Total Synthesis of (+)-Ileabethoxazole via an Iron-Mediated Pauson–Khand [2 + 2 + 1] Carbocyclization

Studies Toward the Synthesis of Leiodolide A

Studies have described a highly convergent plan toward the synthesis of leiodolide A (1), a potent cytotoxic sponge metabolite. The enantiocontrolled preparation of aldehyde 6 is achieved with the application of several advances in methodology for the synthesis of substituted 1,3-oxazoles. Efforts have examined the halogen dance reaction, the selectivity of Stille cross coupling reactions of 4-bromo-1,3-oxazoles, and nucleophilic displacement of the 2-phenylsulfonyl substituent with organolithium reagents as preparatively useful reactions. These techniques have facilitated the efficient synthesis of 6 from the starting bromide 12, alkenylstannane 16, and the primary nonracemic alcohol 25.

Asymmetric Total Syntheses of Hypoestin A, Albolic Acid, and Ceroplastol II

The first asymmetric total synthesis of bioactive diterpenoid hypoestin A with an unprecedented [5-8-5-3] tetracyclic skeleton is accomplished in 15 steps from commercially available (R)-limonene. Furthermore, the second asymmetric total syntheses of sesterterpenoids albolic acid and ceroplastol II in 21 steps are also reported. The synthetically challenging and highly functionalized [X-8-5] (X = 5 or 7) tricarbocyclic ring systems found in hypoestin A, albolic acid, ceroplastol II, and schindilactone A, as well as other natural products, are efficiently and directly constructed via a unique intramolecular Pauson-Khand reaction of an allene-yne. This work represents the first reported use of the Pauson-Khand reaction to access synthetically challenging eight-membered-ring systems in natural product synthesis.

Enantiomeric Isopulegol as the Chiral Pool in the Total Synthesis of Bioactive Agents

Isopulegol, a pool of abundant chiral terpene, has long served as the starting material for the total synthesis of isopulegol-based drugs. As an inexpensive and versatile starting material, this compound continues to serve modern synthetic chemistry. This review highlights the total syntheses of terpenoids in the period from 1980 to 2020 in which with isopulegol applied as a building block.

Total synthesis of sesterterpenoids

Total syntheses of biologically and structurally fascinating sesterterpenoids published between Jan. 2012 and Jan. 2018 are summarized and discussed here.

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

Rhodium-catalyzed [(3+2)+1] carbocyclizations of alkynylidenecyclopropanes with carbon monoxide: construction of polysubstituted bicyclohexa-2,5-dienones

A rhodium-catalyzed carbocyclization reaction of alkynylidenecyclopropanes with carbon monoxide to prepare bicyclohexa-2,5-dienones is described. This protocol offers convenient access to doubly conjugated cyclic enones and is tolerant of functionalized alkynes and cyclopropanes. Furthermore, a photochemical rearrangement of a bicyclohexa-2,5-dienone facilitates the construction of a highly functionalized bicyclopentenone containing two contiguous stereogenic centres, which represents a versatile intermediate for target directed synthesis.

Total Synthesis of Astellatol

A nearly-30-year-old unanswered synthetic puzzle, astellatol, has been solved in an enantiospecific manner. The highly congested pentacyclic skeleton of this rare sesterterpenoid, which possesses a unique bicyclo[4.1.1]octane motif, ten stereocenters, a cyclobutane that contains two quaternary centers, an exo-methylene group, and a sterically encumbered isopropyl trans-hydrindane motif, makes astellatol arguably one of the most challenging targets for sesterterpenoid synthesis. An intramolecular Pauson-Khand reaction was exploited to construct the right-hand side scaffold of this sesterterpenoid. An unprecedented reductive radical 1,6-addition, mediated by SmI₂ , forged the cyclobutane motif. Last, a strategic oxidation/reduction step provided not only the decisive solution for the remarkably challenging late-stage transformations, but also a highly valuable unravelling of the notorious issue of trans-hydrindane synthesis. Importantly, the synthesis of astellatol showcases a rapid, scalable strategy to access diverse complex isopropyl trans-hydrindane sesterterpenoids.

Benzoxazole Alkaloids: Occurrence, Chemistry, and Biology

Benzoxazole alkaloids exhibit a diverse array of structures and interesting biological activities. In spite of the extensive research done on the synthesis and biology, till date there is no concise update on this class of alkaloids. This chapter summarizes the literature on benzoxazole alkaloids till March 2017, which covers their isolation, characterization, possible biosynthetic origins, biological activities, and major synthetic approaches. These alkaloids have been broadly classified in the context of their sources, namely (i) fungal origin, (ii) marine origin, and (iii) plant origin.

Navigating the Chiral Pool in the Total Synthesis of Complex Terpene Natural Products

The pool of abundant chiral terpene building blocks (i.e., "chiral pool terpenes") has long served as a starting point for the chemical synthesis of complex natural products, including many terpenes themselves. As inexpensive and versatile starting materials, such compounds continue to influence modern synthetic chemistry. This review highlights 21st century terpene total syntheses which themselves use small, terpene-derived materials as building blocks. An outlook to the future of research in this area is highlighted as well.

Muscarine, imidazole, oxazole and thiazole alkaloids

Covering: July 2012 to June 2015. Previous review: Nat. Prod. Rep., 2013, 30, 869-915The structurally diverse imidazole-, oxazole-, and thiazole-containing secondary metabolites are widely distributed in terrestrial and marine environments, and exhibit extensive pharmacological activities. In this review the latest progress involving the isolation, biological activities, and chemical and biogenetic synthesis studies on these natural products has been summarized.

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

Synthesis and Photooxidation of the Trisubstituted Oxazole Fragment of the Marine Natural Product Salarin C

The eastern section of the macrocyclic marine natural product salarin C from the sponge Fascaplysinopsis sp. was synthesized employing a halogen dance reaction to assemble the trisubstituted oxazole moiety. The synthesis was inspired by Kashman's hypothesis on the biomimetic oxidation of salarin C to salarin A. Clean conversion to the triacylamine partial structure of salarin A occurred on treatment with photochemically generated singlet oxygen. Thus, a Wasserman-type oxidative rearrangement is chemically possible in this case.

Recent Advances in the Pauson-Khand Reaction

The Pauson-Khand [2+2+1] cycloaddition of alkynes, alkenes, and carbon monoxide has been a vibrant area of research for more than 40 years. This review highlights recent achievements in the Pauson-Khand reaction, particularly in catalytic and asymmetric variants. Discussion of regioselectivity and advances in substrate scope is also presented.

Total Syntheses of (+)-Marrubiin and (-)-Marrulibacetal

A stereoselective total synthesis of (+)-marrubiin has been accomplished starting from a chiral building block via the CyNH2-promoted Pauson-Khand reaction (PKR) followed by oxidative cleavage of the resultant cyclopentenone ring. Two successive oxidations and internal transacetalization culminated in the total synthesis of the antispasmodic labdane diterpenoid (-)-marrulibacetal.

Marine natural products

This review covers the literature published in 2014 for marine natural products (MNPs), with 1116 citations (753 for the period January to December 2014) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1378 in 456 papers for 2014), together with the relevant biological activities, source organisms and country of origin. Reviews, biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.

Iron-promoted C-C bond formation in the total synthesis of natural products and drugs

Iron salts are inexpensive and almost innocuous; they are thus the promoters of choice, even in stoichiometric amounts, for the formation of carbon-carbon bonds in the backbone of complex molecules. This review encompasses the key role of iron complexes in the total synthesis of some natural products or pharmacologically important compounds.

Synthesis and antimicrobial activity of binaphthyl-based, functionalized oxazole and thiazole peptidomimetics

Thirty two new binaphthyl-based, functionalized oxazole and thiazole peptidomimetics and over thirty five novel leucine-containing intermediate oxazoles and thiazoles were prepared in this study. This includes the first examples of the direct C-5 arylation of an amino acid dipeptide-derived oxazole. Moderate to excellent antibacterial activity was observed for all new compounds across Gram positive isolates with MICs ranging from 1-16 μg mL(-1). Results for Gram negative E. coli and A. baumannii were more variable, but MICs as low as 4 μg mL(-1) were returned for two examples. Significantly, the in vitro results with a fluoromethyl-oxazole derivative collectively represent the best obtained to date for a member of our binaphthyl peptide antimicrobials.

Total synthesis of myceliothermophins C, D, and E

The total synthesis of cytotoxic polyketides myceliothermophins E (1), C (2), and D (3) through a cascade-based cyclization to form the trans-fused decalin system is described. The convergent synthesis delivered all three natural products through late-stage divergence and facilitated unambiguous C21 structural assignments for 2 and 3 through X-ray crystallographic analysis, which revealed an interesting dimeric structure between its enantiomeric forms.