The Emergence and Evolution of Organic Synthesis and Why It is Important to Sustain It as an Advancing Art and Science for Its Own Sake

Revolutions in Chemistry: Assessment of Six 20th Century Candidates (The Instrumental Revolution; Hückel Molecular Orbital Theory; Hückel's 4n + 2 Rule; the Woodward-Hoffmann Rules; Quantum Chemistry; and Retrosynthetic Analysis)

Six 20th century candidates for revolutions in chemistry are examined, using a definitional scheme published recently by the author. Six groupings of 13 characteristics of revolutions in science are considered: causes and birthings of revolutions, relationships between the old and the new, conceptual qualities of the candidate revolutions, instrumental and methodological functions, social construction of knowledge and practical considerations, and testimonials. The Instrumental Revolution was judged to be a revolution in chemistry because of the enormous increase in community-wide knowledge provided by the new instruments and the intentionality in the identification of specific target instruments, in the mindfulness in their design, manufacture, testing, use, and ultimately commercialization. The Woodward-Hoffmann rules were judged to precipitate the Quantum Chemistry Revolution because of theoretical, practical, and social construction of knowledge characteristics. Neither Hückel molecular orbital theory nor Hückel's 4n + 2 rule was considered an initiator of a revolution in chemistry but rather participants in the Quantum Chemistry Revolution. Retrosynthetic analysis was not judged to initiate a revolution in chemistry.

Evaluation of the Effects of Heteroaryl Ethylene Molecules in Combination with Antibiotics: A Preliminary Study on Control Strains

The discovery of compounds with antibacterial activity is crucial in the ongoing battle against antibiotic resistance. We developed two QSAR models to design six novel heteroaryl drug candidates and assessed their antibacterial properties against nine ATCC strains, including Enterococcus faecalis, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and also Salmonella enterica and Escherichia coli, many of which belong to the ESKAPE group. We combined PB4, a previously tested compound from published studies, with GC-VI-70, a newly discovered compound, with the best cytotoxicity/MIC profile. By testing sub-MIC concentrations of PB4 with five antibiotics (linezolid, gentamycin, ampicillin, erythromycin, rifampin, and imipenem), we evaluated the combination's efficacy against the ATCC strains. To assess the compounds' cytotoxicity, we conducted a 24 h and 48 h 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay on colorectal adenocarcinoma (CaCo-2) cells. We tested the antibiotics alone and in combination with PB4. Encouragingly, PB4 reduced the MIC values for GC-VI-70 and for the various clinically used antibiotics. However, it is essential to note that all the compounds studied in this research exhibited cytotoxic activity against cells. These findings highlight the potential of using these compounds in combination with antibiotics to enhance their effectiveness at lower concentrations while minimizing cytotoxic effects.

Dearomative logic in natural product total synthesis

Covering: 2011 to 2022The natural world is a prolific source of some of the most interesting, rare, and complex molecules known, harnessing sophisticated biosynthetic machinery evolved over billions of years for their production. Many of these natural products represent high-value targets of total synthesis, either for their desirable biological activities or for their beautiful structures outright; yet, the high sp³-character often present in nature's molecules imparts significant topological complexity that pushes the limits of contemporary synthetic technology. Dearomatization is a foundational strategy for generating such intricacy from simple materials that has undergone considerable maturation in recent years. This review highlights the recent achievements in the field of dearomative methodology, with a focus on natural product total synthesis and retrosynthetic analysis. Disconnection guidelines and a three-phase dearomative logic are described, and a spotlight is given to nature's use of dearomatization in the biosynthesis of various classes of natural products. Synthetic studies from 2011 to 2021 are reviewed, and 425 references are cited.

Recent Developments in Nanocatalyzed Green Synthetic Protocols of Biologically Potent Diverse O-Heterocycles—A Review

The dynamic growth in green organic synthetic methodologies for diverse heterocyclic scaffolds has substantially contributed to the field of medicinal chemistry over the last few decades. The use of hybrid metal nanocatalysts (NCs) is one such benign strategy for ensuring the advancement of modern synthetic chemistry by adhering to the principles of green chemistry, which call for a sustainable catalytic system that converts reacting species into profitable chemicals at a faster rate and tends to reduce waste generation. The metal nanoparticles (NPs) enhance the exposed surface area of the catalytic active sites, thereby making it easier for reactants and metal NCs to have an effective interaction. Several review articles have been published on the preparation of metal NCs and their uses for various catalytic heterocyclic transformations. This review will summarize different metal NCs for the efficient green synthesis of various O-heterocycles. Furthermore, the review will provide a concise overview of the role of metal NCs in the synthesis of O-heterocycles and will be extremely useful to researchers working on developing novel green and simple synthetic pathways to various O-heterocyclic-derived molecules.

Multiple annulations of inert C(sp2)-H bonds with alkynes

Transition-metal catalyzed directing group (DG) assisted annulation of inert C-H bonds leads to the formation of complex molecular frameworks from readily accessible substrates. Thus, multiple annulation of less functionalized substrates with unsaturated species leads to the construction of structurally diverse fused poly(hetero)cycles. The directed inert C(arene)-H bond activation and the mode of TM-migration in this process could enabled obatining L-type [involves DG heteroatom, o-C(arene)-H bond, and C(arene)-H bond of aryl-motif in alkyne], Y-type [involves two heteroatoms of the DG and o-,o'-C(arene)-H bonds], and B-type [involves o-C(arene)-H bond and m-C(arene)-H bond] π-extended annulation products. The coordination preference of the DG heteroatom makes the transformation chemo- and regio-selective. This article underlines the conceptual development of unsymmetrical multiple annulation of arene C(sp²)-H bonds with alkynes, which is exceedingly appealing and highly important.

ACD/Structure Elucidator: 20 Years in the History of Development

The first methods associated with the Computer-Assisted Structure Elucidation (CASE) of small molecules were published over fifty years ago when spectroscopy and computer science were both in their infancy. The incredible leaps in both areas of technology could not have been envisaged at that time, but both have enabled CASE expert systems to achieve performance levels that in their present state can outperform many scientists in terms of speed to solution. The computer-assisted analysis of enormous matrices of data exemplified 1D and 2D high-resolution NMR spectroscopy datasets can easily solve what just a few years ago would have been deemed to be complex structures. While not a panacea, the application of such tools can provide support to even the most skilled spectroscopist. By this point the structures of a great number of molecular skeletons, including hundreds of complex natural products, have been elucidated using such programs. At this juncture, the expert system ACD/Structure Elucidator is likely the most advanced CASE system available and, being a commercial software product, is installed and used in many organizations. This article will provide an overview of the research and development required to pursue the lofty goals set almost two decades ago to facilitate highly automated approaches to solving complex structures from analytical spectroscopy data, using NMR as the primary data-type.

Asymmetric Total Synthesis of (-)-Spirochensilide A, Part 1: Diastereoselective Synthesis of the ABCD Ring and Stereoselective Total Synthesis of 13(R)-Demethyl Spirochensilide A

A concise and diastereoselective construction of the ABCD ring system of spirochensilide A is described. The key steps of this synthesis are a semipinacol rearrangement reaction to stereoselectively construct the AB ring system bearing two vicinal quaternary chiral centers and a Co-mediated Pauson-Khand reaction to form the spiro-based bicyclic CD ring system. This chemistry leads to the stereoselective synthesis of 13(R)-demethyl spirochensilide A, paving the way for the first asymmetric total synthesis of (-)-spirochensilide A.

Power of Biocatalysis for Organic Synthesis

Biocatalysis, using defined enzymes for organic transformations, has become a common tool in organic synthesis, which is also frequently applied in industry. The generally high activity and outstanding stereo-, regio-, and chemoselectivity observed in many biotransformations are the result of a precise control of the reaction in the active site of the biocatalyst. This control is achieved by exact positioning of the reagents relative to each other in a fine-tuned 3D environment, by specific activating interactions between reagents and the protein, and by subtle movements of the catalyst. Enzyme engineering enables one to adapt the catalyst to the desired reaction and process. A well-filled biocatalytic toolbox is ready to be used for various reactions. Providing nonnatural reagents and conditions and evolving biocatalysts enables one to play with the myriad of options for creating novel transformations and thereby opening new, short pathways to desired target molecules. Combining several biocatalysts in one pot to perform several reactions concurrently increases the efficiency of biocatalysis even further.

Directing Group Assisted Unsymmetrical Multiple Functionalization of Arene C-H Bonds

Multiple C-H bond functionalizations promptly install diverse groups on the molecular framework and consequently fabricate complex molecular entities. This review briefly surveys the conceptual development of directing group assisted unsymmetrical multiple functionalization of arene C(sp² )-H bonds, which is exceedingly appealing and highly important.

Making natural products from renewable feedstocks: back to the roots?

This review highlights the utilization of biomass-derived building blocks in the total synthesis of natural products.

Oxa- and Azabenzonorbornadienes as Electrophilic Partners under Photoredox/Nickel Dual Catalysis

Herein, the introduction of oxa- and azabenzonorbornadienes into photoredox/nickel dual catalysis in a regioselective and diastereoselective transformation is disclosed. The inherent advantages of this dual catalytic system allow the use of alkyl motifs forming exclusively cis-1,2-dihydro-1-naphthyl alcohol backbones using readily accessible 4-alkyl-1,4-dihydropyridines (DHPs). Whereas previous studies have emphasized the use of nucleophilic organometallic coupling partners, this protocol grants access to a rather unexplored core featuring alkyl residues, while avoiding the use of highly reactive organometallic species (i.e., M = Al, Mg, Li, Zn, Zr). DFT calculations support a oxidative addition/reductive elimination mechanism, followed by a Curtin-Hammett scenario that controls the regioselectivity of the process, unlike previously reported transformations that proceed via a carbometalation/ β-oxygen elimination mechanism.

Exploration of the chemical space and its three historical regimes

Chemical research unveils the structure of chemical space, spanned by all chemical species, as documented in more than 200 y of scientific literature, now available in electronic databases. Very little is known, however, about the large-scale patterns of this exploration. Here we show, by analyzing millions of reactions stored in the Reaxys database, that chemists have reported new compounds in an exponential fashion from 1800 to 2015 with a stable 4.4% annual growth rate, in the long run neither affected by World Wars nor affected by the introduction of new theories. Contrary to general belief, synthesis has been the means to provide new compounds since the early 19th century, well before Wöhler's synthesis of urea. The exploration of chemical space has followed three statistically distinguishable regimes. The first one included uncertain year-to-year output of organic and inorganic compounds and ended about 1860, when structural theory gave way to a century of more regular and guided production, the organic regime. The current organometallic regime is the most regular one. Analyzing the details of the synthesis process, we found that chemists have had preferences in the selection of substrates and we identified the workings of such a selection. Regarding reaction products, the discovery of new compounds has been dominated by very few elemental compositions. We anticipate that the present work serves as a starting point for more sophisticated and detailed studies of the history of chemistry.

Direct Aromatic Nitration System for Synthesis of Nitrotryptophans in Escherichia coli

Nitrotryptophan and its analogues are useful building blocks for synthesizing bioactive and biotechnologically relevant chemicals, materials, and proteins. However, synthetic routes to enantiopure nitro-containing tryptophan derivatives are either complex and polluting or even unestablished yet. Herein, we describe microbial production of 4-NO₂-l-tryptophan (Nitrotrp) and its analogues by designing and expressing the biosynthetic pathway in Escherichia coli. The biosynthetic pathway comprised one engineered self-sufficient P450 TB14 of Streptomyces origin for direct nitration of the C-4 of l-Trp indole and one nitric oxide synthase from Bacillus subtilis (BsNOS) for the production of nitric oxide (NO) from l-Arg to support the direct aromatic nitration. As both TB14 and BsNOS require reducing agent NADPH for their reactions, we also included one glucose dehydrogenase (GDH) from B. subtilis for in situ NADPH regeneration. The initially designed pathway led to 16.2 ± 2.3 mg/L of Nitrotrp by the engineered E. coli fermented in the M9 minimal medium for 3 days. A combination of the design and screening of three additional pathways, fermentation optimization and the knockout of competitive metabolic pathways together improved the Nitrotrp titer to around 192 mg/L within 20 h. Finally, the whole-cell biotransformation system produced eight Nitrotrp analogues with their titers varying from 2.5 to 61.5 mg/L. This work provides the first microbial direct aromatic nitration processes and sets the stage for the development of biocatalytic routes to other useful nitroaromatics in the future.

The Journey of Schinortriterpenoid Total Syntheses

Plants in the Schisandraceae family are important components of the traditional Chinese herbal medicines and are often used to treat various illnesses. Therefore, these Schisandraceae plants are valuable sources for the discovery of new chemical entities for novel therapeutic development. Considerable progress has been made in the identification of bioactive and structurally novel triterpenoids from the Schisandraceae family in the past two decades. In particular, Sun and co-workers have successfully isolated over 100 nortriterpenoids from the Schisandraceae family. Some of these nortriterpenoids have strong inhibitory activities toward hepatitis, tumors, and HIV-1. However, the natural scarcity of these nortriterpenoids in the Schisandraceae plants has hampered their isolation and further biomedical development, and their biosynthesis has not been fully elucidated. It is therefore important and urgent to develop efficient and streamlined total syntheses of these medicinally important nortriterpenoids. Such syntheses will provide sufficient materials for detailed biological studies as well as new synthetic analogues and probe molecules to improve their biological functions and elucidate their mode of actions. However, because of their structural novelty and complexity, the total syntheses of these nortriterpenoid natural products present a significant challenge for synthetic chemists, despite the progress made in organic synthesis, particularly total synthesis, in the 20th century and since the beginning of the 21st century. New synthetic methodologies and strategies therefore need to be invented and developed to facilitate the total syntheses of these nortriterpenoid natural products. With this in mind, our group has spent the last 15 years, ever since the isolation of micrandilactone A (1) by Sun and co-workers in 2003 ( Sun et al. Org. Lett. 2003 , 5 , 1023 - 1026 ), working on synthetic studies with a view to developing methods and strategies for the total syntheses of schinortriterpenoids. Enabling methods such as a thiourea/Pd-catalyzed alkocycarbonylative annulation and a thiourea/Co-catalyzed Pauson-Khand reaction have been developed under these circumstances to form the key ring systems and stereocenters of these complex target molecules. These methodological advances have led us to the first total syntheses of schindilactone A (2), lancifodilactone G acetate (6a), 19-dehydroxyarisandilactone A (9), and propindilactone G (10) with diverse structural features via a branching-oriented strategy. The chemistry developed during our total synthesis campaign has not only helped us to deal with various challenges encountered in the syntheses of the four target molecules, but has also opened up new avenues for synthesizing other naturally occurring schinortriterpenoids and their derivatives, which will likely result in molecules with improved biological functions and tool compounds to enable elucidation of their mechanism of actions or potential cellular targets. This Account highlights the chemistry evolution of our schinortriterpenoid syntheses.

Total Synthesis in Search of Potent Antibody-Drug Conjugate Payloads. From the Fundamentals to the Translational

The emergence and evolution of antibody-drug conjugates (ADCs) as targeted cancer therapies in recent years is a living example of the "magic bullet" concept of Paul Ehrlich, introduced by him more than a century ago. Consisting of three components, the antibody serving as the delivery system, the payload drug that kills the cancer cell, and the chemical linker through which the payload is attached to the antibody, ADCs represent a currently hotly pursued paradigm of targeted cancer therapies. While the needed monoclonal antibody falls in the domains of biology and biochemistry, the potent payload and the linker belong to the realm of chemistry. Naturally occurring molecules and their derivatives endowed with high cytotoxic properties have proven to be useful payloads for the first approved ADCs (i.e., Mylotarg, Adcetris, Kadcyla, and Besponsa). The latest approaches and intensifying activities in this new paradigm of cancer therapy demands a variety of payloads with different mechanisms of action in order to address the medical needs for the various types of cancers, challenging synthetic organic chemists to enrich the library of potential payloads. Total synthesis of natural and designed molecules not only provides a powerful avenue to replicate rare naturally occurring compounds in the laboratory but also offers a unique opportunity to rationally design and synthesize analogues thereof for biological evaluation and optimization of ADC payloads. In this Account, we describe our efforts in this area highlighting a number of total synthesis endeavors through which we rendered scarce naturally occurring molecules readily available for biological evaluations and, most importantly, employed the developed synthetic strategies and methods to construct, otherwise unavailable or difficult to reach, designed analogues of these molecules. Specifically, we summarize the total syntheses of natural and designed molecules of the calicheamicin, uncialamycin, tubulysin, trioxacarcin, epothilone, shishijimicin, namenamicin, thailanstatin, and disorazole families of compounds and demonstrate how these studies led to the discovery of analogues of higher potencies, yet some of them possessing lower complexities than their parent compounds as potential ADC payloads. The highlighted examples showcase the continuing impact of total synthesis of natural products and their analogues on modern medicine, including the so-called biologics and should prove useful and inspirational in advancing both the fields of total synthesis and biomedical research and the drug discovery and development process.

Synthetic fermentation of bioactive molecules

The concept of synthetic fermentation is to 'grow' complex organic molecules in a controlled and predictable manner by combining small molecule building blocks in water-without the need for reagents, enzymes, or organisms. This approach mimics the production of small mixtures of structurally related natural products by living organisms, particularly microbes, under conditions compatible with direct screening of the cultures for biological activity. This review discusses the development and implementation of this concept, its use for the discovery of protease inhibitors, its basis as a chemistry outreach program allowing non-specialists to make and discover new antibiotics, and highlights of related approaches.