Asymmetric Brønsted Acid-Catalyzed Nazarov Cyclization of Acyclic α-Alkoxy Dienones

Recent Advances in the Catalytic Synthesis of the Cyclopentene Core

Five-membered carbocycles are ubiquitously found in natural products, pharmaceuticals, and other classes of organic compounds. Within this category, cyclopentenes deserve special attention due to their prevalence as targets and as well as key intermediates for synthesizing more complex molecules. Herein, we offer an overview summarizing some significant recent advances in the catalytic assembly of this structural motif. A great variety of synthetic methodologies and strategies are covered, including transition metal-catalyzed or organocatalyzed processes. Both inter- and intramolecular transformations are documented. On this ground, our expertise in the application of C-H functionalization reactions oriented towards the formation of this ring and its subsequent selective functionalization is embedded.

Insights into the regioselectivity and diastereoselectivity of the Nazarov cyclization of 3-alkenyl-2-indolylmethanol with tryptophol

Density functional theory calculations of Nazarov cyclizations showed that 3-alkenyl-2-indolylmethanol and tryptophol formed binary and ternary complexes, respectively, in the presence and absence of H2O to produce C–N and C–C products.

Asymmetric Gold(I)‐Catalyzed Tandem Hydroarylation–Nazarov Cyclization: Enantioselective Access to Cyclopentenones

The asymmetric synthesis of cyclopentachromenones from gold‐catalyzed reaction of readily available skipped alkenynones is described. This cascade reaction involves an initial anti‐Michael hydroarylation of the ynone moiety to form a gold‐functionalized dialkenylketone intermediate, followed by a Nazarov cyclization that proceeds in an unprecedented enantioselective manner. Excellent enantiomeric ratios and chemical yields are obtained under mild reaction conditions.

Enyne diketones as substrate in asymmetric Nazarov cyclization for construction of chiral allene cyclopentenones

The Nazarov cyclization is one of the most powerful tools for the stereoselective synthesis of various cyclopentenone scaffolds. Therefore, developing the new classes substrate of Nazarov reaction is an important endeavor in synthetic chemistry. Herein, we report enyne diketones, enables diastereo- and enantioselective construction of chiral allene cyclopentenones in moderate to good yields with good enantioselectivities (up to 97% ee). Importantly, it is a typical example for asymmetric synthesis of cyclopentanones with allene moiety using Nazarov cyclisation. Mechanistic studies indicate that this metal-organo relay catalysis protocol involves a rhodium-catalyzed tandem oxonium ylide formation/[2,3]-sigmatropic rearrangement/reverse benzylic acid rearrangement, followed by organo-catalyzed asymmetric Nazarov cyclization/alkyne-to-allene isomerization to give the final chiral allene cyclopentenones.

N-Triflylphosphoramides: highly acidic catalysts for asymmetric transformations

N-Triflylphosphoramides (NTPA), have become increasingly popular catalysts in the development of enantioselective transformations as they are stronger Brønsted acids than the corresponding phosphoric acids (PA). Their highly acidic, asymmetric active site can activate difficult, unreactive substrates. In this review, we present an account of asymmetric transformations using this type of catalyst that have been reported in the past ten years and we classify these reactions using the enantio-determining step as the key criterion. This compendium of NTPA-catalysed reactions is organised into the following categories: (1) cycloadditions, (2) electrocyclisations, polyene and related cyclisations, (3) addition reactions to imines, (4) electrophilic aromatic substitutions, (5) addition reactions to carbocations, (6) aldol and related reactions, (7) addition reactions to double bonds, and (8) rearrangements and desymmetrisations. We highlight the use of NTPA in total synthesis and suggest mnemonics which account for their enantioselectivity.

N-Heterocyclic Carbene-Catalyzed Asymmetric Synthesis of Cyclopentenones

N-Heterocyclic carbene-catalyzed asymmetric construction of cyclopentenones using enals and α-diketones is achieved, furnishing a series of highly functionalized cyclopentenones in a highly diastereo- and enantioselective manner. The protocol tolerates substrates with both aromatic and aliphatic groups, and further transformations of the products delivered a range of value-added molecules.

Asymmetric Nazarov Cyclizations of Unactivated Dienones by Hydrogen-Bond-Donor/Lewis Acid Co-Catalyzed, Enantioselective Proton-Transfer

We report an enantioselective Nazarov cyclization catalyzed by chiral hydrogen-bond-donors in concert with silyl Lewis acids. The developed transformation provides access to tri-substituted cyclopentenones in high levels of enantioselectivity (up to 95% e.e.) from a variety of simple unactivated dienones. Kinetic and mechanistic studies are consistent with a reversible 4π-electrocyclization C-C bond-forming step followed by rate- and enantio-determining proton-transfer as the mode of catalysis.

Carbocyclic Ring Closure of Aryl C-Glycosides Promoted by Fluoroboric Acid

A novel transformation from rhamnose-type C-glycosides to 2-cyclopentenones is described. With the promotion of fluoroboric acid, C-glycosides underwent ring opening and subsequent Nazarov cyclization to afford 2-cyclopentenones in good to excellent yields. The solvent and the concentration of acid are crucial to the yield of this transformation.

2,2'-Biphenol/B(OH)3 Catalyst System for Nazarov Cyclization

A novel catalyst system-a combination of the readily available 2,2'-biphenol with the inexpensive, nontoxic, and eco-friendly B(OH)₃-promoted the Nazarov cyclization of activated and inactivated divinyl ketones to afford the corresponding cyclopentenones up to 96% yield under, in a cis-selective manner. Compared with the conventional harsh conditions with hazardous reagents, user-friendly method was established with bench-stable and easy-to-handle reagents.

Strong and Confined Acids Enable a Catalytic Asymmetric Nazarov Cyclization of Simple Divinyl Ketones

We report a catalytic asymmetric Nazarov cyclization of simple, acylic, alkyl-substituted divinyl ketones using our recently disclosed strong and confined imidodiphosphorimidate Brønsted acids. The corresponding monocyclic cyclopentenones are formed in good yields and excellent regio-, diastereo-, and enantioselectivities. Further, the chemical utility of the obtained enantiopure cyclopentenones is demonstrated.

Nazarov reaction: current trends and recent advances in the synthesis of natural compounds and their analogs

Recent applications of the Nazarov reaction to stereoselective synthesis of pharmacology-relevant cyclopentenone scaffolds are summarized.

Efficient synthesis of bicyclo[6.3.0]undecadienones by Nazarov cyclization/regioselective elimination

Easily accessible cyclooctadienyl vinyl ketones can generate fused bicyclic 8-5 (bicyclo[6.3.0]undecadienone) ring systems via the Nazarov cyclization when treated with Lewis acid. The resultant products contain three new stereocenters as well as olefins suitably situated to allow further structural elaboration. This concise sequence allows for the rapid preparation of intermediates that should be applicable to a variety of natural product targets containing this bicyclic core.

Salen promoted enantioselective Nazarov cyclizations of activated and unactivated dienones

A novel class of chiral 5,5'-di(2,4,6-trialkyl)aryl salen-metal complexes have been developed and shown to catalyze highly enantioselective Nazarov cyclization reactions, giving rise to cyclopentenoids in 90:10-98:2 er. Significantly, the catalysts also promote, for the first time, highly enantioselective Nazarov reactions of "unactivated" dienones, producing hydrindenone products having in place three contiguous chiral centers.

α-Aryl-substituted allenamides in an imino-Nazarov cyclization cascade catalyzed by Au(I)

An imino-Nazarov cyclization using α-aryl-substituted allenamides is described. This gold(I)-catalyzed cascade is efficient and regioselective in constructing a diverse array of synthetically useful aromatic-ring fused cyclopentenamides. The success in this transformation represents a solution to the challenge in establishing an imino-Nazarov cyclization process.