A Total Synthesis of (±)-trans-Kumausyne

Selective Transformation of Norbornadiene into Functionalized Azaheterocycles and β-Amino Esters with Stereo- and Regiocontrol

Novel functionalized azaheterocycles with multiple chiral centers have been accessed from readily available norbornene β-amino acids or β-lactams across a stereocontrolled synthetic route, based on ring-opening metathesis (ROM) of the staring unsaturated bicyclic amino esters, followed by selective cyclization through ring-closing metathesis (RCM). The RCM transformations have been studied under various experimental conditions to assess the scope of conversion, catalyst, yield, and substrate influence. The structure of the starting norbornene β-amino acids predetermined the structure of the new azaheterocycles, and the developed synthetic route took place with the conservation of the configuration of the chiral centers.

Advances in Total Synthesis of Some 2,3,5-Trisubstituted Tetrahydrofuran Natural Products

2,3,5-Trisubstituted tetrahydrofuran moiety is ubiquitous in natural products. These have served as appealing candidates for total synthesis due to their varied bio- and pharmaceutical activities. This tutorial review delineates the ingenious efforts by many researchers in the total synthesis of selected natural products based on a common 2,3,5-trisubstituted tetrahydrofuran core structure. Many of the syntheses display nuanced interplay between new methods and the ingenuity of planned strategies achieved through catalysis or cascade chemistry. In some cases, the chiron approach has come quite handy, wherein the structural features and the stereochemistry in select molecules could map well with naturally available starting materials. This compilation also aims to enhance the diversity space based on these natural products and further interest in sustainable total synthesis.

Rapid Construction of the Scaffold of Quorumolide A Enabled by a Tandem ROM/RCM Strategy and a Tandem Oxidative Cyclization Strategy

A synthetically challenging framework in the plant-derived cembranoid quorumolide A was established in a seven-step sequence featuring a ring-opening/-closing metathesis cascade reaction to construct the fused butenolide ring and the 14-membered macroring in a single step. The utilization of a tandem oxidative cyclization strategy is the key to build the tetrahydro-2H-pyran moiety.

Domino ring-opening-ring-closing enyne metathesis vs enyne metathesis of norbornene derivatives with alkynyl side chains. Construction of condensed polycarbocycles

The metathesis of norbornene derivatives with alkynyl side-chain with Grubbs’ ruthenium alkylidine as catalyst has been investigated with the objective of constructing condensed polycyclic structures. This investigation demonstrated that the generally observed domino reaction course involving a ring-opening metathesis of the norbornene unit and a ring-closing enyne metathesis is influenced to a great extent by the nature of the functional group and the substrate structure and may follow a different reaction course than what is usually observed. In cases where ROM–RCEYM occurred, the resulting 1,3-diene reacts in situ with the dienophile to provide condensed tetracyclic systems.

Exploration of Ring Rearrangement Metathesis Reaction: A General and Flexible Approach for the Rapid Construction [5,n]-Fused Bicyclic Systems en Route to Linear Triquinanes

Structurally diverse [5,n] bicyclic systems with cis ring junction stereochemistry were accessed readily through RRM (ring rearrangement metathesis) reaction of properly functionalized [2.2.1]norbornene skeletons. Several bicyclic enones, ketones, alcohols, and ethers acted as the substrates and yielded the respective linearly fused [5,n] bicyclic systems stereoselectively after the RRM reaction. Such [5,5]bicyclic enone scaffolds were then synthetically manipulated to core structural analogue of naturally occurring liner triquinane hirsutene having cis-syn-cis stereochemistry.

A polystyrene supported [PdCl–(SeCSe)] complex: a novel, reusable and robust heterogeneous catalyst for the Sonogashira synthesis of 1,2-disubstituted alkynes and 1,3-enynes

PS-[PdCl–(SeCSe)] has been introduced as a novel and heterogeneous catalyst, which exhibits outstanding catalytic activity in the Sonogashira cross-coupling reactions.

A highly efficient synthesis of the DEFG-ring system of rubriflordilactone B

A highly efficient synthesis of the DEFG-ring system of rubriflordilactone B via polar-radical-crossover cycloaddition and carbonyl allylation/lactonization is described.

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.

O-Monoacyltartaric acid catalyzed enantioselective conjugate addition of a boronic acid to dienones: application to the synthesis of optically active cyclopentenones

Enantioselective conjugate addition of styrylboronic acid to dienones was effectively catalyzed by an O-monoacyltartaric acid to afford monostyrylated products with good enantioselectivity. The RCM of the monostyrylated products using the Hoveyda-Grubbs II catalyst afforded optically active cyclopentenones, including a synthetic intermediate of the antitumor agent TEI-9826. The study shows that a diene additive such as 1,6-heptadiene or diallyl ether was essential for the RCM.

Collective synthesis of humulanolides using a metathesis cascade reaction

A new method has been developed for the concise and asymmetric synthesis of seven humulanolides in 5-7 steps without the need for protecting groups. Notably, the challenging 11-membered ring and bridged butenolide moieties in asteriscunolide D and 6,7,9,10-tetrahydroasteriscunolide were introduced in one step using a ring-opening/ring-closing metathesis cascade reaction. Asteriscunolide D was used as a versatile synthetic precursor to prepare asteriscunolides A-C via a photoinduced isomerization reaction, asteriscanolide via a unique transannular Michael reaction, and 6,7,9,10-tetradehydroasteriscanolide via a transannular Morita-Baylis-Hillman-type reaction. The unique bicyclo[6.3.0]undecane core was introduced diastereoselectively.

Computation and experiment reveal that the ring-rearrangement metathesis of Himbert cycloadducts can be subject to kinetic or thermodynamic control

Unusual observations in the ring-rearrangement metathesis of Himbert arene/allene cycloadducts to form fused polycylic lactams led to a more in-depth experimental study that yielded conflicting results. Differences in reactivity within related systems and unexpected changes in diastereoselectivity among other similar substrates were not readily explained on the basis of the experimental results. Computational investigations demonstrated substrate-dependent changes in reaction pathways (ring-opening metathesis/ring-closing metathesis [ROM/RCM] cascade vs ring-closing metathesis/ring-opening metathesis [RCM/ROM] cascade). Furthermore, some reactions were judged to be under thermodynamic control and others under kinetic control. The greater understanding of the most likely reaction pathways and their energetics provides a reasonable explanation for the previously irreconcilable results.

Stereoselective total synthesis of (-)-kumausallene

A stereoselective total synthesis of (-)-kumausallene was completed in 12 steps from acetylacetone. The hidden symmetry of (-)-kumausallene was recognized, and its skeleton was constructed efficiently from a C(2)-symmetric diol by a palladium-catalyzed cascade reaction. High diastereoselectivity was observed for the DMF-promoted biomimetic 1,4-bromocyclization of a conjugated enyne.

Ring-rearrangement metathesis of nitroso Diels-Alder cycloadducts

Strained nitroso Diels-Alder bicyclo[2.2.1] or [2.2.2] adducts functionalized with alkene side chains of diverse length undergo a ring-rearrangement metathesis process with external alkenes and Grubbs II or Hoveyda-Grubbs II ruthenium catalysts, under microwave irradiation or classical heating, to deliver cis-fused bicycles of various ring sizes, which contain a N-O bond. These scaffolds are of synthetic relevance for the generation of molecular diversity and to the total synthesis of alkaloids. The observation of unexpected reactions, such as epimerization or one-carbon homologation of the alkene side chain, is also reported.

Metathesis Cascade Strategies (ROM-RCM-CM): A DOS approach to Skeletally Diverse Sultams

The development of a ring-opening metathesis/ring-closing metathesis/cross metathesis (ROM-RCM-CM) cascade strategy to the synthesis of a diverse collection of bi- and tricyclic sultams is reported. In this study, functionalized sultam scaffolds derived from intramolecular Diels-Alder (IMDA) reactions undergo metathesis cascades to yield a collection tricyclic sultams. Additional appendage based diversity was achieved by utilizing a variety of CM partners.

Development of a concise and general enantioselective approach to 2,5-disubstituted-3-hydroxytetrahydrofurans

Concise syntheses of 2,5-disubstituted-3-hydroxytetrahydrofurans have been developed that provide access to each configurational isomer of this scaffold from a single aldol adduct. Application of these methods to the rapid preparation of (6S,7S,9S,10S)- and (6S,7S,9R,10R)-6,9-epoxynonadec-18-ene-7,10-diol, two structurally related marine epoxylipids, is reported.

A total synthesis of norhalichondrin B

Four corners: The syntheses of four key building blocks for the total synthesis of norhalichondrin B (see structure) are described. The assembly of these subunits into the natural product is also reported. Key features of the synthesis are the use of the Achmatowicz oxidation/ionic hydrogenation for the synthesis of pyrans and pyranopyrans, and the application of tandem metathesis for the synthesis of pyranopyrans.

A concise total synthesis of largazole, solution structure, and some preliminary structure activity relationships

A total synthesis of largazole that proceeds in 8 steps from commercial materials is reported, along with some structure-activity relationships. A combination of NMR studies and molecular modeling have also provided a preliminary picture of the conformation of largazole.

Ring-Rearrangement Metathesis

Ring-rearrangement metathesis (RRM) refers to the combination of several metathesis transformations into a domino process, in which an endocyclic double bond of a cycloolefin reacts with an exocyclic alkene. RRM has proven to be a powerful method for the rapid construction of complex structures. The extension of the basic ring-opening-ring-closing metathesis process by further metathesis steps as well as an examination of the driving forces, limits, scope, recent advantages, and future perspectives of these domino sequences is presented with various examples, thus reflecting the high efficiency and utility of RRM in organic synthesis.