Tandem Use of Cobalt-Mediated Reactions to Synthesize (+)-Epoxydictymene, a Diterpene Containing a Trans-Fused 5−5 Ring System

Strategies to access the [5-8] bicyclic core encountered in the sesquiterpene, diterpene and sesterterpene series

Terpene compounds probably represent the most diversified class of secondary metabolites. Some classes of terpenes, mainly diterpenes (C20) and sesterterpenes (C25) and to a lesser extent sesquiterpenes (C15), share a common bicyclo[3.6.0]undecane core which is characterized by the presence of a cyclooctane ring fused to a cyclopentane ring, i.e., a [5-8] bicyclic ring system. This review focuses on the different strategies elaborated to construct this [5-8] bicyclic ring system and their application in the total synthesis of terpenes over the last two decades. The overall approaches involve the construction of the 8-membered ring from an appropriate cyclopentane precursor. The proposed strategies include metathesis, Nozaki-Hiyama-Kishi (NHK) cyclization, Pd-mediated cyclization, radical cyclization, Pauson-Khand reaction, Lewis acid-promoted cyclization, rearrangement, cycloaddition and biocatalysis.

Photoinduced Intramolecular Cyclization of Acylsilanes Bearing a Boronate Moiety: Construction of a Highly Strained trans-Fused Bicyclo[3.3.0]octane Skeleton

A reliable strategy for the construction of trans-fused bicyclo[n.3.0] skeletons was explored by utilizing photoinduced cyclization of acylsilanes bearing a boronate. The substrates having an acylsilane and a boronate in a 1,2-trans relationship were prepared via hydroboration of cycloalkene derivatives. The desired cyclization was efficiently promoted by photoirradiation to give the trans-fused bicyclo[n.3.0] derivatives as a single diastereomer. It is noteworthy that this methodology enables the efficient construction of a highly strained trans-5-5 ring system.

Single-atom logic for heterocycle editing

Medicinal chemistry continues to be impacted by new synthetic methods. Particularly sought after, especially at the drug discovery stage, is the ability to enact the desired chemical transformations in a concise and chemospecific fashion. To this end, the field of organic synthesis has become captivated by the idea of 'molecular editing'-to rapidly build onto, change or prune molecules one atom at a time using transformations that are mild and selective enough to be employed at the late stages of a synthetic sequence. In this Review, the definition and categorization of a particularly promising subclass of molecular editing reactions, termed 'single-atom skeletal editing', are proposed. Although skeletal editing applies to both cyclic and acyclic compounds, this Review focuses on heterocycles, both for their centrality in medicinal chemistry and for the definitional clarity afforded by a focus on ring systems. A classification system is presented by highlighting methods (both historically important examples and recent advances) that achieve such transformations, with the goal to spark interest and inspire further development in this growing field.

Total synthesis of cynaropicrin

Cynaropicrin is found in artichoke (Cynara scolymus) and is the source of its bitter taste and it is a sesquiterpene lactone with a 5-7-5 tricyclic skeleton, six chiral centers, and four exo-olefins. This natural product has numerous attractive biological activities including the inhibition of NF-κB activation, antihepatitis C activity, and antitrypanosomal activity. In this study, the first total synthesis of cynaropicrin was achieved starting from (S)-α-pinene. The synthesis involved a stereoselective Favorskii rearrangement and an indium-promoted diastereoselective Barbier reaction.

Synthesis of natural products containing highly strained trans-fused bicyclo[3.3.0]octane: historical overview and future prospects

Owing to high strain energy, molecules with trans-fused bicyclo[3.3.0]octane ring systems are very difficult to synthesize, and there are very few approaches to access them. Recently, a number of natural products with such ring systems have been made by the synthetic community. However, there has been no review in this field before. This review provides a systematic and comprehensive discussion on the synthesis of natural products containing trans-fused bicyclo[3.3.0]octanes and the historical context of this work. The prospects for future research in this field are also discussed. Covering the literature before 2021, this review aims to offer a helpful reference for total synthesis of highly strained natural products containing trans-fused bicyclo[3.3.0]octane ring systems.

Synthesis of Medium-Sized Carbocycles via a Bidentate Lewis Acid-Catalyzed Inverse Electron-Demand Diels-Alder Reaction Followed by Photoinduced Ring-Opening

The combination of a Lewis acid-catalyzed inverse electron-demand Diels-Alder (IEDDA) reaction with a photoinduced ring-opening (PIRO) reaction in a domino process has been established as an efficient synthetic method to access medium-sized carbocycles. From readily available electron-rich and electron-poor phthalazines and enamines, respectively, as starting materials, various 9- and 11-membered carbocycles were prepared. This versatile transition-metal-free tool will be valuable for broadening the structural space in biologically active compounds and functional materials.

Total Syntheses of (-)-Conidiogenone B, (-)-Conidiogenone, and (-)-Conidiogenol

Cyclopianes are novel diterpenes featuring a highly strained 6/5/5/5 tetracyclic core embedded with 6-8 consecutive stereocenters. The concise total syntheses of (-)-conidiogenone B, (-)-conidiogenone, and (-)-conidiogenol have been accomplished in 14-17 steps. The present work features a HAT-mediated alkene-nitrile cyclization to access the cis-biquinane, a Nicholas/Pauson-Khand reaction to construct the linear triquinane, and a Danheiser annulation to afford the congested angular triquinane skeleton.

Biomimetic synthesis of nudicaulins I and II, yellow pigments from the Iceland poppy Papaver nudicaule

Indole and the anthocyanin orientalin proceed through a unique cascade sequence that leads to nudicaulins I and II in 92% yield. This biomimetic synthesis confirms the biosynthesis proposal for these structurally unprecedented flavoalkaloids that play a key role in the colour range displayed by the Iceland poppy.

Total Synthesis of (-)-Indoxamycins A and B

The concise total syntheses of (-)-indoxamycins A and B is reported. The chemistry features a seven-step preparation of a highly congested [5.5.6] tricyclic advanced common intermediate from a readily available R-carvone derivative. Key steps involve a Pauson-Khand reaction for the rapid construction of a basic scaffold bearing a quaternary carbon, a copper-catalyzed Michael addition for the introduction of another adjacent all-carbon quaternary stereocenter, and a tandem retro-oxa-Michael addition/1,2-addition/oxa-Michael addition for the installation of a trisubstituted olefin side chain. This synthetic strategy allows for easy access to both enantiomers of this family of natural products and their analogues from cost-effective starting material through straightforward chemical transformations.

A Complementary Process to Pauson-Khand-Type Annulation Reactions for the Construction of Fully Substituted Cyclopentenones

A complementary process to the Pauson-Khand annulation is described that is well suited to forging densely substituted/oxygenated cyclopentenone products (including fully substituted variants). The reaction is thought to proceed through a sequence of metallacycle-mediated bond-forming events that engages an internal alkyne and a β-keto ester in an annulation process that forges two C-C bonds. A variant of this annulation process has also been established that delivers deoxygenated cyclopentenones that lack the allylic tertiary alcohol.

Phosphine-mediated sequential annulations of allenyl ketone and isocyanide: a bicyclization strategy to access a furan-fused eight-membered ring and a spirocycle

Phosphine-mediated cascade annulations of allenyl ketone and isocyanide have been disclosed. This strategy enables the efficient synthesis of a furan-fused eight-membered ring and a spirocycle.

Natural product syntheses via carbonylative cyclizations

This review summarizes the application of various transition metal-catalyzed/mediated carbonylative cyclization reactions in natural product total synthesis.

Palladium(0)-catalyzed [2 + 2 + 1] cyclization of 1,6-enynes with vinyl bromides: a highly diastereoselective synthesis of tetrahydro-1H-cyclopenta[c]furans bearing two quaternary carbon centers

A new cascade process has been accomplished for the synthesis of tetrahydro-1H-cyclopenta[c]furans through palladium-catalyzed [2 + 2 + 1] cyclization of 1,6-enynes with vinyl bromides. Notably, the key feature of this transformation is the use of vinyl bromides as the C1 building block. Various functionalized tetrahydro-1H-cyclopenta[c]furans bearing two quaternary carbon centers could be obtained in good yields with excellent diastereoselectivities.

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.

Syntheses of Cyclic Guanidine-Containing Natural Products

Naturally occurring guanidine derivatives frequently display medicinally useful properties. Among them, the higher order pyrrole-imidazole alkaloids, the dragmacidins, the crambescidins/batzelladines, and the saxitoxins/tetradotoxins have stimulated the development of many new synthetic methods over the past decades. We provide here an overview of the syntheses of these cyclic guanidine-containing natural products.

Lewis acid catalyzed intramolecular [3 + 2] cross cycloadditions of cobalt-alkynylcyclopropane 1,1-diesters with carbonyls for construction of medium-sized and polycyclic skeletons

A Lewis acid catalyzed intramolecular [3 + 2] cross cycloaddition of cobalt-alkynylcyclopropane 1,1-diesters with carbonyls has been successfully developed. Together with simple and efficient postcycloadditions of the cobalt-alkyne moiety, a general and efficient strategy for construction of structurally complex and diverse medium-sized skeletons and related polycycles was supplied successfully.

Total synthesis of (-)-jiadifenin

As easy as ABCD: (-)-Jiadifenin was synthesized in eighteen reaction steps from 1-[(E)-(4'-bromo-2'-butenyl)oxy]-4-methoxybenzene. Key features of this synthesis include: 1) Ireland-Claisen rearrangement to produce the two contiguous quaternary centers at C5 and C6 simultaneously, 2) intramolecular Pauson-Khand reaction (IMPKR) to concurrently construct the A and B rings, and 3) [2+2] photo-cycloaddition to generate the all-carbon quaternary center at C9.

Diastereoselective total synthesis of (±)-schindilactone A, Part 1: Construction of the ABC and FGH ring systems and initial attempts to construct the CDEF ring system

First-generation synthetic strategies for the diastereoselective total synthesis of schindilactone A (1) are presented and methods for the synthesis of the ABC, FGH, and CDEF moieties are explored. We have established a method for the synthesis of the ABC moiety, which included both a Diels-Alder reaction and a ring-closing metathesis as the key steps. We have also developed a method for the synthesis of the FGH moiety, which involved the use of a Pauson-Khand reaction and a carbonylative annulation reaction as the key steps. Furthermore, we have achieved the construction of the central 7-8 bicyclic ring system by using a [3,3]-rearrangement as the key step. However, unfortunately, when this rearrangement reaction was applied to the construction of the more advanced CDEF moiety, the anticipated annulation reaction did not occur and the development of an alternative synthetic strategy would be required for the construction of this central core.