Ti(III)-catalyzed cyclizations of ketoepoxypolyprenes: control over the number of rings and unexpected stereoselectivities

Asymmetric synthesis of the fully functionalized six-membered A-ring of siphonol A

A synthetic study toward the construction of the fully functionalized six-membered A-ring of siphonol A is described. The salient features include the introduction of a six-membered ring system through a HWE reaction, the construction of a stereocenter at C5 via a hetero-Diels-Alder reaction, and the installation of the fully functionalized six-membered A-ring of siphonol A through photolytic decarboxylation.

Coordination-induced bond weakening and small molecule activation by low-valent titanium complexes

Bond activation of small molecules through coordination to low valent metal complexes in M⋯X-H type interactions (where X = O, N, B, Si, etc.) leads to the formation of unusually weak X-H bonds and provides a powerful approach for the synthesis of target compounds under very mild conditions. Coordination of small molecules like water, amides, silanes, boranes, and dinitrogen to Ti(III) or Ti(II) complexes results in the synergetic redistribution of electrons between the metal orbitals and the ligand orbitals which weakens and enables the facile cleavage of the X-H or N-N bonds of the ligands. This review presents an overview of coordination-induced bond activation of small molecules by low valent titanium complexes. In particular, the applications of low valent titanium-induced bond weakening in nitrogen fixation are presented. The review concludes with potential future directions for work in this area including low-valent Ti-based PCET systems, photocatalytic nitrogen reduction, and approaches to tailoring complexes for optimal bond activation.

Cp2TiCl-Promoted Endo-Trig Radical Cyclization to Six- and Seven-Membered Carbocycles: Synthesis of (±)-Isoclavukerin A

A systematic study is undertaken to investigate the less explored endo-trig radical cyclization in activated olefin-appended epoxides using Cp2TiCl. The radical generated by the Ti(III)-promoted reductive opening of the epoxy ring promptly underwent endo-trig cyclization, giving access to differently 1,3-disubstituted six- and seven-membered carbocycles in good yields and diastereoselectivity. This protocol was successfully employed in the construction of 5,7- and 6,7-fused bicyclic frameworks entailing a de novo synthesis of (±)-isoclavukerin A belonging to tri-nor-guaiane class of sesquiterpene natural products in eight simple steps from commercially available starting materials. Besides the Ti(III)-mediated reaction serving as a key step in the synthesis, a sequential [2,3]-sigmatropic rearrangement/syn-elimination of an allyl sulfenate intermediate successfully rendered the highly constrained diene moiety in the hydroazulene core of the target molecule.

InI3-catalyzed polyene cyclization of allenes and its application in the total synthesis of seven abietane-type diterpenoids

A novel polyene cyclization using the allene group as the initiator has been successfully developed. This methodology provides an efficient strategy for the construction of an abietane-type tricyclic skeleton with a functionalizable C2-C3 double bond and features a wide substrate scope and excellent stereoselectivities. Potential utility of this approach has been well demonstrated by the collective total synthesis of seven abietane-type diterpenoids. Specifically, (±)-2,3-dihydroxyferruginol and (±)-2,3-dihydroxy-15,16-dinor-ent-pimar-8,11,13-triene were synthesized for the first time.

Stereocontrolled Radical Bicyclizations of Oxygenated Precursors Enable Short Syntheses of Oxidized Abietane Diterpenoids

The power of cation-initiated cyclizations of polyenes for the synthesis of polycyclic terpenoids cannot be overstated. However, a major limitation is the intolerance of many relevant reaction conditions toward the inclusion in the substrate of polar functionality, particularly in unprotected form. Radical polycyclizations are important alternatives to bioinspired cationic variants, in part owing to the range of possible initiation strategies, and in part for the functional group tolerance of radical reactions. In this article, we demonstrate that Co-catalyzed MHAT-initiated radical bicyclizations are not only tolerant of oxidation at virtually every position in the substrate, oftentimes in unprotected form, but these functional groups can also contribute to high levels of stereochemical control in these complexity-generating transformations. Specifically, we show the effects of protected or unprotected hydroxy groups at six different positions and their impact on stereoselectivity. Further, we show how multiply oxidized substrates perform in these reactions, and finally, we document the utility of these reactions in the synthesis of three aromatic abietane diterpenoids.

Understanding titanium-catalysed radical-radical reactions: a DFT study unravels the complex kinetics of ketone-nitrile couplings

The computational investigation of a titanium-catalysed reductive radical-radical coupling is reported. The results match the conclusions from an earlier experimental study and enable a further interpretation of the previously observed complex reaction kinetics. Furthermore, the interplay between neutral and cationic reaction pathways in titanium(iii)-catalysed reactions is investigated for the first time. The results show that hydrochloride additives and reaction byproducts play an important role in the respective equilibria. A full reaction profile is assembled and the computed activation barrier is found to be in reasonable agreement with the experiment. The conclusions are of fundamental importance to the field of low-valent titanium catalysis and the understanding of related catalytic radical-radical coupling reactions.

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 concise total synthesis of sespenine, a structurally unusual indole terpenoid from Streptomyces

A ten-step (the longest linear sequence) total synthesis of sespenine was accomplished.

Direct conjugate alkylation of α,β-unsaturated carbonyls by TiIII-catalysed reductive umpolung of simple activated alkenes

The title reaction leads to 1,6-difunctionalized products without the requirement of premetallated reagents. Details on scope, selectivity and mechanism are reported.

Synthesis of substituted γ- and δ-lactams based on titanocene(iii)-catalysed radical cyclisations of trichloroacetamides

Straightforward synthesis of dehalogenated γ- and δ-lactams based on a Cp₂TiCl-catalysed radical cyclisation of trichloroacetamides under mild conditions.

Metal-Catalyzed β-Functionalization of Michael Acceptors through Reductive Radical Addition Reactions

Transition-metal-catalyzed radical reactions are becoming increasingly important in modern organic chemistry. They offer fascinating and unconventional ways for connecting molecular fragments that are often complementary to traditional methods. In particular, reductive radical additions to α,β-unsaturated compounds have recently gained substantial attention as a result of their broad applicability in organic synthesis. This Minireview critically discusses the recent landmark achievements in this field in context with earlier reports that laid the foundation for today's developments.

Organic Photocatalytic Cyclization of Polyenes: A Visible-Light-Mediated Radical Cascade Approach

A visible-light-mediated, organic photocatalytic stereoselective radical cascade cyclization of polyprenoids is described. The desired cascade cyclization products are achieved in good yields and high stereoselectivities with eosin Y as photocatalyst in hexafluoro-2-propanol. The catalyst system is also suitable for 1,3-dicarbonyl compounds, which require only catalytic amounts of LiBr to promote the formation of the corresponding enols.

Recent advances in the synthesis of natural multifunctionalized decalins

This review highlights recent innovative synthetic strategies developed for the stereoselective construction of natural complex decalin systems. It offers an insight into various synthetic targets and approaches and provides information for developments within the area of natural products as well as synthetic methodology.

Cationic Titanocene(III) Complexes for Catalysis in Single-Electron Steps

By exploiting solvent and anion effects, [Cp2Ti](+) complexes for atom-economical catalysis in single-electron steps were developed and applied for the first time. These complexes constitute remarkably stable and active catalysts for radical arylations. The reaction kinetics and catalyst composition were studied by cyclic voltammetry and in situ IR spectroscopy.

Total synthesis and antiviral activity of indolosesquiterpenoids from the xiamycin and oridamycin families

Indolosesquiterpenoids are a growing class of natural products that exhibit a wide range of biological activities. Here, we report the total syntheses of xiamycin A and oridamycins A and B, indolosesquiterpenoids isolated from Streptomyces. Two parallel strategies were exploited to forge the carbazole core: 6π-electrocyclization/aromatization and indole C2–H bond activation/Heck annulation. The construction of their trans-decalin motifs relied on two diastereochemically complementary radical cyclization reactions mediated by Ti(III) and Mn(III), respectively. The C23 hydroxyl of oridamycin B was introduced by an sp³ C–H bond oxidation at a late stage. On the basis of the chemistry developed, the dimeric congener dixiamycin C has been synthesized for the first time. Evaluation of the antiviral activity of these compounds revealed that xiamycin A is a potent agent against herpes simplex virus–1 (HSV-1) in vitro.

Using a single strategy to make a number of related intermediates is a useful strategy in the total synthesis. Here, the authors report the synthesis of a number of natural products, employing two diastereomerically complementary metal-catalyzed cyclizations as the key step to access a number of frameworks.