Bioreduction synthesis

Crocq et al. (1997) have synthesized trimegestone through Bakers yeast mediated reduction of a ketone (this material is a new progestomimetic molecule for the treatment of postmenopausal diseases). The key step of the multistep synthesis is the chemo-, regio- and almost stereospecific bioreduction of a triketone to the desired alcohol. 

The UV-Vis spectral detection of an intermediate in the catalytic reductive alkylation reaction provides only circumstantial evidence of the quinone methide species. If the bioreductive alkylating agent has a 13C label at the methide center, then a 13C-NMR could provide chemical shift evidence of the methide intermediate. Although this concept is simple, the synthesis of such 13C-labeled materials may not be trivial. We carried out the synthesis of the 13C-labeled prekinamycin shown in Scheme 7.5 and prepared its quinone methide by catalytic reduction in an N2 glove box. An enriched 13C-NMR spectrum of this reaction mixture was obtained within 100 min of the catalytic reduction (the time of the peak intermediate concentration in Fig. 7.2). This spectrum clearly shows the chemical shift associated with the quinone methide along with those of decomposition products (Fig. 7.3). 

Lin, A. J. Shansky, C. W. Sartorelli, A. C. Potential bioreductive alkylating agents. 3. Synthesis and antineoplastic activity of acetoxymethyl and corresponding ethyl carbamate derivatives of benzoquinones. J. Med. Chem. 1974, 17, 558-561. 

Oxidoreductases are, after lipases, the second most-used kinds of biocatalysts in organic synthesis. Two main processes have been reported using this type of enzymes-bioreduction of carbonyl groups [39] and biohydroxylation of non-activated substrates [40]. However, in recent few years other processes such as deracemization of amines or alcohols [41] and enzymatic Baeyer-Villiger reactions of ketones and aldehydes [42] are being used with great utility in asymmetric synthesis. 

Whereas several anti-cholestemic drugs are produced wholly by fermentation, the side chain of several others is accessible through enzymatic synthesis. (3R,5S)-Dihydroxyhexanoate, a key intermediate of fluvastatin, is accessible by reduction of the diketo acid, either by bioreduction with whole cells (85% yield, 97% e.e.), or cell extracts (72% yield, 98.5% e.e.), or, for syn-(3h,5.S)-dihydroxy-6-Cl-hexanoate, regioselective and (ft)-specific reduction with ADH to yield (5S)-6-chloro-3-ketohexanoate. 

Candida tropicalis PBR-2, a yeast strain isolated from soil, is capable of carrying out the enantioselective reduction of N,N-dimethyl-3-keto-3-(2-thienyl)-l-propanamine 58 to (S)-N,N-dimethyl-3-hydroxy-3-(2-thienyl)-l-propanamine 59 (Fig. 18.18), a key intermediate in the synthesis of the chiral drug (S)-Duloxetine (Soni and Banerjee, 2005). The organism produced the enantiopure (S)-alcohol with a good yield (>80%) and almost absolute enan-tioselectivity, with an ee >99%. Parameters of the bioreduction reaction were optimized and the optimal temperature and pH for the reduction were found to be 30 °C and 7.0, respectively. The optimized substrate and the resting cell concentration were lg/1 and 250 g/1, respectively. The preparative-scale reaction using resting cells of C. tropicalis yielded the (S)-alcohol at 84-88% conversion and ee >99%. 

Gellis A, Kovacic H, Boufatah N et al (2008) Synthesis and cytotoxicity evaluation of some benzimidazole-4, 7-diones as bioreductive anticancer agents. Eur J Med Chem 43 1858-1864... 

Although reductases play an important role in the in vivo synthesis of many chemicals (see flavour example in Fig. 7.11), little attention has been paid to this type of biocatalyst. In most cases whole microbial or plant cells are used to perform a bioreductive reaction due to the requirement for (expensive) cofactors. Typical examples include the reduction of certain double bonds in terpenes by plant cells [27,41], the reduction of Massoi lactone to R(+)-8-decalactone by Basidiomycetes and S. cerevisiae [28], and the baker s yeast-catalyzed reduction of ketones to (chiral) alcohols [42]. 

Shair, M.D. Yoon, T. Chou, T.-C. Danishefsky, S. J. Enediyne quinone imines truncated biologically active dynemicin congeners. Angew. Chem., Int. Ed. Engl. 1994, 33, 2477-2479 Shair, M.D. Yoon, T.Y. Mosny, K.K. Chou, T.C. Danishefsky, S.J. The total synthesis of dynemicin A leading to development of a fully contained bioreductively activated enediyne prodrug. J. Am. Chem. Soc. 1996. 118, 9509-9525. 

The bioactive benzimidazolequinone 131 has been synthesized by demethylation of the dimethoxybenzimidazole 129 followed by facile oxidation of the intermediate dihydroxy compoimd 130 by ferric chloride to yield the quinone 131 in excellent yield (Scheme 23). Synthesis of the related ben-zimidazolequinones 134 was achieved by dinitration of 132 followed by the reduction of 133 and oxidation as above. The benzimidazole-6,9-dione 134 has been found to be 300 times more cytotoxic towards the human skin fibroblast cell line in the MTT assay than the clinically used bioreductive drug, mitomycin C. Attaching methyl substituents onto the quinone moiety increased reductive potential and decreased cytotoxicity and selectivity towards hypoxia [67]. 

Although planar chirality has not been found in nature so far, biological tools can be used for resolution of planar chiral molecules. The synthesis of enantio-merically pure (S)-4-formyl[2.2]paracyclohane (>99% ee) (S)-155 and (R)-4-hydroxymethyl[2.2]paracyclophane (R)-156 (>78% ee) was achieved by bioreduction in 49 and 34% yield respectively. From several mircoorganisms screened only one strain of the yeast Saccharomyces cerevisiae showed a stereospecific reduction of the planar chiral substrate. Despite the high enantiomeric ratio it was necessary to maintain the conversion of the process at almost 50% in order to obtain high enantiomeric excesses of both substrate and reduction product [101]. 

Here the biotransformation (Fig. 19-6) is preferred over the chemical reduction with commercially available asymmetric catalysts (BH3- or noble-metal-based), since with the chemocatalysts the desired high enantiomeric excess (ee > 98%, 99.8% after purification) is not achievable. Since the ketone has only a very low solubility in the aqueous phase, 1 kg ketone is added as solution in 4 L 0.9 M H2SO4 to the bioreactor. The bioreduction is essentially carried out in a two-phase system, consisting of the aqueous phase and small droplets made up of substrate and product. The downstream processing consists of multiple extraction steps with methyl ethyl ketone and precipitation induced by pH titration of the pyridine functional group (pfCa = 4.66) with NaOH. The (R)-amino alcohol is an important intermediate for the synthesis of (1-3-agonists that can be used for obesity therapy and to decrease the level of associated type II diabetes, coronary artery disease and hypertension. 

Esters of 2-oxo acids are not converted by the HVOR. If esters were the products of the 2-0X0 acid synthesis the hydrolysis was carried out as a separate step by alkaline hydrolysis or by Lipozyme at neutral pH. Especially unstable carboxylates such as the 2,4-dioxo carboxylates 18 and 20 (Table 10) were formed enzymically by Lipozyme from the esters diuing the bioreduction. If ethyl 2,4-dioxo carboxylates are used for bioreductions the ratio of Lipozyme activity and amount of P. vulgaris cells has to be optimised. If too litde lipase is present, the reaction proceeds slowly. If too much lipase is used, labile carboxylate accumulates, and side reactions may occur. 

S)-4-Chloro-3-hydroxybutanoic acid methyl ester 59 (Figure 16.16) is a key chiral intermediate in the total chemical synthesis of 60, a cholesterol antagonist that acts by inhibiting hydroxymethyl glutaryl coenzyme A (HMG-CoA) reductase [89,90]. The reduction of 4-chloro-3-oxobutanoic acid methyl ester 61 to (S)-59 has been demonstrated by cell suspensions of Geotrichum candidum SC 5469. In the biotransformation process, a reaction yield of 95% and EE of 96% were obtained for (S)-59 at 10 g/L substrate input. The EE of (S)-59 was increased to 98% by heat treatment of cell suspensions (55°C for 30 min) prior to conducting the bioreduction of 61 [91]. 

TEM analysis confirms the approximate shape, nanogram, and diameter (< 100 nm) after the bioreduction of green synthesis of gold and silver nanoparticles after 24 h. The shapes of some nanoparticles were roughly circular with smooth edges and... 

Damen EWP, Nevalainen TJ, van den Bergh TJM, de Groot FMH, Scheeren HW (2002) Synthesis of Novel Paclitaxel Prodrugs Designed for Bioreductive Activation in Hypoxic Tumour Tissue. Bioorg Med Chem 10 71... 

Alkyl-4-oxy-3,4-dihydroisocoumarins are enantioselectively prepared by oxylactonization ofo-(alk-l-enyl)benzoates promoted by the in situ-generated chiral lactate-based hypervalent iodine(III) catalysts (13EJ07128). Chemoenzymatic synthesis of 3,4-dialkyl-3,4-dihydroisocoumarins involves one-pot dynamic kinetic reductive resolution processes catalyzed by E. co/i/alcohol desidrogenase. This strategy consists in the bioreduction of various racemic ketones to the corresponding enantiopure alcohols followed by intramolecular acidic cyclization (Scheme 71) (130L3872). 

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