An Approach to Lauroxanes by Iterative Use of Co2(CO)6-Acetylenic Complexes. A Formal Synthesis of (+)-Laurencin

A Reverse Approach to the Total Synthesis of Halichondrin B

A new strategy is described for the total synthesis of halichondrin B featuring reversal of the sequential construction of a number of its cyclic ethers from the classical approach by instead forming C-O bonds first followed by C-C bond formation. Employing the Nicholas reaction to generate linear ethers as precursors for the total synthesis of halichondrin B and other members of the halichondrin and eribulin families of compounds, this novel approach provides new opportunities for the development of improved syntheses of these complex and valuable compounds. In this Article, we report the syntheses of defined fragments I, MN, EFG, and A. Fragments I and MN were then coupled and elaborated to advanced intermediate IJKLMN, which was joined with fragment EFG to afford, after appropriate elaboration and macrolactonization, the more advanced polycyclic intermediate EFGHIJKLMN. Elaboration of the latter and coupling with fragment A followed by further functionalization completed the total synthesis of halichondrin B through a short and convergent pathway.

Enantiodivergent Cyclization by Inversion of the Reactivity in Ambiphilic Molecules

Inverting the reactivity of the functional groups in ambiphilic molecules provides a new synthetic strategy to perform late-stage enantiodivergence. Both enantiomers of the final compound can be obtained from a common chiral precursor. As a proof of concept, the synthesis of substituted five- and six-membered oxacycles is described. The key step is the cyclization of an ambiphilic linear precursor bearing a propargylic alcohol and an epoxide linked through an alkyl chain. Through a slight modification of these linear precursors and employing different reaction conditions, these functional groups can inverse their chemical reactivity, producing one enantiomer or another of the final product. This enantiodivergent cyclization involves three stereogenic centers that can undergo fully controlled retention or inversion of their configuration depending on the cyclization pathway that is activated. The cyclization provides late-stage enantiodivergence, enabling the synthesis of either enantiomers of the oxacycles from a common chiral substrate with total transfer of the enantiomeric purity.

Formal Synthesis of (+)-Laurencin by Gold(I)-Catalyzed Intramolecular Dehydrative Alkoxylation

8-Membered cyclic ethers are found in a wide range of natural products; however, they are challenging synthetic targets due to enthalpic and entropic barriers. The gold(I)-catalyzed intramolecular dehydrative alkoxylation of ω-hydroxy allylic alcohols was explored to stereoselectively construct α,α'-cis-oxocenes and further applied in a formal synthesis of (+)-laurencin. The gold(I)-catalyzed intramolecular dehydrative alkoxylation may constitute an alternative method for the synthesis of molecular building blocks and natural products that contain highly functionalized 8-membered cyclic ethers.

Medium-Sized Cyclic Ethers via Stevens [1,2]-Shift of Mixed Monothioacetal-Derived Sulfonium Ylides: Application to Formal Synthesis of (+)-Laurencin

A novel approach to medium-sized cyclic ethers was devised using a Stevens [1,2]-shift of a sulfonium ylide derived from a readily accessible six-membered mixed-monothioacetal precursor. The concise and efficient transformation offers a surprising degree of chirality transfer with observed retention of stereochemical configuration on the anomeric migrating carbon and has been applied as the key step in an enantioselective formal synthesis of (+)-laurencin.

Alkyne Ligation Handles: Propargylation of Hydroxyl, Sulfhydryl, Amino, and Carboxyl Groups via the Nicholas Reaction

The Nicholas reaction has been applied to the installation of alkyne ligation handles. Acid-promoted propargylation of hydroxyl, sulfhydryl, amino, and carboxyl groups using dicobalt hexacarbonyl-stabilized propargylium ions is reported. This method is useful for introduction of propargyl groups into base-sensitive molecules, thereby expanding the toolbox of methods for the incorporation of alkynes for bio-orthogonal reactions. High-value molecules are used as the limiting reagent, and various propargylium ion precursors are compared.

The Laurencia Paradox: An Endless Source of Chemodiversity

Nature, the most prolific source of biological and chemical diversity, has provided mankind with treatments for health problems since ancient times and continues to be the most promising reservoir of bioactive chemicals for the development of modern drugs. In addition to the terrestrial organisms that still remain a promising source of new bioactive metabolites, the marine environment, covering approximately 70% of the Earth's surface and containing a largely unexplored biodiversity, offers an enormous resource for the discovery of novel compounds. According to the MarinLit database, more than 27,000 metabolites from marine macro- and microorganisms have been isolated to date providing material and key structures for the development of new products in the pharmaceutical, food, cosmeceutical, chemical, and agrochemical sectors. Algae, which thrive in the euphotic zone, were among the first marine organisms that were investigated as sources of food, nutritional supplements, soil fertilizers, and bioactive metabolites.Red algae of the genus Laurencia are accepted unanimously as one of the richest sources of new secondary metabolites. Their cosmopolitan distribution, along with the chemical variation influenced to a significant degree by environmental and genetic factors, have resulted in an endless parade of metabolites, often featuring multiple halogenation sites.The present contribution, covering the literature until August 2015, offers a comprehensive view of the chemical wealth and the taxonomic problems currently impeding chemical and biological investigations of the genus Laurencia. Since mollusks feeding on Laurencia are, in many cases, bioaccumulating, and utilize algal metabolites as chemical weaponry against natural enemies, metabolites of postulated dietary origin of sea hares that feed on Laurencia species are also included in the present review. Altogether, 1047 secondary metabolites, often featuring new carbocyclic skeletons, have been included.The chapter addresses: (1) the "Laurencia complex", the botanical description and the growth and population dynamics of the genus, as well as its chemical diversity and ecological relations; (2) the secondary metabolites, which are organized according to their chemical structures and are classified into sesquiterpenes, diterpenes, triterpenes, acetogenins, indoles, aromatic compounds, steroids, and miscellaneous compounds, as well as their sources of isolation which are depicted in tabulated form, and (3) the biological activity organized according to the biological target and the ecological functions of Laurencia metabolites.

A Tin(IV) Chloride Promoted Tandem C-O Bond Cleavage/Nazarov Cyclization/Nucleophilic Addition Reaction of 1,1-Disubstituted Allylic Ethers toward the Synthesis of Multisubstituted Indenes

A novel SnCl4-promoted tandem reaction toward multisubstituted indenes via a sequential C-O bond cleavage/Nazarov cyclization/nucleophilic addition reaction has been developed to afford a series of multisubstituted indenes with an all-carbon quaternary center in moderate to good yields.

Propargylic cation-induced intermolecular electrophilic addition–semipinacol rearrangement

A novel propargylic electrophile-induced tandem intermolecular addition–semipinacol rearrangement was developed for the synthesis of a series of synthetically useful β-propargyl spirocyclic ketones. Its synthetic application was demonstrated by efficient construction of the key tricyclic moiety of daphlongamine E.

β-Hydroxy-γ-lactones as nucleophiles in the Nicholas reaction for the synthesis of oxepene rings. Enantioselective formal synthesis of (−)-isolaurepinnacin and (+)-rogioloxepane A

The enantioselective formal synthesis of (−)-isolaurepinnacin and (+)-rogioloxepane A has been achieved.

Concise synthetic approaches for the Laurencia family: formal total syntheses of (±)-laurefucin and (±)-E- and (±)-Z-pinnatifidenyne

Herein is presented a cohesive strategy to rapidly fashion diverse members of the lauroxocane family of natural products, leading to the shortest syntheses of any member to date. These efforts include racemic formal total syntheses of laurefucin and E- and Z-pinnatifidenyne as well as a facile preparation of the oxocene core of 3E-dehydrobromolaurefucin. The key elements of the design are novel diastereoselective ring-expanding bromoetherifications of tetrahydrofurans triggered by a unique bromonium source (BDSB, Et(2)SBr·SbBrCl(5)) and strategically positioned nucleophilic traps, where altering the identity and position of these traps affords diverse functionality on the eight-membered ring backbone. Its biogenetic relevance is also discussed in light of the range of substrates that successfully undergo this key rearrangement.

Nonterpenoid C15 acetogenins from Laurencia marilzae

Eight new halogenated C(15) acetogenins, 1-8, were isolated from the organic extract of the red alga Laurencia marilzae. The structure elucidation and the assignments of the relative configurations were established by extensive use of spectroscopic studies, particularly 1D and 2D NMR data, while the absolute configurations of compounds 1 and 5 were determined by single-crystal X-ray diffraction analysis. Compounds 1, 2, 4, 5, and 7, along with the previously reported related cyclic ether obtusallene IV (9), were evaluated against six human solid tumor cell lines. All compounds were found to be essentially inactive (GI(50) > 10 μg/mL).