An Enantioconvergent and Concise Synthesis of Lasonolide A

Toward the Briarane Core via 1,3-Dipolar Cycloaddition

The complete C20 framework of brianthein W was established, featuring hydroboration/allylation, to provide the C1-C2 quaternary/tertiary stereoarray with excellent stereocontrol. Intramolecular nitrile oxide cycloaddition (INOC) was adopted as the key transformation to establish the trans-fused 6/10-bicyclic ring system. Evolution of the second INOC event revealed the intricacies governing regioselectivity, which ultimately led to construction of the highly strained 10-membered carbocycle.

Total Synthesis of Lobatamides A and C

The total synthesis of lobatamides A (1 a) and C (1 c) via a common bislactone intermediate is reported. The allylic aryl moiety including a trisubstituted Z-olefin was constructed by hydroboration of a 1,1-disubstituted allene and subsequent Migita-Kosugi-Stille coupling. Although the seco acid proved to be highly unstable even in the presence of weak bases, Zhao macrolactonization under acidic conditions via the α-acyloxyenamide successfully provided the common bislactone intermediate. Hydrozirconation-iodination of the terminal alkyne and subsequent copper-mediated coupling with primary amides proceeded successfully in the presence of the sensitive bislactone framework. The developed synthetic route enables the late-stage installation of enamide side chains, which are crucial structures for V-ATPase inhibition.

Uncovering Lasonolide A Biosynthesis Using Genome-Resolved Metagenomics

Invertebrates, particularly sponges, have been a dominant source of new marine natural products. For example, lasonolide A (LSA) is a potential anticancer molecule isolated from the marine sponge Forcepia sp., with nanomolar growth inhibitory activity and a unique cytotoxicity profile against the National Cancer Institute 60-cell-line screen. Here, we identified the putative biosynthetic pathway for LSA. Genomic binning of the Forcepia sponge metagenome revealed a Gram-negative bacterium belonging to the phylum Verrucomicrobia as the candidate producer of LSA. Phylogenetic analysis showed that this bacterium, here named "Candidatus Thermopylae lasonolidus," only has 88.78% 16S rRNA identity with the closest relative, Pedosphaera parvula Ellin514, indicating that it represents a new genus. The lasonolide A (las) biosynthetic gene cluster (BGC) was identified as a trans-acyltransferase (AT) polyketide synthase (PKS) pathway. Compared with its host genome, the las BGC exhibits a significantly different GC content and pentanucleotide frequency, suggesting a potential horizontal acquisition of the gene cluster. Furthermore, three copies of the putative las pathway were identified in the candidate producer genome. Differences between the three las repeats were observed, including the presence of three insertions, two single-nucleotide polymorphisms, and the absence of a stand-alone acyl carrier protein in one of the repeats. Even though the verrucomicrobial producer shows signs of genome reduction, its genome size is still fairly large (about 5 Mbp), and, compared to its closest free-living relative, it contains most of the primary metabolic pathways, suggesting that it is in the early stages of reduction. IMPORTANCE While sponges are valuable sources of bioactive natural products, a majority of these compounds are produced in small quantities by uncultured symbionts, hampering the study and clinical development of these unique compounds. Lasonolide A (LSA), isolated from marine sponge Forcepia sp., is a cytotoxic molecule active at nanomolar concentrations, which causes premature chromosome condensation, blebbing, cell contraction, and loss of cell adhesion, indicating a novel mechanism of action and making it a potential anticancer drug lead. However, its limited supply hampers progression to clinical trials. We investigated the microbiome of Forcepia sp. using culture-independent DNA sequencing, identified genes likely responsible for LSA synthesis in an uncultured bacterium, and assembled the symbiont's genome. These insights provide future opportunities for heterologous expression and cultivation efforts that may minimize LSA's supply problem.

Total Synthesis of Leiodermatolide A via Transfer Hydrogenative Allylation, Crotylation, and Propargylation: Polyketide Construction beyond Discrete Allyl- or Allenylmetal Reagents

The total synthesis of leiodermatolide A was accomplished in 13 steps (LLS). Transfer hydrogenative variants of three carbonyl additions that traditionally rely on premetalated reagents (allylation, crotylation, and propargylation) are deployed together in one total synthesis.

Applications of the Horner–Wadsworth–Emmons Olefination in Modern Natural Product Synthesis

The Horner–Wadsworth–Emmons (HWE) reaction is one of the most reliable olefination reaction and can be broadly applied in organic chemistry and natural product synthesis with excellent selectivity. Within the last few years HWE reaction conditions have been optimized and new reagents developed to overcome challenges in the total syntheses of natural products. This review highlights the application of HWE olefinations in total syntheses of structurally different natural products covering 2015 to 2020. Applied HWE reagents and reactions conditions are highlighted to support future synthetic approaches and serve as guideline to find the best HWE conditions for the most complicated natural products.

1 Introduction and Historical Background

2 Applications of HWE

2.1 Cyclization by HWE Reactions

2.2.1 Formation of Medium- to Larger-Sized Rings

2.2.2 Formation of Small- to Medium-Sized Rings

2.3 Synthesis of α,β-Unsaturated Carbonyl Groups

2.4 Synthesis of Substituted C=C Bonds

2.5 Late-Stage Modifications by HWE Reactions

2.6 HWE Reactions on Solid Supports

2.7 Synthesis of Poly-Conjugated C=C Bonds

2.8 HWE-Mediated Coupling of Larger Building Blocks

2.9 Miscellaneous

3 Summary and Outlook

Deciphering the Chameleonic Chemistry of Allenols: Breaking the Taboo of a Onetime Esoteric Functionality

The allene functionality has participated in one of the most exciting voyages in organic chemistry, from chemical curiosities to a recurring building block in modern organic chemistry. In the last decades, a special kind of allene, namely, allenol, has emerged. Allenols, formed by an allene moiety and a hydroxyl functional group with diverse connectivity, have become common building blocks for the synthesis of a wide range of structures and frequent motif in naturally occurring systems. The synergistic effect of the allene and hydroxyl functional groups enables allenols to be considered as a unique and sole functionality exhibiting a special reactivity. This Review summarizes the most significant contributions to the chemistry of allenols that appeared during the past decade, with emphasis on their synthesis, reactivity, and occurrence in natural products.

Synthetic efforts on the road to marine natural products bearing 4-O-2,3,4,6-tetrasubstituted THPs: an update

Scientific literature is inundated with secondary metabolites from marine sources. In this ocean of natural products, the presence of recurring patterns has traditionally led scientists to unravel the biosynthetic mechanisms that naturally yield these products, as well as to imitate Nature to prepare them in the laboratory, especially when promising bioactivities and stimulating molecular architectures are involucrate. For instance, natural products containing multisubstituted oxygenated rings and macrocyclic lactones are recurrently selected as targets for developing total syntheses. Thus, in the last decades a noteworthy number of synthetic works regarding miyakolide, madeirolide A and representative compounds of polycavernosides, lasonolides and clavosolides have come to fruition. Up to now, these families of macrolides are the only marine natural products bearing a tetrasubstituted tetrahydropyran ring with carbon substituents at positions 2, 3 and 6, as well as an oxygen at position 4. Their splendid structures have received the attention of the synthetic community, up to the point of starring in dozens of articles, and even some reviews. This work covers all the synthetic studies towards miyakolide and madeirolide A, as well as the synthetic efforts performed after the previous specialised reviews about lasonolide A, polycavernoside A and clavosolides, published in 2006, 2007 and 2016, respectively. In total, this review summarises 22 articles in which these marine natural products with 4-O-2,3,4,6-tetrasubstituted tetrahydropyrans have the leading role.

Development of New Innovative Synthetic Organic Chemistry Using Lone Pairs of Oxygen Atoms

Oxygen atoms have a lone pair of electrons, so they have high chelation ability, high nucleophilic ability, stabilizing ability of adjacent cations, and take a chelate or oxocarbenium ion structure with Lewis acids and metals. I took advantage of these properties to develop three new reactions, 1) asymmetric synthesis of chiral quaternary carbon centers, 2) asymmetric synthesis using acetal functions, and 3) organic chemistry using acetal-type reactive salt chemical species, and applied them to biologically active natural products synthesis. New reactions described here are all innovative and useful for natural products synthesis. In particular, the first asymmetric synthesis of fredericamycin A, and concise asymmetric synthesis of anthracycline antibiotics, scyphostatin, (+)-Sch 642305, (-)-stenine, clavolonine, (+)-rubrenolide, (+)-rubrynolide, (+)-centrolobine, and decytospolide A and B, etc., are noteworthy. Furthermore, since reactions using acetal-type reactive salt chemical species allow the coexistence of functional groups that normally cannot coexist, the reactions using reactive salts have potential to change the retrosynthesis planned based on conventional reactions.

Palladium-catalyzed intramolecular aerobic alkenylhydroxylation of allenamides with alkenyl iodides

An efficient palladium-catalyzed aerobic alkenylhydroxylation cyclization of allenamide derivatives was developed. Mechanistic studies indicated that the reaction might undergo a radical process.