Total Synthesis of Cortistatins A and J

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.

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.

Layered Double Hydroxides: A Toolbox for Chemistry and Biology

Layered double hydroxides (LDHs) are an emergent class of biocompatible inorganic lamellar nanomaterials that have attracted significant research interest owing to their high surface-to-volume ratio, the capability to accumulate specific molecules, and the timely release to targets. Their unique properties have been employed for applications in organic catalysis, photocatalysis, sensors, drug delivery, and cell biology. Given the widespread contemporary interest in these topics, time-to-time it urges to review the recent progresses. This review aims to summarize the most recent cutting-edge reports appearing in the last years. It firstly focuses on the application of LDHs as catalysts in relevant chemical reactions and as photocatalysts for organic molecule degradation, water splitting reaction, CO2 conversion, and reduction. Subsequently, the emerging role of these materials in biological applications is discussed, specifically focusing on their use as biosensors, DNA, RNA, and drug delivery, finally elucidating their suitability as contrast agents and for cellular differentiation. Concluding remarks and future prospects deal with future applications of LDHs, encouraging researches in better understanding the fundamental mechanisms involved in catalytic and photocatalytic processes, and the molecular pathways that are activated by the interaction of LDHs with cells in terms of both uptake mechanisms and nanotoxicology effects.

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.

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.

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.

Nickel-Catalysed Synthesis of 17-Arylandrosta-5,16-Dienes

An efficient Ni-catalysed coupling reaction between arylboronic acids and 17-trifluoromethanesulfonyl- 3β-acetoxyandrosta-5,16-diene which was obtained by the sulfonylation of 3β-acetoxylandrost-5-ene-17-one was developed to afford 17-aryl- 3β-acetoxy-androsta-5,16-dienes in moderate to good yields (52–85%). The structure of the 3,4-dimethoxyaryl product was confirmed by X-ray crystallography.

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.

A total synthesis of (-)-Hortonone C

A total synthesis of the cytotoxic terpenoid hortonone C was accomplished and its absolute stereochemistry confirmed. Intermediate (+)-4 was synthesized using either an asymmetric conjugate addition strategy, or by elaboration of the Hajos-Parrish ketone. Reduction of (+)-4 under dissolving-metal conditions and trapping the enolate intermediate served to control the cis-stereochemistry at the ring fusion and provide a silyl enol ether necessary for ring expansion. Comparison of optical rotation data confirmed that the absolute configuration of natural hortonone C is (6S,7S,10S).

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.

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.

Constructing densely functionalized Hajos–Parrish-type ketones with six contiguous stereogenic centers and two quaternary carbons in a formal [2 + 2 + 2] cycloaddition cascade

Efficient construction of Hajos–Parrish-type ketones with six contiguous stereogenic centers in a formal [2 + 2 + 2] cycloaddition cascade. The spontaneous resolution of racemic product without the need for chiral agent has been achieved.

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.

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.

Recent Applications and Developments of Organic Azides in Total Synthesis of Natural Products

Organic azides have been exploited since their discovery because of their high reactivities. Various organic reactions using azides have been synthetically applied in chemical biology pharmaceuticals medicinal and agricultural areas. In this review we present some recent applications and developments of organic azides in the total synthesis of natural products (mostly within five years) especially alkaloids. We focus not only on application examples of organic azides but also show their preparation methods including recently reported procedures concerning their decomposing and reducing methods in the syntheses of bioactive molecules.

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.

Creation of readily accessible and orally active analogue of cortistatin a

Syntheses of structurally simplified analogues of cortistatin A (1), a novel antiangiogenic steroidal alkaloid from Indonesian marine sponge, and their biological activities were investigated. The analogues were designed by considering the 3-D structure of 1. Compound 30, in which the isoquinoline moiety was appended to the planar tetracyclic core structure, showed potent antiproliferative activity against human umbilical vein endothelial cells (HUVECs) together with high selectivity and also showed in vivo antiangiogenic activity and significant antitumor effect by oral administration.

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.