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GREEN CHEMISTRY…Reduction of amides without hydride reagents



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Clostridium sporogenes


Essentially all medicines and current drug candidates contain at least one basic nitrogen atom. A common approach to the synthesis of amines is to reduce the corresponding amide with a hydride reagent such as LiAlH4, DIBAL, RedAl, B2H6, Et3SiH, or polymethylhydroxysilane (PMHS).

The reaction survey reported that reduction of amides to amines was used in only 0.6% of chemical transformations; this number would surely be higher if safer methods for use on scale were available. The survey indicated that the number of amide reductions was equally split between diborane and hydride reagents.

Lithium aluminium hydride,

Wireframe model of lithium aluminium hydride


having a molecular weight of 38 and four hydrides per molecule, has the highest hydride density and is frequently used, even though it co-generates an inorganic by-product (lithium aluminum hydroxide) which is difficult to separate from the product. The workup procedure recommended by one bulk supplier (Chemetall) is to precipitate and filter the aluminum hydroxide salts. However, slow filtrations and product loss through occlusion or adsorption are typical problems that can be encountered.

Options for disposal of the cake include dissolving in water and sending to a waste water treatment plant or drying the cake and sending to a chemical waste dump that accepts solids.1  Both options have an environmental impact. Therefore, a generally applicable, safe, environmentally benign and economically viable method for the reduction of amides to amines would have an appreciable benefit to numerous processes.

Hydrogen gas is the ideal reductant because the only by-product is water. Thus, much research has been directed towards discovery of a transition metal catalyst selective for hydrogenation of amides. However, even with the best catalysts, both high temperature ([similar]150 °C) and pressure (>100 bar) are required. These conditions involve expensive high pressure hydrogenation equipment not typically available in a common pharmaceutical manufacturing plant.

The harsh conditions also preclude the use of these catalysts with substrates that contain other reducible or thermally labile functional groups. Recent research has led to the discovery of catalysts that are effective at lower temperature and pressure, giving encouragement that the goal of finding a selective, low pressure/temperature catalyst is realistic.2

Another approach would be to use a biotransformation to reduce the amide. It is notable that a number of bacteria and fungi reduce carboxylic acids to aldehydes or ketones.3  The usual fate of amides in biological pathways is hydrolysis. However, an anaerobic bacteria, Clostridium sporogenes, has been reported to reduce benzamide to benzylamine. 4 

A key challenge in this technology area is gaining a detailed understanding of these complex enzyme-catalysed processes that require ATP/NADPH co-factor recycling, and getting the enzymes cloned and produced on a large scale in suitable expression systems.

The acylation/reduction strategy for N-alkylation avoids the need to handle alkylating agents and would be more widely used if a safer, more atom economical or preferably catalytic method for amide reduction were developed. The solution to this problem could be either chemical or biochemical.

  1. Chemetall brochures, Lithium Aluminum Hydride… strong, concentrated and economical, Oct. 2000, pp. 18–19 Search PubMed  .
  2. A. A. Smith, P. Dani, P. D. Higginson and A. J. Pettman, World Pat., WO2005/066112 A1, 2005 Search PubMed  .
  3. (aA. Hage, H. E. Schoemaker and J. A. Field, Appl. Microbiol. Biotechnol., 1999, 52, 834–838 CrossRef  CAS  Search PubMed  ; (bA. He, T. Li, L. Daniels, I. Fotheringham and J. P. N. Rosazza, Appl. Environ. Microbiol., 2004, 70, 1874–1881 CrossRef  CAS  Search PubMed  .
  4. O. Dipeolu, J. Gardiner and G. Stephens, Biotechnol. Lett., 2005, 27, 1803–1807 CrossRef  CAS  Search PubMed  .

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DR ANTHONY MELVIN CRASTO, Born in Mumbai in 1964 and graduated from Mumbai University, Completed his Ph.D from ICT, 1991,Matunga, Mumbai, India, in Organic Chemistry, The thesis topic was Synthesis of Novel Pyrethroid Analogues, Currently he is working with GLENMARK PHARMACEUTICALS LTD, Research Centre as Principal Scientist, Process Research (bulk actives) at Mahape, Navi Mumbai, India. Total Industry exp 30 plus yrs, Prior to joining Glenmark, he has worked with major multinationals like Hoechst Marion Roussel, now Sanofi, Searle India Ltd, now RPG lifesciences, etc. He has worked with notable scientists like Dr K Nagarajan, Dr Ralph Stapel, Prof S Seshadri, Dr T.V. Radhakrishnan and Dr B. K. Kulkarni, etc, He did custom synthesis for major multinationals in his career like BASF, Novartis, Sanofi, etc., He has worked in Discovery, Natural products, Bulk drugs, Generics, Intermediates, Fine chemicals, Neutraceuticals, GMP, Scaleups, etc, he is now helping millions, has 9 million plus hits on Google on all Organic chemistry websites. His friends call him Open superstar worlddrugtracker. His New Drug Approvals, Green Chemistry International, All about drugs, Eurekamoments, Organic spectroscopy international, etc in organic chemistry are some most read blogs He has hands on experience in initiation and developing novel routes for drug molecules and implementation them on commercial scale over a 30 year tenure till date Dec 2017, Around 35 plus products in his career. He has good knowledge of IPM, GMP, Regulatory aspects, he has several International patents published worldwide . He has good proficiency in Technology transfer, Spectroscopy, Stereochemistry, Synthesis, Polymorphism etc., He suffered a paralytic stroke/ Acute Transverse mylitis in Dec 2007 and is 90 %Paralysed, He is bound to a wheelchair, this seems to have injected feul in him to help chemists all around the world, he is more active than before and is pushing boundaries, He has 9 million plus hits on Google, 2.5 lakh plus connections on all networking sites, 50 Lakh plus views on dozen plus blogs, He makes himself available to all, contact him on +91 9323115463, email, Twitter, @amcrasto , He lives and will die for his family, 90% paralysis cannot kill his soul., Notably he has 19 lakh plus views on New Drug Approvals Blog in 216 countries...... , He appreciates the help he gets from one and all, Friends, Family, Glenmark, Readers, Wellwishers, Doctors, Drug authorities, His Contacts, Physiotherapist, etc

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