Synthesis of delta-tributylstannyl-alpha,beta,gamma,delta-unsaturated aldehydes from pyridines

N-Oxide-to-Carbon Transmutations of Azaarene N-Oxides

Reactions that change the identity of an atom within a ring system are emerging as valuable tools for the site-selective editing of molecular structures. Herein, we describe the expansion of an underdeveloped transformation that directly converts azaarene-derived N-oxides to all-carbon arenes. This ring transmutation exhibits good functional group tolerance and replaces the N-oxide moiety with either unsubstituted, substituted, or isotopically labeled carbon atoms in a single laboratory operation.

Stereoselective synthesis of thailandamide A methyl ester

A convergent strategy for the stereoselective synthesis of the methyl ester of the structurally challenging and highly labile antibacterial polyene polyketide natural product thailandamide A has been developed. The key steps include the Zincke aldehyde reaction, Stille cross coupling, Negishi reaction, Julia-Kocienski olefination, cross metathesis, and the less explored Pd(I)-based Heck coupling to access different unsaturation bonds. Additionally, Urpi acetal aldol, Evans methylation, and Crimmins acetate aldol reactions were employed to construct four out of six asymmetric centers of the molecule.

Unusually cyclized triterpenoids: occurrence, biosynthesis and chemical synthesis

Covering: 2009 to 2021Biosynthetically, most of the syntheses of triterpenes follow the cascade cyclization and rearrangement of the acyclic precursors viz., squalene (S) and 2,3-oxidosqualene (OS), which lead to the very well known tetra- and pentacyclic triterpene skeletons. Aside from these, numerous other triterpenoid molecules are also reported from various natural sources and their structures are derived from "S" and "OS" via some unusual cyclization operations which are different from the usual tetra- and pentacyclic frameworks. Numerous compelling advances have been made and reported in the identification of these unusual cyclized mono-, di-, tri- and tetracyclic triterpenes between 2009 and 2021. Besides a dramatic increase in the newly isolated uncommon cyclized triterpenoids, substantial progress in the (bio)-synthesis of these triterpenes has been published along with significant progress in their biological effects. In this review, 180 new unusual cyclized triterpenoids together with their demonstrated biogenetic pathways, syntheses and biological effects will be categorized and discussed.

On the Structure of Thailandene A: Synthetic Examination of the Cryptic Natural Product Aided by a Theoretical Approach

Phenotype-guided transposon mutagenesis has emerged as a valuable tool to access cryptic metabolites encoded in bacterial genomes. Recently, the method was demonstrated by inducing silent biosynthetic gene clusters in Burkholderia thailandensis. Amongst the isolated metabolic products, thailandene A exhibited promising antibiotic activity. By assignment, the linear polyenic aldehyde contained a labile motif, where an ostensible chiral secondary alcohol was interlaced in an allylic and a homoallylic constellation. Our attention was drawn to the pseudo-symmetric relationship between the heterofunctionalities, indicating the transformation of a dodecapentaenedial scaffold. Centering on an iterative cross-coupling protocol, the assigned all-E-(12R)-hydroxydodecapentaenal moiety was assembled by combining Zincke chemistry with the MIDA-attenuated Suzuki reaction developed in the Burke laboratory. Thus, according to the devised strategy, the mixed 1,2-bisborylated vinyl linchpin was consecutively functionalized with 5-bromodienal derivatives in a doubly orthogonal fashion. However, when the synthetic material was matched against the bacterial isolate, inconsistencies were observed. A re-examination of the cryptic natural product was conducted by juxtaposing analytical data from experiment and density functional theory calculations, in which hydroperoxide was evaluated as a candidate metabolite present in the bacterial isolate.

Deaminative chlorination of aminoheterocycles

Selective modification of heteroatom-containing aromatic structures is in high demand as it permits rapid evaluation of molecular complexity in advanced intermediates. Inspired by the selectivity of deaminases in nature, herein we present a simple methodology that enables the NH₂ groups in aminoheterocycles to be conceived as masked modification handles. With the aid of a simple pyrylium reagent and a cheap chloride source, C(sp²)‒NH₂ can be converted into C(sp²)‒Cl bonds. The method is characterized by its wide functional group tolerance and substrate scope, allowing the modification of >20 different classes of heteroaromatic motifs (five- and six-membered heterocycles), bearing numerous sensitive motifs. The facile conversion of NH₂ into Cl in a late-stage fashion enables practitioners to apply Sandmeyer- and Vilsmeier-type transforms without the burden of explosive and unsafe diazonium salts, stoichiometric transition metals or highly oxidizing and unselective chlorinating agents.

Skeletal remodeling of chalcone-based pyridinium salts to access isoindoline polycycles and their bridged derivatives

Simultaneous deconstructive ring-opening and skeletal reconstruction of an inert, aromatic pyridinium ring is of great importance in synthetic communities. However, research in this area is still in its infancy. Here, a skeletal re-modeling strategy was developed to transform chalcone-based pyridinium salts into structurally intriguing polycyclic isoindolines through a dearomative ring-opening/ring-closing sequence. Two distinct driving forces for the deconstruction of the pyridinium core were involved in these transformations. One was the unprecedented harnessing of the instability of in situ generated cyclic β-aminoketones, and the other was the instability of the resultant N,N-ketals. The desired isoindoline polycycles could undergo the Wittig reaction with various phosphorus ylides to achieve structural diversity and complexity. Notably, by tuning the Wittig conditions by addition of one equivalent of base, an additional bridged ring was introduced. A plausible mechanism was proposed on the basis of control experiments and theoretical calculations.

Total Synthesis of Stelletins through an Unconventional Annulation Strategy

Marine ecosystems present the largest source of biodiversity on the planet and an immense reservoir of novel chemical entities. Sessile marine organisms such as sponges produce a wide range of complex secondary metabolites, many of these with potent biological activity engineered for chemical defense. That such compounds exert dynamic effects outside of their native context is perhaps not surprising, and the realm of marine natural products has attracted considerable attention as a largely untapped repository of potential candidates for drug development. Only a handful of the more than 15 000 marine natural products that have been isolated to date have advanced to the clinic, and more are to be expected. The rich chemical information encoded in the intricate three-dimensional structures of many marine natural products facilitates highly discriminating interactions with cell signaling pathways, and especially within cancer cells such nuanced effects offer an exciting opportunity for the development of targeted therapies that lack the side effects and general toxicity of conventional chemotherapeutics. The isomalabaricanes are a rare class of marine triterpenoids that have been hailed as promising cytotoxic lead compounds for the treatment of cancer, and they have attracted a flurry of excitement from researchers because of their potent cytotoxicity in certain human cancer cell lines along with a range of other antineoplastic effects. Most notably, their inhibitory activity is highly cell-selective, characterized by large deviations from their mean GI₅₀ concentrations across 3 orders of magnitude in the NCI-60 Human Tumor Cell Lines screen, suggesting mechanistic specificity rather than general and unbridled toxicity. Despite these auspicious preliminary reports, the isomalabaricane scaffold remains largely unexplored as a potential anticancer lead because of lack of material. This Account describes our recent efforts to develop a general, modular synthesis of the isomalabaricanes, as exemplified by the successful total syntheses of rhabdastrellic acid A, stelletin E, and stelletin A. The unorthodox trans-syn-trans configuration of their perhydrobenz[e]indene core severely circumscribes the synthetic methods available for its construction and required several generations of strategy to assemble. Ultimately, a series of unconventional transformations were identified that were capable of building this highly strained motif, and the syntheses of rhabdastrellic acid A and stelletin E were completed in racemic fashion. Subsequently, a second-generation approach to these natural products was developed, rendering the synthesis enantioselective as well as providing access to stelletin A. These synthetic efforts were greatly assisted by computational techniques such as ¹³C NMR prediction, which enabled structural assignments of hydrocarbon diastereomers, as well as relaxed surface scan conformational analysis, which informed a campaign for directed hydrogenation of an alkene. High-throughput experimentation methods were brought to bear during optimization of a late-stage Suzuki coupling on stelletin A. Finally, preliminary structure-activity relationship studies in glioblastoma and nonsmall cell lung cancer cell lines were conducted on stelletin A, revealing that the singular trans-syn-trans perhydrobenz[e]indene core is essential for the cytotoxic activity of the isomalabaricane triterpenoids.

Enantioselective synthesis and stereochemical determination of the highly reduced polyketide ishigamide

Ishigamide was isolated as a metabolite of a recombinant strain of Streptomyces sp. MSC090213JE08 and its unsaturated fatty acid moiety has been confirmed in vitro to be synthesized by a type II PKS. Biosynthesis of such a highly reduced polyketide by a type II PKS is worthy of note. However, absolute configuration of ishigamide remained unknown. (R)-Ishigamide was synthesized enantioselectively employing Stille coupling and Wittig reaction between three units, vinyl iodide, stannyldienal, and Wittig salt. Stereochemistry of natural ishigamide was determined to be R by chiral HPLC analysis comparing with the synthesized standard.

Synthesis of Allene-Containing Apocarotenoids by Cross-Coupling Strategy

The stereocontrolled total synthesis of the allene and carbonyl conjugated apocarotenoids, paracentrone and 19-hexanoyloxyparacentrone 3-acetate, was achieved by sequential cross-coupling reactions using boronic acid ester and iodine- or tin-substituted C5 dienes, which were the building blocks for the elongation of the conjugated polyene systems at both terminals.

Recent synthetic advances in pyridine-based thermotropic mesogens

Currently, numerous articles have reported pyridine-based thermotropic mesogens; however, reviews of their synthetic methodologies are rare. Therefore, the present critical review describes the recent synthetic advances in the field of pyridine-based thermotropic mesogens. Also, we discuss the various types of thermotropic mesogens (such as calamitic, discotic, bent-shaped, polycatenar, and polymeric mesogens) consisting of pyridine derivatives and their structure–property relationships.

Currently, numerous articles have reported pyridine-based thermotropic mesogens; however, reviews of their synthetic methodologies are rare.

Effect of counter ions on the mesogenic ionic N-phenylpyridiniums

Two series of ionic liquid crystals (ILCs) derived from N-phenylpyridinium 1–2 were prepared and their mesomorphic properties were investigated.

NMR and experimental reinvestigation of the condensation reaction between γ-methylene-α,β-unsaturated aldehydes and propargyl aldehydes

An experimental and computational study of the reaction between γ-methylene enals and propargyl aldehydes uncovered a deconjugative aldol condensation.

Pyridinium salts: from synthesis to reactivity and applications

This review highlights the pyridinium salts in terms of their natural occurrence, synthesis, reactivity, biological properties, and diverse applications.

Is there no end to the total syntheses of strychnine? Lessons learned in strategy and tactics in total synthesis

From the 19th century to the present, the complex indole alkaloid strychnine has engaged the chemical community. In this Review, we examine why strychnine has been and remains today an important target for directed synthesis efforts. A selection of the diverse syntheses of strychnine is discussed with the aim of identifying their influence on the evolution of the strategy and tactics of organic synthesis.

Unraveling the mechanism of cascade reactions of zincke aldehydes

The thermal pericyclic cascade rearrangement of Zincke aldehydes (5-(dialkylamino)-2,4-pentadienals) to afford Z-α,β,γ,δ-unsaturated amides discovered by the Vanderwal group has been studied in depth using quantum mechanical methods. Two mechanistic possibilities that had previously been put forth to explain this internal redox process, one that had been discounted by experiment and the other that had withstood experimental scrutiny, were evaluated. Both of these mechanisms suffered from energetic barriers that appeared too high to allow rearrangement to proceed under the conditions used; however, computational study of a third possibility that implicates the intermediacy of vinylketenes revealed that it is the most likely pathway of rearrangement. Further computational studies accounted for the relative rates of rearrangement in substituted Zincke aldehydes, predicted the feasibility of related processes for other donor-acceptor dienes, and provided insight into the rearrangement of allylamine-derived Zincke aldehydes that provide either dihydropyridones or polycyclic lactams by further pericyclic processes.

Enhancement of 5-aminopenta-2,4-dienals electrophilicity via activation by O,N-bistrifluoroacetylation. application to an N-acyl Pictet-Spengler reaction

Aminopentadienals resulting from the condensation of tryptamine or homoveratrylamine with glutaconaldehydes were treated with trifluoroacetic anhydride, allowing the formation of tetrahydro-beta-carbolines and tetrahydroisoquinolines bearing an enal function. In this N-acyl Pictet-Spengler reaction the electrophilicity of the aminopentadienals was dramatically increased by O,N-bistrifluoroacetylation. Recovery of the nitrogen nucleophilicity was achieved using a reductive process, and the heterocyclic amines were converted into aminonitriles by a Strecker reaction in the presence of butanal. Cyclization, by intramolecular Michael addition of the in situ generated enamines onto the enal moiety, was achieved in the presence of zinc triflate and involved cyanide ion trapping. In this manner, compounds related to protoemetine and dihydrocorynantheal were obtained, and a reduction step led to a short synthesis of (+/-)-protoemetinol.

Complex polycyclic lactams from pericyclic cascade reactions of Zincke aldehydes

In the course of a simple mechanistic study on the rearrangement of Zincke aldehydes to Z-alpha,beta,gamma,delta-unsaturated amides, a thermally induced pericyclic cascade rearrangement that converts Zincke aldehydes derived from allylic and homoallylic amines into polycyclic lactams was discovered. The key reaction involves an E-Z alkene isomerization, a 6pi electrocyclic ring closure, a [1,5]-sigmatropic shift of hydrogen, a 6pi electrocyclic ring-opening, and a Diels-Alder cycloaddition, and proceeds with excellent stereoselectivity. The unusual observation that furans and an indole serve as dienophiles in this cascade reaction permitted the synthesis of complex, functional group-rich tri- and tetracyclic lactams. In all cases, the rigid polycyclic products are available in only two steps from pyridinium salts and the allylic or homoallylic secondary amines.