Ketopantolactone, reduction

Reduction of ketopantoic acid to D-pantoic acid (0, (4) in Fig. 8). Agrobacterium sp. S-246 is a good source of ketopantoic acid reductase. The yield of D-pantoic acid reached 119 g/1 (molar yield, 90% optical purity, 98% e.e.) on incubation with washed cells of the bacterium [102]. From a practical point of view, ketopantoic acid reduction with Agrobacterium cells has several advantages over ketopantoyl lactone reduction with Candida (or Rhodotorula) cells. The former results in a higher product yield, molar conversion and optical purity of the product than the latter. It is necessary to maintain the substrate level at lower than 3% in the case of the ketopantolactone reduction, but not for the ketopantoic acid reduction. 

R)-Pantolactone (9) is prepared ia either of two ways by resolution of the (R,5)-pantolactone mixture (18), or by stereoselective reduction of ketopantolactone (19) by chemical or microbial methods (19). 

Reduction of ketopantolactone (19) to (R)-pantolactone (9) also was evaluated using microbes (45—52) (Table 4). 

In a first step, JS ocardia asteroides selectively oxidizes only (3)-pantolactone to ketopantolactone (19), whereas the (R)-pantolactone remains unaffected (47). The accumulated ketopantolactone is stereospecificaHy reduced to (R)-pantolactone in a second step with Candidaparapsilosis (product concentration 72 g/L, 90% molar yield and 100% ee) (48). Racemic pantolactone can also be converted to (R)-pantolactone by one single microbe, ie, Jiodococcus erythropolis by enantioselective oxidation to (3)-pantolactone and subsequent stereospecific reduction in 90% yield and 94% ee (product concentration 18 g/L) (40). 

Despite the progress made in the stereoselective synthesis of (R)-pantothenic acid since the mid-1980s, the commercial chemical synthesis still involves resolution of racemic pantolactone. Recent (ca 1997) synthetic efforts have been directed toward developing a method for enantioselective synthesis of (R)-pantolactone by either chemical or microbial reduction of ketopantolactone. Microbial reduction of ketopantolactone is a promising area for future research. 

Ketopantolactone was reduced with diphenylsilane in the presence of the Rh/4 complex in THF at 0 °C followed by desilylation with an acidic methanol affording (S)-pantoyl lactone in 84% optical yield (Scheme 4) [17]. (S)-Ethyl 4-hydrox-ypentanoate was obtained in 80% ee by the reduction of ethyl 4-oxopentanoate with the Rh/4 complex in toluene followed by cleavage of the silyl ether with a neutral methanol [17]. 

Fig. 10. Diversity of microbial reduction of ketopantolactone (a), ketopantoic acid (b), and ethyl 2 -ketopantothenate (c). Symbols A, yeasts O, molds , bacteria , actinomycetes , basidiomycetes...

The enzyme catalyzing the reduction of ketopantolactone to D-pantolactone was isolated in a crystalline form from the cells of Candida parapsilosis and characterized in some detail [106] (see Tables 4 and 5). It is a novel NADPH-dependent carbonyl reductase with a molecular mass of about 40,000. In addition to the reduction of ketopantolactone, the enzyme catalyzes those of a variety of cyclic diketones, including derivatives of ketopantolactone, isatin, camphorquinone and so on, to give the corresponding (R)-alcohols [106, 107], The enzyme was termed conjugated polyketone reductase , since the enzyme catalyzes only the reduction of conjugated polyketones as follows. 

The enzyme yielding the antipode, L-pantolactone, was also isolated from Mucor ambiguus [108]. It is also a kind of conjugated polyketone reductase and consists of two polypeptide chains with an identical molecular mass of about 27,500 (see Tables 4 and 5). The occurrence of two kinds of enzymes that show similar substrate specificity but differ from each other in their stereospecificity may be one of the possible reasons why the reduction of ketopantolactone resulted in the formation of the d- and L-enantiomers in varying ratios as shown in Fig. 10a. 

Reduction of ketopantolactone to D-pantolactone (0 in Fig. 8). This reaction is catalyzed by conjugated polyketone reductase. About 50 or 90 g/1 of D-panto-lactone (98 or 94% e.e., respectively) was produced with a molar yield of nearly 100% on incubation with washed cells of R. minuta or C. parapsilosis, respectively [115, 116]. 

Reduction of ethyl 2 -ketopantothenate to ethyl 2 -d-pantothenate ((g) in Fig. 8). The rate of condensation of ketopantolactone or D-pantolactone with ethyl (3-alanine, yielding ethyl 2 -ketopantothenate (0 in Fig. 8) or ethyl D-panto-thenate, respectively, is quite fast compared to the condensation of ketopantolactone or D-pantolactone with (3-alanine, and the reaction with ethyl (3-alanine proceeds more stoichiometrically [117]. Since the enzymatic hydrolysis of ethyl D-pantothenate has been established [118], if the stereoselective reduction of ethyl 2 -ketopantothenate to ethyl D-pantothenate is possible, both the troublesome resolution and the incomplete condensation might be avoided at the same time. Carbonyl reductase of C. macedoniensis is used for this purpose. Washed cells of the yeast converted ethyl 2 -ketopantothenate (80 g/1) almost specifically to ethyl D-pantothenate (> 98% e.e.), with a molar yield of 97.2% [103]. In a similar manner, 2 -ketopantothenonitrile (50 g/1) was converted to D-pan-tothenonitrile (93.6% e.e.), with a molar yield of 95.6%, on incubation with Sporidiobolus salmonicolor cells as a catalyst [104],... 

Pantolactone (PL) [(i )-(-)-3-hydroxy-4,4-dimethyltetrahydofuran-2-on], 1 (Scheme 7.23.), is an intermediate in the preparation of several biologically important molecules such as D-(+)-pantothenic acid, 2, which is a member of B vitamins (Vitamin B5) and is an important constituent of coenz5me A. The biosynthesis of 2 involves the asymmetric reduction of ketopantolactone, i, (KPL) (4,4-dimethyltetrahydrofuran-2,3-dion) to R- -)-PL, because only the 7 -(-)-enantiomer is biologically active. 

Reduction of ketopantolactone using Baker s yeast gives an ee of approximately 72% but a chiral Rh catalyst has been shown to be superior to Baker s yeast in this process. 

Enantioselective reduction of ketopantolactone and methyl benzoylformate was also examined by the same authors. Quite recently, Bakos and coworkers reported an enantioselective hydrogenation of dehydroalanine using an in situ-produced [Rh (COD)(S)-MonoPhos)2]Bp4 complex on AI2O3 [83,84]. After the optimization of the flow reaction conditions, a highly enantioselective production of (R)-acetylalanine methyl ester (>99% conversion and 96-97% ee) was achieved. 

The reduction of a-keto esters has also been performed using baker s yeast [216]. Thus, different 2-oxo-2-arylacehc acid derivatives gave optically pure a-hydroxy acid derivatives [217, 218]. Ethyl p5Tuvate gave (R)-ethyl lactate efficiently [217]. Of par-hcular interest seems the synthesis of enantiomerically pure (f )-pantoyllactone 49 (Figure 21.15) via enantiospecific reduction of ketopantolactone 48. 

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