One-Pot Transformation of Simple Furans into Octahydroindole Scaffolds

The Rapid Synthesis of Colibactin Warhead Model Compounds Using New Metal-Free Photocatalytic Cyclopropanation Reactions Facilitates the Investigation of Biological Mechanisms

Herein, we report the synthesis of a series of colibactin warhead model compounds using two newly developed metal-free photocatalytic cyclopropanation reactions. These mild cyclopropanations expand the known applications of eosin within synthesis. A halogen atom transfer reaction mode has been harnessed so that dihalides can be used as the cyclopropanating agents. The colibactin warhead models were then used to provide new insight into two key mechanisms in colibactin chemistry. An explanation is provided for why the colibactin warhead sometimes undergoes a ring expansion-addition reaction to give fused cyclobutyl products while at other times nucleophiles add directly to the cyclopropyl unit (as when DNA adds to colibactin). Finally, we provide some evidence that Cu(II) chelated to colibactin may catalyze an important oxidation of the colibactin-DNA adduct. The Cu(I) generated as a result could then also play a role in inducing double strand breaks in DNA.

Synthesis of Pyrrolidine- and γ-Lactam-Containing Natural Products and Related Compounds from Pyrrole Scaffolds

Polycyclic alkaloid natural products featuring pyrrolidine and pyrrolidinone motifs remain enduring targets of total synthesis endeavors. Pyrrole and its derivatives have been exploited to access many such frameworks, including alkaloids belonging to the Aspidosperma, Stemona, and batzelladine families. In this article, a selection of exemplars that highlight the utility of pyrrole-based approaches to facilitate total syntheses of pyrrolidine- and pyrrolidinone-containing alkaloids and related molecules are showcased.

5-Hydroxy-pyrrolone based building blocks as maleimide alternatives for protein bioconjugation and single-site multi-functionalization

Recent dramatic expansion in potential uses of protein conjugates has fueled the development of a wide range of protein modification methods; however, the desirable single-site multi-functionalization of proteins has remained a particularly intransigent challenge. Herein, we present the application of 5-hydroxy-1,5-dihydro-2H-pyrrol-2-ones (5HP2Os) as advantageous alternatives to widely used maleimides for the chemo- and site-selective labeling of cysteine residues within proteins. A variety of 5HP2O building blocks have been synthesized using a one-pot photooxidation reaction starting from simple and readily accessible furans and using visible light and oxygen. These novel reagents display excellent cysteine selectivity and also yield thiol conjugates with superior stability. 5HP2O building blocks offer a unique opportunity to introduce multiple new functionalities into a protein at a single site and in a single step, thus, significantly enhancing the resultant conjugate's properties.

Multi-Photocatalyst Cascades: Merging Singlet Oxygen Photooxygenations with Photoredox Catalysis for the Synthesis of Alkaloid Frameworks

The development of photocascades that rapidly transform simple and readily accessible furan substrates into polycyclic alkaloid frameworks or erythrina natural products is described. Each of the sequences developed makes use of photocatalyzed energy transfer processes, which generate singlet oxygen, to set up the substrates for the second photocatalyzed reaction, wherein electron transfer generates carbon-centered radicals for the cyclizations that give the final complex frameworks. A chemical switch has been developed that can "switch off" one photocatalyst; thus, allowing a second photocatalyst to take over control of the sequence. As a corollary, this strategy represents the first time it has been possible to use multiple photocatalysts in photocascades, and, as such, it expands significantly the reactions that can be included in such cascades and the order in which they can be initiated.

ZnCl2 -Catalyzed [4+1] Annulation of Alkylthio-Substituted Enaminones and Enaminothiones with Sulfur Ylides

Concise construction of furan and thiophene units has played an important role in the synthesis of potentially bioactive compounds and functional materials. Herein, an efficient Lewis acid ZnCl₂ catalyzed [4+1] annulation of alkylthio-substituted enaminones is reported, that is, α-oxo ketene N,S-acetals with sulfur ylides to afford 2-acyl-3-aminofuran derivatives. In a similar fashion, [4+1] annulation of the corresponding enaminothiones, that is, α-thioxo ketene N,S-acetals, with sulfur ylides efficiently proceeded to give multisubstituted 3-aminothiophenes. This method features wide substrate scopes as well as broad functional group tolerance, offering a concise route to highly functionalized furans and thiophenes.

One-Pot Synthesis of Diverse γ-Lactam Scaffolds Facilitated by a Nebulizer-Based Continuous Flow Photoreactor

The use of a modified prototype continuous flow reactor (CFR) as a pivotal part of a number of versatile singlet oxygen-mediated reaction sequences is presented herein. These sequences target rapid access to structural complexity and diversity. The prototype reactor achieves high conversions and productivities by attaining large specific surface areas for these biphasic reactions. In the reactor, the reaction solution is nebulized (using either oxygen or air) and the resulting aerosol is irradiated by an LED jacket that surrounds the Pyrex reaction chamber. The one pot procedures developed herein are, according to many different criteria, both highly efficient and green. The key common intermediates and the source of both the complexity and variety of the final products are N-acyl imminium ions (NAI; protonated N-acyl enamines). The initial substrates are simple and readily accessible furans and the diverse array of products is composed of different complex γ-lactams. Many of the products are of particular interest due to their close relationships to known biologically active molecules.