HLADH-catalyzed reactions

These regeneration systems were combined with the HLADH catalyzed reaction of cyclohexanol to cyclohexanone as a model system (Fig. 17). The catalytic... 

Yet another related approach is to use the strong temperature dependence of microemulsions based on nonionic surfactants. After the reaction is completed in a one-phase microemulsion, the temperature is raised (or lowered) so that a two-phase system forms, consisting of microemulsion in equilibrium with excess water (or oil) phase. If the reaction product is hydrophilic, a temperature increase is chosen and the product is recovered from the aqueous phase. If the product is lipophilic, the temperature is instead decreased, and the product is recovered from the excess hydrocarbon phase. The principle has been applied successfully to an HLADH-catalyzed reaction in microemulsion based on C,2E5 [132],... 

In the case of the (1 )-configured olefinic ketones high conversion rates and enantiomeric excesses were achieved on the analytical and on the semi-preparative scale. With TBADH, which requires a slightly higher reaction temperature, decomposition of the halogenated substrates was observed. In the case of CPCR, which was used as a partially purified enzyme preparation from the parental strain, the formation of up to 50% of the fully saturated alcohols was observed (Scheme 2.2.7.18). Substrate (l )-33 could be converted into enantiopure (S)- and (R)- E)-32, respectively, by recLBADH- and HLADH/CPCR-catalyzed reaction. 

HLADH converts a wide range of substrates. For the predicition of the stereoselectivity of reduction reactions, originally Prelog s diamond lattice model was applied, which is based upon the characteristic properties of the ADH of Curvularia falcata [37]. This model describes the stereospecificity of HLADH catalyzed reductions of simple acyclic substrates such as aldehydes. Later on, for more complex acyclic and cyclic substrates, a cubic-space model of the active site was developed [38,121]. Other models are based upon symmetric properties [122-125] or upon a refined diamond lattice model [126-129]. 

Another efficient method is the electrochemical oxidation of NADH at 0.585 V vs Ag/AgCl by means of ABTS2- (2,2,-azinobis(3-ethylbenzothiazoline-6-sulfonate)) as an electron transfer mediator [96]. Due to the unusual stability of the radical cation ABTS, the pair ABTS2 /ABTS is a useful mediator for application in large-scale synthesis even under basic conditions. Basic conditions are favorable for dehydrogenase catalyzed reactions. This electrochemical system for the oxidation of NADH using ABTS2 as mediator was successfully coupled with HLADH to catalyze the oxidation of a meso-diol (ws >-3,4-dihydroxymethylcyclohex-l-ene) to a chiral lactone ((3aA, 7aS )-3a,4,7,7a-tetrahydro-3//-isobenzofurane- l-one) with a yield of 93.5% and ee >99.5% (Fig. 18). 

By fitting a cyclohexanone in the diamond lattice, Prelog has developed a step-by-step analysis of the HLADH catalyzed reduction. The positions marked (A to D) are forbidden oxidoreduction will not take place if binding of a potential substrate places a group in one of these locations. Positions marked (E to G) are undesirable, although their occupation by part of a substrate does not necessarily preclude the oxidoreduction. The rate of reaction will be very slow. The positions under the lattice (U) are also in this category. The location O (I) is a newly identified unsatisfactory position. Placement of a group here is to be avoided if possible, but slow reaction will still take place if it is occupied. 

Lack of congruence of function and structure A common reaction catalyzed by two enzymes does not imply a common structure or sequence Dehydrogenation is the reaction common to alcohol DH from horse liver (HLADH) and alcohol DH from yeast (YADH), but they have neither a common structure nor a common sequence. 

As for oxidation reactions catalyzed by HLADH, the most frequently reported method is the coupling with FMN [222-224], It has been used for instance for the oxidation of many meso-diols forming lactones [250, 251], or for the oxidation of primary alcohols to obtain chiral aldehydes [252]. Generally, these syntheses were carried out at 12 g scale within a reaction time of a few hours up to 2-3 weeks [247],... 

HLADH is nicotinamide coenzyme (NADVNADH) dependent and catalyzes the redox equilibrium between a large number of alcohols and ketones or aldehydes (Figure 2). The equilibrium is overwhelmingly in favour of the reduction reaction. Phosphate analogs of the coenzymes, NADP+ and NADP may also serve as coenzymes in very limited situations. The oxidation-reduction takes place through a ternary complex in which the substrate and coenzyme are simultaneously bound in the active site of enzyme. 

Panza et al. synthesized a C02-philic amphiphile from the coenzyme nicatinamide adenine dinucleotide (MW 664) and a covalently attached perfluoropolyether (MW 2500) (Figure 7B) (73). The fluorofunctional coenzyme (FNAD) was soluble up to 5 mM in CO2 at room temperature and 1400 psi. The C02-soluble FNAD was able to participate in a cyclic oxidation/reduction reaction catalyzed by the enzyme horse liver alcohol dehydrogenase (HLADH) in CO2 at room temperature and 2600 psi. 

A very elegant reaction sequence was reported by Tanaka and coworkers S4l HLADH was used for the kinetic resolution of a series of racemic fi-hydroxysilanes yielding one enantiomer in ee values ranging from 20 to 97 % in reasonable yields and the corresponding fi-ketosilane. This fi-ketosilane hydrolyzes spontaneously and drives the regeneration of NAD4 catalyzed by HLADH (Fig. 16.2-9). 

Redox reactions catalyzed by alcohol dehydrogenases (e.g., from horse liver, HLADH) may be performed in organic solvents in both the reduction and oxidation mode, if the recycling system is appropriately modified (Sect. 2.2.1). Reduction of aldehydes/ketones and oxidation of alcohols is effected by NADH- or NAD" -recycling, using ethanol or wobutyraldehyde respectively. 

Thus, it has been proved that an ADH catalyzes the redox reaction between acetaldehyde and ethanol stereospecifically and that both YADH and HLADH exhibit the same specificity toward the coenzyme and the substrate. These two enzymes can also catalyze the reduction of other aldehydes. It has been ascertained that these reactions... 

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