Expedient synthesis of a highly substituted tropolone via a 3-oxidopyrylium [5+2] cycloaddition reaction

Carbocycloaddition Strategies for Troponoid Synthesis

Tropone is the prototypical aromatic 7-membered ring, and can be found in virtually any undergraduate textbook as a key example of non-benzenoid aromaticity. Aside from this important historical role, tropone is also of high interest as a uniquely reactive synthon in complex chemical synthesis as well as a valuable chemotype in drug design. More recently, there has been growing interest in the utility of tropones for catalysis and material science. Thus, synthetic strategies capable of synthesizing functional tropones are key to fully exploiting the potential of this aromatic ring system. Cycloaddition reactions are particularly powerful methods for constructing carbocycles, and these strategies in turn have proven to be powerful for generating troponoids. The following review article provides an overview of strategies for troponoids wherein the 7-membered carbocycle is generated through a cycloaddition reaction. Representative examples of each strategy are also provided.

Comparison of (5 + 2) Cycloadditions Involving Oxidopyrylium and Oxidopyridinium Ions: Relative Reactivities

A variety of (5 + 2) cycloaddition reactions involving oxidopyridinium and oxidopyrylium zwitterions are compared to investigate the effects of nitrogen-for-oxygen substitution on reactivity. Activation barriers for nitrogen-containing systems are predicted to be larger than those for analogous oxygen-containing systems. Correlations between barrier heights and synchronicity of C-C bond formation, changes to aromaticity, reactant distortion, and interaction energies between zwitterions and alkenes were assessed, leading to the conclusion that reactivity depends more on distortion effects (including aromaticity loss) than on interaction effects (such as those associated with highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) interactions).

Construction of bridged polycycles through dearomatization strategies

Bridged polycycles are privileged molecular skeletons with wide occurrence in bioactive natural products and pharmaceuticals. Therefore, they have been the pursing target molecules of numerous chemists. The rapid and convenient generation of sp3-rich complex three-dimensional molecular skeletons from simple and easily available aromatics has made dearomatization a highly valuable synthetic tool for the construction of rigid and challenging bridged rings. This review summarizes the-state-of-the-art advances of dearomatization strategies in the application of bridged ring formation, discusses their advantages and limitations and the in-depth mechanism, and highlights their synthetic value in the total synthesis of natural products. We wish this review will provide an important reference for medicinal and synthetic chemists and will inspire further development in this intriguing research area.

Synthesis of Naturally Occurring Tropones and Tropolones

Tropones and tropolones are an important class of seven-membered non-benzenoid aromatic compounds. They can be prepared directly by oxidation of seven-membered rings. They can also be derived from cyclization or cycloaddition of appropriate precursors followed by elimination or rearrangement. This review discusses the types of naturally occurring tropones and tropolones and outlines important methods developed for the synthesis of tropone and tropolone natural products.

The biology and synthesis of α-hydroxytropolones

α-Hydroxytropolones are a subclass of the troponoid family of natural products that are of high interest due to their broad biological activity and potential as treatment options for several diseases. Despite this promise, there have been scarce synthetic chemistry-driven optimization studies on the molecules. The following review highlights key developments in the biological studies conducted on α-hydroxytropolones to date, including the few synthetic chemistry-driven optimization studies. In addition, we provide an overview of the methods currently available to access these molecules. This review is intended to serve as a resource for those interested in biological activity of α-hydroxytropolones, and inspire the development of new synthetic methods and strategies that could aid in this pursuit.

Triflic acid-mediated rearrangements of 3-methoxy-8-oxabicyclo[3.2.1]octa-3,6-dien-2-ones: synthesis of methoxytropolones and furans

Methoxytropolones are useful scaffolds for therapeutic development because of their known biological activity and established value in the synthesis of α-hydroxytropolones. Upon treatment with triflic acid, a series of 3-methoxy-8-oxabicyclo[3.2.1]octa-3,6-dien-2-ones rearrange rapidly and cleanly to form methoxytropolones. Interestingly, bicycles that are derived from dimethyl acetylenedicarboxylate (R(2) = R(3) = CO2Me) instead form furans as the major product.

An oxidopyrylium cyclization/ring-opening route to polysubstituted α-hydroxytropolones

α-Hydroxytropolones are a class of molecules with therapeutic potential against several human diseases. However, structure-activity relationship studies on these molecules have been limited due to a scarcity of efficient synthetic methods to access them. It is demonstrated herein that α-hydroxytropolones can be generated through a BCl(3)-mediated ring-opening/aromatization/demethylation process on 8-oxabicyclo[3.2.1]octenes. Used in conjunction with an improved method based on established oxidopyrylium dipolar cycloadditions, several polysubstituted α-hydroxytropolones can be accessed in three steps from readily available α-hydroxy-γ-pyrones.

A Facile and General Synthesis of Tropolonyl-Substituted Chalcone Derivatives

A facile and general synthesis of a series of new troplonyl-substituted chalcone derivatives by Claisen–Schmidt condensation reaction from 3-acetyltropolone and substituted benzaldehydes as well as pyridine aldehydes is described. The method using 5% aq. KOH as catalyst and 50% aq. methanol as solvent is attractive since it specifically generates (E)-isomers with high yields under mild reaction conditions.

Organometallic enantiomeric scaffolding: general access to 2-substituted oxa- and azabicyclo[3.2.1]octenes via a Brønsted acid catalyzed [5 + 2] cycloaddition reaction

6-Substituted TpMo(CO) 2(eta-2,3,4-pyranyl)- and TpMo(CO) 2(eta-2,3,4-pyridinyl) scaffolds (Tp = hydridotrispyrazolylborato) function as reaction partners in an efficient regio- and stereocontrolled synthesis of functionalized oxa- and azabicyclo[3.2.1]octenes through a novel Brønsted acid catalyzed [5 + 2] cycloaddition reaction. Excellent exo-selectivities are obtained, and the reaction gives products with complete retention of enantiomeric purity when carried out with chiral, nonracemic scaffolds. The substituent at C-6 of the eta (3)-coordinated heterocyclic scaffold not only influences [5 + 2] reactivity but also plays a critical role in the demetalation step directing the reaction to only one of two possible products.

Total synthesis of (-)-colchicine via a Rh-triggered cycloaddition cascade

[reaction: see text] A synthesis of the antimitotic alkaloids (-)-colchicine and (-)-isocolchicine is reported. Important steps are (a) enantioselective transfer-hydrogenation of an alkynone, (b) iodine/magnesium exchange with subsequent aromatic acylation, (c) Rh-catalyzed transformation of an alpha-diazoketone into an oxatetracyclic key intermediate through intramolecular [3 + 2]-cycloaddition of an in situ generated carbonyl ylide, and (d) regioselective conversion of the cycloadduct into a tropolone derivative. The new synthetic strategy opens an efficient enantioselective access to colchicine and structural analogues.

The Cycloaddition Strategy for the Synthesis of Natural Products Containing Carbocyclic Seven-Membered Rings

Highly substituted carbocyclic seven-membered rings are frequently found in natural products and their synthesis represents a significant challenge to the synthetic chemist. Direct intramolecular cyclization of these systems often proves difficult and this fact has catalyzed the development of a variety of strategies based on a convergent intermolecular cycloaddition strategy. This concept article discusses the major cycloaddition approaches utilized to access these types of structures and primarily focuses on examples employed in the synthesis of natural products.