Enzymatic multi-step

Some companies are successfully integrating chemo- and biocatalytic transformations in multi-step syntheses. An elegant example is the Lonza nicotinamide process mentioned earlier (.see Fig. 2.34). The raw material, 2-methylpentane-1,5-diamine, is produced by hydrogenation of 2-methylglutaronitrile, a byproduct of the manufacture of nylon-6,6 intermediates by hydrocyanation of butadiene. The process involves a zeolite-catalysed cyciization in the vapour phase, followed by palladium-catalysed dehydrogenation, vapour-pha.se ammoxidation with NH3/O2 over an oxide catalyst, and, finally, enzymatic hydrolysis of a nitrile to an amide. 

Special attention is given to the integration of biocatalysis with chemocatalysis, i.e., the combined use of enzymatic with homogeneous and/or heterogeneous catalysis in cascade conversions. The complementary strength of these forms of catalysis offers novel opportunities for multi-step conversions in concert for the production of speciality chemicals and food ingredients. In particular, multi-catalytic process options for the conversion of renewable feedstock into chemicals will be discussed on the basis of several carbohydrate cascade processes that are beneficial for the environment. 

The fhioethyl group present in the anchor group of (40) was activated by treatment with N-iodosuccinimide (NIS) followed by displacement with a variety of alcohols (44-46). To prove the possible application of this linker in solid phase carbohydrate synthesis, protected glycosides (47) and (48) were coupled to linker (40) and released enzymatically. Flitsch et al. also described the immobilization and enzymatic cleavage on a variety of amines [41]. Nevertheless, the application of this enzyme-labile hnker group in multi-step syntheses on the solid phase and subsequent enzyme-initiated release from the polymeric support has not been described yet. 

Scheme 9.1 Multi-step enzymatic process for the production of 2 -deoxyribonucleoside from glucose, acetaldehyde and a nucleobase through the reverse reactions of2 -deoxy-ribonucleoside degradation.

One-Pot Multi-Step Enzymatic Synthesis of 2 -Deoxyribonucleoside from Glucose, Acetaldehyde and a Nucleobase... 

The one-pot multi-step enzymatic process described above seemed still impractical on an industrial scale due to the low amount of 2 -deoxyribonucleoside accumula-hon and the low molar yield to added nucleobase (yield to adenine was 33.3%). In addition, it required three kinds of catalyst cells (baker s yeast, deoxyriboaldolase-or phosphopentomutase-expressing E. coli and commercial nucleoside phosphory-lase). It is difficult and complicated to operate the multi-catalysts. If the molar yield of 2 -deoxyribonucleoside to the most expensive material, nucleobase, was improved and the number of catalysts could be reduced, a practical enzymahc process could be developed. 

The formation of covalent substrate-catalyst adducts might occur, e.g., by single-step Lewis-acid-Lewis-base interaction or by multi-step reactions such as the formation of enamines from aldehydes and secondary amines. The catalysis of aldol reactions by formation of the donor enamine is a striking example of common mechanisms in enzymatic catalysis and organocatalysis - in class-I aldolases lysine provides the catalytically active amine group whereas typical organocatalysts for this purpose are secondary amines, the most simple being proline (Scheme 2.2). 

Fig. 16 The multi-step enzymatic formation of morphine from dopamine and 4-hydroxyphenyla-cetaldehyde...

The levels of selectivity achieved in these reactions are amongst the highest reported for non-enzymatic acylative KR, and the scope of the method has been reviewed by Vedejs [40], as has its application in PKR [43]. The PBO catalysts 2a-c are prepared by a multi-step enantioselective synthesis from lactate esters [42, 44] and are air-sensitive hence, the reactions are generally run in de-oxygenated solvents. However, the air-stable tetrafluoroboric acid salts of these catalysts can also be employed with in-situ deprotonation by EtsN these conditions give results comparable with those obtained using the original protocol [45]. (For experimental details see Chapter 14.17.1). 

Peptides are not the only potential drug candidates. In most cases, other kinds of small organic molecules are preferred, because of their reduced susceptibility to enzymatic degradation. The split-mix method is fully applicable in the synthesis of organic libraries. Both sequential type and cyclic libraries can easily be prepared if the reaction conditions for solid phase are well developed. It has to be emphasized, however, that the advantages of the split-mix method can be fully exploited only in the case of multi-step synthetic procedures. For realization of the one-pot procedures suggested by Ugi [10], for example, the parallel procedures are better-suited. 

Multi-step technique (3) This is an indirect/direct method combining unlabeled primary antibodies with directly-conjugated antibodies. The method starts with staining the unlabeled antibody/antibodies with the appropriate detection system, but without performing the final enzymatic staining reaction. The tissue is blocked with normal serum from the host of the first primary antibody before the second, directly-labeled primary antibody is added. The staining ends with the two enzymatic reactions being performed sequentially. 

Catalytic aldol reactions are among the most useful synthetic methods for highly stereo-controlled asymmetric synthesis. In this account we discuss the recent development of a novel synthetic technique which uses tandem enzyme catalysis for the bi-directional chain elongation of simple dialdehydes and related multi-step procedures. The scope and the limitations of multiple one-pot enzymatic C-C bond formations is evaluated for the synthesis of unique and structurally complex carbohydrate-related compounds that may be regarded as metabolically stable mimetics of oligosaccharides and that are thus of interest because of their potential bioactivity. 

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