Synthesis of N-alkyl-octahydroisoquinolin-1-one-8-carboxamide libraries using a tandem Diels-Alder/acylation sequence

A synthetic sequence was developed in which a diene containing an attached secondary amine was reacted with maleic anhydride to afford the title structures in one step. The reaction involves a Diels-Alder reaction combined with a transacylation reaction of the imide group. A series of six scaffolds was constructed using this methodology. Each scaffold was subsequently reacted with 12 amines to afford a library containing 72 compounds.     

Different reaction modes for the oxidative dimerization of epoxyquinols and epoxyquinones. Importance of intermolecular hydrogen-bonding

An oxidative dimerization reaction, involving the three successive steps of oxidation, 6 pi-electrocyclization, and Diels-Alder reaction, has been experimentally and theoretically investigated for the three 2-alkenyl-3-hydroxymethyl-2-cyclohexen-1-one derivatives epoxyquinol 3, epoxyquinone 6, and cyclohexenone 10. Of the sixteen possible modes of the oxidation/6 pi-electrocylization/Diels-Alder reaction cascade for the epoxyquinone 6, and eight for the cyclohexenone 10, only the endo-anti(epoxide)-anti(Me)-hetero and endo-anti(Me)-hetero modes are, respectively, observed, while both endo-anti(epoxide)-anti(Me)-hetero and exo-anti(epoxide)-anti(Me)-homo reaction modes occur with the epoxyquinol 3. Intermolecular hydrogen-bonding is found to be the key cause of formation of both epoxyquinols A and B with 3, although epoxyquinone 6 and cyclohexenone 10 both gave selectively only the epoxyquinol A-type product. In the dimerization of epoxyquinol 3, two monomer 2H-pyrans 5 interact with each other to afford intermediate complex 28 or 29 stabilized by hydrogen-bonding, from which Diels-Alder reaction proceeds. Theoretical calculations have also revealed the differences in the reaction profiles of epoxyquinone 6 and cyclohexenone 10. Namely, the rate-determining step of the former is the Diels-Alder reaction, while that of the latter is the 6 pi-electrocyclization.     

Scaffold diversity of exemplified medicinal chemistry space

The scaffold diversity of 7 representative commercial and proprietary compound libraries is explored for the first time using both Murcko frameworks and Scaffold Trees. We show that Level 1 of the Scaffold Tree is useful for the characterization of scaffold diversity in compound libraries and offers advantages over the use of Murcko frameworks. This analysis also demonstrates that the majority of compounds in the libraries we analyzed contain only a small number of well represented scaffolds and that a high percentage of singleton scaffolds represent the remaining compounds. We use Tree Maps to clearly visualize the scaffold space of representative compound libraries, for example, to display highly populated scaffolds and clusters of structurally similar scaffolds. This study further highlights the need for diversification of compound libraries used in hit discovery by focusing library enrichment on the synthesis of compounds with novel or underrepresented scaffolds.     

Synthesis of epoxyquinol A and related molecules: probing chemical reactivity of epoxyquinol dimers and 2H-pyran precursors

Total syntheses of the epoxyquinoid dimers, epoxyquinols A, B, and epoxytwinol A (RKB-3564 D), have been accomplished employing [4 + 2] and [4 + 4] dimerization of 2H-pyran epoxyquinol monomers. Modifications of 2H-pyran precursors have been explored, including alteration of epoxy alcohol and diene stereochemistry. A stable 2H-pyran prepared by alteration of the epoxyquinol 2H-pyran nucleus was evaluated as a diene in Diels-Alder cycloaddition with reactive dienophiles. Extensive studies for improving the [4 + 4] dimerization of selectively protected 2H-pyran monomers to afford the novel epoxyquinoid dimer epoxytwinol A were carried out, and valuable insight regarding competitive [4 + 2] and [4 + 4] dimerization processes has been obtained. In addition, chemical reactivities and structural modifications of epoxyquinol dimers have been evaluated, including [2 + 2] photocycloaddition and [3,3] sigmatropic rearrangement, indicating the possibility for production of novel structural diversity from dimeric epoxyquinoid natural product frameworks.     

Total synthesis of the epoxyquinol dimer (+)-panepophenanthrin: application of a diastereospecific biomimetic Diels-Alder dimerisation

An asymmetric total synthesis of the novel and structurally complex epoxyquinol natural product (+)-panepophenanthrin has been accomplished, in which a biomimetic Diels-Alder dimerisation is a key step. The key monomeric precursor was assembled by an efficient Stille cross coupling of two readily available building blocks that upon standing underwent a diastereospecific dimerisation cascade in excellent yield.     

Diversity through a branched reaction pathway: generation of multicyclic scaffolds and identification of antimigratory agents

A library of 91 heterocyclic compounds composed of 16 distinct scaffolds has been synthesized through a sequence of phosphine-catalyzed ring-forming reactions, Tebbe reactions, Diels-Alder reactions, and, in some cases, hydrolysis. This effort in diversity-oriented synthesis produced a collection of compounds that exhibited high levels of structural variation both in terms of stereochemistry and the range of scaffolds represented. A simple but powerful sequence of reactions thus led to a high-diversity library of relatively modest size with which to explore biologically relevant regions of chemical space. From this library, several molecules were identified that inhibit the migration and invasion of breast cancer cells and may serve as leads for the development of antimetastatic agents.     

New strategy to construct fused/bridged/spiro carbocyclic scaffolds based on the design of novel 6-C synthon precursor

In this article we report a new strategy to build fused/spiro/bridged carbocyclic systems with a novel 6-C synthon from readily available diallyl diacetates through the sequential Pd-catalyzed double allyl alkylation and Diels-Alder annulation. Further exploration on the application of this strategy can construct useful complex scaffolds.     

Privileged Structures Revisited

Privileged structures inspire compound library design in medicinal chemistry. We performed a comprehensive analysis of 1.4 million bioactive compounds, with the aim of assessing the prevalence of certain molecular frameworks. We used the Shannon entropy formalism to quantify the promiscuity of the most frequently observed atom scaffolds across the annotated target families. This analysis revealed an apparent inverse relationship between hydrogen-bond-acceptor count of a scaffold and its potential promiscuity. The results further suggest that chemically easily accessible scaffolds can serve as templates for the generation of bespoke compound libraries with differing degrees of multiple target engagement, and heterocyclic, sp³-rich frameworks are particularly suited for target-focused library design. The outcome of our study enables us to place some of the many narratives surrounding the concept of privileged structures into a critical context.     

Solution-phase parallel synthesis of hexahydro-1H-isoindolone libraries via tactical combination of Cu-catalyzed three-component coupling and Diels-Alder reactions

Parallel solution-phase synthesis of combinatorial libraries of hexahydro-1 H-isoindolones exploiting a novel "tactical combination" of Cu-catalyzed three-component coupling and Diels-Alder reactions was accomplished. Three distinct libraries consisting of 24 members (library I), 60 members (library II), and 32 members (library III) were constructed. Variation of three substituents on the isoindolone scaffold in library I was exclusively achieved by the choice of the building blocks. In the syntheses of libraries II and III, sublibraries of isoindolone scaffolds were prepared initially in a one-pot/two-step process and were further diversified via Pd-catalyzed Suzuki cross-coupling reaction with boronic acids at two different diversification points. The Lipinski profiles and calculated ADME properties of the compounds are also reported.     

Double domino Knoevenagel hetero Diels-Alder strategy towards bis-pyrano-1,4-benzoquinones

Several bis-pyrano-l,4-benzoquinones have been synthesized by a double domino Knoevenagel hetero Diels-Alder reaction. The synthetic approach is highly efficient allowing the construction of complex polycyclic scaffolds with six new σ-bonds. These reactions performed more efficiently and more rapidly using microwave irradiation. The resulting bis-pyrano-1,4-benzoquinones are the first examples of a double domino Knoevenagel hetero Diels-Alder reaction. Our approach represents a novel contribution to the chemistry of 2,5-dihydroxy-l,4-benzoquinones and the first general method for the synthesis of bis-pyranobenzoquinones.     

Organocatalytic Asymmetric Cascade Reactions Based on Gamma‐Nitro Carbonyl Compound

The significant advancements in asymmetric organocascade reactions have been accomplished during the past decades, paving the way to the efficient and stereoselective construction of structurally complex scaffolds from simple and readily available starting materials. Nitro‐containing cyclic compounds have become a privileged molecular library given their broad and promising activities in various therapeutic areas. In various approaches to build these valuable scaffolds, the utility of γ‐nitrocarbonyl intermediates is one of the most efficient approaches due to its high efficiency, reliability and versatility. The strategies and catalyst systems described here highlight recent advances in the enantioselective synthesis of nitro‐containing cyclic molecules via organocascade strategies based on γ‐nitrocarbonyl intermediates. Various organocatalysts with distinct activation modes have found application in providing these sophisticated compounds. This review is organized according to the types of organocatalyst. These methods are of importance for the construction of complex chiral cyclic frameworks and the design of new pharmaceutical compounds. We believe that compounds based on nitro‐containing cyclic skeletons have the potential to provide novel therapeutic agents and useful biological tools.     

Fluorous mixture synthesis of two libraries with hydantoin-, and benzodiazepinedione-fused heterocyclic scaffolds

Diversity-oriented synthesis (DOS) and fluorous mixture synthesis (FMS) are two aspects of combinatorial chemistry. DOS generates library scaffolds with skeletal, substitution, and stereochemistry variations, whereas FMS is a highly efficient tool for library production. The combination of these two aspects in solution-phase synthesis of two novel heterocyclic compound libraries is presented in this paper. Mixtures of different fluorous amino acids undergo [3 + 2] cycloadditions followed by postcondensation reactions. The mixtures are then demixed by fluorous HPLC. Fluorous tags are removed by cyclization to afford hydantoin- and benzodiazepinedione-fused heterocyclic compounds as individual, pure, and structurally defined molecules. The application of MS-directed HPLC purification and parallel four-channel LC/MS analysis further increases the efficiency of FMS.     

Biomimetic Synthesis of Meroterpenoids by Dearomatization‐Driven Polycyclization

A biomimetic route to farnesyl pyrophosphate and dimethyl orsellinic acid (DMOA)‐derived meroterpenoid scaffolds has yet to be reported despite great interest from the chemistry and biomedical research communities. A concise synthetic route with the potential to access DMOA‐derived meroterpenoids is highly desirable to create a library of related compounds. Herein, we report novel dearomatization methodology followed by polyene cyclization to access DMOA‐derived meroterpenoid frameworks in six steps from commercially available starting materials. Furthermore, several farnesyl alkene substrates were used to generate structurally novel, DMOA‐derived meroterpenoid derivatives. DFT calculations combined with experimentation provided a rationale for the observed thermodynamic distribution of polycyclization products.     

Synthesis of the western half of the lolicines and lolitrems

[reaction: see text] The synthesis of the highly substituted indole portion of the complex tremorgenic natural products lolicine A and B is presented. The Diels-Alder reaction of a quinone monoimine enables the synthesis of an appropriately substituted indole. The key step in the synthesis is a tandem isopropenyl cuprate addition/aldol cyclocondensation which provides the necessary functionality for elaboration to the 2,2,5,5-tetramethyltetrahydrofuran.     

Enantioselective parallel synthesis using polymer-supported chiral Co(salen) complexes

[formula: see text] The kinetic resolution of epoxides with phenols catalyzed by a polymer-supported Co(salen) complex is applied to the first enantioselective catalytic synthesis of parallel libraries. The corresponding 1-aryloxy-2-alcohols are obtained in high yield, purity, and enantiomeric excess. Further elaboration with diversity elements provides highly efficient access to important classes of pharmacologically active compounds.     

Synthetic studies on the preparation of oxygenated spongiane diterpenes from carvone

The paper describes a new diastereoselective approach to oxygenated spongiane diterpenes functionally related to natural dorisenones. The strategy followed for the preparation of the spongiane framework, a B→AB→ABC→ABCD approach, is based on the preparation of epoxydecalone 11 (AB rings) from R-(−)-carvone, followed by an intramolecular Diels-Alder reaction for the construction of the C ring (compound 26). Further manipulation of the Diels-Alder adduct functionality allows the completion of the spongiane framework and the elaboration of several oxygenated spongiane-type compounds. The structures of two compounds 27 and 31, has been established by single-crystal X-ray crystallography.     

An approach to the synthesis of stenine

A type 2 N-acylnitroso intramolecular Diels-Alder reaction followed by reductive N-O bond cleavage formed the B and C rings of the Stemona alkaloid stenine. Further elaboration provided the functionalized tricyclic core.     

Synthesis of new amphiphilic chlorin derivatives from protoporphyrin-IX dimethyl ester

A simple synthesis of new amphiphilic chlorin derivatives from protoporphyrin-IX dimethyl ester is reported. The preparation of such compounds is based in a straightforward methodology, which involves the Diels-Alder reaction of protoporphyrin-IX dimethyl ester with maleic anhydride followed by addition of nucleophilic species to the initially formed cycloadducts, a transformation, which is highly regioselective. Preliminary photophysical studies with the new compounds show that they meet adequate features for PDT applications.     

Synthesis of an epoxyquinol analog: efficient methodology for the insertion of side chains into cyclohexenone cores

A novel epoxyquinol analog was prepared by molecular simplification of monomeric and dimeric scaffolds. A feasible methodology for the insertion of side chains into cyclohexenone skeleton was developed. Insertion of the hydroxymethyl side chain was achieved through α-sulfonylcarbanion chemistry. The alkenyl chain was inserted through palladium cross-coupling strategy.     

Recent Advances in the Synthetic Elaboration of Sultones

New methods for the elaboration of sultones enable a short and highly stereoselective synthesis of methyl nonactate and establish vinylsulfonyl chloride as an allene equivalent for the intramolecular Diels-Alder reaction.