Catalysed enzyme reporter

The ability of enzymes to achieve the selective esterification of one enantiomer of an alcohol over the other has been exploited by coupling this process with the in situ metal-catalysed racemisation of the unreactive enantiomer. Marr and co-workers have used the rhodium and iridium NHC complexes 44 and 45 to racemise the unreacted enantiomer of substrate 7 [17]. In combination with a lipase enzyme (Novozyme 435), excellent enantioselectivities were obtained in the acetylation of alcohol 7 to give the ester product 43 (Scheme 11.11). A related dynamic kinetic resolution has been reported by Corberdn and Peris [18]. hi their chemistry, the aldehyde 46 is readily racemised and the iridium NHC catalyst 35 catalyses the reversible reduction of aldehyde 46 to give an alcohol which is acylated by an enzyme to give the ester 47 in reasonable enantiomeric excess. 

To a much smaller extent non-enzymic processes have also been used to catalyse the stereoselective acylation of alcohols. For example, a simple tripeptide has been used, in conjunction with acetic anhydride, to convert rram-2-acctylaminocyclohexanol into the (K),(R)-Qster and recovered (S),(S)-alcohol[17]. In another, related, example a chiral amine, in the presence of molecular sieve and the appropriate acylating agent, has been used as a catalyst in the conversion of cyclohexane-1(S), 2(/ )-diol into 2(S)-benzoyloxy-cyclohexan-1 f / j-ol1 IS]. Such alternative methods have not been extensively explored, though reports by Fu, Miller, Vedejs and co-workers on enantioselective esterifications, for example of 1-phenylethanol and other substrates using /. vo-propyl anhydride and a chiral phosphine catalyst will undoubtedly attract more attention to this area1191. 

Other examples of efficient enzymatic resolutions by reaction at a remote position from stereocentres have been reported, such as the lipase-catalysed resolution of a synthetic intermediate of escitalopram." This property of enzymes has also been effectively used to resolve sterically hindered compounds by the introduction of a tether so that the enzyme-catalysed reaction can be performed at an artificially created, but less hindered, remote location. An example is the resolution of tertiary alcohols by the introduction of a glyoxylate ester. 

Desymmetrization is not restricted to a single class of enzyme. For example, Madrell et reported the gram-scale preparation of a key intermediate of the lovastatin lactone through the desymmetrization of 3-(benzyloxy)glutaronitrile using whole cells from Brevibacterium R312. The transformation occurs via a dual nitrile hydratase/amidase-catalysed hydrolysis to afford acid in 65 % yield and 88 % ee (Scheme 1.49). 

A new, more general, way to combine ab initio quantum mechanical calculations with classical mechanical free-energy perturbation approach (QM/FE approach) to calculate the energetics of enzyme-catalysed reactions and the same reaction in solution has been reported." The calculated free energies were in fairly good agreement with the experimental data for the activation energies of the first test case, amide hydrolysis in trypsin and in aqueous solution. 

Interestingly, enzymes are chiral catalysts and their potential for enantio-selective polymerization has been investigated [93]. Several examples are reported where a racemic mixture of lactones is polymerized by enzymatic polymerization to afford the corresponding optically active polyester [93]. For instance, lipase CA (Novozym 435) catalyses the ROP of racemic 4-methyl-s-caprolactone and 4-ethyl-s-caprolactone in bulk at 45 °C and 60 °C to afford (S )-eiuiched poly(4-methyl-e-caprolactone) and poly(4-ethyl- -caprolactone) with an enantiomeric purity higher than 95% [153]. 

Recently, Mottola [98] reported a sensor based on the disk-ring principle previously developed by Kamin and Wilson [99], and Wang and Lin [100]. Unlike Mottola s design, its forerunners involved no stationary ring electrode or rotation of the reactor part in addition, their reactor/electrode was located at the cell bottom. In Mottola s assembly, a product of an enzyme-catalysed reaction at a bioreactor rotated at a constant speed was hydrodynamically transported to a stationary ring electrode, where it was electrochemically monitored. The sample was transported to the detection imit by an tm-... 

An enzyme that catalyses ATP-dependent 2 -phosphorylation and acetyl-CoA-dependent 6 -acetylation of the antibacterial aminoglycosides has been reported.260 Because of its complementary spectrum of two enzymic reactions, this bifunctional enzyme has a wide breadth of activity. Pentacoordinated thiophosphorane intermediates such as (292a) and (292b) are involved in the hydrolytic reactions of the monothioate analogues of 5 -O-methyluridine 2 - and 3 -dhnethylphosphates, (293) and (294), which have been studied over a wide range of HC1 acidities, //<, = —1.7 to pH9.261... 

---