High Throughput Synthesis Within Flow Reactors

1. High Throughput Synthesis Within Flow Reactors Paul Watts CPAC, Rome, March 19th 2007

2. A B C D Micro Reactors • Defined as a series of interconnecting channels formed in a planar surface • Channel dimensions of 10-300 mm • Various pumping techniques available • Hydrodynamic flow • Electroosmotic flow • Fabricated from polymers, metals, quartz, silicon or glass • Why glass? • Mechanically strong • Chemically resistant • Optically transparent

3. PET Radiosynthesis • Positron emission tomography (PET) is a radiotracer imaging technique used to provide quantitative information on physiological and biochemical phenomena in vivo • Applications in clinical research and drug discovery • Two of the most desirable radioisotopes are: • 11C (t1/2 20.4 minutes) • 18F (t1/2 109.7 minutes) • Syntheses must be conducted within 2-3 half-lives • Aims of miniaturisation: • Produce the desired quantity of radiotracer (< 1 mg) at point of use • Reduced reaction times will produce the product with enhanced specific activity • The PET ligand will have greater sensitivity in vivo Collaboration with NIH, Washington DC

4. PET Chemistry • Reaction of 3-(3-pyridinyl)propionic acid • Reaction optimised with 12CH3I (10 mM concentration) at RT • Hydrodynamic flow (syringe pump) • Reaction with 11CH3I • At 0.5 ml/min flow rate RCY 88% • Reaction of 18FCH2CH2OTs at 80 oC • At 0.5 ml/min flow rate RCY 10% Lab Chip,2004, 4, 523

5. PET Chemistry • Esterification reaction • Reaction with 11CH3I (10 mM concentration) at RT • RCY 65% at 0.5 ml/min flow rate • Product isolated by preparative HPLC Lab Chip,2004, 4, 523

6. Transient positive ions ‘Double Layer’ Negative glass surface Electroosmotic Flow (EOF) • Advantages of EOF: • No mechanical parts • Reproducible, pulse free flow • Minimal back pressure • Electrophoretic separation • See Chem. Commun.,2003, 2886 for peptide separation

7. 18F PET Chemistry • 18F has a longer half-live than 11C • Produced from H218O • For nucleophilic reactions the fluoride needs to be separated from the water • Azeotropic distillation • Electrophoretic separation • Reaction Electrophoresis 18F- J. Lab. Compd. Radiopharm.,2007, 50, in press

8. Stable Radiosynthesis • Stable isotopes routinely used in drug discovery for drug metabolism studies (500 mg typically needed) • Amide synthesis • Optimise reaction with ‘normal’ (cheap) unlabelled reagents

9. Stable Radiosynthesis • Acetylation of aniline • Reaction efficiency dependent of flow rate • Reaction repeated with other derivatives

10. Stable Radiosynthesis • Once optimised substitute labelled precursor J. Lab. Compd. Radiopharm.,2007, 50, 189-196

11. Electrosynthesis - Kolbe Reaction • Radical dimerisation (Kolbe reaction) • Reactor diameter 1 mm • 1 mm platinum electrodes separated by 1 mm • Surface area in cell ca. 3 mm2 • Current 5 mA cm-2

12. Reaction Efficiency • Reaction conducted continuously for 12 hours • A base is needed to deprotonate the acid • Pyridine most successful • Stops contamination of electrode surface • Also works for other dimerisation reactions

13. Electrochemical Debrominations • Parallel plate electrochemical reactor • Electrode area 25 mm2 • Electrodes 160 mm apart • Flow rate 40 ml min-1

14. Coupling Reactions Flow Rate = 10 ml min-1 Electro. Commun.,2005, 7, 918 Angew. Chem. Int. Ed., 2006, 45, 4146 Green Chem., 2007, 9, 20 Lab. Chip, 2007, 7, 141

15. But always require purification • Generally batch work up required Fine Chemical Synthesis • New methodology for fine chemical synthesis • Enhanced yields of more pure products etc

16. Knoevenagel Reaction • Solution phase Knoevenagel reaction • 1:1 Ratio of reagents (0.5 M) in MeCN • EOF • 100 % conversion • Reaction very ‘atom efficient’ • BUT product contaminated with base!! • Traditional solvent extraction needed • This clearly reduces the advantages of flow reactors

17. Functionally Intelligent Reactors • Fabricate micro reactors which enable catalysts and/or supported reagents to be spatially positioned • Quantitative conversion to analytically pure product

18. A-15 Silica-supported piperazine Multi-Step Synthesis

19. Novozyme 435 (ca. 100 mg) Enzymatic Reactions • Enzymatic esterification Flow reactor Reaction Conditions: • Acid: Hexanoic acid, octanoic acid or lauric acid • Alcohol: Methanol, Ethanol or Butanol • 1:1 ratio in hexane (0.2 M) • Room temperature

20. Synthesis of Butyl Hexanoate • Esterification reaction is equilibrium dependent • With time conversion can increase then decrease • In flow the reaction mixture is removed so equilibrium is controlled • 96% yield • Gain knowledge about substrate specificity • Link solution phase and catalysed reactions

21. 200 mm Scale Out and Catalyst Screening Scale-out of reactions: • 4 channels operating in parallel produces 4 times the product • Larger packed reactors also feasible (5 mm diameter) Synthesise arrays of compounds:

22. Combinations of Acids and Bases

23. Conclusions • Micro reactors allow the rapid optimisation of reactions • High-throughput combinatorial synthesis • Immobilised reagents (catalysts and enzymes) allow the synthesis of analytically pure compounds • Micro reactors are suitable for a wide range of reactions • Electrochemical synthesis • Catalysed reactions • Enzyme screening • Micro reactors generate products in: • Higher purity • Higher conversion • Higher selectivity • In situ formation of reagents P. D. I. Fletcher et al., Tetrahedron,2002, 58, 4735 K. Jahnischet al.,Angew. Chem. Int. Ed., 2004, 43, 406 H. Pennemann et al., OPRD, 2004, 8, 422 P. Watts et al., Chem. Soc. Rev.,2005, 34, 235

24. Dr. Charlotte Wiles Dr. Nikzad Nikbin Dr. Ping He Dr. Victoria Ryabova Dr. Vinod George Dr. Leanne Marle Dr. Joe Dragavon LioniX Astra Zeneca Novartis Mairead Kelly Gareth Wild Tamsila Nayyar Julian Hooper Linda Woodcock Haider Al-Lawati Ben Wahab EPSRC Sanofi-Aventis EU FP6 Research Workers and Collaborators