Plant Process for the Preparation of Cinchona Alkaloid-Based Thiourea Catalysts

Asymmetric organocatalysis in drug discovery and development for active pharmaceutical ingredients

INTRODUCTION

Over the last 20 years, it has become clear that organocatalysis is the third pillar of catalysis. The low reactivity in the early days of organocatalysis has been overcome with the invention of more efficient catalysts, and by harnessing enabling technologies like continuous-flow chemistry and photo-redox catalysis.

AREAS COVERED

The main focus of this review is on the development over the last 10-15 years of key APIs using asymmetric organocatalysis. Due to significant engineering advances, and also due to the need for continuous manufacturing, flow and photo-redox approaches are becoming more widespread.

EXPERT OPINION

Over the last 20 years, organocatalysis has been used on various occasions for accessing chiral drugs. The great advantage of using these catalysts is that the final active pharmaceutical ingredient (API) is metal-free. Also due to their inherent stability in air and water, they are very amenable to recovery via attachment to appropriate solid supports and also application in continuous flow systems. In recent years, more efficient organocatalysts have been developed, which includes the photoredox types, with much potential for chiral API synthesis.

Ortho-Phosphinoarenesulfonamide-Mediated Staudinger Reduction of Aryl and Alkyl Azides

Conventional Staudinger reductions of organic azides are sluggish with aryl or bulky aliphatic azides. In addition, Staudinger reduction usually requires a large excess of water to promote the decomposition of the aza-ylide intermediate into phosphine oxide and amine products. To overcome the challenges above, we designed a novel triaryl phosphine reagent 2c with an ortho-SO₂NH₂ substituent. Herein, we report that such phosphine reagents are able to mediate the Staudinger reduction of both aryl and alkyl azides in either anhydrous or wet solvents. Good to excellent yields were obtained in all cases (even at a diluted concentration of 0.01 M). The formation of B-TAP, a cyclic aza-ylide, instead of phosphine oxide, eliminates the requirement of water in the Staudinger reduction. In addition, computational studies disclose that the intramolecular protonation of the aza-ylide by the ortho-SO₂NH₂ group is kinetically favorable and responsible for the acceleration of Staudinger reduction of the aryl azides.

Cutting edge of diphenyl phosphorazidate (DPPA) as a synthetic reagent – A fifty-year odyssey

Recent developments of diphenyl phosphorazidate (DPPA, (C6H5O)2P(O)N3) has been reviewed.

Enantioselective organocatalytic approaches to active pharmaceutical ingredients – selected industrial examples

Catalysis is, often, the preferred approach to access chiral molecules in enantioenriched form both in academia and in industry; nowadays, organocatalysis is recognised as the third pillar in asymmetric catalysis, along with bio- and metal-catalysis. Despite enormous advancements in academic research, there is a common belief that organocatalysis is not developed enough to be applicable in industry. In this review, we describe a selection of industrial routes and their R&D process for the manufacture of active pharmaceutical ingredients, highlighting how asymmetric organocatalysis brings added value to an industrial process. The thorough study of the steps, driven by economic stimuli, developed and improved chemistry that was, otherwise, believed to not be applicable in an industrial setting. The knowledge discussed in the reviewed papers will be an invaluable resource for the whole research community.

Cinchona Alkaloids-Derivatives and Applications

Major Cinchona alkaloids quinine, quinidine, cinchonine, and cinchonidine are available chiral natural compounds (chiral pool). Unlike many other natural products, these alkaloids are available in multiple diastereomeric forms which are separated on an industrial scale. The introduction discusses in short conformational equilibria, traditional separation scheme, biosynthesis, and de novo chemical syntheses. The second section concerns useful chemical applications of the alkaloids as chiral recognition agents and effective chiral catalysts. Besides the Sharpless ethers and quaternary ammonium salts (chiral PTC), the most successful bifunctional organocatalysts are based on 9-amino derivatives: thioureas and squaramides. The third section reports the main transformations of Cinchona alkaloids. This covers reactions of the 9-hydroxyl group with the retention or inversion of configuration. Specific Cinchona rearrangements enlarging [2.2.2]bicycle of quinuclidine to [3.2.2] products are connected to the 9-OH substitution. The syntheses of numerous esterification and etherification products are described, including many examples of bi-Cinchona alkaloid ethers. Further derivatives comprise 9-N-substituted compounds. The amino group is introduced via an azido function with the inversion of configuration at the stereogenic center C9. The 9-epi-amino-alkaloids provide imines, amides, imides, thioureas, and squaramides. The syntheses of 9-carbon-, 9-sulfur-, and 9-selenium-substituted derivatives are discussed. Oxidation of the hydroxyl group of any alkaloid gives ketones, which can be selectively reduced, reacted with Grignard reagents, or subjected to the Corey-Chaykovsky reaction. The alkaloids were also partially degraded by splitting C4'-C9 or N1-C8 bonds. In order to immobilize Cinchona alkaloids the transformations of the 3-vinyl group were often exploited. Finally, miscellaneous functionalizations of quinuclidine, quinoline, and examples of various metal complexes of the alkaloids are considered.

The Goldilocks Principle in Phase Labeling. Minimalist and Orthogonal Phase Tagging for Chromatography-Free Mitsunobu Reaction

An inexpensive and chromatography-free Mitsunobu methodology has been developed using low molecular weight and orthogonally phase-tagged reagents, a tert-butyl-tagged highly apolar phosphine, and a water-soluble DIAD analogue. The byproduct of the Mitsunobu reactions can be removed by sequential liquid-liquid extractions using traditional solvents such as hexanes, MeOH, water, and EtOAc. Owing to the orthogonal phase labeling, the spent reagents can be regenerated. This new variant of the Mitsunobu reaction promises to provide an alternative and complementary solution for the well-known separation problem of the Mitsunobu reaction without having to resort to expensive, large molecular weight reagents and chromatography.