Application of 1‐butyl‐3‐methylimidazolium hexafluorophosphate to Ir(I)‐catalyzed hydrogen isotope exchange labelling of substrates poorly soluble in dichloromethane

Substrate solubility remains a major limitation in Ir(I)‐catalyzed isotopic hydrogen exchange labelling. In the search for an alternative to the solvent dichloromethane, which is critical to the success of the reaction, we examined a series of ionic liquids for their suitability. Commercially available 1‐butyl‐3‐methylimidazolium hexafluorophosphate (abbreviated to [BMI][PF₆]) was found to support efficient deuterium and tritium exchange labelling of N‐(4‐methoxyphenyl)‐N‐methyl benzamide 1 under standard conditions. The solvent dissolves both polar hydroxyl and carboxylic acid substituted acetanilides, providing isotopomers in unprecedentedly high deuterium incorporation as compared to dichloromethane. We report the application of [BMI][PF₆] and its potential for extending the scope of Ir(I)‐catalyzed H/T exchange to more polar compounds. Copyright © 2003 John Wiley & Sons, Ltd.     

Unusual base‐catalyzed exchange in the synthesis of deuterated PF‐2413873a

The preparation of deuterated PF‐2413873 (4‐[3‐cyclopropyl‐1‐(methanesulfonylmethyl)‐5‐methyl‐1H‐pyrazol‐4‐yl]oxy‐2,6‐dimethylbenzonitrile, 1) is described for use as a bioanalytical standard in clinical trials. Two strategies were investigated. The sulfone‐containing substituent was labelled by base‐catalyzed exchange, but unacceptable deuterium loss was noted under assay conditions. Alternatively, labelling 4‐cyano‐3,5‐dimethylphenol was achieved by heating with deuterium oxide over platinum oxide. After building up the pyrazole ring we discovered that, during the subsequent alkylation to attach the methylthiomethyl group, the base, potassium t‐butoxide, caused unwanted scrambling of deuteriums on the aromatic portion and the methylthiomethyl group. Thus, it was necessary to remove all base‐labile hydrogens to prevent their exchange. This was accomplished by alkylating the pyrazole with per‐deuterated chloromethyl methylsulfide, oxidation to the sulfone, and selective removal of its deuteriums by treatment with sodium hydroxide. The unusual sensitivity and selectivity of these base‐promoted exchange reactions are discussed. Thus, 4‐[3‐cyclopropyl‐1‐(methanesulfonylmethyl)‐5‐methyl‐1H‐pyrazol‐4‐yl]oxy‐[²H₆]2,6‐dimethyl‐[3,5‐²H]benzonitrile (17) was obtained, labelled with eight deuterium atoms and an acceptable D₀/D₈ ratio. Copyright © 2009 John Wiley & Sons, Ltd.     

A simple method for α‐position deuterated carbonyl compounds with pyrrolidine as catalyst

A simple, cost‐effective method for deuteration of carbonyl compounds employing pyrrolidine as catalyst and D₂O as deuterium source was described. High degree of deuterium incorporation (up to 99%) and extensive functional group tolerance were achieved. It is the first time that secondary amines are used as catalysts for H/D exchange of carbonyl compounds, which also allow the deuteration of complex pharmaceutically interesting substrates. A possible catalytic mechanism, based on the hydrolysis of 1‐pyrrolidino‐1‐cyclohexene, for this pyrrolidine‐catalyzed H/D exchange reaction has been proposed.     

Convenient and efficient deuteration of functionalized aromatics with deuterium oxide: catalysis by cycloocta‐1,5‐dienyliridium(I) 1,3‐dionates

Aromatic compounds bearing an ortho‐directing substituent may be deuterated by exchange with deuterium oxide in the presence of a range of cycloocta‐1,5‐dienyliridium(I)1,3‐dionate catalysts. The exchange takes place in several dipolar aprotic solvents and is directly applicable to the deuteration of polar compounds. Isotope incorporation is efficient and regiospecific. The method is applicable to a wide range of ortho‐directing groups some of which are only weak directors for alternative ortho‐labelling approaches. In addition, the application of microwaves enables labelling within minutes even with sub‐stituents which are poor directors. Copyright © 2003 John Wiley & Sons, Ltd.     

Buscopan labeled with carbon‐14 and deuterium

Hyosine butyl bromide, the active ingredient in Buscopan, is an anticholinergic and antimuscarinic drug used to treat pain and discomfort caused by abdominal cramps. A straightforward synthesis of carbon‐14– and deuterium‐labeled Buscopan was developed using scopolamine, n‐butyl‐1‐¹⁴C bromide, and n‐butyl‐²H₉ bromide, respectively. In a second carbon‐14 synthesis, the radioactive carbon was incorporated in the tropic acid moiety to follow its metabolism. Herein, we describe the detailed preparations of carbon‐14– and deuterium‐labeled Buscopan.     

Proton and metal‐ion activation of C–H exchange in five‐membered azoles

Factors influencing C–H isotopic exchange rates in five‐membered azoles, that is imidazoles and thiazoles, under catalysis by H⁺ and Mⁿ⁺, especially transition metals, Pt(II) and Co(III) are discussed. Hydrogen ion catalysis through N(3) protonation of azoles 1–3 is generally the most efficient, with rate enhancements in the range 10²–10⁹ over the neutral process being attained. Metal‐ion coordination also results in effective catalysis, though less so than catalysis by protons. Catalysis of C–H exchange by Mⁿ⁺ can be studied through addition of the metal salts to a buffered solution of the heterocycle in which labile complexes exist, or on synthesized complexes such as 4–13 which are substitution‐inert thus precluding complications from unknown dissociation equilibria. A delicate balance of factors influence the ease of C–H exchange, including: (1) the magnitude of the fractional charge located at N(3) of the heterocycle through Mⁿ⁺–N(3) σ bond polarization; (2) metal‐to‐ligand π back‐bonding; (3) the electronic structure of the metal ions. These considerations have obvious consequences for deuterium‐ and tritium‐labelling of a number of biomolecules, e.g. proteins, enzymes, nucleic acids, some vitamins, as well as drugs which incorporate five‐membered azoles in their structures. Copyright © 2002 John Wiley & Sons, Ltd.     

Iridium‐mediated β‐deuteration of enones

Exposure of captodative enone systems to deuterium in the presence of Crabtree's catalyst ( 1 ) results in deuteration at the vinylic site β‐ to the ketone carbonyl, as well as at any accessible ortho‐position. β‐exchange is also observed during the reduction of ethyl cinnamate ( 3 ) catalyzed by 1 . Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis of [2H]‐labelled phase‐I‐metabolites using 1‐[2H]‐pyridinium hydrochloride

Simultaneous O‐demethylation and hydrogen/deuterium exchange of aryl‐methyl‐ethers can be obtained using 1‐[²H]‐pyridinium hydrochloride as reagent at 220°C for 6 h. This reaction was applied to dextromethorphan and various non‐steroidal anti‐inflammatory drugs. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis of deuterated 5‐n‐alkylresorcinols

Four alternative strategies for the preparation of deuterium poly‐labelled 5‐n‐alkylresorcinols are explored. Ring‐labelled ²H₃‐alkylresorcinols synthesized by acidic H/D exchange are stable under electrospray ionization MS conditions but scrambling occurs in electron bombardment ionization MS. Side chain‐labelled ²H₄‐derivatives prepared by two different total synthesis approaches are contaminated by isotopologues with varying number of deuterium labels due to H/D redistribution and exchange during D₂ gas deuterogenation. The derivative carrying an ω‐²H₃ label is isotopically pure and completely stable under all relevant analytical conditions encountered in quantitation work. Copyright © 2008 John Wiley & Sons, Ltd.     

Deuterium labelling of theaflavin

An efficient deuterium labelling of theaflavin, aiming at radiolabelling of theaflavin, was accomplished for the first time by using 4‐deuterium‐labelled epi‐catechin as the precursor and the resulting D ‐labelled theaflavin was fully characterized by ¹H NMR, ¹³C NMR, IR spectroscopy and ESI‐MS. The synthesis features a novel efficient polyphenol oxidase (PPO) catalyzed benzotropolone formation. PPO was bound onto a selected polymer by a novel efficient method developed in this lab. Copyright © 2009 John Wiley & Sons, Ltd.     

A two‐step synthesis of deuterium labeled 8, 8, 9, 9‐d4‐hexadecane from nonanoic acid

Labeled compounds are essential in elucidating metabolic mechanisms and reaction pathways. A two‐step synthesis of deuterium‐labeled 8, 8, 9, 9‐d₄‐hexadecane from nonanoic acid is described here. The synthesis procedures involved hydrogen–deuterium exchange of nonanoic acid with 3.00 m DCl‐D₂O and then Kolbe electrolysis of the deuterated nonanoic acid to achieve the desired n‐alkane that was confirmed by gas chromatography‐mass spectrometry and ¹H nuclear magnetic resonance. This method might provide an alternative route for the preparation of specifically deuterated alkanes of different chain lengths (C > 4) in which deuterium atoms are located at two adjacent carbons of the alkane's carbon chain. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis of sequentially deuterated 1‐n‐Butyl‐3‐methylimidazolium ionic liquids

Deuterium isotopologues of the ionic liquid (IL) 1–n‐butyl‐3‐methylimidazolium chloride ([C₄mim]Cl) sequentially labeled on the C‐1″, C‐1′, C‐2′, C‐3′, and C‐4′ positions of the N‐alkyl groups were prepared following a strategy that minimizes the number of distinct reactions through the use of analogous synthetic routes. In several cases, good yields after the initial deuterium incorporation reaction were achieved by combining well‐established chemical transformations into efficient single‐step processes. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of a P,N‐ligated iridium(I) catalyst in hydrogen isotope exchange reactions of aryl and heteroaryl compounds

We have developed a novel and efficient iridium‐catalyzed hydrogen isotope exchange reaction method with secondary and tertiary sulfonamides at ambient temperatures. Furthermore N‐oxides and phosphonamides have been successfully applied in hydrogen isotope exchange reactions with moderate to excellent deuterium introduction.     

Labelling of anilines,benzylamines and some N‐heterocyclics using cycloocta‐1,5‐dienyliridium(I)‐1,1,1,5,5,5‐hexafluoro‐pentan‐2,4‐dionate and isotopic hydrogen gas in DMF or DMA

A wide range of anilines, benzylamines and some N‐heterocyclics can be ortho‐deuterated at room temperature using deuterium gas and cycloocta‐1,5‐dienyliridium(I)‐1,1,1,5,5,5‐hexafluoropentan‐2,4‐dionate in DMF or DMA. The method is applicable to labelling with tritium. Copyright © 2004 John Wiley & Sons, Ltd.     

Synthesis and selective 2H‐, 13C‐, and 15N‐labeling of the Tau protein binder THK‐523

A new synthetic route to the Tau binder, THK‐523, is disclosed herein, which can easily be adapted to ¹³C‐ and D‐isotope labeling. The synthesis proceeds via two key reactions, namely, a Pd‐catalyzed carbonylative Sonogashira coupling and a reductive ring‐closing step with hydrogen or deuterium gas. By carrying out these reactions in a 2‐chamber reactor we reported previously, ex situ–generated carbon monoxide and hydrogen/deuterium can be applied in stoichiometric quantities, thereby facilitating isotope labeling of this Tau‐binding compound. Iridium‐catalyzed hydrogen isotope exchange (HIE) reactions were performed on THK‐523 and its ¹³C‐labeled analog providing access to 4 additional analogues labeled with deuterium as well. Finally, by applying a Buchwald‐Hartwig coupling, we were able to prepare a ¹⁵N‐THK‐523 variant with the isotope label in the quinoline ring system.     

Base‐catalyzed deuterium and tritium labelling of 1‐biphenyl‐4‐ylpropane‐1,2‐dione and deuteration of aryl methyl ketones

A synthesis and a base‐catalyzed exchange reaction was developed under mild conditions to deuterate and subsequently tritiate the methyl group of the base sensitive diketone 1‐biphenyl‐4‐ylpropane‐l,2‐dione depicted in Figure 1. Using Et₃N as base, deuterium incorporation of the methyl group was 88.9% and the tritium incorporation gave a specific radioactivity of 119 mCi/mmol. The scope of the exchanges was extended to methyl aryl ketones 2–4 . Copyright © 2004 John Wiley & Sons, Ltd.     

Radiobromination of closo‐carboranes using palladium‐catalyzed halogen exchange

closo‐Carborane derivatives are often proposed as boron carriers for use in boron neutron capture therapy (BNCT) of cancer. A positron emitting radiolabel on a boron atom in such carborane compounds might facilitate pharmacokinetic studies in patients. In this paper the four iodo‐closo‐carboranes, namely 3‐iodo‐ortho‐carborane, 9‐iodo‐ortho‐carborane, 9‐iodo‐meta‐carborane and 2‐iodo‐para‐carborane were chosen as model compounds in a study of [⁷⁶Br]bromine labelling of carboranes using palladium‐catalyzed halogen exchange. It was found that within a reaction time of 40 min, the four compounds were all radio‐brominated in good to excellent yield, using Herrmann's catalyst (HC) in toluene. Copyright © 2005 John Wiley & Sons, Ltd.     

Efficient tritiation of the translocator protein (18 kDa) selective ligand DPA‐714

DPA‐714 (N,N‐diethyl‐2‐(2‐(4‐(2‐fluoroethoxy)phenyl)‐5,7‐dimethylpyrazolo[1,5‐a]pyrimidin‐3‐yl)acetamide) is a recently discovered fluorinated ligand of the translocator protein 18 kDa (TSPO). Labelled with the short‐lived positron emitter fluorine‐18, this structure is today the radioligand of reference for in vivo imaging of microglia activation and neuroinflammatory processes with positron emission tomography. In the present work, an isotopically tritium‐labelled version was developed ([³H]DPA‐714), in order to access high resolution in vitro and ex vivo microscopic autoradiography studies, repeated and long‐lasting receptor binding studies and in vivo pharmacokinetic determination at late time points. Briefly, DPA‐714 as reference, and its 3,5‐dibrominated derivative as precursor for labelling, were both prepared from DPA‐713 in nonoptimized 32% (two steps) and 10% (three steps) yields, respectively. Reductive debromination using deuterium gas and Pd/C as catalyst in methanol, performed at the micromolar scale, confirmed the regioselective introduction of two deuterium atoms at the meta positions of the phenyl ring. Tritiodebromination was analogously performed using no‐carrier tritium gas. HPLC purification provided >96% radiochemically pure [³H]DPA‐714 (7 GBq) with a 2.1 TBq/mmol specific radioactivity. Interestingly, additional hydrogen‐for‐tritium exchanges were also observed at the 5‐methyl and 7‐methyl positions of the pyrazolo[1,5‐a]pyrimidine, opening novel perspectives in the labelling of compounds featuring this heterocyclic core.     

Synthesis of uniformly deuterated n‐dodecyl‐β‐d‐maltoside (d39‐DDM) for solubilization of membrane proteins in TROSY NMR experiments

This work reports the first synthesis of uniformly deuterated n‐dodecyl‐β‐d ‐maltoside (d₃₉‐DDM). DDM is a mild non‐ionic detergent often used in the extraction and purification of membrane proteins and for solubilizing them in experimental studies of their structure, dynamics and binding of ligands. We required d₃₉‐DDM for solubilizing large α‐helical membrane proteins in samples for [¹⁵N–¹H]TROSY (transverse relaxation‐optimized spectroscopy) NMR experiments to achieve the highest sensitivity and best resolved spectra possible. Our synthesis of d₃₉‐DDM used d₇‐d ‐glucose and d₂₅‐n‐dodecanol to introduce deuterium labelling into both the maltoside and dodecyl moieties, respectively. Two glucose molecules, one converted to a glycosyl acceptor with a free C4 hydroxyl group and one converted to a glycosyl donor substituted at C1 with a bromine in the α‐configuration, were coupled together with an α(1 → 4) glycosidic bond to give maltose, which was then coupled with n‐dodecanol by its substitution of a C1 bromine in the α‐configuration to give DDM. ¹H NMR spectra were used to confirm a high level of deuteration in the synthesized d₃₉‐DDM and to demonstrate its use in eliminating interfering signals from TROSY NMR spectra of a 52‐kDa sugar transport protein solubilized in DDM.     

Synthesis of selectively labeled histidine and its methylderivatives with deuterium,tritium, and carbon‐14

Isotopologues of l ‐histidine and its N‐methylderivatives labeled with deuterium and tritium at the 5‐position in the imidazole ring were obtained using the isotope exchange method. The deuterium‐labeled isotopologues [5‐²H]‐l ‐histidine, [5‐²H]‐N^τ‐methyl‐l ‐histidine, [5‐²H]‐N^π‐methyl‐l ‐histidine, and [2,5‐²H₂]‐l ‐histidine were synthesized by isotope exchange method carried out in a fully deuterated medium with. The same reaction conditions were applied to synthesize [5‐³H]‐N^τ‐methyl‐l ‐histidine, [5‐³H]‐N^π‐methyl‐l ‐histidine, and [5‐³H]‐l ‐histidine with specific activity of 2.0, 5.0, and 2.6 MBq/mmol, respectively. The N^π‐[methyl‐¹⁴C]‐histamine was obtained with specific activity of 0.23 MBq/mmol in a one‐step reaction by the direct methylation of histamine by [¹⁴C]iodomethane.