Polyketide stereotetrads in natural products

A Flexible Synthesis of Polypropionates via Diastereodivergent Reductive Ring-Opening of Trisubstituted Secondary Glycidols

Polypropionates, characterized by their alternating sequence of stereocenters bearing methyl- and hydroxy-groups, are structurally diverse natural products of utmost importance.[1] Herein, we introduce a novel concept approach towards polypropionate synthesis featuring a diastereodivergent reductive epoxide-opening as a key step. Readily available and stereochemically uniform trisubstituted sec-glycidols serve as branching points for the highly selective synthesis of all isomers of polypropionate building blocks with three or more consecutive stereocenters. Stereodiversification is accomplished by an unprecedented mechanism-control over the stereochemically complementary modification of the epoxide's tertiary C-atom with excellent control of regio- and stereoselectivity. Since our method is not only suited for the preparation of specific targets but also for compound libraries, it will have a great impact on polypropionate synthesis.

Total Synthesis of the Allenic Macrolide (+)-Archangiumide

Archangiumide is the first known macrolide natural product comprising an endocyclic allene. For the ring strain that this linear substructure might entail, it was planned to unveil the allene at a very late stage of the projected total synthesis; in actual fact, this was achieved as the last step of the longest linear sequence by using an otherwise globally deprotected substrate. This unconventional timing was made possible by a gold catalyzed rearrangement of a macrocyclic propargyl benzyl ether derivative that uses a -PMB group as latent hydride source to unveil the signature cycloallene; the protecting group therefore gains a strategic role beyond its mere safeguarding function. Although the gold catalyzed reaction per se is stereoablative, the macrocyclic frame of the target was found to impose high selectivity and a stereoconvergent character on the transformation. The required substrate was formed by ring closing alkyne metathesis (RCAM) with the aid of a new air-stable molybdenum alkylidyne catalyst.

Catalyst-Free Propargylboration of Ketones with Allenyl-Bpins: Highly Stereoselective Synthesis of tert-Homopropargyl Alcohols Bearing Vicinal Stereocenters

A practical and efficient propargylboration of ketones is presented using general allenylboronic acid pinacol esters (allenyl-Bpins) without a catalyst. This reaction is triggered by in-situ activation of stable allenyl-Bpins through the sequential addition of 1.25 equiv. of n BuLi and the prerequisite 2.0 equiv. of TFAA. Under the optimized reaction conditions, the versatile trisubstituted allenyl-Bpins react with various ketones smoothly to afford a wide range of tert-homopropargyl alcohols bearing vicinal stereocenters in high yields with good to excellent diastereoselectivities. Furthermore, propargylboration of ketones with chiral trisubstituted allenyl-Bpins allows for the asymmetric synthesis of chiral tert-homopropargyl alcohols with a full chirality transfer.

Extraction, Isolation, Characterization, and Bioactivity of Polypropionates and Related Polyketide Metabolites from the Caribbean Region

The Caribbean region is a hotspot of biodiversity (i.e., algae, sponges, corals, mollusks, microorganisms, cyanobacteria, and dinoflagellates) that produces secondary metabolites such as polyketides and polypropionates. Polyketides are a diverse class of natural products synthesized by organisms through a biosynthetic pathway catalyzed by polyketide synthase (PKS). This group of compounds is subdivided into fatty acids, aromatics, and polypropionates such as macrolides, and linear and cyclic polyethers. Researchers have studied the Caribbean region to find natural products and focused on isolation, purification, structural characterization, synthesis, and conducting biological assays against parasites, cancer, fungi, and bacteria. These studies have been summarized in this review, including research from 1981 to 2020. This review includes about 90 compounds isolated in the Caribbean that meet the structural properties of polyketides. Out of 90 compounds presented, 73 have the absolute stereochemical configuration, and 82 have shown biological activity. We expect to motivate the researchers to continue exploring the Caribbean region's marine environments to discover and investigate new polyketide and polypropionate natural products.

Allyl Alcohol as an Acrolein Equivalent in Enantioselective C-C Coupling: Total Synthesis of Amphidinolides R, J, and S

The first systematic study of catalytic enantioselective 1,2-additions to acrolein is described. Specifically, using allyl alcohol as a tractable, inexpensive acrolein proelectrophile, iridium-catalyzed acrolein allylation is achieved with high levels of regio-, anti-diastereo-, and enantioselectivity. This process delivers 3-hydroxy-1,5-hexadienes, a useful compound class that is otherwise challenging to access via enantioselective catalysis. Two-fold use of this method unlocks concise total syntheses of amphidinolide R (9 vs 23 steps, LLS) and amphidinolide J (9 vs 23 or 26 steps, LLS), which are prepared in fewer than half the steps previously possible, and the first total synthesis of amphidinolide S (10 steps, LLS).

Epoxide-Based Synthetic Approaches toward Polypropionates and Related Bioactive Natural Products

Polypropionate units are a common structural feature of many of the natural products in polyketides, some of which have shown a broad range of antimicrobial and therapeutic potential. Polypropionates are composed of a carbon skeleton with alternating methyl and hydroxy groups with a specific configuration. Different approaches have been developed for the synthesis of polypropionates and herein we include, for the first time, all of the epoxide-based methodologies that have been reported over the years by several research groups such as Kishi, Katsuki, Marashall, Miyashita, Prieto, Sarabia, Jung, McDonald, etc. Several syntheses of polypropionate fragments and natural products that employed epoxides as key intermediates have been described and summarized in this review. These synthetic approaches involve enatio- and diastereoselective synthesis of epoxides (epoxy-alcohols, epoxy-amides, and epoxy-esters) and their regioselective cleavage with carbon and/or hydride nucleophiles. In addition, we included a description of the isolation and biological activities of the polypropionates and related natural products that have been synthetized using epoxide-based approaches. In conclusion, the epoxide-based methodologies are a non-aldol alternative approach for the construction of polypropionate.

Stereo- and Site-Selective Crotylation of Alcohol Proelectrophiles via Ruthenium-Catalyzed Hydrogen Auto-Transfer Mediated by Methylallene and Butadiene

Iodide-bound ruthenium-JOSIPHOS complexes catalyze the redox-neutral C-C coupling of primary alcohols with methylallene (1,2-butadiene) or 1,3-butadiene to form products of anti-crotylation with good to excellent levels of diastereo- and enantioselectivity. Distinct from other methods, direct crotylation of primary alcohols in the presence of unprotected secondary alcohols is possible, enabling generation of spirastrellolide B (C9-C15) and leucascandrolide A (C9-C15) substructures in significantly fewer steps than previously possible.

Zinc-Mediated Transformation of 1,3-Diols to Cyclopropanes for Late-Stage Modification of Natural Products and Medicinal Agents

A method for incorporating cyclopropane motifs into complex molecules has been developed. Herein we report a zinc dust-mediated cross-electrophile coupling reaction of 1,3-dimesylates to synthesize cyclopropanes. 1,3-Dimesylates can be readily accessed from 1,3-diols, a functionality prevalent in many natural products and medicinal agents. The reaction conditions are mild, such that functional groups, including amides, esters, heterocycles, and alkenes, are tolerated. Notably, we have demonstrated late-stage cyclopropanation of statin medicinal agents.

Stereo- and Site-Selective Conversion of Primary Alcohols to Allylic Alcohols via Ruthenium-Catalyzed Hydrogen Auto-Transfer Mediated by 2-Butyne

The first enantioselective ruthenium-catalyzed carbonyl vinylations via hydrogen autotransfer are described. Using a ruthenium-JOSIPHOS catalyst, primary alcohols 2a-2m and 2-butyne 1a are converted to chiral allylic alcohols 3a-3m with excellent levels of absolute stereocontrol. Notably, 1°,2°-1,3-diols participate in site-selective C-C coupling, enabling asymmetric carbonyl vinylation beyond premetalated reagents, exogenous reductants, or hydroxyl protecting groups. Using 2-propanol as a reductant, aldehydes dehydro-2a, 2l participate in highly enantioselective 2-butyne-mediated vinylation under otherwise identical reaction conditions. Regio-, stereo-, and site-selective vinylations mediated by 2-pentyne 1b to form adducts 3n, 3o, and epi-3o also are described. The tiglyl alcohol motif obtained upon butyne-mediated vinylation, which is itself found in diverse secondary metabolites, may be converted to commonly encountered polyketide stereodiads, -triads, and -tetrads, as demonstrated by the formation of adducts 4a-4d. The collective mechanistic studies, including deuterium labeling experiments, corroborate a catalytic cycle involving alcohol dehydrogenation to form a transient aldehyde and a ruthenium hydride, which engages in alkyne hydrometalation to form a nucleophilic vinylruthenium species that enacts carbonyl addition. A stereochemical model for carbonyl addition invoking formyl CH···I[Ru] and CH···O≡C[Ru] hydrogen bonds is proposed based on prior calculations and crystallographic data.

Natural Polyether Ionophores and Their Pharmacological Profile

This review is devoted to the study of the biological activity of polyether ionophores produced by bacteria, unicellular marine algae, red seaweeds, marine sponges, and coelenterates. Biological activities have been studied experimentally in various laboratories, as well as data obtained using QSAR (Quantitative Structure-Activity Relationships) algorithms. According to the data obtained, it was shown that polyether toxins exhibit strong antibacterial, antimicrobial, antifungal, antitumor, and other activities. Along with this, it was found that natural polyether ionophores exhibit such properties as antiparasitic, antiprotozoal, cytostatic, anti-mycoplasmal, and antieczema activities. In addition, polyethers have been found to be potential regulators of lipid metabolism or inhibitors of DNA synthesis. Further study of the mechanisms of action and the search for new polyether ionophores and their derivatives may provide more effective therapeutic natural polyether ionophores for the treatment of cancer and other diseases. For some polyether ionophores, 3D graphs are presented, which demonstrate the predicted and calculated activities. The data presented in this review will be of interest to pharmacologists, chemists, practical medicine, and the pharmaceutical industry.

Total Synthesis of the Acetyl CoA Carboxylase Inhibitor Soraphen A: Asymmetric Tsuji Reduction Enables Successive Olefin Metathesis

The total synthesis of soraphen A, a myxobacterial metabolite and inhibitor of acetyl CoA carboxylase, was completed in 11 steps (longest linear sequence), less than half the steps previously required. Seven metal-catalyzed processes were deployed to unlock step-economy (comprising five asymmetric processes and four C-C bond formations). The present route does not utilize chiral auxiliaries, and four of five C-C bond formations exploit non-premetalated partners. To maximize convergency, an asymmetric Tsuji reduction was developed using a Pd-AntPhos catalyst that allows a metathesis-inactive allylic carbonate to serve as a masked terminal olefin, thereby enabling successive olefin metathesis events.

Bioinformatic and Mechanistic Analysis of the Palmerolide PKS-NRPS Biosynthetic Pathway From the Microbiome of an Antarctic Ascidian

Complex interactions exist between microbiomes and their hosts. Increasingly, defensive metabolites that have been attributed to host biosynthetic capability are now being recognized as products of host-associated microbes. These unique metabolites often have bioactivity targets in human disease and can be purposed as pharmaceuticals. Polyketides are a complex family of natural products that often serve as defensive metabolites for competitive or pro-survival purposes for the producing organism, while demonstrating bioactivity in human diseases as cholesterol lowering agents, anti-infectives, and anti-tumor agents. Marine invertebrates and microbes are a rich source of polyketides. Palmerolide A, a polyketide isolated from the Antarctic ascidian Synoicum adareanum, is a vacuolar-ATPase inhibitor with potent bioactivity against melanoma cell lines. The biosynthetic gene clusters (BGCs) responsible for production of secondary metabolites are encoded in the genomes of the producers as discrete genomic elements. A candidate palmerolide BGC was identified from a S. adareanum microbiome-metagenome based on a high degree of congruence with a chemical structure-based retrobiosynthetic prediction. Protein family homology analysis, conserved domain searches, active site and motif identification were used to identify and propose the function of the ∼75 kbp trans-acyltransferase (AT) polyketide synthase-non-ribosomal synthase (PKS-NRPS) domains responsible for the stepwise synthesis of palmerolide A. Though PKS systems often act in a predictable co-linear sequence, this BGC includes multiple trans-acting enzymatic domains, a non-canonical condensation termination domain, a bacterial luciferase-like monooxygenase (LLM), and is found in multiple copies within the metagenome-assembled genome (MAG). Detailed inspection of the five highly similar pal BGC copies suggests the potential for biosynthesis of other members of the palmerolide chemical family. This is the first delineation of a biosynthetic gene cluster from an Antarctic microbial species, recently proposed as Candidatus Synoicihabitans palmerolidicus. These findings have relevance for fundamental knowledge of PKS combinatorial biosynthesis and could enhance drug development efforts of palmerolide A through heterologous gene expression.

Enantioselective Synthesis of Acyclic Stereotriads Featuring Fluorinated Tetrasubstituted Stereocenters

Elaboration of enantioenriched complex acyclic stereotriads represents a challenge for modern synthesis even more when fluorinated tetrasubstituted stereocenters are targeted. We have been able to develop a simple strategy in a sequence of two unprecedented steps combining a diastereoselective aldol-Tishchenko reaction and an enantioselective organocatalyzed kinetic resolution. The aldol-Tishchenko reaction directly generates a large panel of acyclic 1,3-diols possessing a fluorinated tetrasubstituted stereocenter by condensation of fluorinated ketones with aldehydes under very mild basic conditions. The anti 1,3-diols featuring three contiguous stereogenic centers are generated with excellent diastereocontrol (typically >99 : 1 dr). Depending upon the precursors both diastereomers of stereotriads are accessible through this flexible reaction. Furthermore, from the obtained racemic scaffolds, development of an organocatalyzed kinetic resolution enabled to generate the desired enantioenriched stereotriads with excellent selectivity (typically er >95 : 5).

Expeditious Asymmetric Synthesis of Polypropionates Relying on Sulfur Dioxide-Induced C–C Bond Forming Reactions

For a long time, the organic chemistry of sulfur dioxide (SO2) consisted of sulfinates that react with carbon electrophiles to generate sulfones. With alkenes and other unsaturated compounds, SO2 generates polymeric materials such as polysulfones. More recently, H-ene, sila-ene and hetero-Diels–Alder reactions of SO2 have been realized under conditions that avoid polymer formation. Sultines resulting from the hetero-Diels–Alder reactions of conjugated dienes and SO2 are formed more rapidly than the corresponding more stable sulfolenes resulting from the cheletropic additions. In the presence of a protic or Lewis acid catalyst, the sultines derived from 1-alkoxydienes are ionized into zwitterionic intermediates bearing 1-alkoxyallylic cation moieties which react with electro-rich alkenes such as enol silyl ethers and allylsilanes with high stereoselectivity. (C–C-bond formation through Umpolung induced by SO2). This produces silyl sulfinates that react with carbon electrophiles to give sulfones (one-pot four component asymmetric synthesis of sulfones), or with Cl2, generating the corresponding sulfonamides that can be reacted in situ with primary and secondary amines (one-pot four component asymmetric synthesis of sulfonamides). Alternatively, Pd-catalyzed desulfinylation generates enantiomerically pure polypropionate stereotriads in one-pot operations. The chirons so obtained are flanked by an ethyl ketone moiety on one side and by a prop-1-en-1-yl carboxylate group on the other. They are ready for two-directional chain elongations, realizing expeditious synthesis of long-chain polypropionates and polyketides. The stereotriads have also been converted into simpler polypropionates such as the cyclohexanone moiety of baconipyrone A and B, Kishi’s stereoheptad unit of rifamycin S, Nicolaou’s C1–C11-fragment and Koert’s C16–CI fragment of apoptolidin A. This has also permitted the first total synthesis of (-)-dolabriferol.

Mild C-C Bond Formation via Lewis Acid Catalyzed Oxetane Ring Opening with Soft Carbon Nucleophiles

Mild oxetane opening by soft carbon nucleophiles has been developed for efficient C-C bond formation. In the presence of LiNTf₂ or TBSNTf₂ as catalyst, silyl ketene acetals were found to be effective nucleophiles to generate a wide range of highly oxygenated molecules, which are key substructure in natural products like polyketides. Furthermore, intramolecular oxetane opening by a styrene-based carbon nucleophile via a Prins-type process was also achieved with Sc(OTf)₃ as catalyst, leading to efficient formation of the useful 2,3-dihydrobenzo[b]oxepine skeleton.

Regioselective synthesis of salicylates and acetophenones by formal [3+3]-cyclocondensations of 3-oxoorthoesters with 1,3-bis(trimethylsilyloxy)-1,3-butadienes

A variety of 4-methoxysalicylates and related polyketide-type phenols are regioselectively prepared by formal [3+3] cyclocondensations of 1,3-bis(trimethylsilyloxy)-1,3-butadienes with 3-oxo-orthoesters. Cycloalkyl-substituted salicylates were prepared for the first time.

Applications of microbial co-cultures in polyketides production

Polyketides are a large group of natural biomolecules that are normally produced by bacteria, fungi and plants. These molecules have clinical importance due to their anti-cancer, anti-microbial, anti-oxidant and anti-inflammatory properties. Polyketides are biosynthesized from units of acyl-CoA by different polyketide synthases (PKSs), which display wide diversity of functional domains and mechanisms of action between fungi and bacteria. Co-culture of different micro-organisms can produce novel products distinctive from those produced during single cultures. This study compared the new polyketides produced in such co-culture systems and discusses aspects of the cultivation systems, product structures and identification techniques. Current results indicate that the formation of new polyketides may be the result of activation of previously silent PKSs genes induced during co-culture. This review indicated a potential way to produce pure therapeutic polyketides by microbial fermentation and a potential way to develop functional foods and agricultural products using co-co-culture of different micro-organisms. It also pointed out a new perspective for studies on the process of functional foods, especially those involving multiple micro-organisms.

Catalytic strategies towards 1,3-polyol synthesis by enantioselective cascades creating multiple alcohol functions

This review highlights the different enantioselective catalyst-controlled cascades creating multiple alcohol functions through the formation of several carbon-carbon bonds. Through subsequent simple derivatization, these strategies ensure the rapid preparation of 1,3-polyols. Thanks to the use of efficient metal- or organo-catalysts, these cascades enable the selective assembly of multiple substrates considerably limiting operations and waste generation. For this purpose, several mono- or bi-directional approaches have been devised allowing successive C-C bond-forming events. The considerable synthetic economies these cascades enable have been demonstrated in the preparation of a wide variety of complex bioactive natural products, notably polyketides.

Total Synthesis and Structural Validation of Phosdiecin A via Asymmetric Alcohol-Mediated Carbonyl Reductive Coupling

The first total synthesis and structural validation of phosdiecin A was accomplished in 13 steps through asymmetric iridium-catalyzed alcohol-mediated carbonyl reductive coupling. The present route is the shortest among >30 total and formal syntheses of fostriecin family members.

Modular Enantioselective Synthesis of an Advanced Pentahydroxy Intermediate of Antimalarial Bastimolide A and of Fluorinated and Chlorinated Analogues

A short enantioselective catalytic synthesis of the key C15-C27 fragment of bastimolide A, a natural product showing promising antimalarial bioactivity, is disclosed. The strategic insertion of halogen atoms such as fluorine and chlorine by enantioselective organocatalytic halogenations allowed an excellent stereochemical control for the formation of complex acyclic fragments bearing up to four stereogenic centers. Furthermore, besides the formation of the 1,5,7,9,13-pentahydroxy fragment of the natural product, this strategy opens the route to the modulation of the bioactivity by halogenohydrins.

Stereoselective one-pot synthesis of polypropionates

Polypropionates—motifs with alternating methyl and hydroxy groups—are important segments of many natural products possessing high bioactivity and therapeutic value. Synthetic access to these structures remains an area of intensive interest, focusing on the establishment of the contiguous stereocentres and a desire for operational simplicity. Here we report an efficient strategy for the stereoselective assembly of polypropionates with three or four stereocentres through a three-step relay process that include Diels–Alder reaction, silylenol ether hydrolysis and Baeyer–Villiger oxidation. The stereochemistry and functionality of the resulting polypropionates depend on the substitution pattern of the diene and dienophile substrates of the Diels–Alder cycloaddition. More importantly, the relay sequence is effectively performed in one pot, and the product could potentially undergo the same sequence for further elaboration. Finally, the C1–C9 segment of the macrolide etnangien is constructed with four of the six stereogenic centres established using the relay sequence.

Polypropionates are present in many natural products possessing high bioactivity and therapeutic value. Here the authors show a strategy for the stereoselective assembly of polypropionates with three or four stereocentres through a process that includes a Diels–Alder reaction, silylenol ether hydrolysis and Baeyer–Villiger oxidation.

Acetyl-CoA production by encapsulated pyruvate ferredoxin oxidoreductase in alginate hydrogels

Pyruvate ferredoxin oxidoreductase from Citrobacter sp. S-77 (PFOR_S77) was purified in order to develop a method for acetyl-CoA production. Although the purified PFOR_S77 showed high O₂-sensitivity, the activity could be remarkably stabilized in anaerobic conditions. PFOR_S77 was effectively immobilized on ceramic hydroxyapatite (PFOR_S77-HA) with an efficiency of more than 96%, however, after encapsulation of PFOR_S77-HA in alginate, the rate of catalytic acetyl-CoA production was highly reduced to 36% when compared to that of the free enzyme. However, the operational stability of the PFOR_S77-HA in alginate hydrogels was remarkable, retaining over 68% initial activity even after ten repeated cycles. The results suggested that the PFOR_S77-HA hydrogels have a high potential for biotechnological application.

Second Generation of Aldol Reaction

Since the discovery of the Mukaiyama aldol reaction more than 40 years ago, several landmark publications have inspired researchers in the field. The Mukaiyama AR is one of the most significant named reactions in organic synthesis. In the past few decades, development of the modern AR has been at the forefront in addressing the challenges of regio-, chemo-, diastereo-, and enantioselectivity in organic synthesis. All of these selectivity challenges maybe present in a single pair of reactants, thus controlling the outcome of such a process has great practical value. More than 10 years ago, our group became involved in this iconic carbon-carbon bond-forming process and attempted to very closely investigate all possible features of the AR to solve several issues still encountered by chemists, most notably the selectivity challenges mentioned above. In this context, our group initiated the second generation of the AR based on a Lewis or Brønsted acid-catalyzed process in conjunction with the use of a "super silyl" (tris(trimethylsilyl)silyl) directing group, which has demonstrated unrivalled properties in controlling the outcome of the AR. Using the extraordinary power of the super silyl group, we were able to develop new methods and concepts that broadly impacted the ability to control the selectivity attributes and thus allowed for a highly stereoselective construction of polyketide, halogenated polyketide, polypropionate, and polyol scaffolds through inter- and/or intramolecular aldolization protocols. Our diastereoselective ARs of super silyl enol ethers and aldehydes have shown great efficiency and modularity in producing exclusively and preferentially syn- or anti-adducts, creating up to four new adjacent stereocenters in a one-pot sequential manner and under mild reaction conditions. The super silyl-directed AR does not only provide a solution to stereochemistry control challenges, but also offers an efficient, modular and high yielding technique toward nontrivial construction of complex architectures with unprecedented ease. We believe that the new Lewis- or Brønsted-acid-catalyzed super-silyl-directed AR processes chronicled in our laboratories have come to maturity and now offer a "road map" for strategic stereoselective synthesis of polyketide-like units. Herein we report our recent achievements in the diastereoselective C-C bond formation, through the super-silyl-directed AR, toward the synthesis of complex and sophisticated hydroxy aldehydes. We would like to note that due to the extremely broad range of work reported in this field, only stereoselective AR involving aldehyde-derived super SEEs will be discussed in this Account.

Emergent Catalytic Behavior of Self-Assembled Low Molecular Weight Peptide-Based Aggregates and Hydrogels

We report a series of short peptides possessing the sequence (FE)n or (EF)n and bearing l-proline at their N-terminus that self-assemble into high aspect ratio aggregates and hydrogels. We show that these aggregates are able to catalyze the aldol reaction, whereas non-aggregated analogues are catalytically inactive. We have undertaken an analysis of the results, considering the accessibility of catalytic sites, pKa value shifts, and the presence of hydrophobic pockets. We conclude that the presence of hydrophobic regions is indeed relevant for substrate solubilization, but that the active site accessibility is the key factor for the observed differences in reaction rates. The results presented here provide an example of the emergence of a new chemical property caused by self-assembly, and support the relevant role played by self-assembled peptides in prebiotic scenarios. In this sense, the reported systems can be seen as primitive aldolase I mimics, and have been successfully tested for the synthesis of simple carbohydrate precursors.

Iterative catalyst controlled diastereodivergent synthesis of polypropionates

Polypropionates are synthesized using a combination of a copper-catalyzed asymmetric allylic alkylation, ruthenium-catalyzed cross-metathesis and iridium-catalyzed asymmetric allylic etherification.

An Effective Bifunctional Aldehyde Linchpin for Type II Anion Relay Chemistry: Development and Application to the Synthesis of a C16-C29 Fragment of Rhizopodin

The design, synthesis, and validation of a new bifunctional aldehyde linchpin for Type II anion relay chemistry have been achieved. For this linchpin, the initial nucleophilic addition proceeds under Felkin–Anh control to generate the syn-alkoxide, which undergoes a 1,4-Brook rearrangement to relay the negative charge, thus leading to the formation of a dithiane-stabilized carbanion. Subsequent trapping with an electrophile furnishes a tricomponent adduct with an embedded propionate subunit, a ubiquitous structural motif found in polyketides. The utility of this new linchpin is demonstrated with the construction of a potential C16–C29 fragment for the synthesis of rhizopodin, an actin-binding macrolide.

Progress in Understanding the Genetic Information and Biosynthetic Pathways behind Amycolatopsis Antibiotics, with Implications for the Continued Discovery of Novel Drugs

Species of Amycolatopsis, well recognized as producers of both vancomycin and rifamycin, are also known for producing other secondary metabolites, with wide usage in medicine and agriculture. The molecular genetics of natural antibiotics produced by this genus have been well studied. Since the rise of antibiotic resistance, finding new drugs to fight infection has become an urgent priority. Progress in understanding the biosynthesis of metabolites greatly helps the rational manipulation of biosynthetic pathways, and thus to achieve the goal of generating novel natural antibiotics. The efforts made in exploiting Amycolatopsis genome sequences for the discovery of novel natural products and biosynthetic pathways are summarized.

Didecarboxylative Iron-Catalyzed Bidirectional Aldolization towards Diversity-Oriented Ketodiol Synthesis

1,3-Acetonedicarboxylic acid was selectively activated by Fe(acac)3 , providing a synthetic platform for rapid synthesis of keto-3,3'-diols. The bidirectional aldol reaction was efficient for challenging aliphatic aldehydes, providing a rapid route to potentially bioactive complex structures.

Diastereoselective Ni-catalyzed 1,4-hydroboration of chiral dienols

The Ni-catalyzed hydroboration of dienols occurs in a 1,4 fashion and delivers a syn-propionate motif in high diastereoselectivity and with a stereodefined trisubstituted crotylboronic ester. The boronic ester can be further manipulated to provide carbon-carbon or carbon-oxygen bonds.