Benzaldehyde lyase

A biochemically related benzaldehyde lyase (BAL) (EC 4.1.2.38) catalyzes the same carboligation reactions, but with opposite (J )-selectivity (mf-110) [178]. All these enzymes seem to display a rather useful substrate tolerance for variously substituted aldehyde precursors. 

R)-Benzoins and (/ )-2-hydroxypropiophcnonc derivatives are formed on a preparative scale by benzaldehyde lyase (BAL)-catalyzed C-C bond formation from aromatic aldehydes and acetaldehyde in aqueous buffer/DMSO solution with remarkable ease in high chemical yield and high optical purity (Eq. 8.112).303 Less-stable mixed benzoins were also generated via reductive coupling of benzoyl cyanide and carbonyl compounds by aqueous titanium(III) ions.304... 

Miiller and co-workers have developed an enantioselective enzymatic crossbenzoin reaction (Table 2) [43, 44], This is the first example of an enantioselective cross-benzoin reaction and takes advantage of the donor-acceptor concept. This transformation is catalyzed by thiamin diphosphate (ThDP) 23 in the presence of benzaldehyde lyase (BAL) or benzoylformate decarboxylase (BFD). Under these enzymatic reaction conditions the donor aldehyde 24 is the one that forms the acyl anion equivalent and subsequently attacks the acceptor aldehyde 25 to provide a variety of a-hydroxyketones 26 in good yield and excellent enantiomeric excesses without contamination of the other cross-benzoin products 27. The authors chose 2-chlorobenzaldehyde 25 as the acceptor because of its inability to form a homodimer under enzymatic reaction conditions. 

Benzoylformate decarboxylase (BFD EC 4.1.1.7) belongs to the class of thiamine diphosphate (ThDP)-dependent enzymes. ThDP is the cofactor for a large number of enzymes, including pyruvate decarboxylase (PDC), benzaldehyde lyase (BAL), cyclohexane-1,2-dione hydrolase (CDH), acetohydroxyacid synthase (AHAS), and (lR,6] )-2-succinyl-6-hydroxy-2,4-cyclohexadiene-l-carboxylate synthase (SHCHC), which all catalyze the cleavage and formation of C-C bonds [1]. The underlying catalytic mechanism is summarized elsewhere [2] (see also Chapter 2.2.3). 

In the framework of SFB380, two projects dealt extensively with acyloin-con-densing ThDP-dependent enzymes such as pyruvate decarboxylase (PDC), ben-zoylformate decarboxylase (BFD), or benzaldehyde lyase (BAL) (see Chapters 2.2.3 and 2.2.7). Another ThDP-dependent decarboxylase, phosphonopyruvate decarboxylase (PPD) from Streptomyces viridochromogenes, became available only recently and was studied in project B21. We wanted to find out whether this PDC-related enzyme could be a valuable tool in the provision of acyloin condensations involving C-P bonds (see Section 2.2.2.23). 

LB-ADH was used for the synthesis of vic-diols. Starting from benzaldehyde and acetaldehyde, (lS,2S)-l-phenylpropane-l,2-diol (de = 98%) and (lS,2R)-l-phenyl-propane-l,2-diol (de = 99%), respectively, could be produced in a stereoselective two-step enzymatic synthesis using benzaldehyde lyase (BAL) and accordingly benzoylformate decarboxylase (BFD) as well as LB-ADH [8] (Scheme 2.2.4.3). 

Table 3.1.2 Stability of benzaldehyde lyase under process conditions.

Benzoin Condensation with Entrapped Benzaldehyde Lyase... 

Enzymatic reactions forming new carbon-carbon bonds are a further important field of biotransformations in natural product synthesis. The construction of new, often complex carbon frameworks or their decomposition is performed by nature under catalysis of a set of enzymes. For organic chemists some of these enzymes, belonging to the enzyme class of lyases, such as aldolases, decarboxylases, hydro-xynitrile lyases (HNLs), or benzaldehyde lyases (BALs), have been proven to represent versatile amendments to their synthetic toolbox. 

Fig. 43 (a) Benzaldehyde lyase-catalyzed acyloin formation, (b) Application of acyloins in natural product synthesis... 

Demir AS, Sesenoglu O et al (2003) Benzaldehyde lyase-catalyzed enantioselective carbo-ligation of aromatic aldehydes with mono- and dimethoxy acetaldehyde. Org Lett 5 2047-2050... 

The bifunctional nature and the presence of a stereocenter make a-hydroxyketones (acyloins) amenable to further synthetic transformations. There are two classical chemical syntheses for these a-hydroxyketones the acyloin condensation and the benzoin condensation. In the acyloin condensation a new carbon-carbon bond is formed by a reduction, for instance with sodium. In the benzoin condensation the new carbon-carbon bond is formed with the help of an umpolung, induced by the formation of a cyanohydrin. A number of enzymes catalyze this type of reaction, and as might be expected, the reaction conditions are considerably milder [2-4, 26, 27]. In addition the enzymes such as benzaldehyde lyase (BAL) catalyze the formation of a new carbon-carbon bond enantioselectively. Transketolases (TK)... 

Another approach towards immobilization via ionic interactions is the use of tethered metal ions, such as Cu, Co, or Ni, in order to bind the enzyme [53, 54]. This is particularly so when the enzyme contains an easily accessible imidazole residue from histidine [55] or a His tag (Figure 2.6) [56-59], that is a short tag with six histidines. This tag can be readily introduced by genetically modifying the enzyme. Little influence of the tag on the catalytic performance has been noticed. When benzaldehyde lyase [60] was immobilized on an Ni -containing polyvinylpyrrolidinone-based matrix, it could be reused several times for the formation of benzoin (12) (Scheme 2.4) [58]. 

Benzaldehyde lyase Reversible benzoin condensation, a carboligase/lyase C6H5C0CH(0H)C6H5 2C6H5CHO None... 

Scheme 2.201 Regiocomplementary carboligation of aldehydes catalyzed by benzaldehyde lyase...

Milller and co-workers recently developed an enantioselective benzoin dimerization using purified enzymes from Pseudomonas. The thiamine diphosphate (ThDP) dependent enzymes benzaldehyde lyase (BAL) and benzoylformate decarboxylase (BED) were found to catalyze the reversible benzoin condensation of aromatic aldehydes. The reaction is driven in the forward direction by the poor solubility of the benzoin products in aqueous media. A wide variety of aromatic aldehydes are accepted by BAL, and products of the (/ )-configuration are produced in excellent yield and enantiomeric purity. The (S)-enantiomer of benzoin is also available in high enantiomeric purity from a BAL-catalyzed kinetic resolution of rac-benzoin. In the presence of excess acetaldehyde, BAL selectively converts (i )-benzoin into (/ )-2-hydroxy-l-phenylpropanone, while the (iS)-benzoin enantiomer is not a substrate for the enzyme. At 49% conversion, (5)-benzoin is resolved to > 99% ee. BED can produce (i )-benzoin from benzaldehyde in comparable yield and enantiomeric purity with respect to BAL, but the substrate scope appears more limited. ... 

The enzymes benzaldehyde lyase (BAL) and benzoylformate decarboxylase (BFD) have also been shown to catalyze enantioselective cross-benzoin reactions. Aryl-aryl as well as aryl-alkyl products are produced in high yield and enantiomeric purity. In the case of aryl-aryl products, the success of the reaction depends on the empirical identification of suitable donor/acceptor pairs. Aldehydes containing or/Ao-substituents were found to be ideal acceptors in BAL and BFD-catalyzed cross-benzoin reactions. The preparation of (/ )-l-(4-bromophenyl)-2-(2-chlorophenyl)-2-hydroxyethanone (25) from 4-bromobenzaldehyde (23) and 2-chlorobenzaldehyde (24) highlights the high conversion and selectivity possible in this transformation. As with rac-benzoin, rac-l-(4-bromo-phenyl)-2-(2-chlorophenyl)-2-hydroxyethanone is easily resolved by BAL to give the (5)-enantiomer with high enantiomeric purity. 

ThDP-dependent carboligases can catalyze acyloin condensations of aliphatic or aromatic donor aldehydes to aliphatic or aromatic acceptors, thus allowing the production of many useftd building blocks (Tables 10.4-10.7). These are the most common reactions catalyzed by ThDP-dependent carboligases such as the pyruvate decarboxylase (PDC) from Saccharomyces cerevisiae (ScPDC), Zymomonas mobilis (ZmPDC), Acetobacter pasteurianus (ApPDC), benzoylformate decarboxylase (BFD) from P. putida (F BFD), and benzaldehyde lyase (BAL) from Psedomonas. fluorescens BidVar 1, which have been recognized as powerful biocatalysts [6,14,48]. 

Benzaldehyde lyase (BAL) from Pseudomonas fluorescens (Table 10.4, entries 6-15) is one of the most efficient catalysts for the homo- and cross-carboligation reaction of aromatic and aliphatic aldehydes, because of its broad substrate range and its high (P)-stereoselectivity [14,57,58,65]. The ability of BAL to catalyze the cleavage of benzoins can also be instrumental in the resolution of racemic mixtures of these compounds [65]. 

M. Knoll, M. Muller, J. Pleiss, M. Pohl, Factors mediating activity, selectivity, and substrate specificity for the thiamin diphosphate-dependent enzymes benzaldehyde lyase and benzoyl-formate decarboxylase, Chembiochem 7 (2006) 1928-1934. 

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