Synthesis of Natural Products by C-H Functionalization of Heterocycless

Three-component cascade reaction of 3-ketonitriles, 2-unsubstituted imidazole N-oxides, and aldehydes

A three-component condensation of 2-unsubstituted imidazole N-oxides, 3-ketonitriles, and aldehydes is described. The reaction proceeds via sequential Knoevenagel condensation/Michael addition under mild, catalyst-free conditions with various substrates. Furthermore, the corresponding 2-functionalized imidazole N-oxides can be further dehydrated to (Z)-2-aroyl-3-(1H-imidazol-2-yl)-acrylonitriles, which may also be directly prepared by changing the reaction conditions as a cascade of Knoevenagel condensation/Michael addition/dehydration.

Rhodium-catalysed additive-free carbonylation of benzamides with diethyl dicarbonate as a carbonyl source

Phthalimides are prevalent in numerous pharmaceuticals, prompting various phthalimide syntheses through C-H activation. Nevertheless, the necessity for stoichiometric additives limits their practicality and versatility. Herein, we introduced diethyl dicarbonate as a carbonyl source for an additive-free carbonylation of benzamides. This transformation signifies an operationally simple and CO-free phthalimide synthesis.

β-Alkenylation of Saturated N-Heterocycles via a C(sp3)-O Bond Wittig-like Olefination

Although the C-Hα functionalization of N-heterocycles is, in fact, an easy chemical transformation, the C-Hβ functionalization is, on the contrary, a quite difficult chemical process. Here, we present a two-step protocol that allows the ready conversion of pyrrolidines, piperidines, and an azepane into their corresponding 3-exo-alkenyl lactams via the transient formation of 3-alkoxyamino lactams followed by a Wittig-like C(sp3)-O bond olefination with stabilized ylides from phosphonium salts mediated by t-BuOK. Additionally, as a proof of the synthetic effectiveness of this novel methodology, the first synthesis of the natural product callylactam A was achieved through a TiCl4-catalyzed double bond isomerization of a 3-exo-alkenyl 2-piperidone to its endo-isomer.

Direct Heterocycle C-H Alkenylation via Dual Catalysis Using a Palladacycle Precatalyst: Multifactor Optimization and Scope Exploration Enabled by High-Throughput Experimentation

One of the major challenges in developing catalytic methods for C-C bond formation is the identification of generally applicable reaction conditions, particularly if multiple substrate structural classes are involved. Pd-catalyzed direct arylation reactions are powerful transformations that enable direct functionalization of C-H bonds; however, the corresponding direct alkenylation reactions, using vinyl (pseudo) halide electrophiles, are less well developed. Inspired by process development efforts toward GSK3368715, an investigational active pharmaceutical ingredient, we report that a Pd(II) palladacycle derived from tri-tert-butylphosphine and Pd(OAc)2 is an effective single-component precatalyst for a variety of direct alkenylation reactions. High-throughput experimentation identified optimal solvent/base combinations for a variety of HetAr-H substrate classes undergoing C-H activation without the need for cocatalysts or stoichiometric silver bases (e.g., Ag2CO3). We propose this reaction proceeds via a dual cooperative catalytic mechanism, where in situ-generated Pd(0) supports a canonical Pd(0)/(II) cross-coupling cycle and the palladacycle effects C-H activation via CMD in a redox-neutral cycle. In all, 192 substrate combinations were tested with a hit rate of approximately 40% and 24 isolated examples. Importantly, this method was applied to prepare a key intermediate in the synthesis of GSK3368715 on multigram scale.

Synthesis of Bridged Cycloisoxazoline Scaffolds via Rhodium-Catalyzed Coupling of Nitrones with Cyclic Carbonate

Bridged isoxazolidines were synthesized via Rh(III)-catalyzed C-H allylation of α-aryl nitrones with 5-methylene-1,3-dioxan-2-one. The nitrone group serves as a directing group and 1,3-dipole in the C-H activation/[3 + 2] cycloaddition cascade, exhibiting excellent chemo- and stereoselectivity along with good functional group compatibility. The resulting skeletal structure was conveniently modified to produce a range of important chemical frameworks, and the protocol was applied to biologically active molecules.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Chiral Transient Ligand Enabled Enantioselective Synthesis of Atropisomers Decorated with Unactivated Olefins via a Palladium-Catalyzed C-H Olefination

Herein, atroposelective synthesis of axially chiral biaryls with unactivated olefins by a palladium-catalyzed C-H olefination using a chiral transient directing group strategy has been disclosed. This protocol is well compatible with a variety of biaryl-2-aldehydes as well as various olefins such as allyl sulfonamides and allyl sulfones to provide the atroposelective olefinated products in synthetically useful yields with excellent enantioselectivities up to >99% ee. In addition, a wide number of axially chiral biaryl alcohols were synthesized by the simple diversification of the products in excellent enantioselectivity.

Rh(iii)-catalyzed highly site- and regio-selective alkenyl C-H activation/annulation of 4-amino-2-quinolones with alkynes via reversible alkyne insertion

3,4-Fused 2-quinolone frameworks are important structural motifs found in natural products and biologically active compounds. Intermolecular alkenyl C-H activation/annulation of 4-amino-2-quinolone substrates with alkynes is one of the most efficient methods for accessing such structural motifs. However, this is a formidable challenge because 4-amino-2-quinolones have two cleavable C-H bonds: an alkenyl C-H bond at the C3-position and an aromatic C-H bond at the C5-position. Herein, we report the Rh(iii)-catalyzed highly site-selective alkenyl C-H functionalization of 4-amino-2-quinolones to afford 3,4-fused 2-quinolones. This method has a wide substrate scope, including unsymmetrical internal alkynes, with complete regioselectivity. Several control experiments using an isolated key intermediate analog suggested that the annulation reaction proceeds via reversible alkyne insertion involving a binuclear Rh complex although alkyne insertion is generally recognized as an irreversible process due to the high activation barrier of the reverse process.

Ru(II)-Catalyzed Weakly Coordinating Carbonyl-Assisted Dialkynylation of (Hetero)Aryl Ketones

Functionalized aryl(heteroaryl) ketones are present in many natural products as key structural components and serve as basic synthetic building blocks for various organic transformation reactions. Therefore, the development of an effective and sustainable route for making these classes of compounds remains challenging yet highly desirable. Herein, we report a simple and efficient catalytic system for dialkynylation of aromatic/heteroaromatic ketones via a double C-H bond activation in the presence of less expensive ruthenium(II)-salt as a catalyst using the weakly and native carbonyl group as the desired directing group. The developed protocol is highly compatible, tolerant, and sustainable toward various functional groups. The synthetic utility of the developed protocol has been demonstrated through the scale-up synthesis and functional group transformation. Control experiments support the involvement of the base-assisted internal electrophilic substitution (BIES) reaction pathway.

Metal-Free Synthesis of Functionalized Quinolines from 2-Styrylanilines and 2-Methylbenzothiazoles/2-Methylquinolines

A facile functionalization of C(sp³)-H bonds and tandem cyclization strategy to synthesize quinoline derivatives from 2-methylbenzothiazoles or 2-methylquinolines and 2-styrylanilines has been developed. This work avoids the requirement for transition metals, offering a mild approach to activation of C(sp³)-H bonds and formation of new C-C and C-N bonds. This strategy features excellent functional group tolerance and scaled-up synthetic capability, thus providing an efficient and environmentally friendly access to medicinally valuable quinolines.

Diversification of (E,E)-1,6-Dioxo-2,4-Dienes for the Synthesis of (+)-Aspicillin, Isolaurepan, and β-Parinaric Acid

A divergent formal synthesis of polyhydroxylated macrocyclic lactone (+)-aspicillin and polyene bioactive natural product β-parinaric acid and the total synthesis of non-terpenoid metabolite isolaurepan have been achieved using a ruthenium-catalyzed stereo- and chemoselective oxidative coupling reaction of easily accessible vinyl ketones and acrylates. The crucial transformation involves the efficient synthesis and functionalization of stereodefined (E,E)-1,6-dioxo-2,4-dienes using simple reaction protocols, which enabled straightforward access to a diverse range of bioactive natural products.