There is a new accepted manuscript in the Journal of Molecular Catalysis B from Yu-Guo Zheng and co-workers entitled "Industrial production of chiral intermediate of cilastatin by nitrile hydratase and amidase catalyzed one-pot, two-step biotransformation" which describes their research into using a chiral nitrile to carboxylic acid conversion which results from a cascade of NHase from (Rhodococcus boritolerans FW815) then amidase (from Delftia tsuruhatensis ZJB-05174). The compound they are aiming to hydrolyze is rac 2,2-dimethylcyclopropanecarbonitrile to the S-2,2-dimethylcyclopropanecarboxamide which is an intermediate in the synthesis of cilastatin (a booster component to some antibiotics which prolongs their activity by blocking the kidney’s dehydropeptidase activity). This is achieved using a NHase which is notably active but not chirally selective, and an amidase which is only avid for the R enantiomer of the amide. The R acid and S amide are separated using macroporous resin adsorption chromatography and the R acid is converted to the acid chloride. This can than have its chirality scrambled with a bit of heat such that its conversion to amide by addition of ammonia gives a fresh racemic batch of amide to challenge the R selective amidase with. The conditions described are to get it to work on the 100kg scale. Considering that this is a report on a process which has been undergoing continuous optimization since 2005, I guess the choice of amidase to do the chiral resolution was “baked in” before there was literature evidence of significantly stereoselective nitrile hydratases which might have prompted an enzyme discovery effort in that direction. Having said that racemization using the wrong acid chloride and heat rather than on the wrong isomer of the nitrile is a neat touch.
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