Modelling flavoenzymatic charge transfer events: development of catalytic indole deuteration strategies

Au(I)-Catalyzed Regioselective Hydrogen Isotope Labeling of Indoles

The gold(I)-catalyzed hydrogen isotope exchange reaction on indoles and related heterocycles is described under mild conditions and low catalyst loadings, using CD3OD and D2O as readily available deuterium sources. C3-unsubstituted indoles are labeled at the C3 position with exquisite regioselectivity, while C3-substituted indoles are labeled at the C2 position. The method is also applicable to the regioselective tritiation of indoles. Mechanistic studies revealed the involvement of aurated indoles as key intermediates.

Programmable Deuteration of Indoles via Reverse Deuterium Exchange

Methods for selective deuterium incorporation into drug-like molecules have become extremely valuable due to the commercial, mechanistic, and biological importance of deuterated compounds. Herein, we report a programmable labeling platform that allows access to C2, C3, or C2- and C3-deuterated indoles under mild, user-friendly conditions. The C2-deuterated indoles are accessed using a reverse hydrogen isotope exchange strategy which represents the first non-directed C2-deuteration of indoles.

Deuteration of Indole Compounds: Synthesis of Deuterated Auxins, Indole-3-acetic Acid-d5 and Indole-3-butyric Acid-d5

In this study, we describe a practical and facile synthesis of deuterium-labeled indoles via acid-catalyzed hydrogen-deuterium exchange. 3-Substituted indoles were efficiently deuterated through treatment with 20 wt % D₂SO₄ in CD₃OD at 60-90 °C. A deuterium incorporation reaction of 3-unsubstituted indoles was accomplished through treatment with CD₃CO₂D at 150 °C. The in situ preparation of a 20 wt % D₂SO₄/CH₃OD/D₂O solution enabled a large-scale and low-cost synthesis of auxins, indole-3-acetic acid-d5 and indole-3-butyric acid-d5.

Spontaneous conversion of prenyl halides to acids: application in metal-free preparation of deuterated compounds under mild conditions

Here we reveal a simple generation of deuterium halide (DX) from common and inexpensive reagents readily available in a synthetic chemistry laboratory, i.e. prenyl-, allyl-, and propargyl halides, under mild conditions. We envisaged that in situ generation of an acid, deuterium halide, would be useful for acid-catalyzed reactions and could be employed for organocatalytic deuteration. The present work reports a metal-free method for deuterium labeling covering a broad range of substrate including phenolic compounds (i.e. flavonoids and stilbenes), indoles, pyrroles, carbonyl compounds, and steroids. This method was also applied for commonly used drugs such as loxoprofen, haloperidol, stanolone, progesterone, androstenedione, donepezil, ketorolac, adrenosterone, cortisone, pregnenolone, and dexamethasone. A gram-scale chromatography-free synthesis of some deuterated compounds is demonstrated in this work. This work provides a simple, clean and by-product-free, site-selective deuteration, and the deuterated products are obtained without chromatographic separation. When applying these initiators for other acid-catalyzed reactions, the deuterium isotope effects of DX may provide products which are different from those obtained from reactions using common acids. Although the mechanism of the spontaneous transformation of prenyl halides to acid is unclear, this overlooked chemistry may be useful for many reactions.

Comparison of riboflavin-derived flavinium salts applied to catalytic H2O2 oxidations

A series of flavinium salts, 5-ethylisoalloxazinium, 5-ethylalloxazinium, and 1,10-ethylene-bridged alloxazinium triflates, were prepared from commercially available riboflavin. This study presents a comparison between their optical and redox properties, and their catalytic activity in H2O2 oxidations of sulfide, tertiary amine, and cyclobutanone. Reflecting the difference between the π-conjugated ring structures, the flavinium salts displayed very different redox properties, with reduction potentials in the order of: 5-ethylisoalloxazinium > 5-ethylalloxazinium > 1,10-ethylene-bridged alloxazinium. A comparison of their catalytic activity revealed that 5-ethylisoalloxazinium triflate specifically oxidises sulfide and cyclobutanone, and 5-ethylalloxazinium triflate smoothly oxidises tertiary amine. 1,10-Bridged alloxazinium triflate, which can be readily obtained from riboflavin in large quantities, showed moderate catalytic activity for the H2O2 oxidation of sulfide and cyclobutanone.