A Concise Synthesis of the Pentacyclic Framework of Cortistatins

Bio-inspired formal total synthesis of (±)-bisabosqual A

A new approach was developed to construct a hexahydrobenzofurobenzopyran ring system by an oxa-[3+3], Diels–Alder reaction and oxidative aromatization. As a synthetic application, the bio-inspired formal synthesis of bisabosqual A was achieved.

Recent Advances in Oxa-6π Electrocyclization Reactivity for the Synthesis of Privileged Natural Product Scaffolds

The stunning advances in understanding the reactivity and selectivity principles of asymmetric pericyclic reactions have had a profound impact on the synthetic planning of complex natural products. Indeed, electrocyclizations, cycloadditions, and sigmatropic rearrangements enable synthetic chemists to craft highly functionalized scaffolds that would not otherwise be possible with a similar atom-, step-, and redox-economy. In this review, selected examples from the last two decades of research (2003–2020) on tandem processes combining oxa-6π electrocyclic reactions are discussed in terms of reactivity challenges, inherent reversibility, and key structural bond formation in the assembly of natural products. A particular emphasis is given to the electrocyclic ring-closures in the tandem processes featuring Knoevenagel-type condensations, Diels–Alder cycloadditions, Stille couplings, and oxidative dearomatizations. The synthetic manifolds reviewed here illustrate how oxa-6π electrocyclizations are intimately linked to the construction of complex natural product scaffolds and have inspired a number of biomimetic syntheses in the laboratory.

Recent progress in the synthesis of the furanosteroid family of natural products

This review focuses on an overview of recent advances in the synthesis of furanosteroids and illustrates their applications in medicinal chemistry over the period of 2005–present.

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.

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 syntheses of shizukaols A and E

Shizukaols possess a common heptacyclic framework containing more than ten contiguous stereocenters and potential biological activities. Here we report that the total syntheses of shizukaols A (1) and E (2), two lindenane-type dimers from the Chloranthaceae family, are achieved via a modified biomimetic Diels–Alder reaction. The common crucial biomimetic diene 23 and ethylene species (6, 17) are obtained through either a highly Z-selective olefination of α-siloxy ketone with ynolate anions or an intramolecular Horner–Wadsworth–Emmons olefination from commercially available Wieland–Miescher ketone (7). This synthetic approach here opens up practical avenues for the total syntheses of the intriguing Chloranthaceae family members, as well as the understanding of their relevant biological action in nature.

Shizukaols, bioactive dimers naturally occurring in the Chloranthaceae family, possess a complex polycyclic framework with more than ten contiguous stereocenters. Here the authors report the total syntheses of shizukaols A and E achieved via a modified Diels–Alder reaction mimicking the biosynthetic pathway.

Desymmetrisation of meso-diones promoted by a highly recyclable polymer-supported chiral phosphoric acid catalyst

A polystyrene-supported BINOL-derived chiral phosphoric acid has been applied to the desymmetrisation of meso-diones to produce enantioenriched cyclohexenones. The catalytic resin has proven highly active and robust, giving rise to Hajos-Parrish or Wieland-Miescher type products in good yields and enantioselectivities, while allowing for extended recycling.

l-Phenylalanine potassium catalyzed asymmetric formal [3 + 3] annulation of 2-enoyl-pyridine N-oxides with acetone

A highly enantioselective formal [3 + 3] annulation of 2-enoyl-pyridine N-oxides with acetone was developed.

Methodology for the Construction of the Bicyclo[4.3.0]nonane Core

The bicyclo[4.3.0]nonane scaffold, commonly known as a hydrindane, is a common structural motif found in many terpenoid structures and one that remains a challenge for synthetic chemists to elaborate with appropriate regio- and stereo-selectivity. Over the course of the study of terpene natural products, the elaboration of the hydrindane structure has seen progress on the utilization of both old and newer methods to achieve the desired outcomes. This review seeks to serve as a general overview of these methods, and detail specific examples.

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.

Total synthesis and related studies of large, strained, and bioactive natural products

Our chemical syntheses and related scientific investigations of natural products with complex architectures and powerful biological activities are described, focusing on the very large 3 nm-long polycyclic ethers called the ciguatoxins, highly strained and labile chromoprotein antitumor antibiotics featuring nine-membered enediyne cores, and extremely potent anthelmintic macrolides called the avermectins.

Enantioselective synthesis of fused heterocycles with contiguous stereogenic centers by chiral phosphoric acid catalyzed symmetry breaking

Highly enantioselective synthesis of fused-heterocycles was achieved by chiral phosphoric acid catalyzed symmetry breaking.

Total Syntheses of Proposed (±)-Trichodermatides B and C

Total syntheses of putative (±)-trichodermatides B and C are described. These efficient syntheses feature the oxa-[3 + 3] annulation strategy, leading to B and C along with their respective C2-epimers. However, these synthetic samples are spectroscopically very different from the natural products. DFT calculations of C13 chemical shifts are conducted and the predicted values are in good agreement with those of synthetic samples, thereby questioning in the accuracy of structural assignments of trichodermatides B and C.

Iron-catalyzed aerobic oxidation of allylic alcohols: the issue of C═C bond isomerization

An aerobic oxidation of allylic alcohols using Fe(NO3)3·9H2O/TEMPO/NaCl as catalysts under atmospheric pressure of oxygen at room temperature was developed. This eco-friendly and mild protocol provides a convenient pathway to the synthesis of stereodefined α,β-unsaturated enals or enones with the retention of the C-C double-bond configuration.

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.

Constructing the Architecturally Distinctive ABD-Tricycle of Phomactin A through an Intramolecular Oxa-[3 + 3] Annulation Strategy

Our efforts in constructing the ABD-ring of phomactin A through an intramolecular oxa-[3 + 3] annulation strategy is described. This struggle entailed finding a practical and efficient preparation of annulation precursor, and a realization of the unexpected competing regioisomeric pathway. The success entailed accessing the A-ring through Diels-Alder cycloaddition of Rawal's diene. Furthermore, the discovery that the regioisomers from the annulation existed as atropisomers with respect to the D-ring olefin and that they could be equilibrated to the desired ABD-tricycle, allowing large quantities of tricycle to be accessed.

Synthetic studies toward (+)-cortistatin A

We describe herein the synthesis of a late-stage intermediate en route to cortistatin A. Key transformations included a Snieckus-like cascade sequence culminating in a 6π-electrocyclization, an alkylative dearomatization, and the stereoselective functionalization of the cortistatin A-ring. While the total synthesis we sought was not accomplished, the work sets the stage for several approaches to the preparation of novel analogs via diverted total synthesis.

Total synthesis of (±)-phomactin A. Lessons learned from respecting a challenging structural topology

Our struggles and ultimate success in achieving a total synthesis of phomactin A are described. Our strategy features an intramolecular oxa-[3 + 3] annulation to construct its unique ABD-tricyclic manifold. Although the synthesis would constitute a distinctly new approach with the 12-membered D-ring of phomactin A being assembled simultaneously with the 1-oxadecalin at an early stage, the ABD-tricycle represents a unique structural topology that would pose a number of unprecedented challenges. One challenge concerned elaborating this tricycle to have oxygenation at the proper carbon atoms. To overcome this, we would utilize a Kornblum-DeLaMare ring-opening of a peroxide bridge as well as a challenging late-stage 1,3-allylic alcohol transposition. Further, the structural intricacies of the ABD-tricycle were uncovered by a conformational analysis that would be critical for the C5a-homologation.

Total synthesis of (±)-cortistatin J from furan

A concise, diastereoselective total synthesis of (±)-cortistatin J has been completed in 20 steps from furan. Key steps include an intramolecular [4 + 3] cyclization of a disubstituted furan with a (Z)-2-(trialkylsilyloxy)-2-enal to construct the tetracyclic core and a (Z)-vinylsilane/iminium ion cyclization to form the A ring.

Stereoselective synthesis of core structure of cortistatin A

A stereoselective synthesis of the core structure of cortistatin A (1), a novel antiangiogenic steroidal alkaloid from Indonesian marine sponge, is described. An 8-oxabicyclo[3.2.1]octene system, a characteristic B-ring structure of 1, was elaborated by a 7-endo selective intramolecular Heck cyclization and a subsequent acid-mediated oxy-Michael reaction.

Scalable synthesis of cortistatin A and related structures

Full details are provided for an improved synthesis of cortistatin A and related structures as well as the underlying logic and evolution of strategy. The highly functionalized cortistatin A-ring embedded with a key heteroadamantane was synthesized by a simple and scalable five-step sequence. A chemoselective, tandem geminal dihalogenation of an unactivated methyl group, a reductive fragmentation/trapping/elimination of a bromocyclopropane, and a facile chemoselective etherification reaction afforded the cortistatin A core, dubbed "cortistatinone". A selective Δ(16)-alkene reduction with Raney Ni provided cortistatin A. With this scalable and practical route, copious quantities of cortistatinone, Δ(16)-cortistatin A (the equipotent direct precursor to cortistatin A), and its related analogues were prepared for further biological studies.

Concise synthesis of the oxapentacyclic core of cortistatin A

A concise synthetic approach for constructing the oxapentacyclic framework of cortistatin A is described. The synthesis features a furan-oxyallyl [4 + 3] cycloaddition and double-intramolecular aldol reactions. In addition, an interesting core structure was obtained in 11 steps from furan by using our method.

Synthesis of cortistatins A, J, K and L

The cortistatins are a recently identified class of marine natural products characterized by an unusual steroidal skeleton, which have been found to inhibit differentially the proliferation of various mammalian cells in culture by an unknown mechanism. We describe a comprehensive route for the synthesis of cortistatins from a common precursor, which in turn is assembled from two fragments of similar structural complexity. Cortistatins A and J, and for the first time K and L, have been synthesized in parallel processes from like intermediates prepared from a single compound. With the identification of facile laboratory transformations linking intermediates in the cortistatin L synthetic series with corresponding intermediates to cortistatins A and J, we have been led to speculate that somewhat related paths might occur in nature, offering potential sequencing and chemical detail for cortistatin biosynthetic pathways.