Organocatalytic Michael Addition of Aldehydes to Protected 2-Amino-1-Nitroethenes: The Practical Syntheses of Oseltamivir (Tamiflu) and Substituted 3-Aminopyrrolidines

Catalytic asymmetric synthesis of 1,2-diamines

The asymmetric catalytic synthesis of 1,2-diamines has received considerable interest, especially in the last ten years, due to their presence in biologically active compounds and their applications for the development of synthetic building blocks, chiral ligands and organocatalysts. Synthetic strategies based on C-N bond-forming reactions involve mainly (a) ring opening of aziridines and azabenzonorbornadienes, (b) hydroamination of allylic amines, (c) hydroamination of enamines and (d) diamination of olefins. In the case of C-C bond-forming reactions are included (a) the aza-Mannich reaction of imino esters, imino nitriles, azlactones, isocyano acetates, and isothiocyanates with imines, (b) the aza-Henry reaction of nitroalkanes with imines, (c) imine-imine coupling reactions, and (d) reductive coupling of enamines with imines, and (e) [3+2] cycloaddition with imines. C-H bond forming reactions include hydrogenation of CN bonds and C-H amination reactions. Other catalytic methods include desymmetrization reactions of meso-diamines.

NHC-catalyzed [3 + 3] cycloaddition of α-bromoenals with nitroketene aminals or nitroketene N,S-acetals: synthesis of nitro-containing dihydropyridin-2-ones

An N-heterocyclic carbene (NHC)-catalyzed [3 + 3] cycloaddition of α-bromoenals with nitroketene aminals or nitroketene N,S-acetals has been developed. This methodology provides an efficient strategy for the construction of valuable nitro-containing heterocyclic compounds. This protocol features mild reaction conditions, easily available starting materials, broad substrate scope and easy scalability.

Total Synthesis of Polysubstituted γ-Butyrolactone Lignans (-)-Hinokinin, (-)-Bicubebin B, and (-)-Isodeoxypodophyllotoxin via Oxime Carbonate Formation

The diverse structures and profound biological activities of lignan natural products have enticed significant effort in the exploration of new methodologies for their total synthesis. We have prepared γ-butyrolactone oximes from readily available δ-nitro alcohols via Boc2O mediated cyclization. The mild conditions are compatible with a wide range of functional groups, and this methodology has been applied to the total synthesis of five lignan natural products.

Asymmetric Organocatalysis: A Survival Guide to Medicinal Chemists

Majority of drugs act by interacting with chiral counterparts, e.g., proteins, and we are, unfortunately, well-aware of how chirality can negatively impact the outcome of a therapeutic regime. The number of chiral, non-racemic drugs on the market is increasing, and it is becoming ever more important to prepare these compounds in a safe, economic, and environmentally sustainable fashion. Asymmetric organocatalysis has a long history, but it began its renaissance era only during the first years of the millennium. Since then, this field has reached an extraordinary level, as confirmed by the awarding of the 2021 Chemistry Nobel Prize. In the present review, we wish to highlight the application of organocatalysis in the synthesis of enantio-enriched molecules that may be of interest to the pharmaceutical industry and the medicinal chemistry community. We aim to discuss the different activation modes observed for organocatalysts, examining, for each of them, the generally accepted mechanisms and the most important and developed reactions, that may be useful to medicinal chemists. For each of these types of organocatalytic activations, select examples from academic and industrial applications will be disclosed during the synthesis of drugs and natural products.

Enantioselective 1,3-Dipolar Cycloaddition Using (Z)-α-Amidonitroalkenes as a Key Step to the Access to Chiral cis-3,4-Diaminopyrrolidines

The enantioselective 1,3-dipolar cycloaddition between imino esters and (Z)-nitroalkenes bearing a masked amino group in the β-position was studied using several chiral ligands and silver salts. The optimized reaction conditions were directly applied to the study of the scope of the reaction. The determination of the absolute configuration was evaluated using NMR experiments and electronic circular dichroism (ECD). The reduction and hydrolysis of both groups was performed to generate in an excellent enantiomeric ratio the corresponding cis-2,3-diaminoprolinate.

Asymmetric Synthesis of Pyrrolidines via Oxetane Desymmetrization

Asymmetric synthesis of chiral pyrrolidines bearing an all-carbon quaternary stereocenter in the 3-position remains challenging. Herein we report two efficient protocols by means of oxetane desymmetrization, featuring the use of a readily available tert-butylsulfinamide chiral auxiliary and a catalytic system with chiral phosphoric acid as the source of chirality, respectively.

Environmental Modulation of Chiral Prolinamide Catalysts for Stereodivergent Conjugate Addition

Synthetic chiral catalysts generally rely on proximal functional groups or ligands for chiral induction. Enzymes often employ environmental chirality to achieve stereoselectivity. Environmentally controlled catalysis has benefits such as size and shape selectivity but is underexplored by chemists. We here report molecularly imprinted nanoparticles (MINPs) that utilized their environmental chirality to either augment or reverse the intrinsic selectivity of a chiral prolinamide cofactor. The latter ability allowed the catalyst to produce products otherwise disfavored in the conjugate addition of aldehyde to nitroalkene. The catalysis occurred in water at room temperature and afforded γ-nitroaldehydes with excellent yields (up to 94%) and ee (>90% in most cases). Up to 25:1 syn/anti and 1:6 syn/anti ratios were achieved through a combination of catalyst-derived and environmentally enabled selectivity. The high enantioselectivity of the MINP also made it possible for racemic catalysts to perform asymmetric catalysis, with up to 80% ee for the conjugate addition.

Asymmetric Organocatalytic Syntheses of Bioactive Compounds

Background:

The total syntheses of complex natural products have evolved to include new methodologies to save time, simplifying the form to achieve these natural compounds.

Objective:

In this review, we have described the asymmetric synthesis of different natural products and biologically active compounds of the last ten years until the current day.

Results:

An asymmetric organocatalytic reaction is a key to generate stereoselectively the main structure with the required stereochemistry.

Conclusion:

Even more remarkable, the organocatalytic cascade reactions, which are carried out with high stereoselectivity, as well as a possible approximation of the organocatalysts activation with sub-strates are also described.

Chiral recognition coupled with chemometrics using boronate ensembles containing D-π-A cyanostilbenes

Two types of boronic acid-appended D-π-A cyanostilbenes were synthesized to produce chiral boronate ensembles via dehydration with tartaric acid. The aggregation-induced high sensitivity and positional effect of the CN group on the emission properties allowed for chemometrics-coupled chiral recognition.

Asymmetric organocatalysis: an enabling technology for medicinal chemistry

The efficacy and synthetic versatility of asymmetric organocatalysis have contributed enormously to the field of organic synthesis since the early 2000s. As asymmetric organocatalytic methods mature, they have extended beyond the academia and undergone scale-up for the production of chiral drugs, natural products, and enantiomerically enriched bioactive molecules. This review provides a comprehensive overview of the applications of asymmetric organocatalysis in medicinal chemistry. A general picture of asymmetric organocatalytic strategies in medicinal chemistry is firstly presented, and the specific applications of these strategies in pharmaceutical synthesis are systematically described, with a focus on the preparation of antiviral, anticancer, neuroprotective, cardiovascular, antibacterial, and antiparasitic agents, as well as several miscellaneous bioactive agents. The review concludes with a discussion of the challenges, limitations and future prospects for organocatalytic asymmetric synthesis of medicinally valuable compounds.

Rapid Enantioselective and Diastereoconvergent Hybrid Organic/Biocatalytic Entry into the Oseltamivir Core

A formal synthesis of the antiviral drug (-)-oseltamivir (Tamiflu) has been accomplished starting from m-anisic acid via a dissolving metal or electrochemical Birch reduction. The correct absolute stereochemistry is efficiently set through enzyme-catalyzed carbonyl reduction on the resultant racemic α,β-unsaturated ketone. A screen of a broad ketoreductase (KRED) library identified several that deliver the desired allylic alcohol with nearly perfect facial selectivity at the new center for each antipodal substrate, indicating that the enzyme also is able to completely override inherent diastereomeric bias in the substrate. Conversion is complete, with d-glucose serving as the terminal hydride donor (glucose dehydrogenase). For each resulting diastereomeric secondary alcohol, O/N-interconversion is then efficiently effected either by synfacial [3,3]-sigmatropic allylic imidate rearrangement or by direct, stereoinverting N-Mitsunobu chemistry. Both stereochemical outcomes have been confirmed crystallographically. The α,β-unsaturation is then introduced via an α-phenylselenylation/oxidation/pyrolysis sequence to yield the targeted (S)-N-acyl-protected 5-amino-1,3-cyclohexadiene carboxylates, key advanced intermediates for oseltamivir pioneered by Corey (N-Boc) and Trost (N-phthalamido), respectively.

Time and Pot Economy in Total Synthesis

We would all like to make or obtain the materials or products we want as soon as possible. This is human nature. This is true also for chemists in the synthesis of organic molecules. All chemists would like to make their target molecules as soon as possible, particularly when their interest is in the physical or biological properties of those molecules.As demonstrated by today's COVID-19 (SARS-CoV-2) pandemic, rapid synthesis is also crucial to enable chemists to deliver effective therapeutic agents to the community. Several concepts are currently well-accepted as important for achieving this: atom economy, step economy, and redox economy. Considering the importance of synthesizing organic molecules rapidly, I recently proposed adding the concept of time economy.In a multisep synthesis, each step has to be completed within a short period of time to make the desired molecule rapidly. The development of rapid reactions is important but also insufficient. After each step, frequent and repetitive workup operations such as quenching the reaction, extraction, separation of water and organic phases, drying the organic phase, filtration, evaporation, and purification may be required, and the time necessary for these processing operations must be taken into account. Indeed, some of the most time-consuming operations in most syntheses are the purification stages.On the other hand, one-pot reactions are processes in which several sequential reactions are conducted in a single reaction vessel, which avoids the need to purify intermediates. One-pot reactions are a useful way to shorten the total synthesis time, and the approach generally leads to an increase in the yield and a reduction in the amount of chemical waste formed. Thus, I also propose the importance of pot economy.On the basis of these concepts of time and pot economy, we have accomplished efficient syntheses of several natural products and medicines. The key to the success of these syntheses is the use of diphenylprolinol silyl ether as an effective catalyst in a one-pot reaction, in which it does not disturb the subsequent reactions. Our strategy is (1) to construct the chiral key skeletons and/or key components of natural products and medicines directly using organocatalyst-mediated one-pot reactions and (2) to conduct the subsequent transformations to the final molecules in a small number of pots utilizing the internal quench method. By means of this strategy, PGE₁ methyl ester, estradiol methyl ether, and clinprost were synthesized in three, five, and seven pots, respectively. Furthermore, (-)-oseltamivir, ABT-341, baclofen, and Corey lactone were synthesized in a single reaction vessel. Further optimization of the reactions in terms of time economy allowed (-)-oseltamivir and Corey lactone to be synthesized within 60 and 152 min, respectively. These syntheses will be highlighted as case studies. Although the organocatalyst is a key compound in this Account, pot- and time-economical syntheses can be expanded to organometallic chemistry and, indeed, to organic chemistry in general.

Amino-Catalyzed Reactions of Aldehydes with Chiral Nitroalkenes

Chiral nitroalkenes are used for the first time in Michael additions of aldehydes, catalyzed by pyrrolidine derivatives. They yield the same major stereoisomer with either (S)-proline or (R)-proline, but this asymmetric induction does not overcome the effect of sterically more congested catalysts. Nitrocyclobutane intermediates are often formed, which are more stable than those from (E)-1-nitro-2-phenylethene. The cyclobutanes and final products were characterized by 2D NMR and chemical correlations.

In Pursuit of Synthetic Efficiency: Convergent Approaches

The field of total synthesis has reached a stage in which emphasis has been increasingly focused on synthetic efficiency rather than merely achieving the synthesis of a target molecule. The pursuit of synthetic efficiency, typically represented by step count and overall yield, is a rich source of inspiration and motivation for synthetic chemists to invent innovative strategies and methods. Among them, convergent strategy has been well recognized as an effective approach to improve efficiency. This strategy generally involves coupling of fragments with similar complexity to furnish the target molecule via subsequent cyclization or late-stage functionalization. Thus, methodologies that enable effective connection of fragments are critical to devising a convergent plan. In our laboratory, convergent strategy has served as a long-standing principle for pursuing efficient synthesis during the course of planning and implementing synthetic projects. In this Account, we summarize our endeavors in the convergent synthesis of natural products over the last ten years. We show how we identify reasonable bond disconnections and employ enabling synthetic methodologies to maximize convergency, leading to the efficient syntheses of over two-dozen highly complex molecules from eight disparate families.In detail, we categorize our work into three parts based on the diverse reaction types for fragment assembly. First, we demonstrate the application of a powerful single-electron reducing agent, SmI₂, in a late-stage cyclization step, forging the polycyclic skeletons of structurally fascinating Galbulimima alkaloids and Leucosceptrum sesterterpenoids. Next, we showcase how three different types of cycloaddition reactions can simultaneously construct two challenging C-C bonds in a single step, providing concise entries to three distinct families, namely, spiroquinazoline alkaloids, gracilamine, and kaurane diterpenoids. In the third part, we describe convergent assembly of ent-kaurane diterpenoids, gelsedine-type alkaloids, and several drug molecules via employing some bifunctional synthons. To access highly oxidized ent-kaurane diterpenoids, we introduce the hallmark bicyclo[3.2.1]octane ring system at an early stage, and then execute coupling and cyclization by means of a Hoppe's homoaldol reaction and a Mukaiyama-Michael-type addition, respectively. Furthermore, we showcase how the orchestrated combination of an asymmetric Michael addition, a tandem oxidation-aldol reaction and a pinacol rearrangement can dramatically improve the efficiency in synthesizing gelsedine-type alkaloids, with nary a protecting group. Finally, to address the supply issue of several drugs, including anti-influenza drug zanamivir and antitumor agent Et-743, we exploit scalable and practical approaches to provide advantages over current routes in terms of cost, ease of execution, and efficiency.

Nucleation of Tiny Silver Nanoparticles by Using a Tetrafacial Organic Molecular Barrel: Potential Use in Visible-Light-Triggered Photocatalysis

Coordination-driven self-assembly of discrete molecular architectures of diverse shapes and sizes has been well studied in the last three decades. Use of dynamic imine bonds for designing analogous metal-free architectures has become a growing challenge recently. This article reports an organic molecular barrel (OB4^R ) as a potential template for nucleation and stabilization of very tiny (<1.5 nm) Ag nanoparticles (AgNPs). Imine bond condensation of a rigid tetra-aldehyde with a flexible diamine followed by imine-bond reduction yielded the discrete tetragonal organic barrel (OB4^R ). The presence of a molecular pocket ornamented with eight diamine moieties gives the potential for encapsulation of silver(I). The organic barrel was finally used as a molecular vessel for the controlled nucleation of silver nanoparticles (AgNPs) with fine size tuning through binding of Ag^I ions in the confined space of the barrel followed by reduction. Transmission electron microscopy (TEM) analysis of the Ag⁰ @OB4^R composite revealed that the mean particle size is 1.44±0.16 nm. The composite material has approximately 52 wt % silver loading. The barrel-supported ultrafine AgNPs [Ag⁰ @OB4^R ] are found to be an efficient photocatalyst for facile Ullmann-type aryl-amination coupling of haloarenes at ambient temperature without using any additives. The catalyst was stable for several cycles of reuse without any agglomeration. The new composite Ag⁰ @OB4^R represents the first example of discrete organic barrel-supported AgNPs employed as a photocatalyst in Ullmann-type coupling reactions at room temperature.

The evolution of Tamiflu synthesis, 20 years on: Advent of enabling technologies the last piece of the puzzle?

Influenza is a serious respiratory disease responsible for significant morbidity and mortality due to both annual epidemics and pandemics; its treatment involves the use of neuraminidase inhibitors. (-)-Oseltamivir phosphate (Tamiflu) approved in 1999, is one of the most potent oral anti-influenza neuraminidase inhibitors. Consequently, more than 70 Tamiflu synthetic procedures have been developed to date. Herein, we highlight the evolution of Tamiflu synthesis since its discovery over 20 years ago in the quest for a truly efficient, safe, cost-effective and environmentally benign synthetic procedure. We have selected a few representative routes to give a clear account of the past, present and the future with the advent of enabling technologies.

Catalytic Reductive Amination of Aldehydes and Ketones With Nitro Compounds: New Light on an Old Reaction

Reductive amination of carbonyl compounds with primary amines is a well-established synthetic methodology for the selective production of unsymmetrically substituted secondary and tertiary amines. From the industrial and green chemistry perspective, it is attractive to combine reductive amination with the synthesis of primary amines in a single one-pot catalytic process. In this regard, nitro compounds, which are readily available and inexpensive feedstocks, received much attention as convenient precursors to primary amines in such processes. Although the direct reductive coupling of nitro compounds with aldehydes/ketones to give secondary and tertiary amines has been known since the 1940's, due to the development of highly efficient and selective non-noble metal-based catalysts a breakthrough in this area was made in the last decade. In this short overview, recent progress in the methodology of the reductive amination with nitro compounds is summarized together with applications to the synthesis of bioactive amines and heterocycles. Remaining challenges in this field are also analyzed.

Domino and one-pot syntheses of biologically active compounds using diphenylprolinol silyl ether

The successful application of diphenylprolinol silyl ether, which is one of the widely used organocatalysts, to the synthesis of natural products and drugs, is described mostly focusing on the author’s results. The molecules that are explained in this paper are baclofen, telcagepant, oseltamivir, ABT-341, prostaglandins, estradiol, horsfiline and coerulescine.

Nitro compounds as the core structures of promising energetic materials and versatile reagents for organic synthesis

This review addresses some promising areas of chemistry of nitro compounds extensively developed in recent years in Russia (particularly at the N.D.Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences) and worldwide. The most important results in the synthesis of novel energetic N-, C- and O-nitro compounds are summarized. New environmentally friendly approaches to the preparation of known compounds of this series, used as components of energetic compositions, are considered. Methods for selective transformations of various nitro compounds to valuable products of organic synthesis, primarily biologically active products and their precursors, are systematically analyzed.

The bibliography includes 446 references.

Accurate chiral pattern recognition for amines from just a single chemosensor

The current work proposes a novel method for accurate pattern recognition of (mono- and di-) amines and determination of enantiomeric excess (ee) using molecular self-assembly.

Enantioselective Synthesis of Oseltamivir Phosphate (Tamiflu) via the Iron-Catalyzed Stereoselective Olefin Diazidation

We herein report a gram-scale, enantioselective synthesis of Tamiflu, in which the key trans-diamino moiety has been efficiently installed via an iron-catalyzed stereoselective olefin diazidation. This significantly improved, iron-catalyzed method is uniquely effective for highly functionalized yet electronically deactivated substrates that have been previously problematic. Preliminary catalyst structure-reactivity-stereoselectivity relationship studies revealed that both the iron catalyst and the complex substrate cooperatively modulate the stereoselectivity for diazidation. Safety assessment using both differential scanning calorimetry (DSC) and the drop weight test (DWT) has also demonstrated the feasibility of carrying out this iron-catalyzed olefin diazidation for large-scale Tamiflu synthesis.

Formal enantioselective syntheses of oseltamivir and tamiphosphor

An alternative and expedient route for the synthesis of the influenza antiviral drugs oseltamivir and tamiphosphor is described.

Stereoisomers of oseltamivir – synthesis, in silico prediction and biological evaluation

Three diastereoisomers of oseltamivir were synthesized, their properties predicted by quantum-chemical calculations and their antiviral activities evaluated.

Influenza Neuraminidase Inhibitors: Synthetic Approaches, Derivatives and Biological Activity

Despite being a common viral disease, influenza has very negative consequences, causing the death of around half a million people each year. A neuraminidase located on the surface of the virus plays an important role in viral reproduction by contributing to the release of viruses from infected host cells. The treatment of influenza is mainly based on the administration of neuraminidase inhibitors. The neuraminidase inhibitors zanamivir, laninamivir, oseltamivir and peramivir have been commercialized and have been demonstrated to be potent influenza viral neuraminidase inhibitors against most influenza strains. In order to create more potent neuraminidase inhibitors and fight against the surge in resistance resulting from naturally-occurring mutations, these anti-influenza drugs have been used as templates for the development of new neuraminidase inhibitors through structure-activity relationship studies. Here, we review the synthetic routes to these commercial drugs, the modifications which have been performed on these structures and the effects of these modifications on their inhibitory activity.

Anti-Selective Organocatalytic Michael Addition between Phenylacetaldehyde and Nitrostyrene

Using the reaction between phenylacetaldehyde and nitrostyrene catalyzed by pyrrolidine as a simple model, we have studied the diastereochemical outcome of the organocatalytic Michael reactions between benzylic aldehydes and nitrostyrenes. We found that the anti adduct was obtained in high yield and diastereoselection as was demonstrated by the X-ray structure of the product. In situ NMR studies showed a different reaction pathway when compared to aliphatic aldehydes that yield the syn adduct as major isomer.

Pot economy and one-pot synthesis

The one-pot synthesis of a target molecule in the same reaction vessel is widely considered to be an efficient approach in synthetic organic chemistry. In this review, the characteristics and limitations of various one-pot syntheses of biologically active molecules are explained, primarily involving organocatalytic methods as key tactics. Besides catalysis, the pot-economy concepts presented herein are also applicable to organometallic and organic reaction methods in general.