Remodeling of fumagillol: discovery of an oxygen-directed oxidative Mannich reaction

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

Mechanism-Based Approach to Reagent Selection for Oxidative Carbon-Hydrogen Bond Cleavage Reactions

Numerous hydride-abstracting agents generate the same cationic intermediate, but substrate features such as intermediate cation stability, oxidation potential, and steric environment can influence reaction rates in an oxidant-dependent manner. This manuscript provides experimental data to illustrate the role that structural features play in the kinetics of hydride abstraction reactions with commonly used quinone-, oxoammonium ion-, and carbocation- based oxidants. Computational studies of the transition state structures and energies explain these results and energy decomposition analysis calculations reveal unique sensitivities to electrostatic attraction and steric repulsions. Rigorous rate studies of select reactions validated the capacity of the calculations to predict reactivity trends. Additionally, kinetics studies demonstrate the potential for product inhibition in DDQ-mediated reactions. These studies provide a clear guide to select the optimal oxidant for structurally disparate substrates and lead to predictions of reactivity that were validated experimentally.

Limonin as a Starting Point for the Construction of Compounds with High Scaffold Diversity

Structurally complex natural products have been a fruitful source for the discovery and development of new drugs. In an effort to construct a compound collection populated by architecturally complex members with unique scaffolds, we have used the natural product limonin as a starting point. Limonin is an abundant triterpenoid natural product and, through alteration of its heptacyclic core ring system using short synthetic sequences, a collection of 98 compounds was created, including multiple members with novel ring systems. The reactions leveraged in the construction of these compounds include novel ring cleavage, rearrangements, and cyclizations, and this work is highlighted by the discovery of a novel B-ring cleavage reaction, a unique B/C-ring rearrangement, an atypical D-ring cyclization, among others. Computational analysis shows that 52 different scaffolds/ring systems were produced during the course of this work, of which 36 are unprecedented. Phenotypic screening and structure-activity relationships identified compounds with activity against a panel of cancer cell lines.

Experimental and Theoretical Studies on the Mechanism of DDQ-Mediated Oxidative Cyclization of N-Aroylhydrazones

The controversial single-electron-transfer process, frequently proposed in many 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-mediated reactions, was investigated experimentally and theoretically using the oxidative cyclization of aroylhydrazone with DDQ. DDQ-mediated oxadiazole formation involves several processes, including cyclization to form an oxadiazole ring and N-H bond cleavage, either by proton, hydride, or hydrogen atom transfer. The detailed mechanistic study using the M06-2X density functional theory, and the 6-31+G(d,p) basis set, suggests that the pathways involving radical ion pair (RIP) intermediates, which resulted from single-electron transfer (SET), were found to be energetically nearly identical to the pathway without the SET. The substituent-dependent reactivity of oxadiazole formation was consistent with the free energy profiles of both pathways, with or without the SET. This result indicates that in addition to the electron-transfer pathway, the nucleophilic addition/elimination pathway for DDQ should be considered as a possible mechanism of the oxidative transformation reaction using DDQ.

Diversity-oriented synthesis of 17-spirosteroids

A diversity-oriented synthesis (DOS) approach has been used to functionalize 17-ethynyl-17-hydroxysteroids through a one-pot procedure involving a ring-closing enyne metathesis (RCEYM) and a Diels–Alder reaction on the resulting diene, under microwave irradiations. Taking advantage of the propargyl alcohol moiety present on commercially available steroids, this classical strategy was applied to mestranol and lynestrenol, giving a collection of new complex 17-spirosteroids.

Syntheses of (-)-Tripterifordin and (-)-Neotripterifordin from Stevioside

We report short syntheses of (-)-tripterifordin and (-)-neotripterifordin, potent inhibitors of HIV replication, from stevioside, a natural sweetener used worldwide. The key transformations are reduction at C13 through the formation of a tertiary chloride and subsequent three-step lactonization including a selective iodination at C20 by the photoreaction of the C19-alcohol. The title compounds were reliably obtained from stevioside in 9 and 11 steps (with 5-7 isolation steps), respectively. Additionally, the related lactone-containing ent-kaurenes, doianoterpenes A and B, and two more natural products were synthesized.

Oxidative reaction of 2-aminopyridine-3-sulfonyl chlorides with tertiary amines

2-Aminopyridine-3-sulfonyl chlorides undergo a reaction with tertiary amines in the presence of air to produce sulfonylethenamines. The 2-aminopyridine-3-sulfonyl chloride apparently plays a dual role in the process promoting the aerobic oxidation of the amine and electrophilically trapping the resulting enamine.

Access to a Structurally Complex Compound Collection via Ring Distortion of the Alkaloid Sinomenine

Many compound collections used in high-throughput screening are composed of members whose structural complexity is considerably lower than that of natural products. We previously reported a strategy for the synthesis of complex and diverse small molecules from natural products using ring-distortion reactions, called complexity-to-diversity (CtD), and herein, CtD is applied in the synthesis of 16 diverse scaffolds and 65 total compounds from the alkaloid natural product sinomenine. Chemoinformatic analysis shows that these compounds possess complex ring systems and marked three-dimensionality.

One-Pot Evolution of Ageladine A through a Bio-Inspired Cascade towards Selective Modulators of Neuronal Differentiation

A bio-inspired cascade reaction has been developed for the construction of the marine natural product ageladine A and a de novo array of its N1-substituted derivatives. This cascade features a 2-aminoimidazole formation that is modeled after an arginine post-translational modification and an aza-electrocyclization. It can be effectively carried out in a one-pot procedure from simple anilines or guanidines, leading to structural analogues of ageladine A that had been otherwise synthetically inaccessible. We found that some compounds out of this structurally novel library show a significant activity in modulating the neural differentiation. Namely, these compounds selectively activate or inhibit the differentiation of neural stem cells to neurons, while being negligible in the differentiation to astrocytes. This study represents a successful case in which the native biofunction of a natural product could be altered by structural modifications.

Synthesis of rocaglamide derivatives and evaluation of their Wnt signal inhibitory activities

A new class of rocaglamide derivatives were synthesized by [3 + 2] cycloaddition and their Wnt signal inhibitory activities were evaluated.

Design and synthesis of analogues of natural products

In this article strategies for the design and synthesis of natural product analogues are summarized and illustrated with some selected examples.

Rapid synthesis of polyprenylated acylphloroglucinol analogs via dearomative conjunctive allylic annulation

Polyprenylated acylphloroglucinols (PPAPs) are structurally complex natural products with promising biological activities. Herein, we present a biosynthesis-inspired, diversity-oriented synthesis approach for rapid construction of PPAP analogs via double decarboxylative allylation (DcA) of acylphloroglucinol scaffolds to access allyl-desoxyhumulones followed by dearomative conjunctive allylic alkylation (DCAA).

Remodeling natural products: chemistry and serine hydrolase activity of a rocaglate-derived β-lactone

Flavaglines are a class of natural products with potent insecticidal and anticancer activities. β-Lactones are a privileged structural motif found in both therapeutic agents and chemical probes. Herein, we report the synthesis, unexpected light-driven di-epimerization, and activity-based protein profiling of a novel rocaglate-derived β-lactone. In addition to in vitro inhibition of the serine hydrolases ABHD10 and ACOT1/2, the most potent β-lactone enantiomer was also found to inhibit these enzymes, as well as the serine peptidases CTSA and SCPEP1, in PC3 cells.