Macrocyclization by nickel-catalyzed, ester-promoted, epoxide-alkyne reductive coupling: total synthesis of (-)-gloeosporone

Synthetic Applications of Nickel-Catalyzed Cross-Coupling and Cyclisation Reactions

Since the first studies about fifty years ago, the direct formation of C-C bonds, catalyzed by nickel complexes, has appeared as an important research topic, and has re-emerged recently as a renewal of nickel chemistry. This account provides a summary of the use of nickel complexes in catalysis, and highlights the evolution of our own research.

Recent Advances in the Total Synthesis of Natural Products Containing Eight-Membered Carbocycles (2009-2019)

Natural products containing eight-membered carbocycles constitute a class of structurally intriguing and biologically important molecules such as the famous diterpenes taxol and vinigrol. Such natural products are being increasingly investigated because of their fascinating architectural features and potent medicinal properties. However, synthesis of natural products with cyclooctane moieties has proved to be highly challenging. This review highlights the recently completed total syntheses of natural products with eight-membered carbocycles with a focus on strategic considerations. A collection of 27 representative studies from the literature covering the decade from 2009 to 2019 is described in chronological order with relevant studies grouped together, including syntheses of the same natural product by different research groups using different strategies. Finally, a summary and outlook including a discussion of the major features of each strategy used in the syntheses are presented. This review illustrates the diversity and creativity in the elegant synthetic designs of eight-membered carbocycles. We hope this review will provide timely illumination and beneficial guidance for future synthetic efforts for organic chemists who are interested in this area.

Titanium Alkoxide-Based Regioselective Alkyne-Alkyne Reductive Coupling Mediated by In Situ Generated Arylamidate

Titanium alkoxide-based alkyne-alkyne reductive coupling mediated by in situ generated arylamidate is described. A high level of regioselectivity is achieved in 37 examples, where (E,E)-dienes are exclusively formed. To the best of our knowledge, this study represents the first example of an apparent amide and carbamate directing effect in metal-mediated reductive coupling.

An Efficient Synthesis of Benzazocines by Gold(I)-Catalyzed Tandem 1,2-Acyloxy Shift/[3+2] Cycloaddition of Terminal 1,9-Enynyl Esters

An effective synthesis of structurally diverse benzazocines was accomplished in good to excellent chemical yields (55-82 %) through a gold(I)-catalyzed cascade reaction involving tandem 1,2-acyloxy shift/[3+2] cycloaddition of terminal 1,9-enynyl esters. The reaction proceeds under extremely mild conditions and represents one of the relatively few transition-metal-catalyzed intramolecular cycloaddition reactions for the synthesis of benzazocines.

Nickel Catalysis: Synergy between Method Development and Total Synthesis

Nickel(0) catalysts have proven to be powerful tools for multicomponent coupling reactions in our laboratories over the past 15 years. This interest was originally sparked by the ubiquity of allylic alcohol motifs in natural products, such as (-)-terpestacin, which we envisioned assembling by the coupling of two π components (alkyne and aldehyde) with concomitant reduction. Mechanistic investigations allowed us to elucidate several modes of controlling the regioselectivity and stereoselectivity in the oxidative cyclization, and these insights enabled us to leverage combinations of alkenes and phosphine ligands to direct regioselective outcomes. The initial success in developing the first intermolecular reductive alkyne-aldehyde coupling reaction launched a series of methodological investigations that rapidly expanded to include coupling reactions of alkynes with other electrophilic π components, such as imines and ketones, as well as electrophilic σ components, such as epoxides. Aziridines proved to be more challenging substrates for reductive coupling, but we were recently able to demonstrate that cross-coupling of aziridines and alkylzinc reagents is smoothly catalyzed by a zero-valent nickel/phenanthroline system. Moreover, the enantioselective alkyne-aldehyde coupling and the development of novel P-chiral ferrocenyl ligands enabled the total synthesis of (-)-terpestacin, amphidinolides T1 and T4, (-)-gloeosporone, and pumiliotoxins 209F and 251D. We subsequently determined that alkenes could be used in place of alkynes in several nickel-catalyzed reactions when a silyl triflate activating agent was added. We reason that such an additive functions largely to enhance the electrophilicity of the metal center by coordination to the electrophilic π component, such that less nucleophilic alkene π donors can undergo productive combination with nickel complexes. This activation manifold was further demonstrated to be effective for alkene-aldehyde couplings. In a related manner, electrophilic promoters were also successfully employed for allylic substitution reactions of allylic carbonates with simple alkenes and in the Mizoroki-Heck reaction of both benzyl and aryl electrophiles. In these instances, it is proposed that counterion exchange from a more strongly coordinating anion to the weakly or noncoordinating triflate counterion enables reaction at an electrophilic Ni(II) center rather than by coordination to one of the coupling components. Mechanistic insights also played an important role in the development of mixed N-heterocyclic carbene/phosphite ligand systems to overcome challenges in regioselective alkene-aldehyde coupling reactions. We hope that, taken together, the body of work summarized in this Account demonstrates the constructive interplay among total synthesis, methodological development, and mechanistic investigation that has driven our research program.

Exo-selective reductive macrocyclization of ynals

A general protocol for the highly exo-selective macrocyclization of ynals using a nickel/N-heterocyclic carbene catalyst system has been developed. A series of 10- to 21-membered macrocycles bearing an exomethylene substituent was synthesized in good yields with excellent regioselectivity (exo/endo >95:5). Very high levels of long-range diastereocontrol can also be achieved for some classes of macrocycles. Complementary to previously reported endo-selective macrocyclizations, this method provides accesses to exoalkylidene macrocycles from simple ynals in high selectivity.

Gliomasolides A–E, unusual macrolides from a sponge-derived fungus Gliomastix sp. ZSDS1-F7-2

Five unusual macrolides were identified with their structures being established by extensive spectroscopic and single-crystal X-ray diffraction studies.

Gold-catalyzed 1,2-acyloxy migration/intramolecular [3+2] 1,3-dipolar cycloaddtion cascade reaction: an efficient strategy for syntheses of medium-sized-ring ethers and amines

A highly efficient strategy for the formation of medium-sized-ring ethers and amines based on a gold-catalyzed cascade reaction, involving enynyl ester isomerization and intramolecular [3+2] cyclization, has been developed. Various multisubstituted medium-sized-ring unsaturated ethers and amines were obtained through this transformation. This method represents one of the relatively few transition metal catalyzed intramolecular cycloaddition reactions for medium-sized ring synthesis.

Minimal fluorous tagging strategy that enables the synthesis of the complete stereoisomer library of SCH725674 macrolactones

Four mixtures of four fluorous-tagged quasiisomers have been synthesized, demixed, and detagged to make all 16 stereoisomers of the macrocyclic lactone natural product Sch725674. A new bare-minimum tagging pattern needs only two tags--one fluorous and one nonfluorous--to encode four isomers. The structure of Sch725674 is assigned as (5R,6S,8R,14R,E)-5,6,8-trihydroxy-14-pentyloxacyclotetradec-3-en-2-one. Various comparisons of spectra of 32 lactones (16 with tags, 16 without) and 16 ester precursors (8 with tags, 8 without) provide insights into when and why related compounds have the same or different spectra.

Ni(II) salts and 2-propanol effect catalytic reductive coupling of epoxides and alkynes

A Ni-catalyzed reductive coupling of alkynes and epoxides using Ni(II) salts and simple alcohol reducing agents is described. Whereas previously reported conditions relied on Ni(cod)(2) and Et(3)B, this system has several advantages including the use of air-stable and inexpensive Ni(II) precatalysts (e.g., NiBr(2)·3H(2)O) as the source of Ni(0) and simple alcohols (e.g., 2-propanol) as the reducing agent. Deuterium-labeling experiments are consistent with oxidative addition of an epoxide C-O bond that occurs with inversion of configuration.

Protecting-group-free total synthesis of isoquinoline alkaloids by nickel-catalyzed annulation of o-halobenzaldimine with an alkyne as the key step

An efficient short total synthesis of benzo[c]phenanthridine alkaloids including oxyavicine, oxynitidine, and oxysanguinarine is described. Thus, N-methyl-o-bromobenzaldimines 1 b-d undergo regioselective cyclization with 4-(benzo[d][1,3]dioxol-5-yl)but-3-yn-1-ol (2 b) in the presence of [Ni(cod)(2)] (cod=1,5-cyclooctadiene). In situ oxidation of the resultant isoquinolinium salts gives isoquinolinone derivatives 5 b-d with benzo[d][1,3]dioxol-5-yl substitution at the C(3) atom and a (CH(2))(2)OH group at the C(4) atom. Later, oxidation of the alcohol group in 5 b-d to the aldehyde moiety followed by acid-catalyzed cyclization and dehydration completes the total syntheses to give oxyavicine, oxynitidine, and oxysanguinarine in 67, 65, and 60 % yields, respectively. The synthesis requires four steps from o-bromobenzaldehyde derivatives. Transformations of these alkaloids to the other alkaloids in this family are also discussed herein.

Origins of regioselectivity and alkene-directing effects in nickel-catalyzed reductive couplings of alkynes and aldehydes

The origins of reactivity and regioselectivity in nickel-catalyzed reductive coupling reactions of alkynes and aldehydes were investigated with density functional calculations. The regioselectivities of reactions of simple alkynes are controlled by steric effects, while conjugated enynes and diynes are predicted to have increased reactivity and very high regioselectivities, placing alkenyl or alkynyl groups distal to the forming C-C bond. The reactions of enynes and diynes involve 1,4-attack of the Ni-carbonyl complex on the conjugated enyne or diyne. The consequences of these conclusions on reaction design are discussed.