Formal Total Synthesis of the Potent Renin Inhibitor Aliskiren: Application of a SmI2-Promoted Acyl-like Radical Coupling

Growing Utilization of Radical Chemistry in the Synthesis of Pharmaceuticals

Our current unhealthy lifestyle and the exponential surge in the population getting affected by a variety of diseases have made pharmaceuticals or drugs an imperative part of life, making the development of innovative strategies for drug discovery or the introduction of refined, cost-effective and modern technologies for the synthesis of clinically used drugs, a need of the hour. Ever since their discovery, free radicals and radical cations or anions as reactive intermediates have captivated the chemists, resulting in an exceptional utilization of these moieties throughout the field of chemical synthesis, owing to their unprecedented and widespread reactivity. Sticking with the idea of not judging the book by its cover, despite the conventional thought process of radicals being unstable and difficult to control entities, scientists and academicians around the globe have done an appreciable amount of work utilizing both persistent as well as transient radicals for a variety of organic transformations, exemplifying them with the synthesis of significant biologically active pharmaceutical ingredients. This review truly accounts for the organic radical transformations including radical addition, radical cascade cyclization, radical/radical cross-coupling, coupling with metal-complexes and radical cations coupling with nucleophiles, that offers fascinating and unconventional approaches towards the construction of intricate structural frameworks of marketed APIs with high atom- and step-economy; complementing the otherwise employed traditional methods. This tutorial review presents a comprehensive package of diverse methods utilized for radical generation, featuring their reactivity to form critical bonds in pharmaceutical total synthesis or in building key starting materials or intermediates of their synthetic journey, acknowledging their excellence, downsides and underlying mechanisms, which are otherwise poorly highlighted in the literature. Despite great achievements over the past few decades in this area, many challenges and obstacles are yet to be unraveled to shorten the distance between the academics and the industry, which are all discussed in summary and outlook.

Metal-Free Synthesis of N-Aryl Amides using Organocatalytic Ring-Opening Aminolysis of Lactones

Catalytic ring-opening of bio-sourced non-strained lactones with aromatic amines can offer a straightforward, 100 % atom-economical, and sustainable pathway towards relevant N-aryl amide scaffolds. Herein, the first general, metal-free, and highly efficient N-aryl amide formation is reported from poorly reactive aromatic amines and non-strained lactones under mild operating conditions using an organic bicyclic guanidine catalyst. This protocol has high application potential as exemplified by the formal syntheses of drug-relevant molecules.

Discovery of highly potent renin inhibitors potentially interacting with the S3' subsite of renin

To exploit the S3' subsite of renin active site for renin inhibitor design, 42 aliskiren derivatives with modified P2' portion were designed, synthesized and biologically evaluated. Some highly potent renin inhibitors (IC₅₀ < 3 nM) were identified, among which compounds 38 (IC₅₀ = 0.9 nM) and 39 (IC₅₀ = 0.7 nM) were over 2.5-fold more potent than aliskiren (IC₅₀ = 2.3 nM). SAR analysis indicated that incorporation of polar hydrophilic moieties into the P2' portion of renin inhibitors generally enhanced the potency. Consistently with this, molecular modeling study revealed that the triazole part of 39 could provide additional interactions to the S3' subsite of renin active site. Moreover, in vivo evaluation in the double transgenic mouse hypertension model demonstrated that 39 produced greater reduction of the mean arterial blood pressure than ariskiren at the doses of 17.0 and 34.0 μmol/kg, respectively. Taken together, the S3' subsite of renin active site merits further consideration for renin inhibitor design.

The development of a complementary pathway for the synthesis of aliskiren

We present an alternative pathway for the synthesis of aliskiren by applying the Evans chiral auxiliary-aided asymmetric allylation and the Julia–Kocienski olefination as the key transformations.

A new approach to the synthesis of peptidomimetic renin inhibitors: palladium-catalyzed asymmetric allylation of acyclic alkyl aryl ketones

A new approach to the synthesis of Tekturna, a recently marketed drug for hypertension, takes advantage of a modified protocol of the Stoltz palladium-catalyzed asymmetric allylation with a t-BuPHOX ligand for the synthesis of allylated acyclic alkyl aryl ketones. The method led to an α-isopropyl α-allyl aryl ketone in 90% yield and 88 to 91% ee, which was used in the synthesis of an advanced intermediate toward Tekturna. A beneficial effect of protic additives, such as BHT (2,6-di-tert-butyl-p-cresol), on the time and enantioselectivity of the reaction was discovered.

Total synthesis of "aliskiren": the first Renin inhibitor in clinical practice for hypertension

We report a "macrocycle route" toward aliskiren, a drug presently marketed for the treatment of hypertension, using a highly stereocontrolled approach starting from a common "isopropyl chiron". Highlights of the synthesis include a challenging RCM reaction to produce a nine-membered unsaturated lactone, a highly stereoselective catalytic Du Bois aziridination, and a regio- and diastereoselective aziridine ring-opening to a vicinal amino alcohol.

Samarium diiodide mediated reactions in total synthesis

Introduced by Henri Kagan more than three decades ago, samarium diiodide (SmI(2)) has found increasing application in chemical synthesis. This single-electron reducing agent has been particularly useful in C-C bond formations, including those found in total synthesis endeavors. This Review highlights selected applications of SmI(2) in total synthesis, with special emphasis on novel transformations and mechanistic considerations. The examples discussed are both illustrative of the power of this reagent in the construction of complex molecules and inspirational for the design of synthetic strategies toward such targets, both natural and designed.

Expanding the Scope of the Acyl-Type Radical Addition Reactions Promoted by SmI2

N-acyl oxazolidinones of simple carboxylic acids and amino acids were observed to undergo successful SmI2-promoted couplings with substituted acrylamides and acrylates, affording a variety of functionalized gamma-ketoamides and -esters with yields attaining 85%. As many of these reductive couplings were previously found to be ineffective employing the corresponding 4-pyridylthio esters, the applicability of this methodology has been substantially improved. The methodology has been adapted to prepare structures related to two potent aspartate protease inhibitors, the renin inhibitor aliskiren, and the gamma-secretase inhibitor L-685,458. Finally, a convenient two-step procedure for the preparation of N-acyl oxazolidinones of N-protected amino acids, which provides consistently good yields of the corresponding imide, has been devised.

Creating carbon–carbon bonds with samarium diiodide for the synthesis of modified amino acids and peptides

In this perspective, an overview of our experiences on the application of samarium diiodide in organic synthesis for the preparation of amino acid and peptide analogues is presented. Three different carbon-carbon bond forming reactions are discussed, including side chain introductions, gamma-amino acid synthesis and acyl-like radical additions for the construction of C-C mimics of the peptidic bonds.