Methylene Blue as a Photosensitizer and Redox Agent: Synthesis of 5-Hydroxy-1H-pyrrol-2(5H)-ones from Furans

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.

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.

Visible light-mediated applications of methylene blue in organic synthesis

This review presents the manipulation of methylene blue in visible-light-assisted organic synthesis.

Photochemical strategies for the green synthesis of ultrathin Au nanosheets using photoinduced free radical generation and their catalytic properties

Two-dimensional gold nanosheets represent a class of materials with excellent chemical and structural properties, which are often prepared using a template or toxic CO in organic solvents. Here, we report methylene blue (MB) radicals as a reducing agent to grow freestanding hexagonal ultrathin Au nanosheets with well-tuned thicknesses in water. This is the first time that carbon organic radicals have been used as a reducing agent in metal nanosheet synthesis. Notably, no template is used throughout the synthesis process, and the yield of Au nanosheets is very high. It is found that MB is decisive in the growth of Au nanosheets because no Au nanosheets are obtained in the absence of MB with the same reaction parameters. The resulting nanosheets exhibit excellent catalytic activity during H₂O₂ decomposition to generate nontoxic O₂. Thus, folic acid-conjugated oxygen generating nanosheets could detect cancer cells in serum samples with high sensitivity through pressure signals. Furthermore, the nanosheets exhibit highly efficient activity and selectivity toward the hydrogenation of α,β-unsaturated aldehydes. We anticipate that using MB radicals for the high-yield synthesis of 2D materials in this unique system has demonstrated their effectiveness and provides a green alternative route for producing other 2D nanomaterials.

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.

Singlet Oxygen-Induced Furan Oxidation for Site-Specific and Chemoselective Peptide Ligation

A novel chemoselective ligation methodology has been developed for the facile construction of peptide-based fluorescent probes. Furan-containing peptides were activated by singlet oxygen and covalently engaged by nitrogen nucleophiles to yield stable conjugates. Singlet oxygen was compatible with sensitive amino acid residues within the peptides and a range of fluorophores, bearing different functionalities, were successfully incorporated, illustrating the broad scope of the developed strategy.