Total Synthesis of (±)-Platencin

Diels-Alder Reactions of 1-Alkoxy-1-amino-1,3-butadienes: Direct Synthesis of 6-Substituted and 6,6-Disubstituted 2-Cyclohexenones and 6-Substituted 5,6-Dihydropyran-2-ones

We report the cycloaddition reactions of 1-alkoxy-1-amino-1,3-butadienes. These doubly activated dienes are prepared on a multigram scale from crotonic acid chloride and its derivatives. The dienes undergo Diels-Alder (DA) and hetero-Diels-Alder (HDA) reactions under mild reaction conditions with a variety of electron-deficient dienophiles to afford cycloadducts in good yields with excellent regioselectivities. The hydrolysis of the DA cycloadducts provides 6-substituted and 6,6-disubstituted 2-cylohexenones, which are versatile building blocks for complex molecule synthesis. The corresponding HDA cycloadducts afford 6-substituted 5,6-dihydropyran-2-ones.

Intramolecular Formal [4 + 2] Cycloadditions: Synthesis of Spiro Isoindolinone Derivatives and Related Molecules

Acid-catalyzed intramolecular reactions of isoindolinone-derived hydroxylactam derivatives bearing enones or enals that afford spiro isoindolinone derivatives and related molecules have been developed. From the hydroxylactam moieties, N-acylenamides were generated in situ and reacted with the enone and the enal moieties via formal [4 + 2] cycloaddition reactions to construct cyclohexanone- and dihydropyran-fused ring systems and the spiro ring systems.

Semisynthesis and Biological Evaluation of Platensimycin Analogues with Varying Aminobenzoic Acids

Platensimycin (PTM) is an excellent natural product drug lead against various gram-positive pathogens, including methicillin-resistant Staphylococcus aureus and vancomycin-resistant enterococci. In this study, twenty PTM derivatives with varying aminobenzoic acids were semisynthesized. In contrast to all the previous reported inactive aminobenzaote analogues, a few of them showed moderate antibacterial activities against S. aureus. Our study suggested that modification of the conserved aminobenzoic acid remains a viable approach to diversify the PTM scaffold.

Catalytic Enantioselective Formal (4+2) Cycloaddition by Aldol-Aldol Annulation of Pyruvate Derivatives with Cyclohexane-1,3-Diones to Afford Functionalized Decalins

The decalin structure is found in bioactive molecules. We have developed catalytic enantioselective formal (4+2) cycloaddition reactions via aldol-aldol cascade reactions between pyruvate-derived diketoester derivatives and cyclohexane-1,3-dione derivatives that afford highly functionalized decalin derivatives. The reactions were performed using a quinidine-derived catalyst under mild conditions. Decalin derivatives bearing up to six chiral carbon centers including tetrasubstituted carbon centers were synthesized with high diastereo- and enantioselectivities. Five to six stereogenic centers were generated from achiral molecules with the formation of two C-C bonds in a single transformation resulting in the formation of the decalin system.

Short Enantioselective Formal Synthesis of (-)-Platencin

A short enantioselective formal synthesis of the antibiotic natural product platencin is reported. Key steps in the synthesis include enantioselective decarboxylation alkylation, aldehyde/olefin radical cyclization, and regioselective aldol cyclization.

Total synthesis of cyrneines A-B and glaucopine C

The cyrneine diterpenoids represent a structurally intriguing subfamily of cyathane diterpenoids and could significantly induce neurite outgrowth. Therefore, the efficient synthesis of these natural products is of great importance. Herein, we present a route for the collective synthesis of cyrneines A, B, and glaucopine C. As the key precursor, the 5-6-6-tricyclic scaffold is efficiently constructed by employing a mild Suzuki coupling of heavily substituted nonactivated cyclopentenyl triflate and a chelation-controlled regiospecific Friedel-Crafts cyclization as key transformations. The stereoselective installation of the all-carbon quaternary center at C6 ring junction of the tricycle is achieved via Birch reductive methylation. Subsequently, a carbenoid-mediated ring expansion furnishes the essential 5-6-7-tricyclic core. Finally, manipulation of this core by several appropriately orchestrated conversions accomplishes a more step-economic synthesis of cyrneine A (20 steps), and the first synthesis of cyrneine B (24 steps) and glaucopine C (23 steps).

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Scalable synthesis of enaminones utilizing Gold's reagents

Several Gold's reagents were synthesized from cyanuric chloride and N,N-dialkylformamides. These synthetic equivalents of N,N-dimethylformamide dimethyl acetal were used in an optimized and scalable procedure for the regioselective synthesis of a variety of enaminones from ketone starting materials, whose utility was demonstrated by the stereoselective synthesis of Rawal-type dienes.

In vivo instability of platensimycin and platencin: Synthesis and biological evaluation of urea- and carbamate-platensimycin

Platensimycin (PTM) and platencin (PTN), two natural products and promising drug leads that target bacterial and mammalian fatty acid synthases, are known to have unfavorable pharmacokinetic properties. It is not clear, however, what the metabolic fates of PTM and PTN are and no efforts have been reported to address this key roadblock in the development of these compounds as viable drug options. Here we describe the pharmacokinetics of PTM and PTN, and reveal rapid renal clearance as the primary metabolic liability with three additional sites of chemical liability: (i) amide hydrolysis, (ii) glucuronidation, and (iii) oxidation. We determined that hydrolysis is a viable clearance mechanism in vivo and synthesized two PTM analogues to address in vivo hydrolysis. Urea- and carbamate-PTM analogues showed no detectable hydrolysis in vivo, at the expense of antibacterial activity, with no further improvement in systemic exposure. The antibacterial sulfur-containing analogues PTM D1 and PTM ML14 showed significant decreases in renal clearance. These studies set the stage for continued generation of PTM and PTN analogues in an effort to improve their pharmacokinetics while retaining or improving their biological activities.

A Facile Semi-Synthetic Approach towards Halogen-Substituted Aminobenzoic Acid Analogues of Platensimycin

Platensimycin (PTM), produced by several strains of Streptomyces platensis, is a promising drug lead for infectious diseases and diabetes. The recent pilot-scale production of PTM from S. platensis SB12026 has set the stage for the facile semi-synthesis of a focused library of PTM analogues. In this study, gram-quantity of platensic acid (PTMA) was prepared by the sulfuric acid-catalyzed ethanolysis of PTM, followed by a mild hydrolysis in aqueous lithium hydroxide. Three PTMA esters were also obtained in near quantitative yields in a single step, suggesting a facile route to make PTMA aliphatic esters. 1-[Bis(dimethylamino)methylene]-1H-1,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate (HATU)-catalyzed coupling of PTMA and 33 aminobenzoates resulted in the synthesis of 28 substituted aminobenzoate analogues of PTM, among which 26 of them were reported for the first time. Several of the PTM analogues showed weak antibacterial activity against methicillin-resistant Staphylococcus aureus. Our study supported the potential utility to integrate natural product biosynthetic and semi-synthetic approaches for structure diversification.

Aromatic poly(ether ester)s derived from a naturally occurring building block nipagin and linear aliphatic α,ω-diols

Poly(ether ester) materials were synthesized from renewable nipagin and linear aliphatic α,ω-diols, and the thermal, crystalline and mechanical properties were investigated.

Studies Directed toward the Synthesis of Aspidophytine: Construction of Its Perhydroquinoline Core

We have developed an efficient route for the synthesis of the perhydroquinoline core of the indole alkaloid aspidophytine (2), starting from commercially available and inexpensive 3-acetylpyridine. This densely functionalized perhydroquinoline core displays four contiguous stereocenters including an all-carbon quaternary center. The synthetic sequence features a highly effective Diels-Alder reaction using a carbamate-substituted siloxy diene accompanied by a spontaneous intramolecular substitution of the newly formed 3°-alkyl bromide with a carbamate group. The installation of the electron-rich aniline moiety was accomplished via a TBSOTf-mediated intramolecular aza-Michael reaction, and the relative stereochemistry of the aza-Michael product (30) was confirmed by X-ray crystallographic analysis. Among the useful transformations that were developed through this study is a highly enantioselective Diels-Alder reaction of a versatile cyclic carbamate siloxy diene.

Synthesis of tetrasubstituted 1-silyloxy-3-aminobutadienes and chemistry beyond Diels-Alder reactions

Electron-rich dienes have revolutionized the synthesis of complex compounds since the discovery of the legendary Diels–Alder cycloaddition reaction. This highly efficient bond-forming process has served as a fundamental strategy to assemble many structurally formidable molecules. Amino silyloxy butadienes are arguably the most reactive diene species that are isolable and bottleable. Since the pioneering discovery by Rawal, 1-amino-3-silyloxybutadienes have been found to undergo cycloaddition reactions with unparalleled mildness, leading to significant advances in both asymmetric catalysis and total synthesis of biologically active natural products. In sharp contrast, this class of highly electron-rich conjugated olefins has not been studied in non-cycloaddition reactions. Here we report a simple synthesis of tetrasubstituted 1-silyloxy-3-aminobutadienes, a complementarily substituted Rawal's diene. This family of molecules is found to undergo a series of intriguing chemical transformations orthogonal to cycloaddition reactions. Structurally diverse polysubstituted ring architectures are established in one step from these dienes.

Electron-rich dienes have been shown to have multiple uses in synthesis by means of cycloaddition reactions. Here, the authors report a series of non-cycloaddition-based reactions of these compounds, giving access to 4-, 5-, 6- and 7-membered ring systems.

A concise formal synthesis of platencin

A formal synthesis of platencin features an organocatalytic approach to the [2.2.2] bicycle, a radical reductive elimination, a Au-catalyzed rearrangement and a Rh-catalyzed hydrosilylation.

Enantioselective synthesis of bicyclo[2.2.2]octane-1-carboxylates under metal free conditions

A new tandem reaction permits rapid access to bicyclo[2.2.2]octane-1-carboxylates with excellent enantioselectivities under metal free, mild, and operationally simple conditions.

A new route to platencin via decarboxylative radical cyclization

A new approach to platencin, a potent antibiotic isolated from Streptomyces platensis, has been established. The highly congested tricyclic core of the natural product was successfully constructed by decarboxylative radical cyclization of an alkynyl silyl ester with Pb(OAc)4 in the presence of pyridine in refluxing 1,4-dioxane. The key decarboxylation, which likely takes place via lead(IV) esterification followed by carbon-centered radical generation and subsequent capture of the radical with a triple bond, allows the rapid construction of the twisted polycyclic system.

Total synthesis of alkaloid 205B

Concise and highly stereocontrolled total syntheses of racemic and enantiopure frog alkaloid 205B (1) were accomplished in 11 steps from 4-methoxypyridines 6 and 7 in overall yields of 8 and 8%, respectively. The assembly of the core of the natural product relies on a stereoselective Tsuji-Trost allylic amination reaction and a ring-closing metathesis. The synthesis features the use of an N-acylpyridinium salt reaction to introduce the first stereocenter and an unprecedented trifluoroacetic anhydride-mediated addition of an allylstannane to a vinylogous amide with complete facial selectivity. Deoxygenation of the C4 ketone proved difficult but was accomplished via a modified Barton-McCombie reaction in the presence of a catalytic amount of diphenyl diselenide.

The use of platensimycin and platencin to fight antibiotic resistance

Infectious diseases are known as one of the most life-threatening disabilities worldwide. Approximately 13 million deaths related to infectious diseases are reported each year. The only way to combat infectious diseases is by chemotherapy using antimicrobial agents and antibiotics. However, due to uncontrolled and unnecessary use of antibiotics in particular, surviving bacteria have evolved resistance against several antibiotics. Emergence of multidrug resistance in bacteria over the past several decades has resulted in one of the most important clinical health problems in modern medicine. For instance, approximately 440,000 new cases of multidrug-resistant tuberculosis are reported every year leading to the deaths of 150,000 people worldwide. Management of multidrug resistance requires understanding its molecular basis and the evolution and dissemination of resistance; development of new antibiotic compounds in place of traditional antibiotics; and innovative strategies for extending the life of antibiotic molecules. Researchers have begun to develop new antimicrobials for overcoming this important problem. Recently, platensimycin - isolated from extracts of Streptomyces platensis - and its analog platencin have been defined as promising agents for fighting multidrug resistance. In vitro and in vivo studies have shown that these new antimicrobials have great potential to inhibit methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococci, and penicillin-resistant Streptococcus pneumoniae by targeting type II fatty acid synthesis in bacteria. Showing strong efficacy without any observed in vivo toxicity increases the significance of these antimicrobial agents for their use in humans. However, at the present time, clinical trials are insufficient and require more research. The strong antibacterial efficacies of platensimycin and platencin may be established in clinical trials and their use in humans for coping with multidrug resistance may be allowed in the foreseeable future.

The Construction of All-Carbon Quaternary Stereocenters by Use of Pd-Catalyzed Asymmetric Allylic Alkylation Reactions in Total Synthesis

All-carbon quaternary stereocenters have posed significant challenges in the synthesis of complex natural products. These important structural motifs have inspired the development of broadly applicable palladium-catalyzed asymmetric allylic alkylation reactions of unstabilized non-biased enolates for the synthesis of enantioenriched α-quaternary products. This microreview outlines key considerations in the application of palladium-catalyzed asymmetric allylic alkylation reactions and presents recent total syntheses of complex natural products that have employed these powerful transformations for the direct, catalytic, enantioselective construction of all-carbon quaternary stereocenters.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

Platensimycin and platencin congeners from Streptomyces platensis

Platensimycin (1a) and platencin (2) are inhibitors of FabF and FabF/H bacterial fatty acid synthase. The discovery of natural congeners is an approach that can render a better understanding of the structure-function relationships of complex natural products. The isolation and structure elucidation of nine new congeners (11-20) of platensimycin and platencin are described from a fermentation broth of Streptomyces platensis. These hydroxylated congeners are likely derived by cytochrome P450 oxidation of the terpenoid units post-cyclization. Polar groups in the terpenoid portion of the molecule produce negative interactions with the hydrophobic pocket of FabF, resulting in poor activities. However, the discovery of these compounds serves an important purpose, not only to understand structure-function relationships, which cannot be easily accessed by chemical modification, but also to provide access to compounds that could be used for structural identification/confirmation of the oxidative trace metabolites produced in vivo during animal experiments.

Total synthesis of lamellarins D, H, and R and ningalin B

A concise total synthesis of lamellarins D (7 steps), H (7 steps), and R (5 steps) and ningalin B (5 steps) is achieved starting from the corresponding aldehydes and amines. The synthesis features three oxidative reactions as key steps in a biomimetic manner, involving an AgOAc-mediated oxidative coupling reaction to construct the pyrrole core, a Pb(OAc)(4)-induced oxidative cyclization to form the lactone, and Kita's oxidation reaction to form the pyrrole-arene C-C bond.