A process using an engineered phenylalanine ammonia lyase (PAL) enzyme was developed as part of an alternative route to a key intermediate of olodanrigan (EMA401). In the first part of this report, the detailed results from a screening for the optimal reaction conditions are presented, followed by a discussion of several workup strategies investigated. In the PAL-catalyzed reaction, 70−80% conversion of a cinnamic acid derivative to the corresponding phenylalanine derivative could be achieved. The phenylalanine derivative was subsequently telescoped to a Pictet−Spengler reaction with formaldehyde, and the corresponding tetrahydroisoquinoline derivative was isolated in 60−70% yield with >99.9:0.1 er. On the basis of our screenings, carbonate/ carbamate-buffered ammonia at an NH 3 concentration of 9−10 M and pH 9.5−10.5 was found to be optimal. Enzyme loadings down to 2.5 wt % (E:S = 1:40 w/w) could be achieved, and substrate concentrations between 3−9 v/w (1.17−0.39 M) were found to be compatible with the reaction conditions. A temperature gradient was applied in the final process: a pre-equilibrium was established at 45 °C, before making use of the temperature dependence of the entropy term with subsequent cooling to 20 °C to achieve maximum conversion. This temperature gradient also allowed balancing of the enzyme stability (low at 45 °C, high at 20 °C) with the activity (high at 45 °C, low at 20 °C) in order to achieve optimal conversion (low at 45 °C, high at 20 °C). From the various workup operations investigated, a sequence consisting of denaturation of the enzyme, NH 3 /CO 2 removal by distillation, acidification, and telescoping to the subsequent Pictet−Spengler cyclization was our preferred approach. The process presented in this study is a more sustainable, shorter, and more cost-effective alternative to the previous process.
We present the enantioselective synthesis of sodium (3S)-5-(benzyloxy)-2-(diphenylacetyl)-6-methoxy-1,2,3,4-tetrahydroisoquinoline-3-carboxylate (EMA401, olodanrigan), an angiotensin II type 2 antagonist. The manuscript features the process optimizations of the end game used for late phase clinical supplies, an overview of synthetic strategies identified in a route scouting exercise to a key intermediate phenylalanine derivative, and the analytical control strategy of the potentially formed highly toxic impurity bis(chloromethyl) ether (BCME). Starting from the phenylalanine derivative, we describe the optimizations of the end game from early phase to late phase processes with consequent improvements in the PMI factor. This sequence includes a Pictet–Spengler cyclization and an amide coupling as the last bond-forming steps, and the manufacturing process was successfully implemented on a 175 kg scale in a pilot plant setup. The modified process conditions eliminated one step by in situ activation of the carboxylic acid, avoided the REACH listed solvent DMF, and resulted in a PMI improvement by a factor of 3. In the final crystallization, a new, thermodynamically more stable modification of the drug substance was found in the complex solid-state landscape of EMA401 during an extensive polymorph screening. A process suitable for large-scale production was developed to prepare the new polymorph, avoiding the need of any special equipment such as fluidized bed drying required in the early phase process. In the second section, some of the synthetic approaches investigated for the route scouting of the phenylalanine derivative key intermediate are presented. To conclude, we discuss the analytical control strategy for BCME, the formation of which, due to the simultaneous presence of HCl and CH2O in the Pictet–Spengler cyclization, could not be ruled out. The BCME purge factor calculations using the tools of ICH M7 control option 4 are compared to actual results from spiking experiments.
During route scouting for EMA401 (1), an angiotensin II type 2 antagonist, we identified the synthesis of key amino acid intermediate 2 via its cinnamic acid derivative 3 as a streamlined option. In general, cinnamic acids can be synthesized from the corresponding aldehydes by a Knoevenagel−Doebner condensation in pyridine with piperidine as an organocatalyst. We aimed to replace both of these reagents and found novel conditions involving toluene as the solvent and morpholine as the organocatalyst. Scale-up of the process allowed the production of 25 kg of cinnamic acid 3 that was of the quality required for process development of the subsequent phenylalanine ammonia lyase-catalyzed step. The modified conditions were found to be widely applicable to alternative aldehydes and thus are of relevance to practitioners of chemical scale-up.
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