Alloxazinium-Resins as Readily Available and Reusable Oxidation Catalysts

N5-Modified alloxazinium salts including 5-ethyl-1,3-dimethylalloxazinium and 5-ethyl-1,3-dimethyl-8-(trifluoromethyl)alloxazinium salts were readily prepared as alloxazinium-resins from the corresponding N5-unmodified ingredients via the aerobic oxidation—ion exchange protocol, previously introduced by us for the preparation of isoalloxazine analogues, and their catalysis and reusability in H2O2 oxidations were evaluated

Efficient Use of Photons in Photoredox/Enamine Dual Catalysis with a Peptide-Bridged Flavin–Amine Hybrid

An isoalloxazine (flavin) ring system and a secondary amine have been integrated through a short peptide linker with the aim of using photons as efficient as possible in photoredox/enamine dual catalysis. We herein report a peptide-bridged flavin-amine hybrid that can catalyze α-oxyamination of aldehydes with TEMPO under weak blue light irradiation to achieve an extremely high quantum yield of reaction (Φ = 0.80)

Synthesis of Optically Active Polyguanidines by Polyaddition Reaction of Biscarbodiimides with Chiral Diamines

Herein, we present the first study on the polyaddition reaction of biscarbodiimides with chiral diamines, which focuses on a definite case using optically active trans-4a,8a-decahydroquinoxaline and 1,4-phenylenebis(arylcarbodiimide)s, which readily react with each other under ambient and catalyst-free conditions. The specific reactivity allows for facile access to not only the corresponding chiral polyguanidines under balanced stoichiometry but also their oligomeric analogues under imbalanced stoichiometry via a step-by-step procedure. Spectroscopic, chromatographic, and computational characterization of the novel molecular chains containing arrayed guanidines have revealed their structural, optical, and conformational properties as well as the mechanism of polymerization assisted by molecular association. Their potential use as asymmetric catalysts is also described

Noncovalent Modification Strategy with Achiral Phosphoric Acid Diesters for Designing a Chiral Brønsted Base Organocatalyst

A strategy for designing chiral Brønsted base organocatalysts through noncovalent modification of a chiral dibasic molecule with an achiral phosphoric acid diester is introduced for the first time. Such a molecular modification concept utilizing acid-base interactions may facilitate the on-demand design of asymmetric organocatalysts, as preliminarily demonstrated in this work

Enzyme-like Regiodivergent Behavior of a Flavopeptide Catalyst in Aerobic Baeyer-Villiger Oxidation

We recently developed a flavopeptide immobilized on polystyrene resin, Fl-Pep-PS, that could realize the first N5-unmodified neutral flavin (Fl)-catalyzed aerobic oxygenation reactions under non-enzymatic conditions. Although a key active species is assumed to be the corresponding 4a-hydroperoxyflavin (Fl4aOOH) from the unprecedented activity and unique chemoselectivity, further circumstantial support would be helpful to be sure since spectroscopic evidence is difficult to obtain due to the compound’s insolubility. In this article, we report that the aerobic Baeyer-Villiger oxidation of a fused cyclobutanone, (±)-cis-bicyclo[3.2.0]hept-2-en-6-one (1), can be promoted with Fl-Pep-PS in a FMO-like chemoselectivity and regiodivergent manner via Fl-related catalytic intermediates, which delivers strong evidence of the involvement of Fl4aOOH as an active species in Fl-Pep-PS-catalyzed aerobic oxygenation reactions

Nucleophilic Addition to Nitrones Using a Flow Microreactor

Nucleophilic addition reactions of soft carbon nucleophiles to nitrones in a flow microreactor are reported for the first time. Under microflow conditions at 30 °C to 0 °C, a range of nitrones can be efficiently transformed into the corresponding oxyiminium ions by reacting with either acyl halides or trialkylsilyl triflates, which can subsequently undergo the addition of nucleophiles including allyltributylstannane, ketene methyl tert-butyldimethylsilyl acetal, and N-silyl ketene imines to afford the corresponding adducts in high yields, while such reactions at a similar temperature under batch conditions have resulted in lowering the yields due to undesired side reactions

Greener Preparation of 5-Ethyl-4a-hydroxyisoalloxazine and Its Use for Catalytic Aerobic Oxygenations

Isoalloxazine ring systems are found in flavin cofactors in nature, and the simulation of their redox catalyses is an important task for developing sustainable catalytic oxidation reactions. Although 5- ethyl-4a-hydroxyisoalloxazines are among the most promising candidates as catalyst for such purposes, the use of them for laboratorial as well as industrial synthetic chemistry has so far been quite limited presumably due to the lack of their preparation methods readily, safely, and inexpensively available. In this communication, we introduce an environmentally benign and practical preparation of 5-ethyl-4a-hydroxy-3,7,8,10-tetramethylisoalloxazine (1EtOH) from 3,7,8,10-tetramethylisoalloxazine (1), in which conventional synthetic requirements, including (i) operations under inert conditions, (ii) risky or expensive chemicals, and (iii) isolation of labile intermediates, have all been dissolved. In addition, we have presented that 1EtOH could be an effective catalyst for Baeyer-Villiger oxidation as well as sulfoxidation with molecular oxygen (O2) as a terminal oxidant under suitable conditions, which is the first report on aerobic oxygenations catalyzed by 5-alkyl-4a-hydroxyisoalloxazines

An uncommon use of irradiated flavins : Brønsted acid catalysis

We present that thioacetalization of aldehydes can be induced by blue light irradiation in the presence of a catalytic amount of riboflavin tetraacetate (RFTA) under aerobic conditions. Several control experiments have suggested that the reaction is more likely to be catalyzed by acidic species generated in situ during the light irradiation. We have proposed that single electron transfer from a thiol (RSH) to the excited state of RFTA can take place to give a one-electron oxidized thiol (RSH+•) and the one-electron reduced RFTA (RFTA–•), which can be trapped by molecular oxygen to be stabilized as Brønsted acids including the protonated RFTA–• (RFTAH•). Finally, we have demonstrated that such acidic species can be prepared in advance as a solution and used as Brønsted acid catalysts for not only the thioacetalization but also Mannich-type reactions

ユウキ ナノ コウブンシ コウゾウタイ ノ ソウセイ デヒドロアラニン ポリマーゲル ノ ポーラス コウゾウ

A thermoresponsive hydrogel based on poly(methyl 2-isobutyramidoacrylate) (PMIBA) was prepared by free-radical polymerization of the corresponding monomer in the presence of a bis(dehydroalanine) derivative. The PMIBA hydrogel exhibited a volume phase transition within a narrow temperature range around 19 °C. In the equilibrium deswollen state above the transition temperature (Tc), the PMIBA gel contained a certain amount of water, several times the weight of the dry gel. A microscope observation of the equilibrium deswollen PMIBA gel revealed that it has a macroporous structure with 20-50 μm pores. The macroporous nature of the deswollen PMIBA gel resulted in smooth and rapid shrinking when the external temperature was jumped across the Tc