Practical Considerations and Examples in Adapting Amidations to Continuous Flow Processing in Early Development

• Bryan Li*
• Gerald A. Weisenburger
• J. Christopher McWilliams

• Chemical Research & Development, Pharmaceutical Science Small Molecules Division, Worldwide Research and Development, Pfizer Inc., Eastern Point Road, Groton, Connecticut 06340, United States

Amidation is among the most frequently executed reactions in pharmaceutical research and development. We have explored the feasibility of adapting amidations to plug flow reactor (PFR) process conditions for the preparation of early development compounds. Among coupling reagents possessing good thermal stability, carbodiimides and T3P have been selected, as they are readily soluble, require no preactivation, offer excellent reaction kinetics, and enable convenient product isolation. A carbodiimide/2-hydroxylpyridine oxide (HOPO) protocol was demonstrated in four case studies with homogeneous feed and reaction streams that were readily adaptable to a PFR design. In a head-to-head comparison, T3P was also found to be readily adaptable to a PFR flow process and gave comparable yields. The EDC/HOPO method works well for amidations that do not involve substrates that are highly sensitive to racemization; its water compatibility makes it the reagent of choice when the amine reactant is in a salt form, since water can be added as a cosolvent to aid solubility. For substrates that are extremely sensitive to racemization, we have shown one successful example of peptide coupling using TBTU or COMU under PFR continuous flow conditions.

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