Remote Asymmetric Induction Reactions using a E, E-Vinylketene Silyl N, O-Acetal and the Wide Range Stereocontrol Strategy for the Synthesis of Polypropionates

Synthetic Studies on Poecillastrin C: Synthesis of the C14-C35 Segment of the Macrolide Ring Model

A synthesis of the C14-C35 segment of the macrolide ring model of poecillastrin C has been achieved. The C14-C23 segment was synthesized by the stereoselective Mukaiyama aldol reaction between ketene silyl N,O-acetal possessing chiral oxazolidinone auxiliary and O-acetyl lactol prepared by the stereoselective conjugate addition followed by iodolactonization. The C24-C35 segment was constructed by three stereoselective vinylogous Mukaiyama aldol reactions. Both segments were coupled by the aldol reaction followed by diisobutylaluminium hydride (DIBALH) reduction to produce the C14-C35 moiety of the macrocyclic model.

Discovery of a nitroaromatic nannocystin with potent in vivo anticancer activity against colorectal cancer by targeting AKT1

The development of targeted chemotherapeutic agents against colorectal cancer (CRC), one of the most common cancers with a high mortality rate, is in a constant need. Nannocystins are a family of myxobacterial secondary metabolites featuring a 21-membered depsipeptide ring. The in vitro anti-CRC activity of natural and synthetic nannocystins was well documented, but little is known about their in vivo efficacy and if positive, the underlying mechanism of action. In this study we synthesized a nitroaromatic nannocystin through improved preparation of a key fragment, and characterized its in vitro activity and in vivo efficacy against CRC. We first described the total synthesis of compounds 2-4 featuring Heck macrocyclization to forge their 21-membered macrocycle. In a panel of 7 cancer cell lines from different tissues, compound 4 inhibited the cell viability with IC values of 1-6 nM. In particular, compound 4 (1, 2, 4 nM) inhibited the proliferation of CRC cell lines (HCT8, HCT116 and LoVo) in both concentration and time dependent manners. Furthermore, compound 4 concentration-dependently inhibited the colony formation and migration of CRC cell lines. Moreover, compound 4 induced cell cycle arrest at sub-G1 phase, apoptosis and cellular senescence in CRC cell lines. In three patient-derived CRC organoids, compound 4 inhibited the PDO with IC values of 3.68, 28.93 and 11.81 nM, respectively. In a patient-derived xenograft mouse model, injection of compound 4 (4, 8 mg/kg, i.p.) every other day for 12 times dose-dependently inhibited the tumor growth without significant change in body weight. We conducted RNA-sequencing, molecular docking and cellular thermal shift assay to elucidate the anti-CRC mechanisms of compound 4, and revealed that it exerted its anti-CRC effect at least in part by targeting AKT1.

Cu-catalyzed asymmetric addition of alcohols to β,γ-alkynyl-α-imino esters for the construction of linear chiral N,O-ketals

N,O-acetals are part of many synthetic intermediates and important skeletons of numerous natural products and pharmaceutical drugs. The most straightforward method of the synthesis of N,O-acetals is the enantioselective addition of O-nucleophiles to imines. However, using this method for the synthesis of linear chiral N,O-ketals still remains challenging due to the instability of raw materials under acidic or basic conditions. Herein, we developed a Cu-catalyzed asymmetric addition of alcohols to β,γ-alkynyl-α-imino esters under mild conditions, providing the corresponding linear chiral N,O-ketals with up to 96% ee. The method tolerates some variation in the β,γ-alkynyl-α-imino ester and alcohol scope, including some glucose and natural amino acid derivatives. Computational results indicate that the Boc group of the substrates assist in the extraction of hydrogen atoms from the alcohols to promote the addition reactions. These products could be synthesized on a gram-scale and can be used in several transformations. This asymmetric addition system provides an efficient, mild, gram-scale, and transition-metal-catalyzed synthesis of linear chiral N,O-ketals.

N,O-acetals are part of many synthetic intermediates and important skeletons of numerous natural products and pharmaceutical drugs. Here the authors show a Cu-catalyzed asymmetric addition of alcohols to β,γ-alkynyl-α-imino esters, providing the corresponding linear chiral N,O-ketals with up to 96% ee.

Expeditious Asymmetric Synthesis of Polypropionates Relying on Sulfur Dioxide-Induced C–C Bond Forming Reactions

For a long time, the organic chemistry of sulfur dioxide (SO2) consisted of sulfinates that react with carbon electrophiles to generate sulfones. With alkenes and other unsaturated compounds, SO2 generates polymeric materials such as polysulfones. More recently, H-ene, sila-ene and hetero-Diels–Alder reactions of SO2 have been realized under conditions that avoid polymer formation. Sultines resulting from the hetero-Diels–Alder reactions of conjugated dienes and SO2 are formed more rapidly than the corresponding more stable sulfolenes resulting from the cheletropic additions. In the presence of a protic or Lewis acid catalyst, the sultines derived from 1-alkoxydienes are ionized into zwitterionic intermediates bearing 1-alkoxyallylic cation moieties which react with electro-rich alkenes such as enol silyl ethers and allylsilanes with high stereoselectivity. (C–C-bond formation through Umpolung induced by SO2). This produces silyl sulfinates that react with carbon electrophiles to give sulfones (one-pot four component asymmetric synthesis of sulfones), or with Cl2, generating the corresponding sulfonamides that can be reacted in situ with primary and secondary amines (one-pot four component asymmetric synthesis of sulfonamides). Alternatively, Pd-catalyzed desulfinylation generates enantiomerically pure polypropionate stereotriads in one-pot operations. The chirons so obtained are flanked by an ethyl ketone moiety on one side and by a prop-1-en-1-yl carboxylate group on the other. They are ready for two-directional chain elongations, realizing expeditious synthesis of long-chain polypropionates and polyketides. The stereotriads have also been converted into simpler polypropionates such as the cyclohexanone moiety of baconipyrone A and B, Kishi’s stereoheptad unit of rifamycin S, Nicolaou’s C1–C11-fragment and Koert’s C16–CI fragment of apoptolidin A. This has also permitted the first total synthesis of (-)-dolabriferol.

Regio- and Stereoselective Cascade of β,γ-Unsaturated Ketones by Dipeptided Phosphonium Salt Catalysis: Stereospecific Construction of Dihydrofuro-Fused [2,3-b] Skeletons

Chiral (dihydro)furo-fused heterocycles are significant structural motifs in numerous natural products, functional materials and pharmaceuticals. Therefore, developing efficient methods for preparing compounds with these privileged scaffolds is an important endeavor in synthetic chemistry. Herein, we develop an effective, modular method by a dipeptide-phosphonium salt-catalyzed regio- and stereoselective cascade reaction of readily available linear β,γ-unsaturated ketones with aromatic alkenes, affording a wide variety of structurally fused heterocyclic molecules in high yields with excellent stereoselectivities. Moreover, mechanistic investigations revealed that the bifunctional phosphonium salt controlled the regio- and stereoselectivities of this cascade reaction, particularly proceeding through the initial ketone α-addition followed by O-participated substitution; and the multiple hydrogen-bonding interactions between Brønsted acid moieties of catalyst and nitro group of aromatic alkene were crucial in asymmetric induction. Given the generality, versatility, and high efficiency of this method, we anticipate that it will have broad synthetic utilities.

Co-Catalysis for Hydroamidocarbonylation of Alkynes with Amides over a Bifunctional Ligand-Based Pd Catalyst

The hydroamidocarbonylation of alkynes with amides allows for the synthesis of α,β-unsaturated imides with the advantage of 100% atomic economy. Herein, the bifunctional ligand (L1) containing a sulfonic acid group (-SO₃ H) and phosphino-fragment enable the Pd catalyst to accomplish the hydroamidocarbonylation of alkynes with amides. It was found that, due to an intramolecular synergetic effect, the L1-based Pd-catalyst exhibited much higher activity than the individual mechanical mixtures of Xantphos-based Pd-complex and MeSO₃ H. The formation and stability of Pd-H species were promoted by the presence of L1, which was verified by in situ high-pressure FT-IR analysis. Under the optimized conditions, the target products of the branched imides were obtained with yields in the range of 46-87% over the L1-based Pd-catalyst. Advantageously, as an ionic ligand, the L1-based Pd-catalyst could be recycled for 4 runs in the ionic liquid of [Bmim]NTf₂ without any obvious activity loss and detectable metal leaching.

Stereo- and Enantioselective Synthesis of Propionate-Derived Trisubstituted Alkene Motifs

We report a new method for constructing propionate-derived trisubstituted alkene motifs in a stereoselective manner. 1-Substituted 1,1-di(pinacolatoboryl)-(E)-alk-2-enes are generated in situ from 1-substituted 1,1-di(pinacolatoboryl)alk-3-enes through ruthenium(II)-catalyzed double-bond transposition. These species undergo a chiral phosphoric acid catalyzed allylation reaction of aldehydes to produce the E isomers of anti-homoallylic alcohols. On the other hand, the corresponding Z isomers of anti-homoallylic alcohols are obtained when a dimeric palladium(I) complex is employed as the catalyst for this double-bond transposition. Thus, both E and Z isomers can be synthesized from the same starting materials. A B-C(sp² ) bond remaining with the allylation product undergoes the Suzuki-Miyaura cross-coupling reaction to furnish a propionate-derived trisubstituted alkene motif in a stereo-defined form. The present method to construct the motifs with (E)- and (Z)-alkenes are successfully applied to the syntheses of (+)-isotrichostatic acid, (-)-isotrichostatin RK, and (+)-trichostatic acid, respectively.

From Target-Oriented to Motif-Oriented: A Case Study on Nannocystin Total Synthesis

Natural product total synthesis is in essence target-oriented in that a set of organic transformations are orchestrated into a workable process, leading ultimately to the target molecule with a predefined architecture. For a bioactive lead, proof of synthetic viability is merely the beginning. Ensuing effort repurposes the initial synthesis for structural diversification in order to probe structure-activity relationship (SAR). Yet accessibility is not equal to flexibility; moving from convergency to divergency, it is not always feasible to explore the chemical space around a particular substructure of interest simply by tweaking an established route. In this situation, the motif-oriented strategy becomes a superior choice, which gives priority to synthetic flexibility at the concerned site such that a route is adopted only if it is capable of implementing diversification therein. This strategy was recently devised by Fürstner et al., enabling them to achieve total synthesis of both natural and non-natural nannocystins varied at an otherwise challenging position. The present review examines seven distinctive nannocystin total syntheses reported thus far and showcases the merits of conventional (target-oriented) as well as motif-oriented strategies, concluding that these two approaches complement each other and are both indispensable for natural product based drug discovery.

Development of α,α-Disubstituted Crotylboronate Reagents and Stereoselective Crotylation via Brønsted or Lewis Acid Catalysis

The development of α,α-disubstituted crotylboronate reagents is reported. Chiral Brønsted acid-catalyzed asymmetric aldehyde addition with the developed E-crotylboron reagent gave (E)-anti-1,2-oxaborinan-3-enes with excellent enantioselectivities and E-selectivities. With BF₃·OEt₂ catalysis, the stereoselectivity is reversed, and (Z)-δ-boryl-anti-homoallylic alcohols are obtained with excellent Z-selectivities from the same E-crotylboron reagent. The Z-crotylboron reagent also participates in BF₃·OEt₂-catalyzed crotylation to furnish (Z)-δ-boryl-syn-homoallylic alcohols with good Z-selectivities. DFT computations establish the origins of observed enantio- and stereoselectivities of chiral Brønsted acid-catalyzed asymmetric allylation. Stereochemical models for BF₃·OEt₂-catalyzed reactions are proposed to rationalize the Z-selective allyl additions. These reactions generate highly valuable homoallylic alcohol products with a stereodefined trisubstituted alkene unit. The synthetic utility is further demonstrated by the total syntheses of salinipyrones A and B.

Lewis acid-catalyzed asymmetric reactions of β,γ-unsaturated 2-acyl imidazoles

The investigation of diverse reactivity of β,γ-unsaturated carbonyl compounds is of great value in asymmetric catalytic synthesis. Numerous enantioselective transformations have been well developed with β,γ-unsaturated carbonyl compounds as nucleophiles, however, few example were realized by utilizing them as not only nucleophiles but also electrophiles under a same catalytic system. Here we report a regioselective catalytic asymmetric tandem isomerization/α-Michael addition of β,γ-unsaturated 2-acyl imidazoles in the presence of chiral N,N′-dioxide metal complexes, delivering a broad range of optically pure 1,5-dicarbonyl compounds with two vicinal tertiary carbon stereocenters in up to >99% ee under mild conditions. Meanwhile, stereodivergent synthesis is disclosed to yield all four stereoisomers of products. Control experiments suggest an isomerization process involved in the reaction and give an insight into the role of NEt₃. In addition, Mannich reaction and sulfur-Michael addition of β,γ-unsaturated 2-acyl imidazoles proceed smoothly as well under the same catalytic system.

The investigation of reactivity of β,γ-unsaturated carbonyl compounds is of great synthetic value, especially in asymmetric transformations. Here, the authors report a catalytic asymmetric tandem isomerization/α-Michael addition of β,γ-unsaturated 2-acyl imidazoles in presence of chiral N,N′-dioxide metal catalysts.

Using Neighboring-Group Participation for Acyclic Stereocontrol in Diastereoselective Substitution Reactions of Acetals

Neighboring-group participation of an ester enabled stereocontrol in substitution reactions of acyclic acetals. The ester group formed a trans-fused dioxolenium ion intermediate, which underwent a substitution reaction at the acetal carbon atom to afford the product with high diastereoselectivity. Neighboring-group participation was confirmed by isolating dioxolane products resulting from nucleophilic addition at C-2 of a 1,3-dioxolenium ion intermediate. Using a pivaloate ester as the participating group in combination with strong nucleophiles produced substitution products with diastereoselectivities of ≥90:10.

New Developments of the Principle of Vinylogy as Applied to π-Extended Enolate-Type Donor Systems

The principle of vinylogy states that the electronic effects of a functional group in a molecule are possibly transmitted to a distal position through interposed conjugated multiple bonds. As an emblematic case, the nucleophilic character of a π-extended enolate-type chain system may be relayed from the legitimate α-site to the vinylogous γ, ε, ..., ω remote carbon sites along the chain, provided that suitable HOMO-raising strategies are adopted to transform the unsaturated pronucleophilic precursors into the reactive polyenolate species. On the other hand, when "unnatural" carbonyl ipso-sites are activated as nucleophiles (umpolung), vinylogation extends the nucleophilic character to "unnatural" β, δ, ... remote sites. Merging the principle of vinylogy with activation modalities and concepts such as iminium ion/enamine organocatalysis, NHC-organocatalysis, cooperative organo/metal catalysis, bifunctional organocatalysis, dicyanoalkylidene activation, and organocascade reactions represents an impressive step forward for all vinylogous transformations. This review article celebrates this evolutionary progress, by collecting, comparing, and critically describing the achievements made over the nine year period 2010-2018, in the generation of vinylogous enolate-type donor substrates and their use in chemical synthesis.

Synthesis of Karrikinolide Using the Aldol-Type Acetal Addition Reaction

A short step total synthesis of karrikinolide has been achieved. Both α and α' positions of O-acylated acetol were alkylated by the boron-mediated aldol-type acetal addition reaction. The one-pot sequence including the Arbuzov reaction, intramolecular Horner-Wadsworth-Emmons reaction, acidic hydrolysis of acetals, and pyran formation provided karrikinolide. This procedure was applicable to the gram-scale synthesis of karrikinolide.

Very Recent Advances in Vinylogous Mukaiyama Aldol Reactions and Their Applications to Synthesis

It is a challenging objective in synthetic organic chemistry to create efficient access to biologically active compounds. In particular, one structural element which is frequently incorporated into the framework of complex natural products is a β-hydroxy ketone. In this context, the aldol reaction is the most important transformation to generate this structural element as it not only creates new C-C bonds but also establishes stereogenic centers. In recent years, a large variety of highly selective methodologies of aldol and aldol-type reactions have been put forward. In this regard, the vinylogous Mukaiyama aldol reaction (VMAR) became a pivotal transformation as it allows the synthesis of larger fragments while incorporating 1,5-relationships and generating two new stereocenters and one double bond simultaneously. This review summarizes and updates methodology-oriented and target-oriented research focused on the various aspects of the vinylogous Mukaiyama aldol (VMA) reaction. This manuscript comprehensively condenses the last four years of research, covering the period 2016-2019.

Asymmetric Vinylogous Aldol-type Reactions of Aldehydes with Allyl Phosphonate and Sulfone

Two catalytic asymmetric vinylogous aldol-type reactions of aldehydes with allyl phosphonate and allyl sulfone have been uncovered in good to high yields for the first time. The bulky ligand-(R)-DTBM-SEGPHOS-was found to be the key to perfectly control both regio- and enantioselectivities. Transformations of the vinylogous products (including Horner-Wadsworth-Emmons and Julia olefinations) were successfully realized by virtue of the phosphonate and sulfone moieties. Moreover, the present methodology was successfully applied in the asymmetric synthesis of natural products.

Synthesis of C3-C21 Segment of Aflastatin A Using Remote Asymmetric Induction Reactions

The C3-C21 segment of aflastatin A has been synthesized by converging three segments including the C3-C8 segment, the C9-C15 segment, and the C16-C21 segment. Each segment has been synthesized from a vinylketene silyl N,O-acetal possessing a chiral auxiliary by a wide-range stereocontrol strategy. The C3-C8 segment was constructed in seven steps including the stereoselective vinylogous Mukaiyama alkylation, while the C9-C15 segment and the C16-C21 segment were synthesized using the vinylogous Mukaiyama aldol reaction in seven and eight steps, respectively.