accessible by a variety of chemical syntheses, including Strecker reactions, enan-tioselective hydantoinase-catalyzed hydrolysis offers an attractive and general route to chiral amino acid derivatives. Moreover, because hydantoins are easily racemized chemically or enzymatically by appropriate racemases, dynamic kinetic resolution with potential 100% conversion and complete enantioselectivity is theoretically possible. Indeed, a number of such cases have been reported 147). However, if asymmetric induction is poor or if inversion of enantioselectivity is desired, directed evolution can come to the rescue. Such a case has been reported, specifically in the production of L-methionine as part of a whole cell system E. coll) (Figure 22) 148). 

Y. Kurono, and K. Achiwa, Inversion of enantioselectivity in hydrolysis of 1,4-di-hydropyridines by point mutation of lipase PS, Tetrahedron Lett. 1995, 36, 1063-1066. 

A highly enantioselective intramolecular version, catalysed by proline, undergoes an inversion of enantioselectivity on addition of imidazole.173 The imidazole substantially increases the reaction rate it is proposed to act as a co-catalyst, hydrogen bonding to the proline s acid hydrogen, blocking a reactant face which is otherwise available in the proline-only route. 

A highly enantioselective proline-catalysed intramolecular Morita-Baylis-Hillman reaction of hept-2-enedial (111) has been reported. Addition of imidazole to the mixture resulted in an unusual inversion of enantioselectivity.149 The first example of a TiCU-mediated Morita-Baylis-Hillman-type reaction of cy-acetyl cyclic ketene dithioacetals with arylaldehydes has been described.150... 

An interesting inversion of enantioselectivity was observed in this reaction with proline catalyst, when the catalyst was used alone, or with imidazole co-catalyst (Table 5.5) [75]. When hep-2-enedial 66 was submitted to proline-mediated... 

Recent results show that directed evolution can also invert enzyme enantioselectivity [65l The hydantoinase derived from Arthrobacter sp. shows a substrate-dependent inversion of enantioselectivity which limits its use for the production of certain l-amino acids such as L-methionine (for applications of hydantoinases in organic syntheses see Chapter 12). By accumulation of mutations through sequential rounds of error-prone PCR and saturation mutagenesis, the enantioselectivity of the hydantoinase was inverted from ee = 40 % for the D-enantiomer to ee = 20 % for the l-isorner at 30% conversion. Only one amino acid substitution was required for the... 

Ijima Y, Matoishi K, Terao Y et al. (2005) Inversion of enantioselectivity of asymmetric biocat-alytic decarboxylation by site-directed mutagenesis based on the reaction mechanism. Chem Common 877-879... 

INVERSION OF ENANTIOSELECTIVITY DRAMATICALLY IMPROVES CATALYTIC ACTIVITY... 

Y. Terao, Y. Ijima, K. Miyamoto and H. Ohta, "Inversion of enantioselectivity of arylmalonate decarboxylase via site-directed mutation based on the proposed reaction mechanism". Journal of Molecular Catalysis B Enzymatic 45,15-20 (2007). 

---