Meta‐substituent effects on organoiridium‐catalyzed ortho‐hydrogen isotope exchange

Reports in the literature appear to differ on the effects of some C3 substituents on the relative efficiencies of isotope exchange in the nonidentical C2‐ and C6‐positions catalyzed by organoiridium complexes. Controlled experiments were conducted using a set of model substrates in attempts to clarify these effects. The results clearly showed that, in common with most previous findings, alkyl substituents at C3 reduced the rate of isotope incorporation into C2 relative to C6, as expected on steric grounds. In contrast, all substituents possessing electron lone pairs resulted in a lessening of the inhibition of C2‐vs‐C6 labeling or promoted C2 labeling to such a degree that it became faster than that at C6. NMR measurements on equimolar mixtures of active iridium complex with selected substrates revealed that the ratios of C2‐ and C6‐iridacycles present in solution correlated with the relative rates of ortho‐deuteration in the rate studies. The results of the two studies, taken together, suggest that conventional explanations for the origin of the positive meta‐effect may not be adequate for the present system. An alternative hypothesis is advanced. Copyright © 2009 John Wiley & Sons, Ltd.     

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.     

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.     

Synthesis of [15N]t‐butylamine hydrochloride

This report presents an efficient synthesis of [¹⁵N]t‐butylamine hydrochloride. Acylation of [¹⁵N]ammonia with pivaloyl chloride provided [¹⁵N]pivalamide, this was converted to benzyl [¹⁵N]N‐t‐butylcarbamate through a Hofmann rearrangement. Hydrogenolysis of benzyl [¹⁵N]N‐t‐buylcarbamate and acidification afforded [¹⁵N]t‐butylamine hydrochloride in an overall yield of 79.2% in four steps. Copyright © 2010 John Wiley & Sons, Ltd.     

Bimodal t‐ratios: the impact of thick tails on inference

Summary This paper studies the distribution of the classical t‐ratio with data generated from distributions with no finite moments and shows how classical testing is affected by bimodality. A key condition in generating bimodality is independence of the observations in the underlying data‐generating process (DGP). The paper highlights the strikingly different implications of lack of correlation versus statistical independence in DGPs with infinite moments and shows how standard inference can be invalidated in such cases, thereby pointing to the need for adapting estimation and inference procedures to the special problems induced by thick‐tailed (TT) distributions. The paper presents theoretical results for the Cauchy case and develops a new distribution termed the ‘double‐Pareto’, which allows the thickness of the tails and the existence of moments to be determined parametrically. It also investigates the relative importance of tail thickness in case of finite moments by using TT distributions truncated on a compact support, showing that bimodality can persist even in such cases. Simulation results highlight the dangers of relying on naive testing in the face of TT distributions. Novel density estimation kernel methods are employed, given that our theoretical results yield cases that exhibit density discontinuities.     

Synthesis of highly pure 14C‐labelled DL‐allantoin and 13C NMR analysis of labelling integrity

A number of synthetic approaches are assessed to prepare allantoin labelled with ¹⁴C given certain requirements and technical limitations. A method that fulfils these criteria is described to achieve the synthesis of highly pure ¹⁴C‐labelled allantoin with the label introduced to the ureido carbonyl group in the final step by reaction of 5‐chlorohydantoin with [¹⁴C]urea. The chosen method favours high purity at the expense of radiochemical yield, which is achieved at a level of 8%. The integrity of the label is then investigated by performing an NMR analysis of ¹³C‐labelled allantoin synthesized by the same method. The ¹³C NMR spectrum confirms partial scrambling of the label to the C‐2 position by equilibration of the product via a putative bicyclic intermediate, which had been suggested by other workers. The ¹⁴C‐labelled allantoin synthesized by this method is therefore assigned as DL‐[H₂N¹⁴CO/¹⁴C‐2]allantoin. This study also includes the first full characterization of a side product, 5‐hydroxy‐5‐methoxyhydantoin, obtained by the reaction of a 5‐hydroxyhydantoin intermediate with the methanol solvent. Copyright © 2009 John Wiley & Sons, Ltd.     

3‐butyl‐6‐fluoro‐1(3H)‐isobenzofuranone (6‐F‐NBP), a derivative of dl‐n‐butylphthalide,inhibits glutamate‐induced cytotoxicity in PC12 cells

It is widely recognized that glutamate (Glu)‐induced cytotoxicity, intracellular calcium overload and the excessive free radical production are key events in the development and progression of ischemic brain injury. dl‐3‐n‐butylphthalide (NBP), an anti‐ischemic agent, has therapeutic effects in animal models of vascular dementia. The aim of the present study was to investigate the protective effect of 3‐butyl‐6‐fluoro‐1(3H)‐isobenzofuranone (6‐F‐NBP), a derivative of NBP on Glu‐induced cytotoxicity in rat pheochromocytoma (PC12) cells, and to compare this action with NBP. The results showed that after 24‐h incubation with Glu (5 mM), cell viability and mitochondrial membrane potential (MMP) were decreased. In contrast, the content of reactive oxygen species (ROS), activity of nitric oxide synthase (NOS), and apoptosis rate, as well as intracellular accumulation of [Ca²⁺]_i, were increased, 6‐F‐NBP inhibited the damage induced by Glu in a dose‐dependent manner and exerted a more potent effect than NBP, indicating that 6‐F‐NBP exhibited a protective effect against Glu‐induced cytotoxicity in cultured PC12 cells. Drug Dev Res 73: 11–17, 2012. © 2011 Wiley Periodicals, Inc.     

Stereoselective deuterium labeling of proRβ‐protons in the NMR structure determination of a helix‐turn‐helix turn peptide mimic

Abstract: The NMR structure of a small, side‐chain‐cyclized tripeptide mimic of the turn in the helix‐turn‐helix (HTH) motif was determined. The four β‐protons were stereospecifically assigned by stereoselective deuterium replacement of only the proRβ‐protons. All 24 of 30 NOESY cross‐peaks not involving chemically defined or freely rotating protons, and six of seven coupling constants from the P.COSY were used as distance and angle constraints in molecular modeling. MacroModel found 33/1000 structures in the NMR constrained search and 263/1000 structures in the unconstrained search, indicating meaningful constraint by the NMR data. However, the 10 lowest‐energy structures from the unconstrained and constrained searches are very similar, so modeling alone was able to find the experimentally determined structure.     

Base‐catalyzed deuterium and tritium labeling of aryl methyl sulfones

A method is presented for conveniently tritiating the aryl methyl sulfones of compounds identified as potent and selective inhibitors of human Cox‐2 and as DP receptor antagonists. A base‐catalyzed exchange reaction was conducted with deuterated water and the total deuterium incorporation, ranging from 46 to 99%, was calculated using mass spectrometry. Results from these exchanges were used as guidelines for tritium labeling giving specific radioactivities in the range of 28–120 mCi/mmol (1.03–4.43 GBq/mmol). Copyright © 2004 John Wiley & Sons, Ltd.     

Palladium‐catalyzed hydroxycarbonylation as the key step in the synthesis of a carbon‐14 labeled Maxi‐K channel blocker

Carbon‐14 labeled sodium formate was utilized for the preparation of a radiolabeled tracer to support the development of a Maxi‐K channel blocker. Key to the process was a palladium‐catalyzed hydroxycarbonylation of a bromoarene for installation of the radiolabel. Copyright © 2010 John Wiley & Sons, Ltd.     

Facile synthesis of high specific activity 4‐[1‐14C]butyl‐1,2‐diphenylpyrazolidine‐3,5‐dione (phenylbutazone) using nucleophilic substitution

Metabolism, environmental fate, and low concentration food residue studies would be facilitated by the use of radiolabeled test articles that can be readily quantified within complex matrices. However, radiochemical approaches for such studies require high specific activities to allow analytical detection of correspondingly low masses of test article. The synthesis of high specific activity (>50 μCi/μmol) [¹⁴C]‐radiolabeled phenylbutazone presents a challenge using existing methodology, mainly due to the low solvent volumes required and vigorous refluxing needed to close the pyrazolidinedione ring. Herein, we report on the significant modification of an existing method that allows the synthesis of low masses of high specific activity (>50 μCi/μmol) [¹⁴C]‐phenylbutazone under mild conditions with simple purification and high yield. The closure of the pyrazolidinedione ring of 1,2‐diphenyl‐3,5‐pyrazolidinedione was accomplished as a first step with unlabeled 1,2‐diphenylhydrazine and diethyl malonate in 32% yield under gram‐scale conditions, which avoided the challenges of low solvent use and vigorous refluxing. Low mass of high specific activity n‐[1‐¹⁴C]‐butyl bromide was then added via a nucleophilic substitution reaction as a final step. Yields ranged from 65% to 92% during multiple synthetic attempts with unlabeled butyl bromide and were greatly influenced by reaction stoichiometry and the selection of base.     

An unusual detection of tert‐butyl‐4‐anilinopiperidine‐1‐carboxylate in seizures of falsified ‘Xanax’ tablets and in items in a suspected heroin seizure submitted by Irish law enforcement

The identification of tert‐butyl‐4‐anilinopiperidine‐1‐carboxylate (4‐anilinopiperdine‐t‐BOC or 4‐AP‐t‐BOC) in many seized falsified ‘Xanax’ tablets has been reported after being encountered in forensic casework in late 2019 and early 2020 in Ireland. This substance was also detected in a pink powder submitted for analysis in March 2020. The pink powder was part of a larger seizure comprising brown powders which contained morphine or diamorphine (heroin) or a type of counterfeit heroin or heroin adulterant (known as ‘bash’). Novel benzodiazepines and other substances are being detected as ingredients in falsified benzodiazepine tablets more frequently on the illicit market. The detection of 4‐AP‐t‐BOC in benzodiazepine tablets is noteworthy and 4‐AP‐t‐BOC is added to the list of adulterants found in benzodiazepine tablets emerging in Europe. The presence of 4‐AP‐t‐BOC in both falsified ‘Xanax’ and powdered seizures is unusual, and analytical data are presented to assist with the identification of this compound in suspected illicit substances. The presence of 4‐AP‐t‐BOC in the tablets was confirmed using gas chromatography–mass spectrometry and liquid chromatography–mass spectrometry analyses, and spectral fragmentation pathways were suggested. To the authors' best knowledge, information about the biological activity of 4‐AP‐t‐BOC is not available. The removal of the t‐BOC protecting group yields 4‐anilinopiperidine which has been reported to be involved in the synthesis of fentanyl.     

X‐ray and 13C solid‐state NMR studies of N‐benzoyl‐phenylalanine

Abstract: A crystalline sample of N‐benzoyl‐dl ‐phenylalanine 1 and a polycrystalline sample of N‐benzoyl‐l ‐phenylalanine 2 were studied using ¹³C high‐resolution solid‐state NMR spectroscopy. The X‐ray structure of the dl form was established. Sample 1 crystallizes in a monoclinic form with a P2₁/c space group, a = 11.338(1) Å, b = 9.185(1) Å, c = 14.096(2) Å, β = 107.53(3)°, V = 1400(3) ų, Z = 4 and R = 0.053. The principal elements of the ¹³C chemical shift tensors δ_ii for 1 and 2 , selectively ¹³C (99%) labeled at the carboxyl groups were calculated. On the basis of ¹³C δ_ii analysis the hydrogen bonding pattern for sample 2 was deduced. Enriched samples were used to establish the intermolecular distance between chemically equivalent nuclei for 1 and spatial proximity in heterogeneous domain for 2 , employing the ODESSA pulse sequence. The consistence of the complementary approach covering X‐ray data, analysis of the ¹³C δ_ii parameters and ODESSA results is revealed.     

Asymptotic local power of pooled t‐ratio tests for unit roots in panels with fixed effects

Summary We derive analytically the local asymptotic power of two pooled t‐ratio tests for the presence of a unit root in a panel with fixed effects. We consider two statistics which differ according to the method used to remove the bias of the pooled OLS estimator. We show that when we bias‐correct the numerator only, the resulting test has significant local power in n^?1/4T^?1 neighbourhoods of the null of a panel unit root, while when the entire estimator is corrected for bias, the resulting statistic has local asymptotic power in neighbourhoods shrinking at the faster rate of n^?1/2T^?1 . This latter test is equivalent to the well‐known pooled t test proposed by Levin et al. (2002, Journal of Econometrics 108, 1–24), and its power depends only on the mean of the local‐to‐unity parameters. This implies that it has the same power against homogeneous and heterogeneous alternatives with the same mean autoregressive parameter. We then compare these tests to a panel version of the Sargan‐Bhargava (1983, Econometrica 51, 153–74) statistic for a unit root and the common point‐optimal test of Moon et al. (2007, Journal of Econometrics 141, 416–51). Monte Carlo simulations confirm the usefulness of our local‐to‐unity framework.     

Preparation of α‐labeled aldehydes by base‐catalyzed exchange reactions

A simple, efficient protocol for the preparation of α‐labeled aldehydes based on H/D exchange catalyzed by 4‐(N,N‐dimethylamino)pyridine or Et₃N is described. High chemical yields and ratios of isotope incorporation were obtained even when small amounts (～1 mmol) of aldehyde were used. Copyright © 2010 John Wiley & Sons, Ltd.     

Feasibility of palladium‐catalyzed isotopic exchange between sodium [125I]I and 2‐iodo‐para‐carborane

Many ortho‐/meta‐/para‐closo‐carborane derivatives have been proposed for boron neutron capture therapy. However, it is difficult to follow their pharmacokinetics in patients, which creates a risk of suboptimal treatment. Adding a radioactive label to closo‐carboranes may simplify pharmacokinetic studies. This paper reports on a study of the feasibility of palladium‐catalyzed isotopic exchange of iodinated closo‐carborane with radioisotopes of iodine. 2‐iodo‐para‐carborane was selected as a model compound. It was shown that such isotopic exchange is possible and provides a high yield (83±4.2%) after 40 min of reaction time. The reaction conditions were optimized, and it was demonstrated that the presence of tetra n‐butylammonium hydrogensulfate is important in order to stabilize the catalyst and to give reproducibility of the labeling. Copyright © 2003 John Wiley & Sons, Ltd.     

Enzymatic synthesis of l‐tryptophan and 5prime‐hydroxy‐l‐tryptophan labeled with deuterium and tritium at the alpha‐carbon position

The enzymatic synthesis of l‐tryptophan and its derivative 5′‐hydroxy‐l‐tryptophan labeled with deuterium and tritium at the α‐carbon position is reported. The mixture containing S‐methyl‐l‐cysteine, indole or 5‐hydroxyindole dissolved in deuteriated or tritiated water has been converted to [2‐²H]‐l‐tryptophan, [2‐³H]‐l‐tryptophan, 5′‐hydroxy‐[2‐²H]‐l‐tryptophan, and 5′‐hydroxy‐[2‐³H]‐l‐tryptophan, respectively, in a one‐pot reaction using the enzyme tryptophanase. The same reaction carried out in heavy water with THO added yielded either doubly labeled [2‐²H/³H]‐l‐tryptophan or 5′‐hydroxy‐[2‐²H/³H]‐l‐tryptophan. Copyright © 2003 John Wiley & Sons, Ltd.     

Synthesis of tritiated Cyclosporin A and FK‐506 by metal‐catalyzed hydrogen isotope exchange

Tritium‐labeled Cyclosporin A ( I ) and FK‐506 ( II ) have been prepared using a metal‐catalysed hydrogen isotope exchange procedure and high specific activity tritiated water. Specific activities of the labeled compounds were 0.15 and 0.59 TBq/mmol, respectively. Copyright © 2004 John Wiley & Sons, Ltd.     

NMR‐based approach to the analysis of radiopharmaceuticals: radiochemical purity,specific activity,and radioactive concentration values by proton and tritium NMR spectroscopy

Compounds containing tritium are widely used across the drug discovery and development landscape. These materials are widely utilized because they can be efficiently synthesized and produced at high specific activity. Results from internally calibrated ³H and ¹H nuclear magnetic resonance (NMR) spectroscopy suggests that at least in some cases, this calibrated approach could supplement or potentially replace radio‐high‐performance liquid chromatography for radiochemical purity, dilution and scintillation counting for the measurement of radioactivity per volume, and liquid chromatography/mass spectrometry analysis for the determination of specific activity. In summary, the NMR‐derived values agreed with those from the standard approaches to within 1% to 9% for solution count and specific activity. Additionally, the NMR‐derived values for radiochemical purity deviated by less than 5%. A benefit of this method is that these values may be calculated at the same time that ³H NMR analysis provides the location and distribution of tritium atoms within the molecule. Presented and discussed here is the application of this method, advantages and disadvantages of the approach, and a rationale for utilizing internally calibrated ¹H and ³H NMR spectroscopy for specific activity, radioactive concentration, and radiochemical purity whenever acquiring ³H NMR for tritium location.     

Enantioseparation of cetirizine by chromatographic methods and discrimination by 1H‐NMR

Cetirizine is an antihistaminic drug used to prevent and treat allergic conditions. It is currently marketed as a racemate. The H1‐antagonist activity of cetirizine is primarily due to (R)‐levocetirizine. This has led to the introduction of (R)‐levocetirizine into clinical practice, and the chiral switching is expected to be more selective and safer. The present work represents three methods for the analysis and chiral discrimination of cetirizine. The first method was based on the enantioseparation of cetirizine on silica gel TLC plates using different chiral selectors as mobile phase additives. The mobile phase enabling successful resolution was acetonitrile‐water 17: 3, (v/v) containing 1 mM of chiral selector, namely hydroxypropyl‐β‐cyclodextrin, chondroitin sulphate or vancomycin hydrochloride. The second method was a validated high performance liquid chromatography (HPLC), based on stereoselective separation of cetirizine and quantitative determination of its eutomer (R)‐levocetirizine on a monolithic C18 column using hydroxypropyl‐β‐cyclodextrin as a chiral mobile phase additive. The resolved peaks of (R)‐levocetirizine and (S)‐dextrocetirizine were confirmed by further mass spectrometry. The third method used a ¹H‐NMR technique to characterize cetirizine and (R)‐levocetirizine. These methods are selective and accurate, and can be easily applied for chiral discrimination and determination of cetirizine in drug substance and drug product in quality control laboratory. Moreover, chiral purity testing of (R)‐levocetirizine can also be monitored by the chromatographic methods. Copyright © 2009 John Wiley & Sons, Ltd.