Total synthesis of (±)-cortistatin J from furan

Concise and collective total syntheses of 2,4-disubstituted furan-derived natural products from hydroxyoxetanyl ketones

The furan moiety, prevalent in bioactive natural products and essential drugs, presents intriguing structural features that have spurred our exploration into streamlined chemical synthesis routes for related natural products. In this study, we demonstrate the concise total synthesis of eight 2,4-disubstituted furan-derived natural products (including methylfuroic acid, rabdoketones A and B, paleofurans A and B, tournefolin C, and shikonofurans A and B). Our methodology revolves around the utilization of hydroxyoxetanyl ketones as pivotal intermediates. The approach encompasses transformations such as selective organo-catalyzed cross-ketol addition, synthesis of hydroxymethyl-tethered furans through Bi(OTf)3 catalyzed dehydrative cycloisomerization of α-hydroxyoxetanyl ketones, and a hydrogen atom transfer (HAT)-mediated oxidation of primary alcohols into the corresponding acids. This comprehensive synthetic strategy highlights the versatility of hydroxyoxetanyl ketones as invaluable building blocks in the synthesis of furan-containing natural products.

α-Functionally Substituted α,β-Unsaturated Aldehydes as Fine Chemicals Reagents: Synthesis and Application

α-Functionalized α,β-unsaturated aldehydes is an important class of compounds, which are widely used in fine organic synthesis, biology, medicine and pharmacology, chemical industry, and agriculture. Some of the 2-substituted 2-alkenals are found to be the key metabolites in plant and animal cells. Therefore, the development of efficient methods for their synthesis attracts the attention of organic chemists. This review focusses on the recent advances in the synthesis of 2-functionally substituted 2-alkenals. The approaches to the preparation of α-alkyl α,β-unsaturated aldehydes are not included in this review.

Concise Synthesis of 9,11-Secosteroids Pinnigorgiols B and E

Pinnigorgiols B and E are 9,11-secosteroids with a unique tricyclic γ-diketone framework. Herein, we report the first synthesis of these natural products from inexpensive, commercially available ergosterol. This synthesis features a semipinacol rearrangement and an acyl radical cyclization/hemiketalization cascade; the latter efficiently assembled the tricyclic γ-diketone skeleton, with two rings and three contiguous stereogenic centers being formed in a single step.

Synthetic approaches towards cortistatins: evolution and progress through its ages

Cortistatins are a family of steroidal alkaloids with a unique pentacyclic skeleton, having immensely potent anti-angiogenetic activities. Given the scarcity in the natural availability of these compounds, their syntheses became major attractions in organic chemistry. Along with total synthesis of the most potent congeners in the family: cortistatins A and J, the synthesis of two other members have been successfully completed, while various other analogues have also been designed with variable degrees of biological activities. This review is an exhaustive coverage of the significant attempts towards constructing this highly challenging molecule and also aims to highlight the deep understanding of the structure-activity relationships of these compounds, which have been garnered over time.

Construction of key building blocks towards the synthesis of cortistatins

This work reports the construction of key building blocks towards the synthesis of cortistatins; a family of steroidal alkaloids. Cortistatin A, being a primary target due to its superior biological properties over other congeners, has been prepared by two different synthetic routes. Synthesis of the precursor to the heavily substituted A-ring starting from d-glucose and construction of the DE-ring junction employing a Hajos-Parrish ketone as a chiral pool have been demonstrated. Efforts are underway to assemble these key fragments and build towards the total synthesis of cortistatin A.

Synthesis of Furo[3,2-b]quinolines and Furo[2,3-b:4,5-b']diquinolines through [4 + 2] Cycloaddition of Aza-o-Quinone Methides and Furans

An approach for the construction of furo[3,2-b]quinolines and furo[2,3-b:4,5-b']diquinolines is developed through a metal-free [4 + 2] cycloaddition of easily available in situ generated aza-o-quinone methides and furans. The reaction tolerates a wide range of aza-o-quinone methides and substituted furans to afford the corresponding dihydro- or tetrahydrofuroquinolines in good to excellent yields. Mechanistic studies reveal that the reaction involves a concerted [4 + 2] cycloaddition pathway and shows a high regioselectivity of cycloaddition for a furan ring. The present method features mild reaction conditions, dearomatization of furans, high regio- and diastereoselectivity, gram-scalable preparations, and diversity of furoquinolines.

(4 + 3) cycloadditions of allenyl ether-derived oxygen-stabilized oxyallyls with furans

(4 + 3) Cycloadditions between allenyl ethers and furans are described. The reaction features anin situformation of oxygen-stabilized oxyallylsviaepoxidations of allenyl ethers in the presence of H₂PO₄⁻.

Facile Access to Bridged Ring Systems via Point-to-Planar Chirality Transfer: Unified Synthesis of Ten Cyclocitrinols

Bridged ring systems are found in a wide variety of biologically active molecules including pharmaceuticals and natural products. However, the development of practical methods to access such systems with precise control of the planar chirality presents considerable challenges to synthetic chemists. In the context of our work on the synthesis of cyclocitrinols, a family of steroidal natural products, we herein report the development of a point-to-planar chirality transfer strategy for preparing bridged ring systems from readily accessible fused ring systems. Inspired by the proposed pathway for biosynthesis of cyclocitrinols from ergosterol, our strategy involves a bioinspired cascade rearrangement, which enabled the gram-scale synthesis of a common intermediate in nine steps and subsequent unified synthesis of 10 cyclocitrinols in an additional one to three steps. Our work provides experimental support for the proposed biosynthetic pathway and for the possible interrelationships between members of the cyclocitrinol family. In addition to being a convenient route to 5(10→19) abeo-steroids, our strategy also offers a generalized approach to bridged ring systems via point-to-planar chirality transfer. Mechanistic investigations suggest that the key cascade rearrangement involves a regioselective ring scission of a cyclopropylcarbinyl cation rather than a direct Wagner-Meerwein rearrangement.

Construction of seven- and eight-membered carbocycles by Lewis acid catalyzed C(sp3)–H bond functionalization

Concise construction of seven- or eight-membered carbocycles was accomplished by Lewis acid catalyzed C(sp³)–H bond functionalization.

In(OTf)3-Catalyzed Cascade Cyclization for Construction of Oxatricyclic Compounds

A highly diastereoselective cascade cyclization reaction has been developed for establishing a series of oxatricyclic compounds using Chan's diene and simple keto alkynal substrates with only 1 mol % of In(OTf)₃ as the catalyst in 82-92% yields. The potential utility of this synthetic strategy has been demonstrated in model studies for the construction the core structures of 1α,8α:4α,5α-diepoxy-4,5-dihydroosmitopsin and cortistatin A.

Total Synthesis of Aplysiasecosterol A

Aplysiasecosterol A (1) is a structurally unusual 9,11-secosteroid isolated from the sea hare Aplysia kurodai. We have accomplished the first and asymmetric total synthesis of 1 in a convergent fashion. The left-hand segment bearing three adjacent stereocenters was constructed through desymmetrizing reduction, ketalization, and radical cyclization. A strategy of asymmetric 2-bromoallylation followed by spontaneous desymmetrizing lactolization enabled a more expeditious access to this segment. The right-hand segment was prepared through two different approaches: one featuring Myers alkylation and Suzuki-Miyaura coupling and the other relying upon Aggarwal lithiation-borylation and Zweifel-Evans olefination. The two fragments were coupled by a Reformatsky type reaction. The three consecutive stereocenters embedded in the central domain of 1 were generated by an iron-mediated, hydrogen atom transfer based radical cyclization reaction.

Direct Synthesis of Polycyclic Tropinones by a Condensation-[4+3]-Cycloaddition Cascade Reaction

A concise method of constructing polycyclic tropinone frameworks was developed. The single-step synthesis of polycyclic tropinone consists of an intramolecular [4+3] cycloaddition reaction of N-nosyl-pyrrole with oxyallyl cation that was generated in situ by an intermolecular condensation reaction of the nucleophilic functional groups on a tethered pyrrole with the aldehyde of 2-(silyloxy)-acrolein. This cascade reaction afforded various polycyclic tropinones including tri-, tetra-, and pentacyclic systems in high yields as single diastereomers.

Application of (4+3) cycloaddition strategies in the synthesis of natural products

This review summarizes the applications of (4+3) cycloadditions, both classical and formal, in the syntheses of natural products in the last two decades.

NHC-Cu(I)-Catalyzed Friedländer-Type Annulation of Fluorinated o-Aminophenones with Alkynes on Water: Competitive Base-Catalyzed Dibenzo[b,f][1,5]diazocine Formation

An efficient, easily scalable synthesis of 4-trifluoromethylquinolines and naphthydrines (as well as their difluoro- and perfluoro-analogues) as a result of tandem direct catalytic alkynylation/dehydrative condensation of o-aminofluoromethylketones (o-FMKs), for the first time catalyzed by NHC-copper(I) complexes on water, is reported. A wide range of terminal alkynes is tolerated under the reaction conditions, including β-lactam-, steroid-, and sugar-derived ones, leading to desired quinolines and naphthydrines with good yields. Further investigations proved that o-FMKs could be efficiently transformed into a rare class of heterocyclic compounds-dibenzo[b,f][1,5]diazocines-by a base-catalyzed condensation, also on water. The developed method was applied for gram-scale synthesis of a fluorinated analogue of G protein-coupled receptor antagonist (GPR91).

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.

The synthesis of cycloalka[b]furans via an Au(i)-catalyzed tandem reaction of 3-yne-1,2-diols

A rapid and convenient approach toward substituted cycloalka[b]furan compounds has been developed via an Au(i)-catalyzed cyclization/1,2-rearrangement/aromatization cascade.

Total synthesis of 1α,25-dihydroxyvitamin D3 (calcitriol) through a Si-assisted allylic substitution

A total synthesis of calcitriol through a Si-assisted substitution to generate the trans-hydrindane core and an asymmetric CuH-catalyzed 1,4-reduction of α,β,γ,δ-unsaturated ester to access the steroid configuration at C20 is described.

Search for Anti-angiogenic Substances from Natural Sources

As angiogenesis is critical for tumor growth and metastasis, potent and selective anti-angiogenic agents with novel modes of action are highly needed for anti-cancer drug discovery. In this review, our studies focusing on the search for anti-angiogenic substances from natural sources, such as bastadins, globostellatic acid X methyl esters and cortistatins from marine sponges, and pyripyropenes from marine-derived fungus, together with senegasaponins from medicinal plant, are summarized.

Synthesis of the Cortistatin Pentacyclic Core by Alkoxide-Directed Metallacycle-Mediated Annulative Cross-Coupling

The pentacyclic core skeleton of the cortistatins has been prepared in a stereoselective fashion by strategic use of an alkoxide-directed metallacycle-mediated annulative cross-coupling. This metal-centered tandem reaction delivers a polyunsaturated hydrindane and establishes the C13 stereodefined quaternary center with high levels of stereocontrol. Subsequent regio- and stereoselective global hydroboration results in the realization of the DE-trans ring fusion and a tertiary alcohol at C8. Establishment of the ABC-tricyclic subunit was then accomplished through phenolic oxidation/trans-acetalization, chemoselective reduction, regioselective cleavage, and intramolecular alkylation at C5.

Mechanisms and origins of the switchable regioselectivity of FeBr3-catalyzed [1,2]-aryl and [1,2]-alkyl shifts of α-aryl aldehydes

With the aid of DFT calculations, the FeBr3-catalyzed skeletal rearrangements of 2-cyclohexanal,2-p-C6H4OMe-propylaldehyde (1A) and 2-phenyl,2-p-C6H4OMe-propylaldehyde (1B) were investigated theoretically. As compared to mono-FeBr3 as a catalyst, the bis-FeBr3 serving as a catalyst is found to be not only enhancing the catalytic efficiency but also improving the product selectivity. For the reaction starting from 1A, the [1,2]-group shift (first step) is rate-determining, and why the Cy shift is the most favored is rationalized in comparison with the p-C6H4OMe and Me shifts. For the reaction starting from 1B, the [1,2]-H shift (second step) is rate-determining although the [1,2]-p-C6H4OMe shift is favored over the [1,2]-phenyl shift. In contrast to the experimental proposal, the newly established H2O/Br(-) joint-assisted H-shift mechanism explains the partial α-H source of the [1,2]-Cy shift product. In addition, we discussed the inherent mechanism that explains why both the [1,2]-p-C6H4OMe and [1,2]-p-C6H4CF3 shifts are more facile than the [1,2]-phenyl shift although the substituents -OMe and -CF3 have opposite electronic behaviors.

Towards a general diastereoselective route to oxabicyclo[3.2.1]octanes via a gold-catalysed cascade reaction

The development of an efficient diastereoselective synthesis of the oxabicyclo[3.2.1]octane ring system bearing two oxygenated quaternary chiral centres represents a significant challenge. This motif can be found in a wide range of natural products with significant biological activities. Here we report the synthesis of such kind of scaffold using a cyclohexane-trans-1,4-diol with an alkyne side chain in the presence of Au(I) catalyst. This is a domino process in which two C–H, two C–O and one C–C bond is assembled through a sequence of cyclization/semi-pinacol rearrangements. This strategy has been successfully applied to the asymmetric formal total synthesis of (+)-cortistatins.

Oxygenated bicyclic cores are common to many natural and bioactive compounds, but their efficient synthesis can be difficult. Here, the authors report a gold-catalysed diastereoselective process for the synthesis of oxabicyclo[3.2.1]octanes via a domino process.

Concerted Ring Opening and Cycloaddition of Chiral Epoxy Enolsilanes with Dienes

Silyl-triflate-catalyzed (4+3) cycloadditions of epoxy enolsilanes with dienes provide a mild and chemoselective synthetic route to seven-membered carbocycles. Epoxy enolsilanes containing a terminal enolsilane and a single stereocenter undergo cycloaddition with almost complete conservation of enantiomeric purity, a finding that argues against the involvement of oxyallyl cation intermediates which have been previously proposed for these types of reactions. Reported are theoretical and experimental investigations of the cycloaddition mechanism. The major enantiomers of the cycloadducts are derived from S(N)2-like reactions of the silylated epoxide with the diene, in which stereospecific ring opening and formation of the two new C-C bonds occur in a single step. Calculations predict, and experiments confirm, that the observed small losses of enantiomeric purity are traced to a triflate-mediated double S(N)2 cycloaddition pathway.

Structure-Based Design of Type II Inhibitors Applied to Maternal Embryonic Leucine Zipper Kinase

A novel Type II kinase inhibitor chemotype has been identified for maternal embryonic leucine zipper kinase (MELK) using structure-based ligand design. The strategy involved structural characterization of an induced DFG-out pocket by protein-ligand X-ray crystallography and incorporation of a slender linkage capable of bypassing a large gate-keeper residue, thus enabling design of molecules accessing both hinge and induced pocket regions. Optimization of an initial hit led to the identification of a low-nanomolar, cell-penetrant Type II inhibitor suitable for use as a chemical probe for MELK.

Mild arylboronic acid catalyzed selective [4 + 3] cycloadditions: access to cyclohepta[b]benzofurans and cyclohepta[b]indoles

The first example of arylboronic acid catalyzed [4 + 3] cycloaddition reaction is reported.

The Chemistry and Biology of Nakiterpiosin - C-nor-D-Homosteroids

Isolated from the sponge Terpios hoshinota that causes coral black disease, nakiterpiosin was the first C-nor-D-homosteroid discovered from a marine source. We provide in this account an overview of the chemistry and biology of this natural product. We also include a short history of the synthesis of C-nor-D-homosteroids and the results of some unpublished biological studies of nakiterpiosin.