Development of novel Lewis acid catalyzed cycloisomerizations: synthesis of bicyclo[3.1.0]hexenes and cyclopentenones

Acid-Promoted Iso-Nazarov Cyclization of Conjugated trans-Dienones and Dienals for the Synthesis of 2-Cyclopentenones

The acid-promoted cyclization of all-trans linearly conjugated dienones and dienals constitutes a synthetic strategy for the construction of 2-cyclopentenones.

The role of halogen bonds in the catalytic mechanism of the iso-Nazarov cyclization reaction: a DFT study

With the continuous development of halogen bonds, halogen bond donors have been used as clean and efficient catalysts in organic reactions. In this work, with inorganic halides (I₂, IBr, ICl, and ICl₃) as catalysts and the iso-Nazarov cyclization as the benchmark reaction, we aim at investigating the role of the halogen bond in the catalytic mechanism. The halogen bond catalyzed iso-Nazarov cyclization reaction involves three steps: carbon-carbon coupling process, [1,2]-H shift process, and [1,4]-H shift process. The halogen-bonding interaction promotes the charge accumulation of the oxygen atom in the carbonyl group and decreases the activation energy of the reaction. The catalytic activity of the halogen bond donor is enhanced in the order of I₂ < IBr < ICl < ICl₃, and it could be predicted that the partial covalent interaction of the I⋯O halogen bond between the catalyst ICl₃ and the oxygen atom of the reactant may exhibit good catalytic activity in the experiments. In the [1,4]-H shift process, the two-step hydrogen bond/halogen bond co-catalyzed mechanism exhibits the lowest reaction energy barrier than the one-step water co-catalyzed proton transfer mechanism and the direct one.

Transition-Metal Catalysis of Triene 6π Electrocyclization: The π-Complexation Strategy Realized

Triene 6π electrocyclization, wherein a conjugated triene undergoes a concerted stereospecific cycloisomerization to a cyclohexadiene, is a reaction of great historical and practical significance. In order to circumvent limitations imposed by the normally harsh reaction conditions, chemists have long sought to develop catalytic variants based upon the activating power of metal-alkene coordination. Herein, we demonstrate the first successful implementation of such a strategy by utilizing [(C₅ H₅ )Ru(NCMe)₃ ]PF₆ as a precatalyst for the disrotatory 6π electrocyclization of highly substituted trienes that are resistant to thermal cyclization. Mechanistic and computational studies implicate hexahapto transition-metal coordination as responsible for lowering the energetic barrier to ring closure. This work establishes a foundation for the development of new catalysts for stereoselective electrocyclizations.

Iodine-Catalyzed Iso-Nazarov Cyclization of Conjugated Dienals for the Synthesis of 2-Cyclopentenones

Molecular iodine was identified as an efficient catalyst for the cycloisomerization of conjugated dienals to substituted 2-cyclopentenones. DFT calculations suggested an unexpected concerted character for this cyclization.

The iso-Nazarov reaction

The acid-promoted cycloisomerization of conjugated dienals and linearly-conjugated dienones for the synthesis of five-membered ring systems is reviewed.

Direct construction of vicinal all-carbon quaternary stereocenters in natural product synthesis

Molecules containing vicinal all-carbon quaternary stereocenters are found in many secondary metabolites, and they exhibit a variety of biological and pharmacological activities.

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.

Oxidation-initiated Nazarov cyclization of vinyl alkoxyallenes

A mild method for the diastereoselective formation of C(4), C(5)-disubstituted cyclopentenones has been developed, involving formation of a pentadienyl cation via diastereoselective oxidation of a vinyl alkoxyallene. Conrotatory electrocyclization provides the cyclopentenone product. The broad scope, mild conditions, and uncommon substitution pattern accessible through this transformation make it a useful addition to the existing repertoire of cyclopentenone synthetic methods.

A mechanistic spectrum of chemical reactions

The mechanism of chemical reactions between electron donors and acceptors continuously changes with the power of the donors and the acceptors. The interaction between the HOMO (d) of the donors and the LUMO (a*) of the acceptors or delocalization of electrons is important for the reactions. The electron d-to-a* transferred configuration mixes to a significant extent. As the donors and/or the acceptors are strong, their excited configurations appreciably mixes together with the transferred configuration. The d-a and d*-a* orbital interactions are important in addition to the d-a* interaction. Reactions have features characteristic of the excited-state reactions although the donor-acceptor system is not really excited, but in the ground state. This process is termed pseudoexcitation. The a-d-a*-d* interaction is important. For much stronger donors and acceptors, the electron transferred configuration is stable and predominant. Covalent bonds do not form but instead ionic pairs, and electron transfer results. A mechanistic spectrum of chemical reactions is composed of the delocalization, pseudoexcitation, and electron transfer bands.

Bond formations by intermolecular and intramolecular trappings of acylketenes and their applications in natural product synthesis

The reactive intermediates known as acylketenes exhibit a rich chemistry and have been extensively utilized for many types of inter- and intramolecular bond-forming reactions within the field of organic synthesis. Characteristic reactions of acylketenes include cycloadditions, carbon-carbon bond-forming reactions, and nucleophilic capture with alcohols or amines to give beta-keto acid derivatives. In particular, the intramolecular capture of acylketene intermediates with pendant nucleophiles represents a powerful method for forming both medium-sized rings and macrocycles, often in high yield. This tutorial review examines the history, generation, and reactivity of acylketenes with a special focus on their applications in the synthesis of natural products.

Unprecedented Rearrangement of a 4-Alkoxy-5-bromoalk-2-en-1-ol to a Cyclopentenone via an Iso-Nazarov Cyclization Process

We report the structure determination of the product 9 of the rearrangement of the allylic alcohol 3 under very mild conditions, probably promoted by an acidic substance, and propose a reasonable mechanism for its formation.

Metal-Catalyzed Chemoselective Cycloisomerization of cis-2,4-Dien-1-als to 3-Cyclopentenones and 4-Alkylidene-3,4-dihydro-2H-pyrans

[reaction: see text] PtCl(2) (5 mol %) catalyst effected cycloisomerization of cis-2,4-dien-1-al (1) to 3-cyclopentenone (3) efficiently in hot toluene. In the presence of p-TSA, this PtCl(2) catalysis gave 2-cyclopentenone (5) exclusively because of the secondary isomerization reaction. Although the 1-2 equilibrium state greatly favors aldehyde (1), PdCl(2)(PhCN)(2) (5 mol %) catalyzed cycloisomerization of aldehyde (1) to 4,6,7,8-tetrahydro-3H-isochromene (4) smoothly in hot toluene. A plausible mechanism is proposed on the basis of reaction observation and isotope-labeled experiment.

Cycloisomerization of Activated (2E,4Z)-Heptatrienoate and Its Relevance to Crispatene (Bio)synthesis. A Case of Concerted and Stepwise Uncertainty

A single transition structure was located on the potential energy surface of the cycloisomerization of protic and Lewis acid activated (2E,4Z)-heptatrienal and the corresponding methyl ester to provide the bicyclo[3.1.0]hexene derivatives, the central skeleton of the crispatene natural products. A two-dimensional scan of the C-C bond-forming reactions revealed a barrierless cyclopropane closure following the pentadienyl cycloisomerization, with preservation of the stereochemical information.

Stereoselective Synthesis of Cyercene A and the Placidenes

Members of a family of alpha-methoxy-gamma-pyrone-containing polypropionate natural products have been stereoselectively synthesized. Two key iodovinyl pyrone building blocks were coupled to appropriately selected vinyl stannanes to assemble the highly substituted polyene side chains of the natural products. [structure: see text]

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2003 is provided. The emphasis on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure conformation or stereochemical assignments.

The total synthesis of (-)-crispatene

The total synthesis of the molluscan polypropionate (-)-crispatene is described. The synthesis features a palladium-catalyzed cross-coupling to establish a sensitive conjugated tetraene and its Lewis acid-catalyzed cycloisomerization to yield the bicyclo[3.1.0]hexene core of the natural product. The absolute configuration of (-)-crispatene and related molecules is established.