3- hydroxyglutaronitrile, prochiral

The biocatalytic differentiation of enantiotopic nitrile groups in prochiral or meso substrates has been studied by several research groups. For instance, the nitrilase-catalyzed desymmetrization of 3-hydroxyglutaronitrile [92,93] followed by an esterification provided ethyl-(Jl)-4-cyano-3-hydroxybutyrate, a useful intermediate in the synthesis of cholesterol-lowering dmg statins (Figure 6.32) [94,95]. The hydrolysis of prochiral a,a-disubstituted malononitriles by a Rhodococcus strain expressing nitrile hydratase/amidase activity resulted in the formation of (R)-a,a-disubstituted malo-namic acids (Figure 6.33) [96]. 

An alternative approach, explored by Dowpharma, involved the desymmetrization of prochiral dinitrile (meso)-3-hydroxyglutaronitrile (3-HGN) with a nitrUase provided by Diversa (Figure 6.2b). The resulting (R)-3-hydroxy-4-cyanobutyrate can in several steps be converted into ethyl 6-cyano-3R,5R-dihydroxyhexanoate, an advanced intermediate to atorvastatin. 3-HGN could be easily prepared from... 

A nitrilase enzyme (BD9570,0.1 unit/mg, Diversa) was used to desymmetrize prochiral substrate 14 to afford (R)-15. In this process, an aqueous solution of 100 mM NaH PO at pH 7.5 (510 mL) was added to lyophilized nitrilase enzyme powder (15.15 g). The mixture was stirred at 27°C (to rehydrate the lyophilized enzyme powder) for 40 min. 3-Hydroxyglutaronitrile (252.5 g) was then charged over 10 min. The mixture was stirred at 27°C for 16 h and then cooled to 2°C prior to acidification to give pH 2. Celite was charged (25 g) and the slurry filtered. The filtrate was extracted with methyl ethyl ketone. The combined methyl ethyl ketone extracts were evaporated in vacuo (15 mbar, 40°C) to yield the product 15 as a brown liquid (240.8 g, 81% yield ee 98.8%). 

Figure 15 Asymmetric synthesis of (5)-3-hydroxyglutaronitrile acids from the prochiral dinitriles by an (5)-selective nitrile hydratase from immobilized Rhodococcus rhodochrous sp, SP 361.

Besides the racemic resolution of 2-arylpropionitriles (Fig. 16), R. rhodochrous ATCC 21197 additionally catalyzes the asynunetric hydrolysis of prochiral 3-substituted glutaronitriles [56]. The highest enantioselectivity was found in the conversion of the benzoate ester of 3-hydroxyglutaronitrile to the corresponding nitrile monocarboxylic acid (e.e. more than 99%) by an enzyme system consisting of an (/ )-specific nitrile hydratase and an (5)-specific amidase. The enantioselectivity depended on the number of oxygen atoms between the carbon chain and the aryl substituent (Fig. 21). 

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