Transition‐Metal‐Free Carbon Isotope Exchange of Phenyl Acetic Acids

A transition‐metal‐free carbon isotope exchange procedure on phenyl acetic acids is described. Utilizing the universal precursor CO₂, this protocol allows the carbon isotope to be inserted into the carboxylic acid position, with no need of precursor synthesis. This procedure enabled the labeling of 15 pharmaceuticals and was compatible with carbon isotopes [¹⁴C] and [¹³C]. A proof of concept with [¹¹C] was also obtained with low molar activity valuable for distribution studies.     

[11C]Carbonyl Difluoride—a New and Highly Efficient [11C]Carbonyl Group Transfer Agent

Herein, the synthesis and use of [¹¹C]carbonyl difluoride for labeling heterocycles with [¹¹C]carbonyl groups in high molar activity is described. A very mild single‐pass gas‐phase conversion of [¹¹C]carbon monoxide into [¹¹C]carbonyl difluoride over silver(II) fluoride provides easy access to this new synthon in robust quantitative yield for labeling a broad range of cyclic substrates, for example, imidazolidin‐2‐ones, thiazolidin‐2‐ones, and oxazolidin‐2‐ones. Labeling reactions may utilize close‐to‐stoichiometric precursor quantities and short reaction times at room temperature in a wide range of solvents while also showing high water tolerability. The overall radiosynthesis protocol is both simple and reproducible. The required apparatus can be constructed from widely available parts and is therefore well suited to be automated for PET radiotracer production. We foresee that this straightforward method will gain wide acceptance for PET radiotracer syntheses across the radiochemistry community.     

Determination of the uncertainties in the theoretical mass isotopomer distribution of molecules

A procedure for the determination of the uncertainties in the theoretical mass isotopomer distribution of molecules due to natural variations in the isotope composition of their constituting elements is described here for the first time. For this purpose, a Visual Basic macro for Microsoft Excel was written by adapting the direct stepwise calculation algorithm published by Kubinyi (Anal. Chim. Acta 1991, 247, 107-119, [11] Fig. 1). In our procedure no pruning threshold factors were used to eliminate round up errors for large molecules. Then, the Kragten [13] procedure of uncertainty propagation (Analyst 1994, 119, 2161-2165) was applied taking into account the correlation coefficients between the isotope abundances of the corresponding atoms. For bi-isotopic elements (C, H, N, Cl, Br) the correlation coefficients were given the value of −1. For tri- and tetra-isotopic elements the correlation coefficients were calculated using the mass dependent fractionation law used in stable isotope geochemistry and values of +1 or −1 were obtained depending on the isotope system considered. It was observed that for small organic molecules of natural isotope abundances, such as phenol or polybrominated diphenylethers, the method provided relatively small propagated uncertainties similar in magnitude to those measured experimentally. For ¹³C-labelled molecules the calculated uncertainties were mainly due to the uncertainties in the isotope enrichment of ¹³C and were much larger than the experimental uncertainties. For large molecules of natural isotope abundances, such as peptide C₆₈H₁₀₇N₁₇O₂₅ (NIST 8327 RM), the uncertainties in their mass isotopomer distributions were much larger and their source could be assigned mainly to the uncertainty of the natural isotope composition of carbon. When the size of the molecule was even larger, such as bovine insulin (C₂₅₄H₃₇₇N₆₅O₇₅S₆), Kragten procedure provided a good estimate for the uncertainty when the most probable isotope composition of carbon in mammals was used in the calculations.     

Late‐Stage Isotopic Carbon Labeling of Pharmaceutically Relevant Cyclic Ureas Directly from CO2

A robust, click‐chemistry‐inspired procedure for radiolabeling of cyclic ureas was developed. This protocol, suitable for all carbon isotopes (¹¹C, ¹³C, ¹⁴C), is based on the direct functionalization of carbon dioxide: the universal building block for carbon radiolabeling. The strategy is operationally simple and reproducible in different radiochemistry centers, exhibits remarkably wide substrate scope with short reaction times, and demonstrates superior reactivity as compared to previously reported systems. With this procedure, a variety of pharmaceuticals and an unprotected peptide were labeled with high radiochemical efficiency.     

Fast-atom bombardment tandem mass spectrometry of [13C]arachidonic acid labeled phospholipid molecular species

A method employing stable isotope labeling and fast-atom bombardment (FAB) tandem mass spectrometry has been developed to directly assess events of biosynthesis and metabolism of arachidonic acid containing phospholipid molecular species by cells carried in culture. Mast cells, cultured with [¹³C]linoleic acid, converted this precursor into arachidonic acid which was then incorporated into cellular phospholipids. Over a 24 hour period, the extent of label enrichment in each arachidonate-containing phospholipid molecular species was monitored by using negative FAB ionization with selected reaction monitoring. Specific incorporation of [¹³C₁₇] labeled arachidonate was determined from the ratio of the carboxylate anions at m/z 320 and 303, which correspond to [¹³C₁₇]arachidonate and unlabeled arachidonate, respectively, produced by collision-induced dissociation of each specific molecular anion. The use of [¹³C]linoleic acid as a precursor of arachidonic acid avoids the problem of changing the endogenous pool size by directly adding labeled arachidonic acid. Measurement of the [¹³C₁₇]label also avoids interferences from endogenous isobaric fatty acids that are naturally present at low levels.     

Precise and accurate compound-specific carbon and nitrogen isotope analysis of RDX by GC-IRMS

A new analytical method is presented for the compound-specific carbon and nitrogen isotope ratio analysis of a thermo-labile nitramine explosive hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) by gas chromatograph coupled to an isotope ratio mass spectrometer (GC-IRMS). Two main approaches were used to minimise thermal decomposition of the compound during gas chromatographic separation: programmed temperature vaporisation (PTV) as an injection technique and a high-temperature ramp rate during the GC run. δ¹⁵N and δ¹³C values of RDX measured by GC-IRMS and elemental analyser (EA)-IRMS were in good agreement within a standard deviation of 0.3‰ and 0.4‰ for nitrogen and carbon, respectively. Application of the method for the isotope analysis of RDX during alkaline hydrolysis at 50°C revealed isotope fractionation factors ε _carbon?=??7.8‰ and ε _nitrogen?=??5.3‰.     

One-Pot Synthesis of 11C-Labelled Primary Benzamides via Intermediate [11C]Aroyl Dimethylaminopyridinium Salts

Electrophilic ¹¹C-labelled aroyl dimethylaminopyridinium salts, obtained by carbonylative cross-coupling of aryl halides with [¹¹C]carbon monoxide, were prepared for the first time and shown to be valuable intermediates in the synthesis of primary [¹¹C]benzamides. The methodology furnished a set of benzamide model compounds, including the two poly (ADP-ribose) polymerase (PARP) inhibitors niraparib and veliparib, in moderate to excellent radiochemical yields. In addition to providing a convenient and practical route to primary [¹¹C]benzamides, the current method paves the way for future application of [¹¹C]aroyl dimethylaminopyridinium halide salts in positron emission tomography (PET) tracer synthesis.     

Understanding the Mechanisms of Unusually Fast HH,CH,and CC Bond Reductive Eliminations from Gold(III) Complexes

Carbon–carbon bond reductive elimination from gold(III) complexes are known to be very slow and require high temperatures. Recently, Toste and co‐workers have demonstrated extremely rapid C?C reductive elimination from cis‐[AuPPh₃(4‐F‐C₆H₄)₂Cl] even at low temperatures. We have performed DFT calculations to understand the mechanistic pathway for these novel reductive elimination reactions. Direct dynamics calculations inclusive of quantum mechanical tunneling showed significant contribution of heavy‐atom tunneling (>25 %) at the experimental reaction temperatures. In the absence of any competing side reactions, such as phosphine exchange/dissociation, the complex cis‐[Au(PPh₃)₂(4‐F‐C₆H₄)₂]⁺ was shown to undergo ultrafast reductive elimination. Calculations also revealed very facile, concerted mechanisms for H?H, C?H, and C?C bond reductive elimination from a range of neutral and cationic gold(III) centers, except for the coupling of sp³ carbon atoms. Metal–carbon bond strengths in the transition states that originate from attractive orbital interactions control the feasibility of a concerted reductive elimination mechanism. Calculations for the formation of methane from complex cis‐[AuPPh₃(H)CH₃]⁺ predict that at ?52 °C, about 82 % of the reaction occurs by hydrogen‐atom tunneling. Tunneling leads to subtle effects on the reaction rates, such as large primary kinetic isotope effects (KIE) and a strong violation of the rule of the geometric mean of the primary and secondary KIEs.     

Intermolecular insertion of an N,N-heterocyclic carbene into a nonacidic C-H bond: Kinetics, mechanism and catalysis by (K-HMDS)2 (HMDS = Hexamethyldisilazide)

The reaction of 2‐[¹³C]‐1‐ethyl‐3‐isopropyl‐3,4,5,6‐tetrahydropyrimidin‐1‐ium hexafluorophosphate ([¹³C₁]‐ 1 ‐PF₆) with a slight excess (1.03 equiv) of dimeric potassium hexamethyldisilazide (“(K‐HMDS)₂”) in toluene generates 2‐[¹³C]‐3‐ethyl‐1‐isopropyl‐3,4,5,6‐tetrahydropyrimid‐2‐ylidene ([¹³C₁]‐ 2 ). The hindered meta‐stable N,N‐heterocyclic carbene [¹³C₁]‐ 2 thus generated undergoes a slow but quantitative reaction with toluene (the solvent) to generate the aminal 2‐[¹³C]‐2‐benzyl‐3‐ethyl‐1‐isopropylhexahydropyrimidine ([¹³C₁]‐ 14 ) through formal C? H insertion of C(2) (the “carbene carbon”) at the toluene methyl group. Despite a significant pK_a mismatch (ΔpK_a 1 ⁺ and toluene estimated to be ca. 16 in DMSO) the reaction shows all the characteristics of a deprotonation mechanism, the reaction rate being strongly dependent on the toluene para substituent (ρ=4.8(±0.3)), and displaying substantial and rate‐limiting primary (k_H/k_D=4.2(±0.6)) and secondary (k_H/k_D=1.18(±0.08)) kinetic isotope effects on the deuteration of the toluene methyl group. The reaction is catalysed by K‐HMDS, but proceeds without cross over between toluene methyl protons and does not involve an HMDS anion acting as base to generate a benzyl anion. Detailed analysis of the reaction kinetics/kinetic isotope effects demonstrates that a pseudo‐first‐order decay in 2 arises from a first‐order dependence on 2 , a first‐order dependence on toluene (in large excess) and, in the catalytic manifold, a complex noninteger dependence on the K‐HMDS dimer. The rate is not satisfactorily predicted by equations based on the Brønsted salt‐effect catalysis law. However, the rate can be satisfactorily predicted by a mole‐fraction‐weighted net rate constant: ?d[ 2 ]/dt=({x ₂ k_uncat}+{(1?x ₂ ) k_cat})[ 2 ]¹[toluene]¹, in which x ₂ is determined by a standard bimolecular complexation equilibrium term. The association constant (K_a) for rapid equilibrium–complexation of 2 with (K‐HMDS)₂ to form [ 2 (K‐HMDS)₂] is extracted by nonlinear regression of the ¹³C NMR shift of C(2) in [¹³C₁]‐ 2 versus [(K‐HMDS)₂] yielding: K_a=62(±7) M ^?1; δ_C(2) in 2 =237.0 ppm; δ_C(2) in [ 2 (K‐HMDS)₂]=226.8 ppm. It is thus concluded that there is discrete, albeit inefficient, molecular catalysis through the 1:1 carbene/(K‐HMDS)₂ complex [ 2 (K‐HMDS)₂], which is found to react with toluene more rapidly than free 2 by a factor of 3.4 (=k_cat/k_uncat). The greater reactivity of the complex [ 2 (K‐HMDS)₂] over the free carbene ( 2 ) may arise from local Brønsted salt‐effect catalysis by the (K‐HMDS)₂ liberated in the solvent cage upon reaction with toluene.     

Determination of Deuterated Phenylalanine and Tyrosine in Egg Protein by GCQ

In order to study protein digestibility by means of noninvasive tracer techniques (stable isotopes), a representative oral tracer, i.e. a stable isotope labeled protein, is needed. Therefore, egg white containing L-[ring-²H₅]phenylalanine and L-[ring-²H₄]tyrosine was prepared. The aim of this study was to measure the isotopic enrichment of the labeled amino acids in the egg white. The use of a standard GC-MS, based on ion trap technology was found to be a reliable technique. The enrichment of L-[ring-²H₅]phenylalanine and L-[ring-²H₄]tyrosine, expressed in Molar Percent (MP) amounted to 23.2 MP and 2.8 MP respectively.     

Evaluation of the sealed‐tube low‐temperature combustion method for the 13C/12C and 2H/1H ratio determinations of cellulose nitrate

Traditionally‐suggested combustion time of 1 h at 550°C) with the sealed‐tube combustion method for determining the ¹³C/ ¹²C ratio of cellulose nitrate or other nitrogen‐containing components could produce large negative deviation up to 1°. Three types of cellulose are used to ascertain possible causes. The presence of nitrous oxide (N₂O) formed during combustion is most likely responsible for this deviation. Prolongation of the combustion time (at least 5 h at 550°C) and intimate contact between copper oxide and organic matter can greatly improve the analysis precision and effectively reduce this deviation to an acceptable level. Regardless of scattered carbon isotope data, hydrogen isotope data are all reproducible within 2° when this method is coupled with the high temperature uranium reduction method. Thus, care should be taken for determining carbon and nitrogen isotope compositions of nitrogen‐containing substances using the low temperature sealed‐tube combustion method.     

Rapid synthesis of C-labeled FK506 for positron emission tomography

The present study describes a rapid synthesis method for labeled [¹¹C]FK506 for positron emission tomography (PET). A one-pot reaction from [¹¹C]CH₃I, involving a Wittig reaction as the key carboncarbon bond formation was developed. The chemical process was accomplished using a designed, fully automated synthetic apparatus, and an injectable solution of [¹¹C]FK506 was obtained in only 34 min from [¹¹C]CH₃I. The decay-corrected radiochemical yield based on [¹¹C]CH₃I was 11.9%, and the specific activity was 39.8 GBq/μmol.     

Theoretical Study on the Gas‐Phase SN2 Reaction of Microhydrated Fluoride with Methyl Fluoride

The rate constants for the gas‐phase S_N2 reaction of F^?(H₂O) with CH₃F have been calculated using the dual‐level variational transition state theory including multidimensional tunneling from 50 to 500 K. Tunneling was found to dominate the reaction below 200 K. The deuterium, ¹³C, and ¹⁴C kinetic isotope effects (KIEs) and solvent (D₂O) isotope effects (SKIEs) were also calculated in the same temperature range. The results indicated that the deuterium and heavy water substitutions resulted in inverse KIEs (0.6～0.8 ) while the ¹³C and ¹⁴C substitutions resulted in normal KIEs (1.0～1.2) at room temperature. The calculated carbon KIEs increased significantly below 80 K due to the differences in the magnitude of the tunneling effects for different isotopic substitutions.     

Spark source mass spectrometric calibration of the local vibrational mode absorption of carbon in gallium arsenide on arsenic sublattice sites

An appropriate calibration of the local vibrational mode absorption of carbon in GaAs on arsenic sublattice sites, C_As, at wavenumber 582 cm^–1 (77 K) is presented. Integrated absorptions I_α of C_As, the dominant acceptor in undoped monocrystalline GaAs, and calibrated carbon concentrations [C] were measured in single crystals using the methods FTIR and SSMS, respectively. A calibration factor f_C = [C]/I_α of (7.1 ± 0.2) × 10¹⁵ cm^–1 has been derived for 2.8 × 10¹⁴ cm^–3≤ [C] ≤ 1.4 × 10¹⁶ cm^–3 above a SSMS detection limit of [C]_DL? 1.4 × 10¹³ cm^–3. The carbon concentrations [C] = [C]_SSMS/RSC_C were calibrated with a relative sensitivity coefficient RSC_C = [C]_SSMS/ [C]_TRUE of 3.1 ± 0.1. CPAA was used as a reference method for [C]_CPAA≥ 4.4 × 10¹⁴ cm^–3 in order to approximate [C]_TRUE.     

Synthesis of the dopamine D2/D3 receptor agonist (+)-PHNO via supercritical fluid chromatography: preliminary PET imaging study with [3-C]-(+)PHNO

Carbon-11 labeled (+)-4-[1-¹¹C]propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol ([1-¹¹C]-(+)-PHNO) is a dopamine D₃-preferring agonist radiopharmaceutical used for medical imaging by positron emission tomography (PET). We report the synthesis of (+)-PHNO using supercritical fluid chromatography for enantiomeric resolution of its norpropyl derivative, HNO, followed by propylation. (+)-HNO was used to prepare the radiolabeling precursor, (+)-trans-4-acetyl-9-triisopropylsilyloxy-2,3,4a,5,6,10b-hexahydro-4H-naphth[1,2b][1,4]oxazine, in 12 steps. Modifications to the labeling procedure were made to ensure consistent preparation of [3-¹¹C]-(+)-PHNO via [¹¹C]CH₃I. A preliminary PET imaging study was carried out with this tracer in an attempt to image dopamine receptors in brown adipose tissue (brown fat) in vivo.     

Performances of a TRACERlab FX C synthesis module using a Ni-nanopowder/molecular sieves mixed catalyst

The progress of positron emission tomography goes together with an increasing demand for new radiopharmaceuticals: among these, the development of radiopharmaceuticals labelled with carbon-11 is particularly interesting because these compounds are biologically indistinguishable from their stable analogues. These radiotracers are prepared starting from [¹¹C]carbon dioxide, the most common and versatile primary labelling precursor, or from secondary labelling precursors such [¹¹C]methyl iodide produced by “wet” or “gas-phase” method. The gas-phase is the most used method and consists in the radical reaction of iodine vapours with [¹¹C]methane, produced in target or from [¹¹C]carbon dioxide by reduction with hydrogen on nickel catalyst at high temperature. This second approach is frequently adopted in commercial automatic methylation modules, such as the TRACERlab FX C. When not performed in target, [¹¹C]CH₄ production represents a key step for the [¹¹C]CH₃I synthesis from which the outcome of the whole radiolabelling process depends. In order to improve the performance of the module, a new reduction catalyst made of a mixture of metallic Ni (nanopowder) and molecular sieves mixed in different ratios has been tested. Preliminary results demonstrated that not only the mixture of nanopowder-Ni and molecular sieves represents a valid reduction catalyst but also permits to trap [¹¹C]CO₂ and subsequently use it as labelling reagent, making TRACERlab FX C a module for both methylation and carboxylation.     

Biosynthesis of the antibiotic verrucarin E use of [1-13C]-, [2-13C]-, [1,2-13C]- and [2-13C, 2-2H3]-acetates. Verrucarins and roridins, 37th communication [1]

The origin of the carbon skeleton of verrucarin E (1) from acetate as precursor is confirmed. Incorporation studies with [1,2-¹³C]-acetate have demonstrated that two acetoacetate units couple together as shown in pattern A (Scheme 2) and not as in B . Analysis of the deuterium distribution in both verrucarin E (1) isolated after the incorporation of [2-¹³C,2-²H₃]-acetate and in sodium acetate obtained after Kuhn-Roth oxidation of the metabolite demonstrated that C(7) is derived from the starter unit of one of the acetoacetate moieties. The deuterium exchange in verrucarin E (1) occurring during fermentation was investigated.     

[C]Lithium trimethylsilyl ynolate: a new [C] precursor and its application in heterocyclic synthesis

[¹¹C]Lithium trimethylsilyl ynolate was characterized to be a new precursor in carbon-11 chemistry. It was obtained from [¹¹C]carbon monoxide and lithiated silyldiazomethane. The new precursor might be of high potential for the incorporation of carbon 11 into heterocyclic ring systems for which labeling methods are not well developed. More examples will certainly follow if other researchers will apply this useful method.     

A new technique for labeling of [11C]-choline, a positron-emitting tracer for tumor imaging

Summary [¹¹C]-choline has been reported as a potential tracer for imaging a variety of human tumors with positron emission tomography (PET). A new labeling technique for [¹¹C]-choline was established depending on parameters optimized, such as reaction time, volume, temperature, and the quantity of DMAE.The synthesis yield was improved from 82.0% to 96.5% (EOB), while the consumption of DMAE precursor decreased from 60 to 2 mg. Absolute yield of [¹¹C]-choline was 2500 MBq for a 10-minute irradiation at15mA, and a total synthesis time of less than 8 minutes from [¹¹C]-CH₃I to [¹¹C]-choline.     

Carbon-14 kinetic isotope effect in the decarbonylation of lactic acid[1-14C]

The carbon-14 kinetic isotope effect for the decarbonylation of lactic acid[1-¹⁴C] in sulfuric acid has been measured in the temperature interval of 20–90°C. The experimental values of (k_12C/k_14C) are compared with the theoretical¹⁴C kinetic isotope effect calculated assuming that one carbon-oxygen stretching vibration is lost in the rate-determining step. The discrepancy between experimentally observed temperature dependence of the¹⁴C kinetic isotope effect and the theoretical one is explained by the possible side reactions which change the apparent degrees of decarbonylation and isotopic composition of CH₃CHOHCOOH[1-¹⁴C] used in experiments aiming at the determination of carbon-14 kinetic isotope effect in the decarbonylation process itself.