Stefan Abele

2-Imino-thiazolidin-4-one Derivatives as Potent, Orally Active S1P1 Receptor Agonists

Sphingosine-1-phosphate (S1P) is a widespread lysophospholipid which displays a wealth of biological effects. Extracellular S1P conveys its activity through five specific G-protein coupled receptors numbered S1P(1) through S1P(5). Agonists of the S1P(1) receptor block the egress of T-lymphocytes from thymus and lymphoid organs and hold promise for the oral treatment of autoimmune disorders. Here, we report on the discovery and detailed structure-activity relationships of a novel class of S1P(1) receptor agonists based on the 2-imino-thiazolidin-4-one scaffold. Compound 8bo (ACT-128800) emerged from this series and is a potent, selective, and orally active S1P(1) receptor agonist selected for clinical development. In the rat, maximal reduction of circulating lymphocytes was reached at a dose of 3 mg/kg. The duration of lymphocyte sequestration was dose dependent. At a dose of 100 mg/kg, the effect on lymphocyte counts was fully reversible within less than 36 h. Pharmacokinetic investigation of 8bo in beagle dogs suggests that the compound is suitable for once daily dosing in humans.

Industrial Applications of the Diels–Alder Reaction

The Diels-Alder reaction is one of the most popular transformations for organic chemists to generate molecular complexity efficiently. Surprisingly, little is known about its industrial application for the synthesis of pharmacologically active ingredients, agrochemicals, and flavors and fragrances. This Review highlights selected examples, with a focus on large-scale applications (>1 kg) from a process research and development perspective.

Scalable Synthesis of Enantiomerically Pure Bicyclo[2.2.2]octadiene Ligands

An operationally simple and scalable synthesis of enantiomerically pure bicyclo[2.2.2]octadiene (bod*) ligands relying on an organocatalytic one-pot Michael addition-aldol reaction with cheap 2-cyclohexenone and phenylacetaldehyde is presented. The crystalline bicyclic product 4a (6-hydroxy-5-phenylbicyclo[2.2.2]octan-2-one) is transformed into phenylbicyclo[2.2.2]oct-5-en-2-one 2, a versatile starting material for the 2-step synthesis of both symmetrical, such as Hayashis Ph-bod* ligand, as well as novel unsymmetrical chiral dienes.

Novel S1P1 Receptor Agonists − Part 3: From Thiophenes to Pyridines

In preceding communications we summarized our medicinal chemistry efforts leading to the identification of potent, selective, and orally active S1P1 agonists such as the thiophene derivative 1. As a continuation of these efforts, we replaced the thiophene in 1 by a 2-, 3-, or 4-pyridine and obtained less lipophilic, potent, and selective S1P1 agonists (e.g., 2) efficiently reducing blood lymphocyte count in the rat. Structural features influencing the compounds receptor affinity profile and pharmacokinetics are discussed. In addition, the ability to penetrate brain tissue has been studied for several compounds. As a typical example for these pyridine based S1P1 agonists, compound 53 showed EC50 values of 0.6 and 352 nM for the S1P1 and S1P3 receptor, respectively, displayed favorable PK properties, and penetrated well into brain tissue. In the rat, compound 53 maximally reduced the blood lymphocyte count for at least 24 h after oral dosing of 3 mg/kg.

Novel S1P1 Receptor Agonists – Part 1: From Pyrazoles to Thiophenes

From a high-throughput screening campaign aiming at the identification of novel S1P1 receptor agonists, the pyrazole derivative 2 emerged as a hit structure. Medicinal chemistry efforts focused not only on improving the potency of the compound but in particular also on resolving its inherent instability issue. This led to the discovery of novel bicyclo[3.1.0]hexane fused thiophene derivatives. Compounds with high affinity and selectivity for S1P1 efficiently reducing the blood lymphocyte count in the rat were identified. For instance, compound 85 showed EC50 values of 7 and 2880 nM on S1P1 and S1P3, respectively, had favorable pharmacokinetic properties in rat and dog, distributed well into brain tissue, and efficiently and dose dependently reduced the blood lymphocyte count in the rat. After oral administration to spontaneously hypertensive rats, the S1P1 selective compound 85 showed no effect on mean arterial blood pressure and affected the heart rate during the wake phase of the animals only.

A Practical Synthesis of Renin Inhibitor MK-1597 (ACT-178882) via Catalytic Enantioselective Hydrogenation and Epimerization of Piperidine Intermediate

A practical enantioselective synthesis of renin inhibitor MK-1597 (ACT-178882), a potential new treatment for hypertension, is described. The synthetic route provided MK-1597 in nine steps and 29% overall yield from commercially available p-cresol (7). The key features of this sequence include a catalytic asymmetric hydrogenation of a tetrasubstituted ene-ester, a highly efficient epimerization/saponification sequence of 4 which sets both stereocenters of the molecule, and a short synthesis of amine fragment 2.

Discovery of Highly Potent Dual Orexin Receptor Antagonists via a Scaffold-Hopping Approach.

Starting from suvorexant (trade name Belsomra), we successfully identified interesting templates leading to potent dual orexin receptor antagonists (DORAs) via a scaffold‐hopping approach. Structure–activity relationship optimization allowed us not only to improve the antagonistic potency on both orexinu20051 and orexinu20052 receptors (Ox1 and Ox2, respectively), but also to increase metabolic stability in human liver microsomes (HLM), decrease time‐dependent inhibition of cytochrome P450 (CYP) 3A4, and decrease P‐glycoprotein (Pgp)‐mediated efflux. Compound 80u2009c [{(1S,6R)‐3‐(6,7‐difluoroquinoxalin‐2‐yl)‐3,8‐diazabicyclo[4.2.0]octan‐8‐yl}(4‐methyl‐[1,1′‐biphenyl]‐2‐yl)methanone] is a potent and selective DORA that inhibits the stimulating effects of orexin peptides OXA and OXB at both Ox1 and Ox2. In calcium‐release assays, 80u2009c was found to exhibit an insurmountable antagonistic profile at both Ox1 and Ox2, while displaying a sleep‐promoting effect in rat and dog models, similar to that of the benchmark compound suvorexant.

One-Pot Synthesis of Trifluoromethylated Quinazolin-4(3H)-ones with Trifluoroacetic Acid as CF3 Source

A novel and convenient one-pot sequential cascade method for the preparation of 2-trifluoromethylquinazolin-4(3 H)-ones is described. Trifluoroacetic acid (TFA) was employed as inexpensive and readily available CF3 source, which in the presence of T3P was condensed with a variety of anthranilic acids and amines to provide the products in up to 75% yield. The protocol was proved to be robust on 80 g scale, and the synthetic versatility of the prepared quinazolinon-4-ones was demonstrated by derivatization to further useful building blocks.

Daring the Challenge and Thinking Big: The Value of Early Process R&D.

The production of the L/T channel blocker ACT-280778 required the enantiomerically pure 5-phenylbicyclo[2.2.2]oct-5-en-2-one (1) as key building block. As the published routes towards 1 are very low yielding (<0.5% yield) and comprise many steps that are not acceptable for scale-up, a series of processes to 1 was developed to match the increasing requirements from first kg-batches to clinical supplies. The three routes are characterized by an individual asset. (1) The first route contains a scale-up of a Diels-Alder reaction with highly reactive reagents and afforded 90 kg enantiomerically pure 1. To mitigate safety risks, a flow reactor was developed for the high-temperature Diels-Alder reaction. This route relied on an efficient enantiomer separation on a ¼-ton scale by HPLC. (2) A Crystallization Induced Diastereomer Transformation (CIDT) during an intramolecular aldol reaction was the pivotal step of a first enantioselective route that starts with the Shibasaki reaction. (3) The 2(nd) enantioselective route represents a rare example of organocatalysis on scale and allowed to skip six out of nine steps with a significant impact on the cost of goods. This simple way to 1 opened up a short synthesis of Hayashis chiral diene ligands (bod*) that were so far lacking an affordable access. Some of these novel C1-symmetrical dienes have shown very high enantioselectivities in Rh-catalyzed additions of arylboronates.The production of the L/T channel blocker ACT-280778 required the enantiomerically pure 5-phenylbicyclo[2.2.2]oct-5-en-2-one (1) as key building block. As the published routes towards 1 are very low yielding (<0.5% yield) and comprise many steps that are not acceptable for scale-up, a series of processes to 1 was developed to match the increasing requirements from first kg-batches to clinical supplies. The three routes are characterized by an individual asset. (1) The first route contains a scale-up of a Diels-Alder reaction with highly reactive reagents and afforded 90 kg enantiomerically pure 1. To mitigate safety risks, a flow reactor was developed for the high-temperature Diels-Alder reaction. This route relied on an efficient enantiomer separation on a ¼-ton scale by HPLC. (2) A Crystallization Induced Diastereomer Transformation (CIDT) during an intramolecular aldol reaction was the pivotal step of a first enantioselective route that starts with the Shibasaki reaction. (3) The 2nd enantioselective route represents a rare example of organocatalysis on scale and allowed to skip six out of nine steps with a significant impact on the cost of goods. This simple way to 1 opened up a short synthesis of Hayashis chiral diene ligands (bod*) that were so far lacking an affordable access. Some of these novel C1-symmetrical dienes have shown very high enantioselectivities in Rh-catalyzed additions of arylboronates.