Combined rhodium-catalyzed carbon-hydrogen activation and beta-carbon elimination to access eight-membered rings

Diastereo- and Enantioselective Synthesis of Tetracyclic Cycloheptanols through (4+3) Annulation via C-C/C-H Activation Cascade

Transition metal-catalyzed annulations of four-membered rings via C-C activation are powerful tools to construct complex fused and bridged ring systems. Despite significant progress in (4+1), (4+2) and (4+4) annulations, the (4+3) annulation remains unexplored. Herein, we develop an asymmetric Rh-catalyzed intramolecular (4+3) annulation of α-arylalkene-tethered benzocyclobutenols for the synthesis of dihydrofuran-annulated dibenzocycloheptanols with two discontinuous chiral carbon centers via a C-C and C-H activation cascade. The reaction features excellent diastereo- and enantioselectivities and 100 % atom economy, and is applicable to late-stage modification of complex molecules.

Current developments in the synthesis of 4-chromanone-derived compounds

The chroman-4-one framework is a significant structural entity that belongs to the class of oxygen-containing heterocycles. It acts as a major building block in a large class of medicinal compounds, and synthetic compounds exhibit a broad variety of remarkable biological and pharmaceutical activities. Several studies have been performed to improve the methodologies of 4-chromanone-derived compounds. This review focuses on the major synthetic methods of preparation reported on chroman-4-one derivatives from 2016 to 2021.

Construction 7-membered ring via Ni-Al bimetal-enabled C-H cyclization for synthesis of tricyclic imidazoles

The construction of 7-membered ring via direct C7-H cyclization of benzoimidazoles with alkenes would provide a more atom- and step-economical route to tricyclic imidazoles and derivatives that widely exist in a broad range of bioactive molecules. However, transition metal-catalyzed C-H cyclization for medium-ring synthesis has been limited to reactive C-H bonds, instead, the activation of unreactive C-H bonds towards medium synthesis still remains an elusive challenge. Herein, we report a direct construction of 7-membered rings via Ni-Al co-catalyzed unreactive C7-H cyclization of benzoimidazoles with alkenes, providing a series of tricyclic imidazoles in 40-98% yield and with up to 95:5 er.

A Synthetic Strategy toward Eight-Membered Cyclic Amines by Cycloetherification and Claisen Rearrangement

Eight-membered nitrogen-containing heterocycles were straightforwardly produced by a nickel-catalyzed cycloetherification and subsequent Claisen rearrangement of secondary and tertiary alcohols. In particular, a one-pot transformation was achieved with tertiary alcohols in moderate to good yields. This operationally simple reaction is tolerant of many functional groups and applicable to the synthesis of various medium-sized ring nitrogen-containing heterocycles.

Cleavage of Carbon-Carbon σ-Bonds of Four-Membered Rings

This article reviews synthetic transformations involving cleavage of a carbon-carbon bond of a four-membered ring, with a particular focus on the examples reported during the period from 2011 to the end of 2019. Most significant is the progress of catalytic reactions involving oxidative addition of carbon-carbon bonds onto transition metals or β-carbon elimination of transition metal alkoxides. When they are looked at from synthetic perspectives, they offer unique and efficient methods to build complex natural products and structures that are difficult to construct by conventional methods. On the other hand, β-scission of radical intermediates has also attracted increasing attention as an alternative elementary step to cleave carbon-carbon bonds. Its site-selectivity is often complementary to that of transition metal-catalyzed reactions. In addition, Lewis acid-mediated and thermally induced ring-opening of cyclobutanone derivatives has garnered renewed attention. On the whole, these examples demonstrate unique synthetic potentials of structurally strained four-membered ring compounds for the construction of organic skeletons.

Synthesis of indoline-fused eight-membered azaheterocycles through Zn-catalyzed dearomatization of indoles and subsequent base-promoted C-C activation

A cascade reaction involving the Zn-catalyzed dearomatization of indoles, base-promoted ring-expansion and intramolecular S_NAr reaction has been developed. This process realized a novel, atom economical and efficient synthesis of indoline-fused eight-membered azaheterocycles in a one pot manner.

Synthesis of Aza-Eight-Membered Ring-Fused Indolines Initiated by Zn-Catalyzed C2 Alkylation of Indoles and Subsequent Base-Promoted Ring Expansion

A novel and efficient synthesis of aza-eight-membered ring-fused indolines has been developed. This process is realized by zinc-catalyzed C2 alkylation of indoles and subsequent base-promoted ring expansion of the newly formed six-membered ring with alkynes. Easily accessible starting materials, good functional group tolerance, and high atom economy make this procedure attractive.

Rhodium catalyzed C-C bond cleavage/coupling of 2-(azetidin-3-ylidene)acetates and analogs

The C-C bond cleavage/coupling of 2-(azetidin-3-ylidene)acetates with aryl boronic acids catalyzed by a rhodium complex was studied with a "conjugate addition/β-C cleavage/protonation" strategy.

Convergent Stereoselective Synthesis of the C16-C37 Subunit of Sorangicin A

A convergent stereoselective route to the C16-C37 fragment of sorangicin A is disclosed using an α-chloro sulfide for C-C bond formation. The key intermediate, an α,β-unsaturated ketone, is revealed by a [2,3] sigmatropic rearrangement of a propargylic sulfoxide. Three disparate approaches are detailed to create the C25 carbinol stereocenter. The cis-2,6-disubstitution of the THP ring is secured by ionic hydrogenation. A cross-metathesis reaction and Julia-Kocienski olefination furnish the C16-C37 fragment of sorangicin A.

Synthesis of 2-(2-Oxo-2-phenylethyl)cyclopentanone by Rhodium-Catalyzed Tandem Alkynyl Cyclobutanols Hydroacylation and Semipinacol Rearrangement

A rhodium-catalyzed tandem reaction of alkynyl cyclobutanols with salicylaldehydes has been developed. The reaction offers a new and atom-economical approach for the selective preparation of multisubstituted 2-(2-oxo-2-phenylethyl)cyclopentanone in high yields under mild reaction conditions with tolerance of a broad range of substituted alkynyl cyclobutanols and salicylaldehyes. The isolation of intermediate suggests that the reaction proceeds through a sequential process of intermolecular hydroacylation and semipinacol rearrangement.

Synthesis of dibenzo[a,d]cycloheptanoids via aryne insertion into 2-arylidene-1,3-indandiones

A cascade synthetic strategy for the direct synthesis of dibenzo[a,d]cycloheptanoids and dibenz[a,c]anthracene-9,14-dione derivatives from aryne precursors has been developed.

Nickel-catalyzed exo-selective hydroacylation/Suzuki cross-coupling reaction

The first nickel-catalyzed intramolecular hydroacylation/Suzuki cross coupling cascade of o-allylbenzaldehydes with a broad range of phenylboronic acid neopentyl glycol esters has been developed.

Iron- or Palladium-Catalyzed Reaction Cascades Merging Cycloisomerization and Cross-Coupling Chemistry

A conceptually novel reaction cascade is presented, which allows readily available enynes to be converted into functionalized 1,3-dienes comprising a stereodefined tetrasubstituted alkene unit; such compounds are difficult to make by conventional means. The overall transformation is thought to commence with formation of a metallacyclic intermediate that evolves via cleavage of an unstrained C-X bond in its backbone. This non-canonical cycloisomerization process is followed by a cross-coupling step, such that reductive C-C bond formation regenerates the necessary low-valent metal fragment and hence closes an intricate catalytic cycle. The cascade entails the formation of two new C-C bonds at the expense of the constitutional C-X entity of the substrate: importantly, the extruded group X must not be a heteroelement (X=O, NR), since activated β-C-C bonds can also be broken. This concern was reduced to practice in two largely complementary formats: one procedure relies on the use of alkyl-Grignard reagents in combination with catalytic amounts of Fe(acac)_3, whereas the second method amalgamates cycloisomerization with Suzuki coupling by recourse to arylboronic acids and phosphine-ligated palladium catalysts.

Oxidative Cascade Reaction of N-Aryl-3-alkylideneazetidines and Carboxylic Acids: Access to Fused Pyridines

A versatile silver-promoted oxidative cascade reaction of N-aryl-3-alkylideneazetidines with carboxylic acids is reported, providing a very efficient pathway to functionalized fused pyridines. This method allows introduction of fused pyridine ring systems to heterocycles, drugs, and natural products. A mechanistic study revealed that silver salt is essential for the chemo- and regioselective ring expansion, sequential oxidative nucleophilic additions, and oxidative aromatization. This approach represents the first example of the strained N-heterocycles undergoing a cascade reaction with a π bond and a nucleophile together.

Stereoselective Access to Alkylidenecyclobutanes through γ-Selective Cross-Coupling Strategies

Alkylidenecyclobutanes (ACBs) containing all-carbon quaternary stereocenters were simply and efficiently synthesized by combining boron-homologation and γ-selective cross-coupling strategies. This unique sequence led to excellent regio- and diastereoselectivities in the generation of targeted four-membered rings with up to 99% enantiomeric excess using chiral substrates. In addition to the original synthesis of ACBs, the first asymmetric catalytic formation of quaternary stereocenters based on γ-selective cross-coupling reactions is finally shown.

Stereoselective Sequence toward Biologically Active Fused Alkylidenecyclobutanes

Combining an efficient preparation of cyclobutenylmetal species, high-yielding cross-coupling reactions, and highly diastereoselective [4 + 2]-cycloaddition led to opening a new route toward the synthesis of fused alkylidenecyclobutanes containing up to five consecutive stereocenters. New complex architectures, analogues to protoilludane skeletons, were obtained in a very efficient manner and with a minimum number of steps starting from commercial sources and were tested for their cytotoxicity against leukemia cell lines HL60.

Recent Methodologies That Exploit C-C Single-Bond Cleavage of Strained Ring Systems by Transition Metal Complexes

In this review, synthetic and mechanistic aspects of key methodologies that exploit C-C single-bond cleavage of strained ring systems are highlighted. The focus is on transition-metal-catalyzed processes that are triggered by C-C bond activation and β-carbon elimination, with the review concentrating on developments from mid-2009 to mid-2016.

Unsaturated Four-Membered Rings: Efficient Strategies for the Construction of Cyclobutenes and Alkylidenecyclobutanes

Our recent studies of the diastereo- and enantioselective formation of strained alkylidenecycloalkanes drove us to more-thoroughly investigate the formation of four-membered rings for which only few efficient methods are described. We first developed a strategy to diversify the saturated part of the four-membered ring and applied it to a highly diastereoselective synthesis of more-elaborate alkylidenecyclobutanes, which completed our precedent studies. In parallel, cyclobutene structures were built employing simple organometallic methods and further functionalized to give a diverse range of new substitution patterns, which consequently enriched the pool of cyclobutene-based building blocks.

Construction of Azabicyclo[6.4.0]dodecatrienes Based on Rhodium(I)-Catalyzed Intramolecular [6+2] Cycloaddition between Azetidine, Allene, and Alkynes

Treatment of the allenylazetidine-alkynes with a catalytic amount of [RhCl(CO)dppp]2 (dppp: 1,3-bis(diphenylphosphino)propane) effected the intramolecular hetero-[6+2]-type ring-closing reaction via the C-C bond cleavage of the azetidine ring to produce azabicyclo[6.4.0]dodecatriene derivatives in good to excellent yields. The formation of the oxa analogue could also be achieved.

Single-Pot Asymmetric Approach toward Enantioenriched Quaternary Stereocenter-Containing Alkylidenecyclobutanes

Enantioenriched alkylidenecyclobutanes possessing a quaternary stereogenic center, usually difficult to access, have been synthesized by combining a double boron-homologation and an allylboration through a highly efficient and diastereoselective one-pot process. Starting from commercially available substrates, this protocol represents a simple way of accessing chiral unsaturated four-membered ring systems with excellent stereoisomeric ratios.

N-Heterocyclic carbene-catalysed intramolecular hydroacylation to form basic nitrogen-containing heterocycles

N-Heterocyclic carbene-catalysed intramolecular hydroacylation of N-allylimidazole-2-carboxaldehydes and N-allylbenzimidazole-2-carboxaldehydes is reported. These exo-selective hydroacylations of unactivated alkenes enable the synthesis of a variety of basic, polycyclic nitrogen heterocycles in good-to-excellent yields.

Highly Diastereoselective Synthesis of Methylenecyclobutanes by Merging Boron-Homologation and Boron-Allylation Strategies

A highly diastereoselective synthesis of methylenecyclobutanes possessing a quaternary stereocenter is reported, in which boron homologation of an easily-generated cyclobutenylmetal species is performed, followed by an allylation reaction. Combining three steps in a one-pot process further optimized the method, which afforded the expected adducts in excellent yields and stereoselectivity, starting from commercially available 4-bromobutyne.