Sequential ring-closing metathesis/Pd-catalyzed, Si-assisted cross-coupling reactions: general synthesis of highly substituted unsaturated alcohols and medium-sized rings containing a 1,3-cis–cis diene unit

Modular synthesis of 2-furyl carbinols from 3-benzyldimethylsilylfurfural platforms relying on oxygen-assisted C–Si bond functionalization

3-Silylated furfurals, readily prepared in three steps from biomass-derived furfural and 5-methylfurfural, are converted into 3-silylated 2-furyl carbinols upon condensation with organomagnesium or organolithium reagents. The hydroxy unit of the carbinol adducts can be exploited to promote C3(sp²)–Si bond functionalization through intramolecular activation. Two approaches were contemplated for this purpose. Activation by alkoxides of the C3–SiEt₃ or C3–SiMe₂t-Bu bonds was ineffective. Conversely, treatment of the C3-benzyldimethylsilyl-appended derivatives with tetrabutylammonium fluoride led to cyclic siloxanes, which revealed to be competent donors for copper-catalyzed cross-coupling reactions, such as arylation reactions catalyzed by Pd₂(dba)₃/CuI, as well as allylation and methylation reactions catalyzed by CuI⋅PPh₃. C3-Benzyldimethylsilyl-appended furfurals are thus introduced as versatile platforms, providing a modular access to 3-substituted 2-furyl carbinols from renewable feedstock.

Iron-catalyzed alkyne-carbonyl metathesis for the synthesis of 6,7-dihydro-5H-dibenzo[c,e]azonines

The synthesis of nine-membered rings via alkyne-carbonyl metathesis is reported. The alkyne and acetal units contained in a biaryl substrate undergo the intramolecular alkyne-carbonyl metathesis reaction under FeCl₃-catalysis to furnish the unexplored 6,7-dihydro-5H-dibenzo[c,e]azonines. The method provides an alternative to existing routes to nine-membered rings with cheap, non-toxic iron catalysts and milder conditions.

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.

A Modular, Enantioselective Synthesis of Resolvins D3, E1, and Hybrids

Resolvins D3 and E1 are important signaling molecules in the resolution of inflammation. Here, we report a convergent and flexible strategy to prepare these natural products using Hiyama-Denmark coupling of five- and six-membered cyclic alkenylsiloxanes to connect three resolvin fragments, and control the stereochemistry of the natural product (Z)-alkenes. The modular nature of this approach enables the synthesis of novel resolvin hybrids, opening up opportunities for more-extensive investigations of resolvin biology.

Pd–Co Nanoparticles Supported on Calcined Mg–Fe Hydrotalcites for the Suzuki–Miyaura Reaction in Water with High Turnover Numbers

We reveal the Suzuki–Miyaura reaction catalyzed by a Pd–Co nanocatalyst supported by Mg–Fe–CHT (calcined hydrotalcites). A variety of boronic acids and aryl halides were transformed into functionalized biphenyls in excellent yields using water as a solvent. The reaction could proceed under mild conditions with a simple operation and high turnover numbers. The excellent catalytic activities are reasonably attributed to the Co-doping, which forms a Pd–Co alloy on the surface of CHT.

Synthesis of Cyclic Alkenyl Dimethylsiloxanes from Alkynyl Benzyldimethylsilanes and Application in Polyene Synthesis

Cyclic dimethylalkenylsiloxanes, useful motifs for (Z)-selective Hiyama cross-coupling, are accessed from alkynyl benzyldimethylsilanes featuring adjacent allylic or homoallylic oxygen substituents by semihydrogenation/debenzylation/cyclization. While formation of 5- and 6-membered rings can be achieved from the free alcohols using fluoride or silanolate, allylic acetate precursors to 5-membered rings display distinct modes of activation. The utility of these compounds is demonstrated through the preparation of a variety of (Z)-alkene-containing polyenes and application to a concise total synthesis of leukotriene B₃.

Synthesis of substituted Z-styrenes by Hiyama-type coupling of oxasilacycloalkenes: application to the synthesis of a 1-benzoxocane

Several Hiyama cross-coupling reactions of oxasilacycloalkenes and aryl iodides are described that produce trisubstituted Z-styrenes in moderate to excellent yields. Both electron-rich and electron-poor aryl iodides are tolerated in the cross-coupling reaction. The oxasilacycloalkene coupling partners were prepared by ruthenium-catalyzed intramolecular anti-hydrosilylation of alkynols. One of the cross-coupling products was converted to a 1-benzoxocane, albeit in low yield, using an intramolecular Buchwald–Hartwig etherification. The cyclic ether produced contains the carbon skeleton of heliannuol A.

Heteromultimetallic catalysis for sustainable organic syntheses

Fully complementary bimetallic catalysis has been identified as an increasingly powerful tool for molecular transformations, which was largely inspired by early examples of sequential catalytic transformations.

(5Z,11Z,15R)-15-Hydroxyeicosa-5,11-dien-13-ynoic acid: A stable isomer of 15(S)-HETE that retains key vasoconstrictive and antiproliferative activity

15(S)-Hydroxyeicosa-(5Z,8Z,11Z,13E)-tetraenoic acid (15(S)-HETE) is a metabolite of arachidonic acid that elicits a number of biological effects including vasoconstriction and angiogenesis. (5Z,11Z,15R)-15-Hydroxyeicosa-5,11-dien-13-ynoic acid (HETE analog 1) is a synthetic isomer of 15(S)-HETE that is much more stable to autoxidation. Using isometric recording of isolated pulmonary arteries from male and female rabbits, HETE analog 1 and 15(S)-HETE were found to elicit concentration-dependent contractions that were slightly greater in females compared to males. The maximal response in females was greater with 15(S)-HETE. HETE analog 1 and 15(S)-HETE increased [(3)H]-thymidine incorporation in vascular smooth muscle cells cultured from male rabbit pulmonary arteries; both the maximal response and potency were greater with 15(S)-HETE. In contrast, HETE analog 1 produced a concentration-dependent inhibition in proliferation and migration of human hormone-independent prostate carcinoma PC-3 cells. The protocol for synthesis of HETE analog 1 is reported. The stability of this substance and its similar biological profile to 15(S)-HETE support future studies in eicosanoid research.

A two-step, stereoselective synthesis of nine- and ten-membered carbocycles from phthalates

A two-step, stereoselective procedure for the synthesis of nine- and ten-membered carbocycles from readily available phthalates is described. A variety of dialkyl phthalates have been transformed into [6,n]-fused bicyclo systems (n = 5, 6, 7) by a dearomatization/cyclization process and then converted into cyclonona- and cyclodecadienes through a bond cleavage reaction, whereby both processes are promoted by alkaline metals in THF.

Identification of a novel NAMPT inhibitor by combinatorial click chemistry and chemical refinement

The identification of compounds able to inhibit the NAD salvage pathway is experiencing a growing popularity as it has been proposed to be a novel target for antitumoral and anti-inflammatory drugs.

Synthesis of cyclic alkenylsiloxanes by semihydrogenation: a stereospecific route to (Z)-alkenyl polyenes

Cyclic alkenylsiloxanes were synthesized by semihydrogenation of alkynylsilanes-a reaction previously plagued by poor stereoselectivity. The silanes, which can be synthesized on multigram scale, undergo Hiyama-Denmark coupling to give (Z)-alkenyl polyene motifs found in bioactive natural products. The ring size of the silane is crucial: five-membered cyclic siloxanes also couple under fluoride-free conditions, whilst their six-membered homologues do not, enabling orthogonality within this structural motif.

9-Membered carbocycle formation: development of distinct Friedel-Crafts cyclizations and application to a scalable total synthesis of (±)-caraphenol A

Explorations into a series of different approaches for 9-membered carbocycle formation have afforded the first reported example of a 9-exo-dig ring closure via a Au(III)-promoted reaction between an alkyne and an aryl ring as well as several additional, unique Friedel-Crafts-type cyclizations. Analyses of the factors leading to the success of these transformations are provided, with the application of one of the developed 9-membered ring closures affording an efficient and scalable synthesis of the bioactive resveratrol trimer caraphenol A. That synthesis proceeded with an average yield of 89% per step (7.8% overall yield) and has provided access to more than 600 mg of the target molecule.

Palladium-catalysed cross-coupling of organosilicon reagents

The palladium catalysed cross-coupling of organosilicon reagents with organo halides and pseudo-halides has developed over the past 30 years into an efficient and attractive carbon-carbon bond forming strategy. Extensive research within this field to expand and diversify on the scope of the organosilicon coupling reaction will continue to promote its use in the synthesis of biologically and pharmaceutically important organic molecules. The recent advances made within this area are explored in this critical review (199 references).

Ring strain-promoted allylic transposition of cyclic silyl ethers

Relief of the ring strain of medium-sized rings promotes a regioselective allylic transposition of a C-O bond when catalyzed by rhenium oxide. Through the allylic transposition, eight-membered cyclic silyl ethers undergo ring contraction to the corresponding six-membered siloxacycles.

Stereocontrolled total syntheses of isodomoic acids G and H via a unified strategy

Marine neuroexcitatory compounds isodomoic acids G and H were efficiently synthesized from a common intermediate using a silicon-based cross-coupling reaction. Dividing each target compound into the core fragment and the side-chain fragment enabled the synthesis to be convergent. The trans-2,3-disubstituted pyrrolidine core fragment was accessed through a diastereoselective rhodium-catalyzed carbonylative silylcarbocyclization reaction of a vinylglycine-derived 1,6-enyne. A stereochemically divergent desilylative iodination reaction was developed to convert the cyclization product to both E- and Z-alkenyl iodides, which would eventually lead to isodomoic acid G and isodomoic acid H, respectively. The late-stage alkenyl-alkenyl silicon-based cross-coupling reaction uniting the core alkenyl iodides and the side-chain alkenylsilanol was achieved under mild conditions. Finally, two mild deprotections afforded the target molecules.

Sequential Processes in Palladium-Catalyzed Silicon-Based Cross-Coupling

Although developed somewhat later, silicon-based cross-coupling has become a viable alternative to the more conventional Suzuki-Miyaura, Stille-Kosugi-Migita, and Negishi cross-coupling reactions because of its broad substrate scope, high stability of silicon-containing reagents, and low toxicity of waste streams. An empowering and yet underappreciated feature unique to silicon-based cross-coupling is the wide range of sequential processes available. In these processes, simple precursors are first converted to complex silicon-containing cross-coupling substrates, and the subsequent silicon-based cross-coupling reaction affords an even more highly functionalized product in a stereoselective fashion. In so doing, structurally simple and inexpensive starting materials are quickly transformed into value-added and densely substituted products. Therefore, sequential processes are often useful in constructing the carbon backbones of natural products. In this review, studies of sequential processes involving silicon-based cross-coupling are discussed. Additionally, the total syntheses that utilize these sequential processes are also presented.

Silicon-based cross-coupling reactions in the total synthesis of natural products

Unlike other variants of transition-metal-catalyzed cross-coupling reactions, those based on organosilicon donors have not been used extensively in natural product synthesis. However, recent advances such as: 1) the development of mild reaction conditions, 2) the expansion of substrate scope, 3) the development of methods to stereoselectively and efficiently introduce the silicon-containing moiety, 4) the development of a large number of sequential processes, and 5) the advent of bifunctional bis(silyl) linchpin reagents, signify the coming of age of silicon-based cross-coupling reactions. The following case studies illustrate how silicon-based cross-coupling reactions play a strategic role in constructing carbon-carbon bonds in selected target molecules.

The interplay of invention, discovery, development, and application in organic synthetic methodology: a case study

This Perspective chronicles the conceptual development, proof of principle, exploration of scope, mechanistic investigations, and applications in natural product total synthesis of palladium-catalyzed cross-coupling reactions of silicon derivatives. The explication of how this new class of cross-coupling reactions evolved from problem formulation to use in complex molecule synthesis serves as one goal of the essay. The other goal is the presentation of the various stages of this methodological enterprise such that the reader gleans a first hand look at one approach to the creation of new synthetic reactions. These two goals are woven together such that the underlying thought processes that guide a program of reaction development emerge in clear view and imbue the chemical tapestry with a cohesive logic.

Conjugated imines and iminium salts as versatile acceptors of nucleophiles

Growing interests in nitrogen-containing molecules involving bioactive and functional materials have stimulated the recent development of synthetic methodologies where nucleophilic addition reactions to imino carbons are utilized in crucial steps. This article summarizes double nucleophilic addition reactions with alpha,beta-unsaturated aldimines, addition reactions using alkynyl imines, "umpoled" reactions of alpha-imino esters, and the use of iminium salts as reactive electrophiles.

A relay ring-closing metathesis synthesis of dihydrooxasilines, precursors of (Z)-iodo olefins

A convenient Grubbs II metathesis provides dihydrooxasilines by relay RCM (RRCM). Dihydrooxasilines undergo ring opening to give Z-vinyl silanes. These can then be converted to Z-vinyl iodides. This sequence provides a short, high yield, and convenient route to trisubstituted Z-vinyl iodides, useful intermediates for the preparation of polypropionate antibiotics.

Vinylation of aromatic halides using inexpensive organosilicon reagents. Illustration of design of experiment protocols

The preparation of styrenes by palladium-catalyzed cross-coupling of aromatic iodides and bromides with divinyltetramethyldisiloxane (DVDS) in the presence of inexpensive silanolate activators has been developed. To facilitate the discovery of optimal reaction conditions, Design of Experiment (DoE) protocols were used. By the guided selection of reagents, stoichiometries, temperatures, and solvents, the vinylation reaction was rapidly optimized with three stages consisting of ca. 175 experiments (of a possible 1440 combinations). A variety of aromatic iodides undergo cross-coupling at room temperature in the presence of potassium trimethylsilanoate using Pd(dba)2 in DMF in good yields. Triphenylphosphine oxide is needed to extend catalyst lifetime. Application of these conditions to aryl bromides was accomplished by the development of two complementary protocols. First, the direct implementation of the successful reaction conditions using aryl iodides at elevated temperature in THF provided the corresponding styrenes in good to excellent yields. Alternatively, the use of potassium triethylsilanolate and a bulky "Buchwald-type" ligand allows for the vinylation reactions to occur at or just above room temperature. A wide range of bromides underwent coupling in good yields for each of the protocols described.

Platinum-Catalyzed Multistep Reactions of Indoles with Alkynyl Alcohols

PtCl2 effectively catalyzes the multistep reaction of N-methyl indole (1 a) with pent-3-yn-1-ol (2 a) in THF at room temperature for 2 h to give indole derivative 3 a, which contains a five-membered cyclic ether group at C3 in 93% yield. Under similar reaction conditions, various substituted N-methyl indoles 1 b-h and indole (1 i) reacted efficiently with 2 a to afford the corresponding indole derivatives 3 b-h and 3 i in 48-91 and 72% yields. The results showed that N-methyl indoles with electron-donating substituents were more reactive affording higher product yields than those with electron-withdrawing groups. Likewise, various substituted but-3-yn-1-ols 2 b-e and other longer chain alkynyl alcohols 2 f-i also underwent a cyclization-addition reaction with N-methyl indole (1 a) to provide the corresponding cyclization-addition products 3 j-m and 3 a, 3 j, and 3 n-o in good to excellent yields. The present platinum-catalyzed cyclization-addition reaction can be further extended into N-methyl pyrrole. Mechanistically, the catalytic reaction proceeds by an intramolecular hydroalkoxylation of alkynyl alcohol to afford cyclic enol ether followed by the addition of the C--H bond of indole to the unsaturated moiety of cyclic enol ether providing the final product. Experimental evidence to support this proposed mechanism is provided.

Sequential silylcarbocyclization/silicon-based cross-coupling reactions

A sequential rhodium-catalyzed silylcarbocyclization of enynes parlayed with a palladium-catalyzed, silicon-based cross-coupling reaction has been developed for the synthesis of highly substituted cyclopentanes. 1,6-Enynes reacted with benzyldimethylsilane in the presence of rhodium catalysts to afford five-membered rings bearing a (Z)-alkylidenylbenzylsilyl group. A variety of substitution patterns and heteroatom substituents were compatible. The silylcarbocyclization in which an unsaturated ester participated was also achieved. The resulting alkylidenylsilanes underwent palladium-catalyzed cross-coupling using tetra-n-butylammonium fluoride. This cross-coupling reaction displayed a broad substrate scope. A wide variety of substitution patterns, electronic properties, and heteroatoms were compatible. All of the cross-coupling reactions proceeded in high yields under very mild conditions and with complete retention of double bond configuration, resulting in densely functionalized 3-(Z)-benzylidenecyclopentanes and heterocycles.

Synthesis of 3,4,5-trisubstituted isoxazoles via sequential [3 + 2] cycloaddition/silicon-based cross-coupling reactions

[reaction: see text] A [3 + 2] cycloaddition reaction between alkynyldimethylsilyl ethers and aryl and alkyl nitrile oxides to produce isoxazolylsilanols has been developed. The cross-coupling reactions of these heterocyclic silanols with a variety of aryl iodides affords 3,4,5-trisubstituted isoxazoles. This sequential process allows for rapid variation of substituents at the 3, 4, and 5 positions of the isoxazole.