Continuous flow Aza-Michael reaction for preparing the fast-acting synthetic opioid drug Remifentanil

Abstract Remifentanil is a modern fentanyl analogue with ultrashort-action granted by an esterase-labile methyl propanoate chain. Here, we present the development of a continuous flow methodology for the key N-alkylation step of remifentanil preparation in a biphasic, “slug-flow” regime. We screened parameters under microwave-assisted reactions, translated conditions to flow settings, and obtained remifentanil under 15-min residence time in a 1-mL microreactor, with a space-time yield of 89 mg/mL·h and 94% yield.


INTRODUCTION
Remifentanil (Figure 1, structure 2) is a fast-acting synthetic µ-opioid that can provide rapid onset and offset of anesthetic effects (Bürkle, Dunbar, Van Aken, 1996).This is important for the clinical management of pain, especially during preoperative analgesia and mechanical ventilation of intubated patients.This drug and fentanyl are estimated to have equivalent potency (Figure 1, structure 1), but, like other µ-opioids, remifentanil causes relevant side effects such as bradycardia and respiratory depression.Nevertheless, thanks to its fast in-vivo metabolization, remifentanil offers patients much faster recovery times, a feature that may arguably diminish its potential for abuse.Remifentanil has similar structure to carfentanil (Figure 1, structure 3), one of the most potent fentanyl analogues designed by Janssen in the 1960s (Janssen, Gardocki, 1964;Raffa et al., 2018).However, unlike carfentanil, remifentanil, as devised by Feldman et al. at Glaxo Inc. in the 1990s, bears a methyl propanoate chain instead of an alkylaryl moiety at the nitrogen of the piperidine ring (Feldman, 2020), so it is rapidly metabolized by nonspecific esterases in the bloodstream and tissues and converted to the inactive remifentanil acid.
Continuous flow technologies have gained growing attention from organic and medicinal chemists and from the fine chemistry industry and producers of active pharmaceutical ingredients (APIs) (de Souza et al., 2018;Porta, Benaglia, Puglisi, 2016;Aguillon et al., 2020;Murie et al., 2021).Continuous flow techniques have allowed chemists to develop up-scalable setups aimed at producing important APIs and natural products on demand (Adamo et al., 2016;Pastre, Browne, Ley, 2013), promoting faster processes with lower footprint, especially during the multistep preparation of bioactive compounds (Bloemendal et al., 2020).Regarding our studies on the synthesis, stability, and metabolism of important analgesics belonging to the class of fentanyl opioids, we have been searching for an innovative methodology to convert norcarfentanil hydrochloride (Figure 2, structure 4) to remifentanil through an aza-Michael addition reaction carried out in flow in a multiphase solvent system.In this work, we have developed a continuous flow methodology that focuses on a key step of remifentanil preparation.The methodology allowed highly pure API to be prepared from norcarfentanil in a biphasic, "slug-flow" regime.

RESULTS AND DISCUSSION
During our search for a novel and efficient methodology for preparing fentanyl analogues, we investigated reaction conditions under microwave irradiation that could be the translated to continuous flow settings, in what has been called a "microwave to flow" investigative approach (Glasnov, Kappe, 2011).An important aspect of continuous flow methodologies is that, while in flow, reactions are carried out in microchannels, which can greatly enhance mass and heat transfer processes (Kockmann et al., 2008;Mandrelli et al., 2017).However, this limits flow reactions conducted in homogeneous media: the presence, or formation, of solids in the reaction mixture likely results in setup clogging and failure (Pieber, Gilmore, Seeberger, 2017).Therefore, a sound strategy when developing preparation methods in continuous flow is to screen reaction conditions in conventional batch settings and searching for optimal combinations of solvent systems, reagent concentrations, and heating that could then be employed in a flow setup.To accomplish this, we began our investigation by reacting a simple model substrate, 4-piperidinone, with different equivalents of methyl acrylate in the presence of inorganic bases, in various combinations of solvent systems.We carried out this reaction, an example of an aza-Michael addition, in batch under reflux for several hours, to achieve full conversion.This procedure can be greatly accelerated if conducted under microwave irradiation.Thus, after screening for the reaction conditions, we found that 4-piperidone hydrochloride could be reacted with methyl acrylate in a water/THF biphasic system, in the presence of two equiv. of a base, in a biphasic solvent system under microwave irradiation ranging from 50 to 75 °C, to obtain 76% isolated yield.It is important to mention that, in batch processes of greater volumes, the presence of biphasic systems can be highly detrimental to reaction yields because contact between reactants in different phases is limited, which often requires the use of phase transfer catalysts (PTCs), i.e. surfactants (Malet-Sanz, Susanne, 2012).Under flow, however, biphasic solvent systems result in a "slug-flow" regime, which can greatly increase the contact surface between phases and enhance such reactions (Figure 1, C).Therefore, we proposed that this type of reaction could be successfully performed in flow in the absence of a PTC.After we established the reaction conditions under microwave heating and successfully accomplished the alkylation of our model substrate, we devised a dualchannel setup to investigate the aza-Michael reaction under flow conditions for application in remifentanil preparation.With this setup, we pumped a stock aqueous solution of norcarfentanil hydrochloride (0.125 M) and K 2 CO 3 (0.250 M) and a stock THF solution of methyl acrylate (0.15 M) through dedicated channels.After Under a residence time of only 20 min, we achieved an isolated yield of 94% for remifentanil.Under this condition, we obtained a calculated STY of 67 mg/mL•h (Figure 2, entry 5).We were able to achieve an almost fivefold STY of 319 mg/mL•h under shorter residence times (Figure 2, entry 1).Nevertheless, reaction yields under these conditions were considerably lower, an aspect that may have to be accounted for, especially regarding the overall E-factor of the process if upscaling is desired (Dallinger, Kappe, 2017;Sheldon, 2017).These results prompted us to explore how a flow system comprised of three channels, dedicated to stock solutions with higher concentrations, could improve reaction outcomes.Thus, we devised a new setting, equipped with a 4-mL stainless-steel coil reactor heated to 75 °C (Figure 3).meeting in a regular T-Union, the reaction mixture was pumped through a glass microchip reactor (1-mL internal volume), heated to 75 °C.We attached a 75-psi rated spring BPR at the end of the system to prevent the reaction mixture from boiling (Figure 2, top diagram).We set the flow rates of the system with a view to evaluating total residence times ranging from 2.5 to 20 min.At each flow rate, we allowed sufficient system runtime (>1.3x residence times) so that steady-system states would be achieved.Then, we collected reaction volumes and neutralized and extracted them to evaluate outcomes after workup.FAPESP-Grants 16/12718-1, 17/50188-7, 22/05327-7, Conselho Nacional de Desenvolvimento e Tecnológico (CNPq), and Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES).With this setup, we configured the combined flow rate of the three pumps so that we would achieve a residence time of 20 min, and that stock solutions with fourfold reagent concentrations could be used without any pump stall.This allowed us to achieve STY of 112 mg/mL•h for remifentanil, with an isolated yield of 80% (Figure 3).
In summary, we have developed an efficient flowbased methodology that allows high-purity remifentanil to be continuously prepared from norcarfentanil hydrochloride.The use of continuous flow reactors in a biphasic "slug-flow" regime enables precise control of the aza-Michael reaction, providing faster and more efficient production of the API when compared to the corresponding batch process.The scope of the developed methodology and its application toward the synthesis of other important opioid drugs is currently being investigated in our laboratories.

FIGURE 1 -
FIGURE 1 -A: structures of fentanyl, remifentanil, and carfentanil.B: a glass chip microreactor with a slug-flow reaction being carried out.C: diagram of a slug-flow regime.

FIGURE 2 -
FIGURE 2 -Flow setup diagram for the aza-Michael reaction for preparing remifentanil and space-time yields under different reaction residence times.

FIGURE 3 -
FIGURE 3 -Continuous flow setup scheme for preparing remifentanil.