A Catalytic Asymmetric Three-Component 1,4-Addition/Aldol Reaction: Enantioselective Synthesis of the Spirocyclic System of Vannusal A

Unique Reactivity of the 1,4-Bis(silyloxy)-1,3-cyclopentadiene Moiety: Application to the Synthesis of 7-Norbornanone Derivatives

We describe a method for the synthesis of various 2-silyloxy-2-norbornen-7-ones by exploiting the specific reactivity of the 1,4-bis(silyloxy)-1,3-cyclopentadiene framework, which is generated by the silylation of a 2,2-disubstituted-1,3-cyclopentanedione bearing a picolinoyloxy group at the 2' position of its C-2 side chain. The release of the acyloxy group during the reaction generates carbocations that are then attacked by silyloxy-substituted carbons in the 1,4-bis(silyloxy)-1,3-cyclopentadiene moiety skeleton, forming a 4,5-cis-fused ring skeleton. Skeletal rearrangement of the bicyclic core results in the formation of the corresponding 2-silyloxy-2-norbornen-7-one. This novel transformation of 1,3-cyclopentanedione moieties can be used to synthesise other cyclopentenone-fused bicyclic frameworks.

Diastereo- and Enantioselective Construction of Stereochemical Arrays Exploiting Non-Classical Hydrogen Bonding in Enolborates

We report a copper-catalyzed reductive aldol addition to aldehydes and ketones, with pinacolborane as stoichiometric reductant, that results in the generation of stereodefined syn-aldol products. Cyclic, acyclic, fused and spirocyclic aldols bearing contiguous stereocenters are obtained with excellent yields and diastereoselectivities. Moreover, enantioselective reactions could be carried out with cycloalkenones to deliver aldols bearing three contiguous stereocenters and with up to 98 % ee. Computations reveal that the enolborate intermediate undergoes the syn-aldol reaction via a twist-boat transition state that is stabilized by non-classical hydrogen bonding interactions.

Recent Advances in Catalysis Involving Bidentate N-Heterocyclic Carbene Ligands

Since the discovery of persistent carbenes by the isolation of 1,3-di-l-adamantylimidazol-2-ylidene by Arduengo and coworkers, we witnessed a fast growth in the design and applications of this class of ligands and their metal complexes. Modular synthesis and ease of electronic and steric adjustability made this class of sigma donors highly popular among chemists. While the nature of the metal-carbon bond in transition metal complexes bearing N-heterocyclic carbenes (NHCs) is predominantly considered to be neutral sigma or dative bonds, the strength of the bond is highly dependent on the energy match between the highest occupied molecular orbital (HOMO) of the NHC ligand and that of the metal ion. Because of their versatility, the coordination chemistry of NHC ligands with was explored with almost all transition metal ions. Other than the transition metals, NHCs are also capable of establishing a chemical bond with the main group elements. The advances in the catalytic applications of the NHC ligands linked with a second tether are discussed. For clarity, more frequently targeted catalytic reactions are considered first. Carbon-carbon coupling reactions, transfer hydrogenation of alkenes and carbonyl compounds, ketone hydrosilylation, and chiral catalysis are among highly popular reactions. Areas where the efficacy of the NHC based catalytic systems were explored to a lesser extent include CO2 reduction, C-H borylation, alkyl amination, and hydroamination reactions. Furthermore, the synthesis and applications of transition metal complexes are covered.

Diastereo-, Enantio-, and Z-Selective α,δ-Difunctionalization of Electron-Deficient Dienes Initiated by Rh-Catalyzed Conjugate Addition

Metal-catalyzed enantioselective conjugate additions are highly reliable methods for stereoselective synthesis; however, multicomponent reactions that are initiated by conjugate arylation of acyclic π-systems are rare. These reactions generally proceed with poor diastereoselectivity while requiring basic, moisture sensitive organometallic nucleophiles. Here, we show that Rh-catalysts supported by a tetrafluorobenzobarrelene ligand (Ph-tfb) enable the enantio-, diastereo-, and Z-selective α,δ-difunctionalization of electron-deficient 1,3-dienes with organoboronic acid nucleophiles and aldehyde electrophiles to generate Z-homoallylic alcohols with three stereocenters. The reaction accommodates diene substrates activated by ester, amide, ketone, or aromatic groups and can be used to couple aryl, alkenyl, or alkyl aldehydes. Diastereoselective functionalization of the Z-olefin unit in the addition products allows for the generation of compounds with five stereocenters in high dr and ee. Mechanistic studies suggest aldehyde allylrhodation is the rate-determining step, and unlike reactions of analogous Rh-enolates, the Rh-allyl species generated by δ-arylation undergoes aldehyde trapping rather than protonolysis, even when water is present as a cosolvent. These findings should have broader implications in the use of privileged metal-catalyzed conjugate addition reactions as entry points toward the preparation of acyclic molecules containing nonadjacent stereocenters.

Synthetic Study Aiming at the Tricyclic Core of 12-epi-JBIR-23/24

The synthetic study toward highly enantio- and diastereoselective synthesis of the tricyclic framework of 12-epi-JBIR-23/24, a natural product analogue showing inhibitory activity against four malignant pleural mesothelioma cell lines, is presented herein. In this synthesis, a rhodium-catalyzed asymmetric three-component Michael/aldol reaction introduces three consecutive tertiary carbon centers, while the unique epoxyquinol core motif is successfully forged via [3,3]-sigmatropic rearrangement of an allylic xanthate, vinylogous Pummerer rearrangement, and a selective mesylation/epoxidation cascade of a triol.

Biomimetic Approaches to the Synthesis of Natural Disesquiterpenoids: An Update

Natural disesquiterpenoids represent a small group of secondary metabolites characterized by complex molecular scaffolds and interesting pharmacological profiles. In the last decade, more than 400 new disesquiterpenoids have been discovered and fully characterized, pointing out once more the "magic touch" of nature in the design of new compounds. The perfect blend of complex and unique architectures and biological activity has made sesquiterpene dimers an attractive and challenging synthetic target, inspiring organic chemists to find new and biomimetic approaches to replicate the efficiency and the selectivity of natural processes under laboratory conditions. In this work, we present a review covering the literature from 2010 to 2020 reporting all the efforts made in the total synthesis of complex natural disesquiterpenoids.

Asymmetric Conjugate Addition of Chiral Secondary Borylalkyl Copper Species

We report the diastereo- and enantioselective conjugate addition of chiral secondary borylalkyl copper species derived from borylalkenes in situ to α,β-unsaturated diesters. In the presence of a chiral bisphosphine-ligated CuH catalyst, the conjugate addition provides a direct access to enantioenriched alkylboron compounds containing two contiguous carbon stereogenic centers in good yield with high diastereo- and enantioselectivity (up to >98:2 dr, >99:1 er) by assembling readily available starting alkenyl reagents in a single operation without using preformed organometallic reagents or chiral auxiliaries. The resulting products were used in various organic transformations. The utility of the synthetic approach was highlighted by the synthesis of (-)-phaseolinic acid.

Are Organozirconium Reagents Applicable in Current Organic Synthesis?

The search for mild, user-friendly, easily accessible, and robust organometallic reagents is an important feature of organometallic chemistry. Ideally, new methodologies employing organometallics should be developed with respect to practical applications in syntheses of target compounds. In this short review, we investigate if organozirconium reagents can fulfill these criteria. Organozirconium compounds are typically generated via in situ hydrozirconation of alkenes or alkynes with the Schwartz reagent. Alkyl and alkenylzirconium reagents have proven to be convenient in conjugate additions, allylic substitutions, cross-coupling reactions, and additions to carbonyls or imines. Furthermore, the Schwartz reagent itself is a useful reducing agent for polar functional groups.

1 Introduction

2 Synthesis and Generation of the Schwartz Reagent

3 Structure and Properties of Cp2Zr(H)Cl

4 Reactivity of Organozirconium Reagents

4.1 Asymmetric Conjugate Addition

4.2 Asymmetric Allylic Alkylations

4.3 Desymmetrization Reactions

4.4 Cross-Coupling Reactions

4.5 1,2-Additions

5 Conclusions

Total Synthesis Endeavors and Their Contributions to Science and Society:A Personal Account

The advent of organic synthesis in the 19th century, serendipitous as it was, set in motion a revolution in science that continues to evolve into increasing levels of sophistication and to expand into new domains of science and technology for the benefits of science and society. Its evolution was always driven by the challenges posed by natural products, whose structures were becoming increasingly complex and diverse. In response to these challenges, synthetic organic chemists were prompted to sharpen their art to reach their target molecules, whose structures were often confirmed only after their synthesis in the laboratory through the art and science of total synthesis. The latter became the “locomotive” and the “flagship” of organic synthesis, for through this practice novel synthetic methods were discovered and invented, and also tested for their generality, applicability, and scope with regard to molecular complexity and diversity. The purpose of total synthesis has also evolved over the years to include aspects beyond the synthesis of the molecule and confirmation of its structure. In this article, we briefly review the evolution of total synthesis in terms of its power and reach and demonstrate its current state of the art that combines fundamentals with translational aspects through examples from our laboratories. The highlighted examples reflect the newly emerged paradigm of the discipline that includes—in addition to the total synthesis of the target molecule—structural elucidations, method discovery and development, design, synthesis, and biological evaluation of analogues for biology and medicine, and training of young students, preparing them for academic and industrial careers in the various disciplines that require knowledge and skills to practice the central science of chemical synthesis. Such disciplines include chemical biology, drug discovery and development, materials science and nanotechnology, and other endeavors whose fundamentals depend and rely on the structure of the molecule and its synthesis.

N-Heterocyclic Carbene-Cu-Catalyzed Enantioselective Allenyl Conjugate Addition

A catalytic enantioselective conjugate addition with commercially available allenylboronic acid pinacol ester as nucleophile promoted by a chiral copper complex of N-heterocyclic carbene (NHC) is disclosed. This process constitutes an unprecedented instance of the conjugate addition that introduces an allenyl group into α,β-unsaturated carbonyl compounds, affording products that are otherwise difficult to access in up to 92% yield, >98% allenyl addition selectivity and 96:4 enantiomeric ratio. DFT calculations were performed to elucidate the origins of enantioselectivity.

Construction of β to carbonyl stereogenic centres by asymmetric 1,4-addition of alkylzirconocenes to dienones and ynenones

A new asymmetric addition of alkylzirconium species to dienones, a dienthioate, ynenones, and ynethionates with good yields and excellent ee's is reported.

Synthetic Strategies toward Natural Products Containing Contiguous Stereogenic Quaternary Carbon Atoms

Strategies for the total synthesis of complex natural products that contain two or more contiguous stereogenic quaternary carbon atoms in their intricate structures are reviewed with 12 representative examples. Emphasis has been put on methods to create quaternary carbon stereocenters, including syntheses of the same natural product by different groups, thereby showcasing the diversity of thought and individual creativity. A compendium of selected natural products containing two or more contiguous stereogenic quaternary carbon atoms and key reactions in their total or partial syntheses is provided in the Supporting Information.

Activation of Marginally Reactive Boron Enolates by MeLi for the Formation of Enol Phosphates and Synthesis of the Δ(9)-THC Intermediate

The addition of MeLi to boron enolates produced by the 1,4-addition of Ar2Cu(CN)Li2 to BF3·OEt2-activated enones was followed by the reaction with ClP(O)(OEt)2 to afford the corresponding enol phosphates in moderate to good yields. The scope of this method was examined with sterically hindered or electronically biased enones and/or reagents. This activation of boron enolates was successfully applied to the synthesis of the methyl ether of Δ(9)-tetrahydrocannabinol.

Palladium-catalyzed enantioselective Heck alkenylation of acyclic alkenols using a redox-relay strategy

We report a highly enantioselective intermolecular Heck reaction of alkenyl triflates and acyclic primary or racemic secondary alkenols. The mild reaction conditions permit installation of a wide range of alkenyl groups at positions β, γ, or δ to a carbonyl group in high enantioselectivity. The success of this reaction is attributed to the use of electron-withdrawing alkenyl triflates, which offer selective β-hydride elimination followed by migration of the catalyst through the alkyl chain to give the alkenylated carbonyl products. The synthetic utility of the process is demonstrated by a two-step modification of a reaction product to yield a tricyclic core structure, present in various natural products.

Prodigiosin alkaloids: recent advancements in total synthesis and their biological potential

The present review article is focused on the medicinal potential and total synthesis of prodigiosins witnessed in the last decade. The aim will be to provide an inspiration to the marvels and pit falls of constructing the polypyrrole heterocycles with in the complex systems.

Total syntheses of natural products containing spirocarbocycles

The spiro motif is becoming an increasingly prevalent structure in medicinal and organic chemistry. The total syntheses of natural products containing all-carbon spirocycles is reviewed.

Rhodium and copper-catalyzed asymmetric conjugate addition of alkenyl nucleophiles

Since the initial reports in the mid-90s, metal catalyzed asymmetric conjugate addition (ACA) reactions evolved as an important tool for the synthetic chemist. Most of the research efforts have been done in the field of rhodium and copper catalyzed ACA reactions employing aryl and alkyl nucleophiles. Despite the great synthetic value of the double bond, the addition of alkenyl nucleophiles remains insufficiently explored. In this account, an overview of the developments in the field of rhodium and copper catalyzed ACA reactions with organometallic alkenyl reagents (B, Mg, Al, Si, Zr, Sn) will be provided. The account is intended to give a comprehensive overview of all the existing methods. However, in many cases only selected examples are displayed in order to facilitate comparison of different ligands and methodologies.

Constructing molecular complexity and diversity: total synthesis of natural products of biological and medicinal importance

The advent of organic synthesis and the understanding of the molecule as they occurred in the nineteenth century and were refined in the twentieth century constitute two of the most profound scientific developments of all time. These discoveries set in motion a revolution that shaped the landscape of the molecular sciences and changed the world. Organic synthesis played a major role in this revolution through its ability to construct the molecules of the living world and others like them whose primary element is carbon. Although the early beginnings of organic synthesis came about serendipitously, organic chemists quickly recognized its potential and moved decisively to advance and exploit it in myriad ways for the benefit of mankind. Indeed, from the early days of the synthesis of urea and the construction of the first carbon-carbon bond, the art of organic synthesis improved to impressively high levels of sophistication. Through its practice, today chemists can synthesize organic molecules--natural and designed--of all types of structural motifs and for all intents and purposes. The endeavor of constructing natural products--the organic molecules of nature--is justly called both a creative art and an exact science. Often called simply total synthesis, the replication of nature's molecules in the laboratory reflects and symbolizes the state of the art of synthesis in general. In the last few decades a surge in total synthesis endeavors around the world led to a remarkable collection of achievements that covers a wide ranging landscape of molecular complexity and diversity. In this article, we present highlights of some of our contributions in the field of total synthesis of natural products of biological and medicinal importance. For perspective, we also provide a listing of selected examples of additional natural products synthesized in other laboratories around the world over the last few years.

Total synthesis of the G2/M DNA damage checkpoint inhibitor psilostachyin C

A concise total synthesis of the G2/M DNA damage checkpoint inhibitor psilostachyin C is reported using a 1,4-addition-aldol condensation-ring-closing metathesis (RCM) strategy. Initial biological studies indicate that psilostachyin C could enhance the sensitivity of the HeLa cell toward camptothecin (CPT) treatment via the activation of the caspase-3 mediated apoptosis pathway.

Formation of vinyl-, vinylhalide- or acyl-substituted quaternary carbon stereogenic centers through NHC-Cu-catalyzed enantioselective conjugate additions of Si-containing vinylaluminums to β-substituted cyclic enones

A catalytic method for enantioselective conjugate addition (ECA) of Si-containing vinylaluminum reagents to β-substituted cyclopentenones and cyclohexenones is described. Reactions are promoted by 1.0-5.0 mol % of a bidentate NHC-Cu complex, which is prepared from air-stable CuCl(2)•2H(2)O and used in situ, and typically proceed to completion within 15-20 min. The requisite vinylmetals are generated efficiently by a site-selective hydroalumination of an alkyne with dibal-H. The desired products, containing a quaternary carbon stereogenic center, are obtained in 48-95% yield after purification and in 89:11 to >98:2 enantiomer ratio (er). The vinylsilane moiety within the products can be functionalized to afford acyl, vinyliodide, or desilylated alkenes in 67% to >98% yield and with >90% retention of the alkene's stereochemical identity. The utility of the catalytic process is illustrated in the context of a concise enantioselective synthesis of riccardiphenol B.

Synthesis of quaternary carbon stereogenic centers through enantioselective Cu-catalyzed allylic substitutions with vinylaluminum reagents

Catalytic enantioselective allylic substitution (EAS) reactions, which involve the use of alkyl- or aryl-substituted vinylaluminum reagents and afford 1,4-dienes containing a quaternary carbon stereogenic center at their C-3 site, are disclosed. The C-C bond-forming transformations are promoted by 0.5-2.5 mol % of sulfonate bearing chiral bidentate N-heterocyclic carbene (NHC) complexes, furnishing the desired products efficiently (66-97% yield of isolated products) and in high site (>98% S(N)2')- and enantioselectivity [up to 99:1 enantiomer ratio (er)]. To the best of our knowledge, the present report puts forward the first cases of allylic substitution reactions that result in the generation of all-carbon quaternary stereogenic centers through the addition of a vinyl unit. The aryl- and vinyl-substituted vinylaluminum reagents, which cannot be prepared in high efficiency through direct reaction with diisobutylaluminum hydride, are accessed through a recently introduced Ni-catalyzed reaction of the corresponding terminal alkynes with the same inexpensive metal-hydride agent. Sequential Ni-catalyzed hydrometalations and Cu-catalyzed C-C bond-forming reactions allow for efficient and selective synthesis of a range of enantiomerically enriched EAS products, which cannot be accessed by previously disclosed strategies (due to inefficient vinylmetal synthesis or low reactivity and/or selectivity with Si-substituted derivatives). The utility of the protocols developed is demonstrated through a concise enantioselective synthesis of natural product bakuchiol.

Alternatives to organoboron reagents in rhodium-catalyzed conjugate additions

The rhodium-catalyzed conjugate addition of organoboron reagents to alkene acceptors has emerged as an important methodology in organic synthesis. The process results in the formation of new carbon-carbon bonds, and in principle, can create new stereocenters at both the alpha and beta carbon atoms of the acceptor. Other organometallics are known to participate in the key transmetalation to rhodium and are becoming more established in stereoselective synthesis (notably arylzinc reagents). This Focus Review will describe recent developments in the use of alternative organometallic donors to organoboron reagents in rhodium-catalyzed conjugate addition reactions and their growing applications in synthesis.

Structures, biological activities and phylogenetic relationships of terpenoids from marine ciliates of the genus Euplotes

In the last two decades, large scale axenic cell cultures of the marine species comprising the family Euplotidae have resulted in the isolation of several new classes of terpenoids with unprecedented carbon skeletons including the (i) euplotins, highly strained acetylated sesquiterpene hemiacetals; (ii) raikovenals, built on the bicyclo[3.2.0]heptane ring system; (iii) rarisetenolides and focardins containing an octahydroazulene moiety; and (iv) vannusals, with a unique C₃₀ backbone. Their complex structures have been elucidated through a combination of nuclear magnetic resonance spectroscopy, mass spectrometry, molecular mechanics and quantum chemical calculations. Despite the limited number of biosynthetic experiments having been performed, the large diversity of ciliate terpenoids has facilitated the proposal of biosynthetic pathways whereby they are produced from classical linear precursors. Herein, the similarities and differences emerging from the comparison of the classical chemotaxonomy approach based on secondary metabolites, with species phylogenesis based on genetic descriptors (SSU-rDNA), will be discussed. Results on the interesting ecological and biological properties of ciliate terpenoids are also reported.

Total synthesis and structural revision of vannusals A and B: synthesis of the originally assigned structure of vannusal B

The total synthesis of the originally assigned structure of vannusal B (2) and its diastereomer (d-2) are described. Initial forays into these structures with model systems revealed the viability of a metathesis-based approach and a SmI(2)-mediated strategy for the key cyclization to forge the central region of the molecule, ring C. The former approach was abandoned in favor of the latter when more functionalized substrates failed to enter the cyclization process. The successful, devised convergent strategy based on the SmI(2)-mediated ring closure utilized vinyl iodide (-)-26 and aldehyde fragment (+/-)-86 as key building blocks, whose lithium-mediated coupling led to isomeric coupling products (+)-87 and (-)-88 (as shown in Scheme 17 in the article). Intermediate (-)-88 was converted, via (-)-89 and (-)-90/(+)-91, to vannusal B structure 2 (as shown in Scheme 18 in the article), whose spectroscopic data did not match those reported for the natural product. Similarly, intermediate (+)-25, obtained through coupling of vinyl iodide (-)-26 and aldehyde (+/-)-27 (as shown in Scheme 13 in the article) was transformed via intermediates (-)-97 and (+)-98 (as shown in Scheme 19 in the article) to diastereomeric vannusal B structure (+)-d-2 (as shown in Scheme 19 in the article) which was also proven spectroscopically to be non-identical to the naturally occurring substance. These investigations led to the discovery and development of a number of new synthetic technologies that set the stage for the solution of the vannusal structural conundrum.