Enamide-olefin ring-closing metathesis

NHC-Au-Catalyzed Isomerization of Propargylic B(MIDA)s to Allenes and Double Isomerization of Alkynes to 1,3-Dienes

The synthesis of allenyl boronates is an important yet challenging topic in organic synthesis. Reported herein is an NHC-gold-catalyzed 1,3-H shift toward allenyl boronates synthesis from simple propargylic B(MIDA)s. Mechanistic studies suggest dual roles of the boryl moiety in the reaction: to activate the substrate for isomerization and at the same time, to prevent the allene product from further isomerization. These effects should be a result of α-anion stabilization and α-cation destabilization conferred by the B(MIDA) moiety, respectively. The NHC-Au catalyst, which is commercially available, is also found to be reactive in alkyne-to-1,3-diene isomerization reactions in an atom-economic and base-free manner.

Atom-Economical Syntheses of Dihydropyrroles Using Flavin-Iodine-Catalyzed Aerobic Multistep and Multicomponent Reactions

Herein, we report facile, atom-economical syntheses of multisubstituted 2,3-dihydropyrroles using flavin-iodine-catalyzed aerobic oxidative multistep transformations of chalcones with β-enamine ketones or 1,3-dicarbonyl compounds and amines. Exploiting coupled flavin-iodine catalysis, the multistep reaction, including C-C and C-N bond formation, is promoted only by the consumption of O₂ (1 atm), thus allowing aerobic oxidative synthesis that generates green H₂O as the only waste.

Manganese(III)-promoted highly stereoselective phosphorylation of acyclic tertiary enamides to synthesize E-selective β-phosphoryl enamides

A concise manganese(III)-promoted stereoselective β-phosphorylation of acyclic tertiary enamides and diverse H-phosphine oxides was achieved. This reaction proceeds with absolute E-selectivity in contrast to Z-selectivity obtained in other previous works and affords various E-selective β-phosphorylated tertiary enamides in high efficiency. To the best of our knowledge, this is the first case of E-selective β-phosphorylation of tertiary enamides through C-H functionalization. In addition, the method features broad substrate scope, good functional group compatibility and efficient scale-up.

Asymmetric Total Synthesis of Shagenes A and B

We report the first total synthesis of shagenes A and B, which are tricyclic terpenoids containing a cis-substituted cyclopropane, via ring-closing metathesis of an enamide and Ir-catalyzed double-bond isomerization of an alkylidenecyclopropane. Chemo- and diastereoselectivity in the distorted cis-substituted structures were controlled by the alkylidenecyclopropane reactivity and using the ketone functionality as a remote directing group for the Ir catalyst, respectively. The total synthesis suggested the absolute configuration of shagenes.

Ring-closing metathesis of N-alkenyl-cyanamides

An entry to new classes of nitrogen-containing heterocycles featuring both a cyanamide and an unsaturated 5-, 6- or 7-membered ring is reported by RCM of the corresponding N-alkenyl-cyanamides, new partners for olefin metathesis.

Fused N-Heterocycles with Contiguous Stereogenic Centers Accessed by an Asymmetric Catalytic Cascade Reaction of Tertiary Enamides

We report in this article a cascade reaction strategy for the synthesis of complex N-heterocyclic compounds with contiguous and tetrasubstituted stereogenic carbons. Under the sequential catalysis of a chiral binol-Ti complex and BF₃ , cyclopentanone-derived tertiary enamides undergo an enantioselective enamine addition to ketone carbonyls followed by diastereoselective trapping of the resulting acyliminiums by electron-rich aryl moieties to furnish four- and five-membered ring-fused N-heterocyclic products as the sole diastereomers in high yields with up to 99 % ee.

Palladium-catalyzed intermolecular [4 + 2] formal cycloaddition with (Z)-3-iodo allylic nucleophiles and allenamides

A highly chemo- and regioselective [4 + 2] formal cycloaddition of (Z)-3-iodo allylic nucleophiles and allenamides catalyzed by palladium is reported. The methodology proceeds under mild reaction conditions and is tolerant of alkyl and aryl functional groups. The SN2' substitution at the proximal C[double bond, length as m-dash]C bond performed against the Heck or SN2 pathway delivered a variety of 2-amino-dihydropyrans and 2-amino-tetrahydropiperidines in moderate to satisfactory yields. The [4 + 2] formal cycloaddition derivatives are convertible to interesting scaffolds 2,6,7,7a-tetrahydropyrano[2,3-b]pyrrole and 2,6,7,7a-tetrahydro-1H-pyrrolo[2,3-b]pyridine derivatives via ring-closing metathesis (RCM) with Grubbs catalyst II.

Direct synthesis of dialkylarylvinylsilane derivatives: metathesis of dialkylaryl-iso-propenylsilane and its application to tetracyclic silacycle dye synthesis

The metathesis of dialkylarylvinylsilane, which has not been accomplished to date, is achieved using dialkylaryl-iso-propenylsilane as a substrate.

Ruthenium-Catalyzed Intramolecular Double Hydroalkoxylation of Internal Alkynes

Intramolecular double hydroalkoxylation of internal alkynes could be achieved using a Grubbs-type ruthenium carbene complex or its modified species to deliver a series of bridged- and spiroacetal derivatives in moderate to good yields. This study represents a new example of nonmetathetic reactions catalyzed by Grubbs-type ruthenium carbene complexes.

Direct metal-catalyzed regioselective functionalization of enamides

Enamides are stable enamine surrogates and provide key intermediates for the synthesis of small but complex nitrogen-containing compounds. Metal-catalyzed regioselective functionalization of enamides provides a rapid method to synthesize useful nitrogen containing heterocycles. This review discloses the recent progress made in the development of the C-H functionalization of enamides involving efficient and atom-economical routes. Syntheses of different heterocycles are classified based on the site reactivity of enamides and key mechanistic insights are given for each transformation.

Monobenzofused 1,4-azaborines: synthesis, characterization, and discovery of a unique coordination mode

We report the first general synthesis of boron-substituted monobenzofused 1,4-azaborines using ring-closing metathesis of an enamine-containing diene as a key synthetic strategy. As part of our investigations, we discovered that the B-C3 moiety in a 1,4-azaborine can serve uniquely as a η(2)-L-type ligand. This functionality is exemplified by two κ(2)-N-η(2)-BC Pt complexes of a boron-pyridyl-substituted monobenzofused-1,4-azaborine. Single-crystal X-ray diffraction analysis of the Pt complexes shows a strong structural contribution from the iminium resonance form of the monobenzofused-1,4-azaborine ligand. We also demonstrate that a palladium(0) complex supported by a 1,4-azaborine-based phosphine ligand can catalyze hydroboration of 1-buten-3-yne with unique selectivity. In view of the importance of arene-metal π-interactions in catalytic applications, this work should open new opportunities for ligand design involving the 1,4-azaborine motif as an arene substitute.

Cu-mediated enamide formation in the total synthesis of complex peptide natural products

Cu-mediated C(sp(2))-N bond formation has received intense interest recently, and has been applied to the total synthesis of a wide variety of structurally complex natural products. This review covers the synthetic assembly of peptide natural products in which Cu-mediated enamide formation is the key transformation. The total syntheses of cyclopeptide alkaloids, pacidamycin D, and yaku'amide A exemplify the versatility of the Cu-catalyzed cross-coupling reaction in comparison to other synthetic methods.

Easy access to enamides: a mild nickel-catalysed alkene isomerization of allylamides

The first Ni-catalysed alkene isomerization of allylamides for the synthesis of enamides was demonstrated. Various substituted N-allylamides were found to be suitable substrates for this isomerization. Isotopic labelling experiments showed that it is an intramolecular hydrogen transfer process.

Stereoselective synthesis of enamides by Pd-catalyzed hydroamidation of electron deficient terminal alkynes

Hydroamidation of electron-deficient terminal alkynes by amides in presence of Pd-catalyst has been exploited for the stereoselective synthesis of Z-enamides. The possible intramolecular hydrogen bonding between the amido proton and carbonyl oxygen of ester group provides the extra stability to the Z-isomer of vinyl-palladium complex, which subsequently undergoes protodepalladation and leads to the Z-enamide selectively. This process is found to be mild and operationally simple with broad substrate scope.