Organic Chemistry and Synthesis Rely More and More upon Catalysts

A few months before the COVID-19 pandemic, Pierre Vogel and Kendall N. Houk published with a new textbook Wiley-VCH, “Organic Chemistry: Theory, Reactivity, and Mechanisms in Modern Synthesis”, with a foreword from the late Roberts H. Grubbs. The book demonstrates how catalytic processes dominate all fields of modern organic chemistry and synthesis, and how invention combines thermodynamics, kinetics, spectroscopy, quantum mechanics, and thermochemical data libraries. Here, the authors present a few case studies that should be of interest to teachers, practitioners of organic and organometallic chemistry, and the engineers of molecules. The Vogel–Houk book is both textbook and reference manual; it provides a modern way to think about chemical reactivity and a powerful toolbox to inventors of new reactions and new procedures.

Synthetic Access to Aromatic α-Haloketones

α-Haloketones play an essential role in the synthesis of complex N-, S-, O-heterocycles; of which some exhibit a remarkable biological activity. Research further illustrated that α-bromo-, α-chloro-, and α-iodoketones are key precursors for blockbuster pharmacological compounds. Over the past twenty years, substantial advances have been made in the synthesis of these industrially relevant building blocks. Efforts have focused on rendering the synthetic protocols greener, more effective and versatile. In this survey, we summarised and thoroughly evaluated the progress of the field, established in the past two decades, in terms of generality, efficacy and sustainability.

Radical-Mediated Functionalization of Internal Alkenes: Synthesis of Multisubstituted Allylic and Homoallylic Azides

Radical-mediated functionalization of alkenes provides a powerful tool for transformation of simple alkenes into numerous value-added products. The precedent radical functionalization of alkenes is mainly restricted to terminal alkenes, while the conversion of internal alkenes generally remains challenging, as the increased steric congestion on alkenes significantly conflicts with the intermolecular addition of radicals. Herein, we describe an efficient photoredox catalytic functionalization of internal trisubstituted alkenes, leading to a plethora of valuable multifunctionalized allylic and homoallylic azides, which are otherwise difficult to obtain. The azide products serve as versatile feedstock for construction of useful heterocycles. Allylic or homoallylic azides are selectively generated in the transformation, regulated by the regioselective deprotonation process. This method also features mild reaction conditions and high product diversity.

Applications of Halogen-Atom Transfer (XAT) for the Generation of Carbon Radicals in Synthetic Photochemistry and Photocatalysis

The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.

Iron-Catalyzed Asymmetric Decarboxylative Azidation

The first iron-catalyzed asymmetric azidation of benzylic peresters has been reported with trimethylsilyl azide (TMSN₃) as the azido source. Hydrocarbon radicals that lack of strong interactions were capable to be enantioselectively azidated. The reaction features good functional group tolerance, high yields, and mild conditions. The chiral benzylic azides can further be used in click reaction, phosphoramidation, and reductive amination, which demonstrate the synthetic values of this reaction.

The azidosulfonylation of terminal alkynes leading to β-azidovinyl sulfones

We report herein the first example of the N₃ radical-mediated azidosulfonylation of alkynes, affording the β-azidovinyl sulfone products with a broad substrate scope, excellent functional group compatibility, and high yield. DFT calculations suggest that the mechanism of the reaction proceeds through an unprecedented sequential N₃ radical addition and sulfonyl radical coupling pathway.

Preparation of Antiproliferative Terpene-Alkaloid Hybrids by Free Radical-Mediated Modification of ent-Kauranic Derivatives

A convenient strategy for molecular editing of available ent-kauranic natural scaffolds has been developed based on radical mediated C-C bond formation. Iodine atom transfer radical addition (ATRA) followed by rapid ionic elimination and radical azidoalkylation were investigated. Both reactions involve radical addition to the exo-methylenic double bond of the parent substrate. Easy transformations of the obtained adducts lead to extended diterpenes of broad structural diversity and artificial diterpene-alkaloid hybrids possessing lactam and pyrrolidine pharmacophores. The cytotoxicity of selected diterpenic derivatives was examined by in vitro testing on several tumor cell lines. The terpene-alkaloid hybrids containing N-heterocycles with unprecedented spiro-junction have shown relevant cytotoxicity and promising selectivity indexes. These results represent a solid basis for following research on the synthesis of such derivatives based on available natural product templates.

Inhibitory activities of indolizine derivatives: a patent review

INTRODUCTION

Indolizines are structural isomers with indoles. Although several indole-based commercial drugs are available in the market, none of the indolizine-based drugs are available up-to-date. Natural and synthetic indolizines have a wide-range of pharmaceutical importance such as antitumor, antimycobacterial, antagonist, and antiproliferative activities. This prompted us to search and collect all possible data about the pharmacological importance of indolizine to open an avenue to the researchers in exploring more medicinal applications of such biologically important compounds.

AREAS COVERED

The current review article covers the advancements in the biological and pharmacological activities of indolizine-based compounds during the last decade. The covered areas of this work involved anticancer, anti-HIV-1, anti-inflammatory, antimicrobial, anti-tubercular, larvicidal, anti-schizophrenia, CRTh2 antagonist's activities in addition to enzymatic inhibitory activity.

EXPERT OPINION

The discovery of indolizine drugs will be a major breakthrough as compared with their widely available drug-containing indole isosteres. Major work collected here was focused on anticancer, anti-tubercular, anti-inflammatory, and enzymatic inhibitory activities. The SAR study of the reported biologically active indolizines is summarized throughout the review whenever highlighted to the rationale the behavior of inhibitory action. Several indolizines with certain functions provided great enhancement in the therapeutic activities comparing with reference drugs.

Two-Step Azidoalkenylation of Terminal Alkenes Using Iodomethyl Sulfones

The radical azidoalkylation of alkenes that was initially developed with α-iodoesters and α-iodoketones was extended to other activated iodomethyl derivatives. By using iodomethyl aryl sulfones, the preparation of γ-azidosulfones was easily achieved. Facile conversion of these azidosulfones to homoallylic azides using a Julia–Kocienski olefination reaction is reported, making the whole process equivalent to the azidoalkenylation of terminal alkenes.

Catalytic Enantioselective Total Synthesis of the Picrotoxane Alkaloids (-)-Dendrobine, (-)-Mubironine B, and (-)-Dendroxine

A concise enantioselective total synthesis of three sesquiterpenoid alkaloids-(-)-dendrobine, (-)-mubironine B, and (-)-dendroxine-is presented, which highlights the state-of-art catalytic methods, including enantioselective Diels-Alder cycloaddition, iron-catalyzed aerobic lactonization, copper-catalyzed cycloisomerization, and free-radical-initiated hydroazidation.

Azidoheteroarylation of unactivated olefins through distal heteroaryl migration

A novel azidoheteroarylation of unactivated olefins by means of intramolecular distal heteroaryl migration is herein reported.

Fe-Catalyzed three-component carboazidation of alkenes with alkanes and trimethylsilyl azide

Iron out the difference. Carboazidation of alkenes using cycloalkanes, CH₂Cl₂, CHCl₃ and CCl₄ as alkylating reagents proceeded smoothly in the presence of TMSN₃, DTBP and a catalytic amount of Fe(acac)₃ to afford chain extended alkyl azides and γ-azido chloroalkanes in good to high yields.

Synthesis of Aminoindolizidines through the Chemoselective and Diastereoselective Catalytic Hydrogenation of Indolizines

Indolizidines are bioactive heterocyclic compounds of great potential normally prepared following multistep routes. However, to the best of our knowledge, the synthesis of 1-aminoindolizidines has never been reported. Herein, 1-(dialkylamino)-3-substituted indolizidines have been straightforwardly synthesized using an atom-economic protocol that involves a copper-catalyzed three-component synthesis of indolizines followed by heterogeneous catalytic hydrogenation. The latter was found to be chemoselective using platinum(IV) oxide as the catalyst at 3.7 atm, providing the aminoindolizidines in modest-to-high yields (35-95%) and high diastereoselectivity (92:8 to >99:1). It has been experimentally demonstrated that the hydrogenation occurs through the intermediate 5,6,7,8-tetrahydroindolizine, which contains a pyrrole moiety. Moreover, the diastereomerically pure 1-(dibenzylamino)-3-substituted indolizidines could be further transformed into the corresponding monobenzylated or fully debenzylated aminoindolizidines by selective hydrogenolysis catalyzed by Pt/C or Pd/C, respectively, under ambient conditions.

Radicals: Reactive Intermediates with Translational Potential

This Perspective illustrates the defining characteristics of free radical chemistry, beginning with its rich and storied history. Studies from our laboratory are discussed along with recent developments emanating from others in this burgeoning area. The practicality and chemoselectivity of radical reactions enable rapid access to molecules of relevance to drug discovery, agrochemistry, material science, and other disciplines. Thus, these reactive intermediates possess inherent translational potential, as they can be widely used to expedite scientific endeavors for the betterment of humankind.

Simple Indolizidine and Quinolizidine Alkaloids

This review of simple indolizidine and quinolizidine alkaloids (i.e., those in which the parent bicyclic systems are in general not embedded in polycyclic arrays) is an update of the previous coverage in Volume 55 of this series (2001). The present survey covers the literature from mid-1999 to the end of 2013; and in addition to aspects of the isolation, characterization, and biological activity of the alkaloids, much emphasis is placed on their total synthesis. A brief introduction to the topic is followed by an overview of relevant alkaloids from fungal and microbial sources, among them slaframine, cyclizidine, Steptomyces metabolites, and the pantocins. The important iminosugar alkaloids lentiginosine, steviamine, swainsonine, castanospermine, and related hydroxyindolizidines are dealt with in the subsequent section. The fourth and fifth sections cover metabolites from terrestrial plants. Pertinent plant alkaloids bearing alkyl, functionalized alkyl or alkenyl substituents include dendroprimine, anibamine, simple alkaloids belonging to the genera Prosopis, Elaeocarpus, Lycopodium, and Poranthera, and bicyclic alkaloids of the lupin family. Plant alkaloids bearing aryl or heteroaryl substituents include ipalbidine and analogs, secophenanthroindolizidine and secophenanthroquinolizidine alkaloids (among them septicine, julandine, and analogs), ficuseptine, lasubines, and other simple quinolizidines of the Lythraceae, the simple furyl-substituted Nuphar alkaloids, and a mixed quinolizidine-quinazoline alkaloid. The penultimate section of the review deals with the sizable group of simple indolizidine and quinolizidine alkaloids isolated from, or detected in, ants, mites, and terrestrial amphibians, and includes an overview of the "dietary hypothesis" for the origin of the amphibian metabolites. The final section surveys relevant alkaloids from marine sources, and includes clathryimines and analogs, stellettamides, the clavepictines and pictamine, and bis(quinolizidine) alkaloids.

Cu- or Fe-catalyzed C–H/C–C bond nitrogenation reactions for the direct synthesis of N-containing compounds

This tutorial account summarizes the recent progress in Cu- or Fe-catalyzed C–H/C–C bond nitrogenation reactions for the direct synthesis of N-containing compounds.

Transition-metal-free oxidative carboazidation of acrylamides via cascade C–N and C–C bond-forming reactions

A novel transition-metal-free oxidative carboazidation of acrylamides using inexpensive NaN₃ and K₂S₂O₈ was achieved, which not only provided an efficient method to prepare various N₃-substituted oxindoles, but also represented a novel strategy for C–N and C–C bond formation via a free-radical cascade process.

Copper-catalyzed intermolecular azidocyanation of aryl alkenes

A copper-catalyzed Markovnikov-type intermolecular azidocyanation of aryl alkenes has been developed for the first time.

A Mn(iii)/TEMPO-co-mediated tandem azidation–1,2-carbon migration reaction of allylic silyl ethers

A novel Mn(iii)/TEMPO-mediated azidation–1,2-migration reaction was explored.

Rhodium-catalyzed hydroformylation in fused azapolycycles synthesis

N-Heterocycles, including fused ones, have proven to be an important class of compounds since they possess biological and pharmacological activities themselves and serve as valuable intermediates for synthetic drug discovery. My interest in the synthesis of these compounds stems from studies dealing with the hydroformylation (oxo) of olefins. The dihydroindolizines and benzofused ones are easily generated via rhodium-catalyzed hydroformylation of N-allylpyrroles and indoles: the butanal intermediate undergoes an intramolecular cyclodehydration giving the final polycyclic compound. This chapter reports my results in the area of the conversions of oxo aldehydes with additional C,C-bond-forming reactions together with relevant work from other laboratories on additional C,N-bond-forming reactions, encountered in the field of Azapolycycles synthesis over the last 5 years or so. The intramolecular sequences for polycylization will be especially emphasized using rhodium complexes to effect these transformations, under both conventional and microwave heating.

Convenient Ambient Temperature Generation of Sulfonyl Radicals

Presented herein is a novel method for the efficient, ambient temperature generation of sulfonyl radicals from aryl and alkyl sulfonylbromides upon autoxidation of triethylborane (Et3B). The resultant radicals were regioselectively trapped via addition to terminal alkynes, generating a secondary vinyl radical that selectively abstracts a Br atom from RSO2Br, yielding the (E)-bromo vinylsulfones. Sensitivity towards Lewis basic groups was observed, presumably due to the disruptive coordination to Et3B before atom-transfer.

Kinetic Study of the Radical Azidation with Sulfonyl Azides

Rate constants for the reaction between a secondary alkyl radical and two different sulfonyl azides were determined using bimolecular competing radical reactions. The rates of azidation were determined by competition with hydrogen atom transfer from tris(trimethylsilyl)silane ((TMS)3SiH) of the 4-phenylcyclohexyl radical. 3-Pyridinesulfonyl azide and trifluoromethanesulfonyl azide were found to have rate constants for azidation of 2 × 105 M–1 s–1 and 7 × 105 M–1 s–1 at 80°C, respectively.