Purification in multistep flow synthesis of APIs

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current batch manufacturing would inevitably have to produce pharmaceuticals in the common dosage forms of tablets and capsules as well as sterile injectable solutions, which would require advances in downstream processing. Specifically, classical unit operations of crystallization, drying, powder transport, solids blending, and tableting would have to be miniaturized and integrated.’^[137] That means the main problem is basically the connection of the synthesis with the formulation step, which could not be done unless the purity of the APIs meets the requirements of the authorities, which sometimes means purity over 99%.

In the next section, I give a brief overview of the existing continuous-flow purification methods and their limitations.

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In-line work-up can be achieved by filtration of co-products, liquid-liquid phase separation (LLPS), gas-liquid phase separation (GLPS) or by the use of solid phase supported scavengers (SPSS).

Filtration can only be categorized as a truly continuous in-line purification method, when the product remains in the mother liquor (Fig 2.26.), while the solid impurities are retained on the filter material and then discarded by an appropriate mechanism.^[146]

Figure 2.26. In-line work-up by filtration of solid co-products (BPR: back pressure regulator).

There are several examples of extraction methods using LLPS,^[147,148] to eliminate the excess of the reagents,^[149,150] co-products,^[150] traces of solvents^[151,152] or co-solvents,^[153]

which have been used in a previous step. Complete removal of water may require a column filled with solid MgSO₄ desiccant after the phase separation, as exemplified in the flow preparation of milnacipran analogs.^[151] The robust nature of LLPS allows removal of multiple impurities at the same time.[137,152–155]

Insoluble gases cause irreproducible residence time (due to the expansion of bubbles) and potential side reactions in downstream steps. Excess of gaseous reagents or co-products can be removed conveniently by using GLPS units. Hydrogen is commonly used in flow synthesis,^[76,93] usually in excess amounts. When hydrogenation is followed by other steps,[153,156–160]

a simple buffer flask allows the outgassing of hydrogen, after the pressure is reduced. However, in some cases the applied chemistry did not tolerate this method.

Semipermeable Teflon^® AF2400 membrane (Fig 2.27.) was used to remove excess of diazomethane as well as the formed nitrogen.^[161] A similar set-up was employed to eliminate excess CO2 in the lithiation sequence leading to the core of amitriptyline.^[162]

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Figure 2.27. In-line work-up by gas-liquid phase separation using Teflon^® AF2400 membrane (BPR: back pressure regulator).

Columns filled with SPSS resins are widely used for purification of the stream.^[163,164]

The combination of multiple, differently functionalized scavenger resins is a common practice for the removal of several different impurities.[158,159,165–169]

In an illustrative example,^[155,163]

removal of homogeneous transition metal complexes was possible this way (Fig. 2.28.), which prevented harmful catalytic activity in downstream heterogeneous nitro group reductions in the flow synthesis of the fungicide Boscalid^®.^[163]

Figure 2.28. Application of polymer bound thiourea (QuadraPure^TM TU) scavenger resin to remove homogeneous transition metal catalyst before the next catalytic step in the flow synthesis of Boscalid^®.

A similar concept was applied in the early steps towards olanzapine.^[155] The drawbacks of this method are the wide dispersion of reactants on the column^[170] and the exhaustion of the resin by time, principally when the stoichiometric amounts of the immobilized reagent is

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consumed. These issues can be solved by engineering solutions, such as intelligent pumping^[170] and switching between multiple columns.^[171]

2.4.2. Continuous-flow final product purification

After the final synthetic step, the API has to be purified to meet the standards of regulatory agencies, which sometimes means purity over 99%. The hitherto discussed separation techniques, which could be ideally used between intermediate steps, are not adequate for this goal by themselves. Final products can be conveniently purified using simulated moving bed (SMB) chromatography,^[172,173] crystallization[71,137,174,175]

or recrystallization^[137], although the latter two usually require semi-batch processing.

Continuous crystallization was employed in case of rufinamide^[174] and aliskiren,^[71,141]

while semi-batch recrystallization of the crude products was necessary to provide pure diphenhydramine,^[137] fluoxetine^[137] and lidocaine.^[137,176] Purification of the final product can also be aided by catch and release chromatographic methods[169,177–179]

or salt formation - neutralization sequence using multiple extraction steps.^[137] A salt formation – neutralization sequence using multiple extraction steps, followed by precipitation and recrystallization yielded highly pure diazepam.^[137]

Catch and release chromatography^[156] enables stepwise introduction of a continuous stream of the reaction mixture to a single bed of stationary phase, followed by elution of the product. High purity can be achieved, as it was demonstrated in case of iloperidone^[177] and flurbiprofen.^[178,179] However, the capacity of the employed stationary phase determines the amount of the adsorbed product during the ‘catch’ phase and thus limits the achievable throughput.

High purity standards in API production may require the combination of these techniques, as demonstrated in the case of artemisinin.^[180,181] The complexity of artemisinin- derived medicinal compounds call for sophisticated systems consisting of the combination of in-line work-up and multiple final purification techniques. A continuous three-stage purification method was developed for α-artesunate, including of precipitation and filtration of by-products, followed by chromatography in a multi-column arrangement and finally crystallization of the product.^[181] In a more recent study, a closely related system was constructed, based on continuously operated simulated moving bed (SMB) chromatography (Fig 2.29.).^[172,173] Following the synthesis of artemisinin, first an SMB chromatographic

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separation brought the product mixture to 92% purity, then crystallization from the concentrated mixture provided the API in a purity of 99.9%.^[180]

Figure 2.29. General scheme of a continuously operated simulated moving bed (SMB) chromatography system integrated with crystallization for the purification of artemisinin following its continuous flow synthesis. The circular arrow marks the simulated counterclockwise movement of the bed (this is achieved by clockwise switching of the inlet and outlet ports).

With this knowledge in hand, multistep processes can be designed to facilitate the purification procedure between the reaction steps, which was demonstrated in the process leading to efavirenz,^[182] in which the appropriate choice of the trifluoroacetylating reagent allowed easy removal of the co-product by a simple scavenging column (Fig. 2.30.).

Figure 2.30. Design of the flow chemical synthetic procedure towards Efavirenz, which facilitates in-line purification.

46 2.4.3. Summary of continuous-flow purification

To sum up, the chemical knowledge and technology for the realisation of multistep flow syntheses and subsequent purification exist, but connecting individual chemical steps holds hidden traps.The utility of the available continuous purification methods (Table 2.10.) can be maximized by a proper design of the synthetic process; however, we hope that our proposal for a novel purification method of using centrifugal partition chromatography will be given wide applicability for the synthesis of complex molecules and APIs even on large scale, due to its advantages that are presented in the last chapter of the introduction.

Table 2.10. Summary of purification methods in flow.

In-line workup Final product purification

’Co-’

product type

1. Side-product filtration 2. Liquid-liquid phase separation

3. Gas-Liquid phase separation