Bioreductive processes

Another class of DNA alkylating agents, the Mitomycins, proved to be most promising in clinical trials. Among these, mitomycin C, shown in Fig. 6.1, exhibits significant anti-tumor activity. Its mechanism of activation consists of a complex bioreductive process. The first step is the reduction to hydroquinone, followed by a loss of methanol. This reaction fa-... 

The fermentation of S. paucimobilis SC 16113 culture was carried out in a 750-liter fermentor. From each fermentation batch, about 60 kg of wet cell paste was collected. Cells harvested from the fermentor were used to conduct the biotransformation in 1-, 10-, and 210-liter preparative batches under aerobic or anaerobic conditions. The cells were suspended in 80 mM potassium phosphate buffer (pH 6.0) to 20% (w/v, wet cells) concentration. Compound (6) (1-2 g/ liter) and glucose (25 g/liter) were added to the fermentor and the reduction reaction was carried out at 37°C. In some batches, at the end of the fermentation cycle, the cells were concentrated sevenfold by ceramic crossflow microfiltration using a 0.2-pm filter, diafiltered using 10 mM potassium phosphate buffer (pH 7.0), and used directly in the bioreduction process. In all batches of biotransformation, the reaction yield of >85% and the e.e. of >98% were obtained (Table 4). The isolation of compound (7) from the 210-liter preparative batch was carried out to obtain 100 g of product (7). The isolated (7) gave 83% chemical purity and an e.e. of 99.5%. 

What are the bottelnecks for bioreduction The drawbacks of a bioreduction process involving whole cells of microoganisms can be summarized i) Microbial strains possessing both carbonyl reductase activity and cofactor (NAD(P)H)-regenerating activity are necessary to obtain a highmolar yield, because a stoichiometric amount of cofactor is required for substrate reduc-... 

One of the prominent industrial bioreduction processes, run by cofactor regeneration, is performed by leucine dehydrogenase. This enzyme can catalyze the reductive amination of trimethylpyruvic acid, using ammonia (see Fig. 3.45). For this process the cofactor regeneration takes place by using formate as the reductant and formate dehydrogenase as the second enzyme. The advantage of... 

Phosphorous trioxide, P4O6, is present in the atmosphere of Venus [34]. Spectroscopic molecules such as PH, PO, PN, PC and more recently HC P, have all been detected in interstellar space. Recent studies [36] have supported earlier views [37] that phosphorus may have originally been present on Earth in a reduced oxidation state, for example, as derivatives of phosphonic (phosphorous) acid H3PO3 rather than of phosphoric acid H3PO4. Recent studies have confirmed that condensed phosphates (which are needed for the production of evolutionary DNA) can be produced by oxidation of phosphonates or phosphinates [38]. The identification of sulphate-reducing bacteria (Chapter 11.7), also suggests a means by which P could have become fully oxidised during the course of planetary evolution. Natural phosphine gas, PH3, may also result from bioreduction processes (Chapter 4.4). 

The discovery in 1993 that both phosphine and diphosphine could be produced in faeces by bioreduction processes offered a possible explanation of legendary spontaneous human combustion [15,15a]. (The fire from the mythological dragon could perhaps be explained on the same basis ) Some doubts have, however, been raised about earlier reports of bioreduction. Energy requirements for some processes, for example, PH3 formation from phosphite, may be unfavourable [17]. 

Figure 4.2 Examples of natural diketones used in ADH-catalyzed bioreduction processes.

The Pd( 11)-catalyzed Wacker-type oxidation reaction is particularly useful to obtain prochiral ketones in aqueous reaction medium, which would enable a subsequent selective bioreduction process (Scheme 4.16) [66]. In one example, Schnapperelle... 

Xu and coworkers reported several robust carbonyl reductases discovered via in silico data rnining approaches and developed a set of water-based bioreduction processes for efficient synthesis of functionalized optically active a- and P-hydroxy esters [2-4]. As shown in Table 9.1, several a- and P-keto esters were asymmetrically reduced at high loads using a carbonyl reductase mined from Candida abrata... 

The most established of bioreduction processes focuses on carbon-oxygen double bond reduction. For example, alcohol dehydrogenases (ADHs), which can even catalyze the reduction of sterically demanding substrates such as 2-hydroxy ketones, can be used effectively [8]. The mild conditions of the enzymatic reaction mean that... 

As discussed earlier in this chapter, it is clear that one of the most important design decisions to be taken when scaling up and developing a new bioreduction process is... 

Relevant Practical Applications of Bioreduction Processes in the Synthesis of Active Pharmaceutical Ingredients... 

Others have driven the bioreduction process to completion by employing an in situ product removal (ISPR) technique to overcome the inherent equilibrium problem associated with the coupled-substrate approach to biocatalytic carbonyl reduction. This technique allowed the valuable chiral alcohol, (S)-2-bromo-2-cyclohexen-l-ol, to be isolated in 88% yield and 99.8% ee without the need for further purification, validating the general applicability of this experimental setup shown in Figure 6.3 [6]. 

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