Hydrogen-Bonding-Induced Fluorescence: Water-Soluble and Polarity-Independent Solvatochromic Fluorophores

Fluorophores with emission wavelengths that shift depending on their hydrogen-bonding microenvironment in water would be fascinating tools for the study of biological events. Herein we describe the design and synthesis of a series of water-soluble solvatochromic fluorophores, 2,5-bis­(oligoethylene glycol)­oxybenzaldehydes (<b>8</b>–<b>11</b>) and 2,5-bis­(oligoethylene glycol)­oxy-1,4-dibenzaldehydes (<b>14</b>–<b>17</b>), based on a push–pull strategy. Unlike typical examples in this class of fluorophores, the fluorescence properties of these compounds are independent of solvent polarity and become fluorescent upon intermolecular hydrogen-bonding, exhibiting high quantum yields (up to ϕ = 0.55) and large Stokes shifts (up to 134 nm). Furthermore, their emission wavelengths change depending on their hydrogen-bonding environment. The described fluorophores provide a starting point for unprecedented applications in the fields of chemical biology and medicinal chemistry

Understanding the Reactivity of Enol Ether Radical Cations: Investigation of Anodic Four-Membered Carbon Ring Formation

The reactivity of enol ether radical cations was investigated in anodic four-membered carbon ring formations, advancing the mechanistic understanding of these reactions. The mono-ring-containing aromatic cations were reduced through inter- or intramolecular electron transfer to give mono- or bis-ring-containing compounds, respectively. Small structural changes in the hydrocarbon linkers tethering two aromatic rings exerted a powerful effect on the efficiency of such electron transfer events

Soluble Tag-Assisted Peptide Head-to-Tail Cyclization: Total Synthesis of Mahafacyclin B

A soluble tag-assisted liquid-phase method was successfully applied to peptide head-to-tail cyclization, leading to the total synthesis of antimalarial cyclic heptapeptide, mahafacyclin B (<b>1</b>). The cyclization was carried out in the liquid phase with the tag remaining, which allowed rapid reaction workup and product isolation

Total Synthesis of Elastin Peptide Using High Pressure–Liquid Phase Synthesis Assisted by a Soluble Tag Strategy

A highly aggregating elastin peptide was prepared efficiently using a high pressure–liquid phase synthesis approach assisted by a soluble tag strategy. Two standard syringes were connected to each other to construct a reactor. This simple reactor was used to apply high pressure to the highly viscous reaction mixture thereby maintaining its fluidity. The reactions were completely inhibited due to aggregation when conducted in a standard flask reactor, whereas our high pressure approach accelerated the couplings to realize complete conversion within 5–7 min. All steps were conducted at 0.10 M concentration, affording grams of the desired product

Role of Al-Based Additives in Controlling Ash Adhesion

In combustion plants, ash particles adhere to the plant walls because of liquid bridging induced by the melting of alkali-metal compounds. The presence of adhered ash particles can hinder the efficient and stable operation of the plant. Therefore, adopting appropriate adhesion control methods that suit the combustion conditions is critical. Additives, which are one method to deal with ash adhesion, increase the slag formation temperature and prevent the formation of liquid bridges by changing the composition of the ash. However, with the recent expansion of biomass utilization, various types of ash are generated, and additives that are effective irrespective of the ash composition are desirable. This study aims to investigate the effect of three different Al salts on adhesion reduction using synthetic ashes that exhibit high-temperature adhesion caused by the melting of Na and K components. The study revealed the influence of the counteranions of the Al salts on the adhesion of ash particles. The results indicate that the pyrolysis temperature for the Al salts and the reactivity of the heat treatment intermediates play critical roles in controlling adhesion

Phosphorus-Related Ash Chemistry at High Temperatures: Role of Aluminum on Particle Adhesion

Phosphorus-containing ash generated from the thermochemical conversion of biomass is expected to be applied as a phosphorus resource. Handling of ash is important in biomass utilization. In this study, the effect of Al, a coexisting element in ash, on P-related particle adhesion at high temperatures was investigated using synthetic ashes, which are model compounds of ashes, to understand phosphorus-related ash chemistry. Synthetic ashes containing P, Al and Si (P–Al–Si) were prepared, and their adhesiveness was quantified as tensile strength at 500–900 °C. Below 20 wt % of P, tensile strength increased slightly with increasing P concentration. At P concentrations of 20 wt % or higher, the tensile strength increased rapidly with increasing P concentration. P easily reacted with Al derived from ash, resulting in the formation of P–Al or P–Al–Si compounds. Since the slag formation temperatures of these systems were higher than that of the P–Si system, Al in P-containing ash suppressed the increase in adhesion. On the other hand, excess P in P–Al–Si synthetic ashes increased particle adhesion due to the formation of P–Si compounds. Addition of Al2O3 nanoparticles showed the strongest effect for decreasing adhesion because the formation of a P–Al phase was promoted

TiO₂ Photocatalysis in Aromatic “Redox Tag”-Guided Intermolecular Formal [2 + 2] Cycloadditions

Since the pioneering work by Macmillan, Yoon, and Stephenson, homogeneous photoredox catalysis has occupied a central place in new reaction development in the field of organic chemistry. While heterogeneous semiconductor photocatalysis has also been studied extensively, it has generally been recognized as a redox option in inorganic chemistry where such “photocatalysis” is most often used to catalyze carbon–carbon bond <i>cleavage</i> and not in organic chemistry where bond <i>formation</i> is usually the focal point. Herein, we demonstrate that titanium dioxide photocatalysis is a powerful redox option to construct carbon–carbon bonds by using intermolecular formal [2 + 2] cycloadditions as models. Synergy between excited electrons and holes generated upon irradiation is expected to promote the overall net redox neutral process. Key for the successful application is the use of a lithium perchlorate/nitromethane electrolyte solution, which exhibits remarkable Lewis acidity to facilitate the reactions of carbon-centered radical cations with carbon nucleophiles. The reaction mechanism is reasonably understood based on both intermolecular and intramolecular single electron transfer regulated by an aromatic “redox tag”. Most of the reactions were completed in less than 30 min even in aqueous and/or aerobic conditions without the need for sacrificial reducing or oxidizing substrates generally required for homogeneous photoredox catalysis

Soluble-support-assisted Electrochemical Reactions: Application to Anodic Disulfide Bond Formation

A soluble-support-assisted technique was successfully applied to electrochemical reactions, leading to anodic disulfide bond formation. The support-bound peptide was soluble in electrolyte solution, allowing electron transfer at the surface of the electrodes. After completion of the reaction, the support-bound product was recovered as a precipitate by simple dilution of the reaction mixture with poor solvent

Acid-Triggered Colorimetric Hydrophobic Benzyl Alcohols for Soluble Tag-Assisted Liquid-Phase Synthesis

Simple screening of acid-triggered reactions of methoxybenzyl alcohols led to the development of a novel colorimetric hydrophobic benzyl alcohol (HBA) tag. HBA tag-3 (<b>14</b>) retained high solubility in less polar solvents and excellent precipitation properties in polar solvents. Our routine procedure for tag-assisted liquid phase peptide synthesis was applied using HBA tag-3 (<b>14</b>), and an effective synthesis of β-sheet breaker peptide iAβ5 (<b>4</b>) was achieved. The tagged peptides showed a vivid blue color under acidic conditions both on TLC plates and in solution, enabling quantitative assay

Anodic Substitution Reaction of Proline Derivatives Using the 2,4,6-Trimethoxyphenyl Leaving Group

An efficient method for modifying a proline moiety through anodic carbon–carbon bond cleavage is developed. Use of the 2,4,6-trimethoxyphenyl (TMP) moiety as a leaving group at the 5-position allows the incorporation of various functional groups for modification in both the <i>N</i>- and <i>C</i>-terminal direction due to the stability of the N1–C5–C linkage. This approach also enables anodic substitution reactions using reactants with lower oxidation potential compared to <i>N</i>-carbonyl bonds