Halide trapping of the Nazarov intermediate in strained polycyclic systems: a new interrupted Nazarov reaction

Asymmetric Total Synthesis of Illisimonin A

The discovery of illisimonin A in 2017 extended the structural repertoire of the Illicium sesquiterpenoids─a class of natural products known for their high oxidation levels and neurotrophic properties─with a new carbon backbone combining the strained trans-pentalene and norbornane substructures. We report an asymmetric total synthesis of (-)-illisimonin A that traces its tricyclic carbon framework back to a spirocyclic precursor, generated by a tandem-Nazarov/ene cyclization. As crucial link between the spirocyclic key intermediate and illisimonin A, a novel approach for the synthesis of tricyclo[5.2.1.0^1,5]decanes via radical cyclization was explored. This approach was applied in a two-stage strategy consisting of Ti(III)-mediated cyclization and semipinacol rearrangement to access the natural product's carbon backbone. These key steps were combined with carefully orchestrated C-H oxidations to establish the dense oxidation pattern.

New Twists in Nazarov Cyclization Chemistry

The defining feature of the Nazarov cyclization is a 4π-conrotatory electrocyclization, resulting in the stereospecific formation of functionalized cyclopentanones. The reaction provides access to structural motifs that are found in many natural products and drug targets. Harnessing the full potential of the Nazarov cyclization broadens its utility by enabling the development of new methodologies and synthetic strategies. To achieve these goals through efficient cyclization design, it is helpful to think of the reaction as a two-stage process. The first stage involves a 4π-electrocyclization leading to the formation of an allylic cation, and the second stage corresponds to the fate of this cationic intermediate. With a complete understanding of the discrete events that characterize the overall process, one can optimize reactivity and control the selectivity of the different Stage 2 pathways.In this Account, we describe the development of methods that render the Nazarov cyclization catalytic and chemoselective, focusing specifically on advances made in our lab between 2002 and 2015. The initial discovery made in our lab involved reactions of electronically asymmetric ("polarized") substrates, which cyclize efficiently in the catalytic regime using mild Lewis acidic reagents. These cyclizations also exhibit selective eliminative behavior, increasing their synthetic utility. Research directed toward catalytic asymmetric Nazarov cyclization led to the serendipitous discovery of a 4π-cyclization coupled to a well-behaved Wagner-Meerwein rearrangement, representing an underexplored Stage 2 process. With careful choice of promoter and loading, it is possible to access either the rearrangement or the elimination pathway. Additional experimental and computational studies provided an effective model for anticipating the migratory behavior of substiutents in the rearrangements. Problem-solving efforts prompted investigation of alternative methods for generating pentadienyl cation intermediates, including oxidation of allenol ethers and addition of nucleophiles to dienyl diketones. These Nazarov cyclization variants afford cyclopentenone products with vicinal stereogenic centers and a different arrangement of substituents around the ring. A nucleophilic addition/cyclization/elimination sequence can be executed enantioselectively using catalytic amounts of a nonracemic chiral tertiary amine.In summary, the discovery and development of several new variations on the Nazarov electrocyclization are described, along with synthetic applications. This work illustrates how strongly substitution patterns can impact the efficiency of the 4π-electrocyclization (Stage 1), allowing for mild Lewis acid catalysis. Over the course of these studies, we have also identified new ways to access the critical pentadienyl cation intermediates and demonstrated strategies that exploit and control the different cationic pathways available post-electrocyclization (Stage 2 processes). These advances in Nazarov chemistry were subsequently employed in the synthesis of natural product targets such as (±)-merrilactone A, (±)-rocaglamide, and (±)-enokipodin B.

Novel d-Annulated Pentacyclic Steroids: Regioselective Synthesis and Biological Evaluation in Breast Cancer Cells

The acid-catalyzed cyclization of benzylidenes based on 16-dehydropregnenolone acetate (16-DPA) was studied. It was found that these compounds readily undergo regioselective interrupted Nazarov cyclization with trapping chloride ion and an efficient method of the synthesis of d-annulated pentacyclic steroids based on this reaction was proposed. The structures of the synthesized pentacyclic steroids were determined by NMR and X-ray diffraction. It was found that the reaction affords a single diastereomer, but the latter can crystallize as two conformers depending on the structure. Antiproliferative activity of synthesized compounds was evaluated against two breast cancer cell lines: MCF-7 and MDA-MB-231. All tested compounds showed relatively high antiproliferative activity. The synthetic potential of the protocol developed was illustrated by the gram-scale experiment.

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.

Interrupting the Nazarov Cyclization with Bromine

The generation of dibrominated cyclopentenones via an interrupted Nazarov cyclization is reported. The installation of two bromine atoms occurs at the α and α' positions of the cyclopentenyl scaffold via successive nucleophilic and electrophilic bromination of the 2-oxidocyclopentenyl cation and its resulting enolate. Notably, the reaction proceeds with good diastereoselectivity, favoring the symmetrical product.

Assembly of functionalized carbocycles or N-heterocycles through a domino electrocyclization-[1,2] migration reaction sequence

The development of processes that streamline the synthesis of complex, functionalized carbocycles and heterocycles remains a hotly pursued topic because their scaffolds are present in a range of bioactive molecules and electronic materials. Although the Nazarov reaction has emerged to be useful in the synthesis of carbocycles and heterocycles, using an electrocyclization to trigger a migration remains underdeveloped. By constructing several bonds in one operation, domino reaction sequences are particularly effective at improving the efficiency of synthesis. The use of transition metal catalysts has the potential to render these processes stereoselective. This review examines the use of electrocyclization-[1,2] migrations to construct molecules and is organized by the type of ring constructed and the order of the two steps in this process.

Cooperative benzylic-oxyallylic stabilized cations: regioselective construction of α-quaternary centers in ketone-derived compounds

We describe a novel reactivity of benzylic-stabilized oxyallyl cations towards regioselective construction of carbon quaternary centers. These synthetically useful intermediates were readily generated upon ionization of aryl substituted α-hydroxy methylenol ethers with catalytic, mild Brønsted acid. The emerging unsymmetrical oxyallyl cations were then directly captured by indoles and other nucleophiles with exquisite control of regioselectivity, predictably at the electrophilic carbon bearing the alkyl substituent to produce highly functionalized, value-added enol ethers.

Efficient catalytic-free method to produce α-aryl cycloalkanones through highly chemoselective coupling of aryl compounds with oxyallyl cations

Efficient catalytic-free method to produce α-aryl cycloalkanones through highly chemoselective coupling of aryl compounds with oxyallyl cations at room temperature.

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.

Formal synthesis of (±)-methyl rocaglate using an unprecedented acetyl bromide mediated Nazarov reaction

To date the prototype Nazarov cyclization of a cross-conjugated pentadienone to the core structure of the rocaglate natural products has not been successful (9 into 12). It has been found that this conversion can be achieved by the use of acetylbromide in excellent yield and results in a strategically very direct route to these antitumor agents.

Understanding the fate of the oxyallyl cation following Nazarov electrocyclization: sequential Wagner-Meerwein migrations and the synthesis of spirocyclic cyclopentenones

A general reaction sequence is described that involves Nazarov cyclization followed by two sequential Wagner-Meerwein migrations, to afford spirocyclic compounds from divinyl ketones in the presence of 1 equiv of copper(II) complexes. A detailed investigation of this sequence is described including a study of substrate scope and limitations. It was found that after 4π electrocyclization, two different pathways are available to the oxyallyl cation intermediate: elimination of a proton can give the usual Nazarov cycloadduct, or ring contraction can give an alternative tertiary carbocation. After ring contraction, either [1,2]-hydride or carbon migration can occur, depending upon the substitution pattern of the substrate, to furnish spirocyclic products. The rearrangement pathway is favored over the elimination pathway when catalyst loading is high and the copper(II) counterion is noncoordinating. Several ligands were found to be effective for the reaction. Thus, the reaction sequence can be controlled by judicious choice of reaction conditions to allow selective generation of richly functionalized spirocycles. The three steps of the sequence are stereospecific: electrocyclization followed by two [1,2]-suprafacial Wagner-Meerwein shifts, the ring contraction and then a hydride, alkenyl, or aryl shift. The method allows stereospecific installation of adjacent stereocenters or adjacent quaternary centers arrayed around a cyclopentenone ring.

Synthesis of 5-hydroxycyclopent-2-enones from allenyl vinyl ketones via an interrupted Nazarov cyclization

Treatment of an allenyl vinyl ketone with trifluoroacetic acid leads to Nazarov cyclization, and the intermediate carbocation is trapped efficiently by trifluoroacetate. Hydrolysis of the ester with methanol and basic alumina provides, in good to excellent overall yield, a 5-hydroxycyclopent-2-enone in which the alcohol is predominantly trans to a substituent at C-4.

Interrupting the Nazarov cyclization with indoles

A carbon-terminated interrupted Nazarov reaction is described. The trimethylsilyl enol ethers derived from propargyl vinyl ketones undergo selective proton transfer at the distal acetylenic carbon atom to provide allenyl vinyl ketones as transient intermediates. In the presence of indoles, a cascade of reactions is initiated that converts the initially formed pentadienyl cations to cyclopentenones bearing a quaternary alpha carbon atom.

Construction of aryl-substituted triquinanes through the interrupted Nazarov reaction

The first examples of intermolecular trapping of Nazarov cyclopentenyl cation intermediates by simple arenes to furnish alpha-arylcyclopentanones are described.

Polarizing the Nazarov cyclization: the impact of dienone substitution pattern on reactivity and selectivity

The impact of dienone substitution on the Nazarov cyclization has been examined in detail. Substrates bearing different substituents at each of four positions on the dienone backbone were systematically probed in order to identify trends leading to higher reactivity and better selectivity. Desymmetrization of the pentadienyl cation and oxyallyl cation intermediates through placement of polarizing groups at both the C-2 and C-4 positions was found to be particularly effective. These modifications allowed cyclizations to occur in the presence of catalytic amounts of mild Lewis acids. It was also found that stereoconvergent cyclization of mixtures of E and Z isomers of alkylidene beta-ketoesters occurred via an efficient isomerization process that occurred under the reaction conditions.

Highly Stereoselective 6π Electrocyclization of Bridged Bicyclic 1,3,5-Trienes

Conjugated 1,3,5-hexatrienes encased in bridged bicyclic skeletons are prepared by cross-coupling followed by half-reduction of the resulting dienynes. The trienes undergo 6pi electrocyclization at an ambient or elevated temperature to furnish complex, polycyclic cyclohexadienes. In all cases, complete selectivity in favor of cyclization from the exo face of the bridged bicyclic system was seen, in contrast to the corresponding 4pi Nazarov cyclizations.