Synthesis of the perdeuterated cellulose solvents N‐methylmorpholine N‐oxide (NMMO‐d11) and N,N‐dimethylacetamide (DMAc‐d9)

The synthesis of the perdeuterated cellulose solvents NMMO‐d₁₁ (9) and N,N‐dimethylacetamide‐d₉ (14) is described. NMMO‐d₁₁ was obtained according to a five‐step approach from non‐labeled diglycolic acid (1) via diethylene glycole‐d₈ (4) and its bis‐tosylate (5), which underwent cyclization with benzylamine to N‐benzylmorpholine (6). The removal of the benzyl protecting group, methylation and N‐oxidation completed the synthesis. DMAc‐d₉ (14) was obtained from deuterated acetic acid (10) and dimethylamine–carbon dioxide complex (17) with acidic alumina as the catalyst according to a solvent‐free gas–solid reaction. Copyright © 2008 John Wiley & Sons, Ltd.     

C‐Terminal 18F‐fluoroethylamidation exemplified on [Gly‐OH9] oxytocin

The no‐carrier‐added (n.c.a.) ¹⁸F‐fluoroethylamidation of the acid function of the protected nonapeptide Boc–Cys–Tyr(tBu)–Ile–Gln(Mtt)–Asn(Mtt)–Cys–Pro–Leu–Gly–OH forming the labelled peptide hormone derivative [Gly‐(2‐[¹⁸F]fluoroethyl)NH⁹]‐oxytocin is described. The labelling conditions were elaborated using a protected tripeptide, identical to the C‐terminal sequence of oxytocin. The prosthetic group n.c.a. 2‐[¹⁸F]fluoroethylamine was synthesised via cryptate mediated n.c.a. ¹⁸F‐fluorination of N‐Boc‐2‐(p‐toluenesulfonyloxy)ethylamine in DMSO (RCY: ca. 60%) and subsequent deprotection with a radiochemical yield of 46±5%. [¹⁸F]Fluoroethylamine was reacted with Z–Pro–Leu–Gly–OH in presence of the coupling reagent TBTU or with activated esters of the model‐tripeptide. The activated ester method as well as the condensation in presence of TBTU yielded ?90% of the ¹⁸F‐fluoroethyl‐amidated tripeptide. TBTU‐mediated condensation of n.c.a. 2‐[¹⁸F]fluoro‐ethylamine with the C‐terminal free acid group of protected oxytocin gave the radiochemical yield of about 75%. Deprotection under acidic conditions led to the formation of [Gly–(2‐[¹⁸F]fluoroethyl)NH⁹]oxytocin within 75 min with a radiochemical yield of about 30% as measured by analytical HPLC. Copyright © 2002 John Wiley & Sons, Ltd.     

Preparation of the very acid-sensitive Fmoc-Lys(Mtt)-OH Application in the synthesis of side-chain to side-chain cyclic peptides and oligolysine cores suitable for the solid-phase assembly of MAPs and TASPs

N ^α-9-Fluorenylmethoxycarbonyl-N^ε-4-methyltrityl-lysine, [Fmoc-Lys(Mtt)-OH], was prepared in two steps from lysine, in 42% overall yield. The N^ε-Mtt function can be quantitatively removed upon treatment with 1% TFA in dichloromethane or with a 1:2:7 mixture of acetic acid/trifluoroethanol/dichloromethane for 30 min and 1 h at room temperature, respectively. Under these conditions, groups of the tert-butyl type and peptide ester bonds to TFA-labile resins, such as the 2-chlorodiphenylmethyl- and the Wang-resin, remained intact. The utility of the new derivative in peptide synthesis has been exemplified with the synthesis of a cyclic cholecystokinin analog. As an example of further application, five types of lysine cores suitable for the solid-phase synthesis of one, two or three epitopes containing antigenic peptides or template-assembled synthetic proteins have been synthesized on Merrifield, Wang and 2-chlorodiphenylmethyl resin. © Munksgaard 1995.     

Synthesis of enantiopure non‐natural α‐amino acids using tert‐butyl (2S)‐2‐[bis‐(tert‐butoxycarbonyl)amino]‐5‐oxopentanoate as key‐intermediate:the first synthesis of(S)‐2‐amino‐oleic acid

Abstract: A general method for the synthesis of enantiopure non‐natural α‐amino acids is described. The key intermediate tert‐butyl (2S)‐2‐[bis(tert‐butoxycarbonyl)amino]‐5‐oxopentanoate was obtained from l ‐glutamic acid after suitable protection and selective reduction of the γ‐methyl ester group by DIBALH. Wittig reaction of this chiral aldehyde with various ylides led to a variety of δ,ε‐unsaturated α‐amino acids. This methodology was applied to the synthesis of (S)‐2‐amino‐oleic acid.     

Cyclodehydrogenation of aniline derivatives carrying carboxamide or urea functions (author's transl)

Cyclodehydrogenation of Aniline Derivatives Carrying Carboxamide or Urea Functions The synthesis of o-tert. aminobenzamides is described. Dehydrogenation of these compounds with mercury(II)-EDTA yields condensed quinazolinones. The analogous reaction of o-tert. aminophenylureas leads to condensed benzimidazoles with simultaneous removal of the aminocarbonyl group.     

A first synthesis of 18F‐radiolabeled lapatinib: a potential tracer for positron emission tomographic imaging of ErbB1/ErbB2 tyrosine kinase activity

Fluorine‐18‐labeled lapatinib has been successfully synthesized for the first time by the reaction of a dimethylformamide solution of meta‐[¹⁸F]fluorobenzylbromide with a Boc‐protected lapatinib precursor fragment. The reaction proceeded in the presence of K₂CO₃ at 110 °C for 10 min in a microwave and was followed by Boc‐group deprotection with trifluoroacetic acid. The overall radiochemical yield of the reaction starting from the radiofluorination of the iodonium salt was 8–12% (uncorrected, n = 6) in a 140‐min synthesis time.     

Use of the excluded protecting group (EPG) method for peptide synthesis

Abstract: The excluded protecting group (EPG) method has been used for the solution synthesis of several peptides including Merrifield's Model Tetrapeptide, linear antamanide and an analogue of magainin‐1, [Ala¹⁹, Asn²²]magainin‐1. In the approach reported, the C‐terminal amino acid is esterified to the 2‐position of cholestane as the [2s,3s]iodohydrin ester and the penultimate amino acid added to the aminoacyl‐steroid as the Fmoc‐pentafluorophenyl‐ester. The Fmoc group is removed with Et₂NH/DMF (～15% v/v) and, after evaporation to ～10 mL, the solution chromatographed on Sephadex LH‐20 in DMF. The dipeptidyl‐steroid elutes as the free amine well separated from other reaction mixture components. Fractions containing the dipeptide, as determined by counting and TLC, are pooled and reacted with the next Fmoc‐amino acid‐pentafluorophenyl ester in the sequence. Repetition of the deprotection/purification/reaction cycle yields the fully protected peptide.On completion of the synthesis, the cholestane iodohydrin ester is selectively removed by treatment with Zn°/AcOH to yield the peptide with intact α‐amino and side chain protecting groups. Global deprotection is achieved with HF. All intermediates from the syntheses reported were characterized. The magainin analogue was shown to have full biologic activity. The Fmoc iodohydrin esters of 16 of the 20 proteogenic amino acids have been prepared and characterized for use as the C‐terminal amino acids in other EPG syntheses.     

Chemical synthesis of allyl‐[13C6]‐glucuronate

The synthesis of allyl‐[¹³C₆]‐glucuronate from α‐D‐[¹³C₆]‐glucose in five steps is described. Selective protection of the primary alcohol in the glucose with the bulky trityl group followed by acetylation in the same pot gave the fully protected sugar. Removal of the trityl group followed by oxidation of the primary alcohol to the acid and removal of the acetate groups using catalytic amounts of sodium methoxide gave the glucuronic acid. Reaction with allyl bromide using resin‐bound fluoride gave the title compound. Copyright © 2011 John Wiley & Sons, Ltd.     

An efficient one‐step radiosynthesis of [18F]FE‐PE2I,a PET radioligand for imaging of dopamine transporters

The aim of this study was to develop a direct fluorination method for the preparation of [¹⁸F]‐(E)‐N‐(3‐iodoprop‐2‐enyl)‐2β‐carbofluoroethoxy‐3β‐(4′‐methyl‐phenyl)nortropane ([¹⁸F]FE‐PE2I (VI). The synthesis procedure relies on the conventional Kryptofix‐mediated nucleophilic ¹⁸F‐substitution of the tosylate group in the precursor, TsOE‐PE2I (V). Out of reaction conditions tested, the highest fluorination efficiency was obtained in dimethyl sulfoxide at 140°C. The reaction mixture was purified by semi‐preparative HPLC, followed‐up by a standard Sep‐Pak SPE procedure. On average, 1.0 GBq of [¹⁸F]FE‐PE2I was produced from 5‐min irradiation at 35 μA (dimethyl sulfoxide, 5 min/140°C). Decay‐uncorrected yield of the product after HPLC purification and formulation was in the order of 20%. Specific radioactivity of [¹⁸F]FE‐PE2I at 15 min after EOS was 3.3–5.1 Ci/µmol (n = 3); radiochemical purity was >98% (n = 4). This direct nucleophilic fluorination strategy is well suited for the automation of the entire synthesis of [¹⁸F]FE‐PE2I in a modern PET synthesizer for human PET application. In addition, the ¹⁸F‐incorporation rate into TsOE‐PE2I was evaluated using radio‐thin layer chromatography (TLC) and radio‐HPLC. The suggested HPLC method (ACE 5 C18‐HL column and acetonitrile/0.1 M NH₄CO₂ (80:20)) was found to be suitable for evaluation of ‘free’ ¹⁸F‐fluoride in the reaction mixture; in addition, this method allowed the detection of three radiolabelled by‐products that were not discernable with the TLC approach. Therefore, we conclude that the HPLC approach may serve as a good alternative to traditional radio‐TLC technique as it provides more detailed information about the fluorination process in the reaction kinetics or optimization studies. Copyright © 2012 John Wiley & Sons, Ltd.     

Improved method for the synthesis of Nα-9-fluorenylmethyloxycarbonyl-Nδ,ω bis-adamantyloxycarbonyl-L-arginine

A modified method is reported for the prepartion of N^α-9-fluorenylmethyloxycarbonyl-N^δ,ω bis-adamantyloxycarbonyl-L-arginine, giving an overall yield of 60% over three steps based on N^α-benzyloxycarbonyl-L-arginine. Commercially available adamantyl fluoroformate for guanidine function protection of N^α benzyloxycarbonyl-L-arginine, catalytic transfer hydrogenation with formic acid on palladium black for removal of the benzyloxycarbonyl protecting group, and fluorenylmethylsuccinimidyl carbonate for the final synthesis, were introduced to simplify and reduce the cost of preparation of this arginine derivative. The reaction conditions have been accurately studied at each step in order to optimize the yields.     

Preparation of 4‐[11C]methylmetaraminol,a potential PET tracer for assessment of myocardial sympathetic innervation

The false adrenergic neurotransmitter [¹¹C]meta‐hydroxyephedrine ([¹¹C]HED) is currently the PET tracer of choice for assessment of myocardial sympathetic innervation. The molecule is metabolised in the 4‐position of the aromatic ring. The resulting radiolabelled metabolites need to be measured in order to obtain an arterial input function. Our aim was the development of a PET tracer with an increased metabolic stability relative to [¹¹C]HED. We selected 4‐methylmetaraminol as a candidate molecule for radiolabelling with ¹¹C (t_1/2 20.4 min). Our radiosynthetic approach towards 4‐[¹¹C]methylmetaraminol involved a palladium‐catalyzed cross‐coupling reaction of a protected 4‐trimethylstannyl derivative of metaraminol with [¹¹C]methyl iodide followed by removal of the protective groups. 4‐[¹¹C]methylmetaraminol was obtained in a final decay‐corrected radiochemical yield of 20–25% within a synthesis time of 60–80 min. The specific radioactivity at the end of the synthesis ranged from 18–37 to GBq/μmol. The unlabelled reference molecule, 4‐methylmetaraminol, was prepared in a 5‐step synthesis starting from metaraminol. A biological evaluation of 4‐[¹¹C]methylmetaraminol is in progress and the results will be reported elsewhere. Copyright © 2002 John Wiley & Sons, Ltd.     

Preparation of fluorine‐18‐labelled fluoromisonidazole using two different synthesis methods

¹⁸F‐labelled fluoromisonidazole [1H‐1‐(3‐[¹⁸F]fluoro‐2‐hydroxypropyl)‐2‐nitroimida‐zole; ([¹⁸F]FMISO)] is used as an in vivo marker of hypoxic cells in tumours and ischaemic areas of the heart and the brain. The compound plays an important role in evaluating the oxygenation status in tumours during radiotherapy. In this paper, we report experiments carried out in our laboratory in synthesizing [¹⁸F]FMISO using two different methods. The first method (I) for the [¹⁸F]FMISO synthesis was the fluorination of (2R)‐(?)‐glycidyl tosylate to [¹⁸F]epifluorohydrin. The subsequent nucleophilic ring opening, achieved with 2‐nitroimidazole, leads to labelled FMISO. The second method (II) was the fluorination of the protected precursor 1‐(2′‐nitro‐1′‐imidazolyl)‐2‐O‐tetrahydropyranyl‐3‐O‐toluenesulphonyl‐propanediol, followed by a rapid removal of the protecting group. With the first method, the radiochemical yield was about 10% at the end of the synthesis (EOS), and the radiochemical purity was over 99%. The radiochemical yield in the second method was 21% (EOS) on an average, and the radiochemical purity was over 97%. When an automated commercial synthesis module was used with method II, slightly better and more reproducible yields were achieved. The improvement in the synthesis yield with the automated apparatus will be valuable when working with high activities, and therefore it is under further development. Copyright © 2003 John Wiley & Sons, Ltd.     

Automated production of [18F]FTHA according to GMP

14‐(R,S)‐[¹⁸F]fluoro‐6‐thia‐heptadecanoic acid is a tracer for fatty acid imaging by positron emission tomography. High demand for this tracer required us to replace semiautomatic synthesis with a fully automated procedure. An automated synthesis device was constructed in‐house for multistep nucleophilic ¹⁸F‐fluorination and a control system was developed. The synthesis device was combined with a sterile filtration unit and both were qualified. 14‐(R,S)‐[¹⁸F]fluoro‐6‐thia‐heptadecanoic acid was produced according to good manufacturing practice guidelines set by the European Union. The synthesis includes an initial nucleophilic labelling reaction, deprotection, preparative HPLC separation, purification of the final product, and formulation for injection. The duration and temperature of the reaction and hydrolysis were optimized, and the radiochemical stability of the formulated product was determined. The rotary evaporator used to evaporate the solvent after HPLC purification was replaced with solid phase extraction purification. We also replaced the human serum albumin used in the earlier procedure with a phosphate buffer‐ascorbic acid mixture in the final formulation solution. From 2011 to 2016, we performed 219 synthesis procedures, 94% of which were successful. The radiochemical yield of 14‐(R,S)‐[¹⁸F]fluoro‐6‐thia‐heptadecanoic acid, decay‐corrected to the end of bombardment, was 13% ± 6.3%. The total amount of formulated end product was 1.7 ± 0.8 GBq at end of synthesis.     

The synthesis of a benzamidine‐containing NR2B‐selective NMDA receptor ligand labelled with tritium or fluorine‐18

A novel tritium or flourine‐18‐labelled benzamidine‐containing NR2B‐selective NMDA receptor ligand has been synthesized. This compound was designed to contain the fluoromethoxy group to allow for the synthesis of a high specific activity, fluorine‐18‐labelled PET tracer for imaging studies of the NR2B receptor. In addition to the fluorine‐18‐labelled compound, this compound was also tritium labelled. The tritiated ligand (11 Ci/mmol) was synthesized by a gas tritiation reaction of an aryl bromide precursor. The fluorine‐18 ligand (2916 Ci/mmol), which was deuterated in the fluoromethoxy group to aid in metabolic stability, was synthesized by alkylating a phenolic precursor with [¹⁸F]fluoromethylbromide‐d₂. Copyright © 2004 John Wiley & Sons, Ltd.     

Evaluation of new base‐labile2‐(4‐nitrophenylsulfonyl)ethoxycarbonyl (Nsc)‐amino acids for solid‐phase peptide synthesis

Abstract: The 2‐(4‐nitrophenylsulfonyl)ethoxycarbonyl (Nsc) group is a new base‐labile protecting group for solid‐phase peptide synthesis, completely interchangeable with the fluorenylmethoxycarbonyl (Fmoc) protecting group, but with certain advantages. In this paper, we report a methodology with N^α‐Nsc‐protected amino acids for the synthesis of some melanotropins important to our research, namely, γ‐melanocyte‐stimulating hormone (γ‐MSH), its [Nle³]‐analogue, and a cyclic α‐MSH/β‐MSH hybrid. We developed an efficient protocol for the synthesis of the cyclic MSH analogue that yielded this peptide in > 98% purity. The γ‐MSH synthesis, which gave problems with both the Boc and Fmoc strategies, yielded the desired peptide by Nsc‐chemistry but was accompanied by side products. Finally, the Nle³‐γ‐MSH analogue was synthesized more efficiently using the Fmoc strategy, suggesting that Nsc‐chemistry might not be the best methodology for certain sequences.     

Fully automated high yield synthesis of (R)‐ and (S)‐[11C]verapamil for measuring P‐glycoprotein function with positron emission tomography

Racemic (±) verapamil is a well characterized substrate for P‐glycoprotein (P‐gp). However, the in vivo pharmacokinetics and pharmacodynamics of both enantiomers are reported to be different. In the preparation of evaluation studies of both enantiomers in animals and humans, the purpose of the present study was to optimize and automate the synthesis of (R)‐ and (S)‐[¹¹C]verapamil. (R)‐ and (S)‐[¹¹C]verapamil were prepared from (R)‐ and (S)‐desmethyl‐verapamil, respectively, by methylation with no‐carrier added [¹¹C]methyliodide or [¹¹C]methyltriflate. Different conditions of the methylation reaction were studied: reaction time, temperature, base and solvent, and chemical form of the precursor using either the hydrochloric acid salt or the free base of the starting material. After optimization, the synthesis was fully automated using home‐made modules and performed according to GMP guidelines. Optimal yields of 60–70% for the methylation reaction were obtained using 1.5 mg of the free base of (R)‐ or (S)‐desmethyl‐verapamil in 0.5 ml of acetonitrile at 50°C for 5 min with [¹¹C]methyltriflate as methylating agent. Under the same reaction conditions, but with a reaction temperature of 100°C, the radiochemical yield starting with [¹¹C]methyliodide as methylation reagent was 40%. The specific activity of (R)‐ and (S)‐[¹¹C]verapamil was >20 GBq/μmol and the radiochemical purity was >99% for both methods. The total synthesis time was 45 min. The automated high yield synthesis of (R)‐ and (S)‐[¹¹C]verapamil provides the means for evaluating both enantiomers as in vivo tracers of P‐gp function. Copyright © 2002 John Wiley & Sons, Ltd.     

Enzymatic synthesis of [1‐14C‐N‐acetyl,P18O2] cytidine monophosphate neuraminic acid

A method for the enzymatic synthesis of [1‐¹⁴C‐N‐acetyl, P¹⁸O₂] cytidine monophosphate neuraminic acid (CMP‐NeuAc) is described. Central to the synthesis of [1‐¹⁴C‐N‐acetyl, P¹⁸O₂]CMP‐NeuAc was the enzymatic preparation of [γ‐P¹⁸O₃]ATP for use in a reaction with uridine kinase and cytidine to provide 5′‐[P¹⁸O₃]CMP. The [1‐¹⁴C‐N‐acetyl, P¹⁸O₂]CMP‐NeuAc isotopomer was then synthesized from a reaction involving nucleoside monophosphate kinase, pyruvate kinase and CMP‐NeuAc synthetase. The isolated reaction yield was 35%. Copyright © 2004 John Wiley & Sons, Ltd.     

Dual‐labeled prostate‐specific membrane antigen (PSMA)‐targeting agent for preoperative molecular imaging and fluorescence‐guided surgery for prostate cancer

The objective of this study was to report the synthesis and characteristics of a dual modality imaging agent, Tc‐99m GRFLTGGTGRLLRIS‐GHEG‐ECG‐K(‐5‐carboxy‐X‐rhodamine)‐NH₂ (GRFLT‐ECG‐ROX), and to verify its feasibility as both molecular imaging and intraoperative guidance agent. GRFLT‐ECG‐ROX was synthesized using Fmoc solid‐phase peptide synthesis. Radiolabeling of GRFLT‐ECG‐ROX with Tc‐99m was accomplished using ligand exchange via tartrate. Binding affinity and in vitro cellular uptake studies were performed. Gamma camera imaging, biodistribution, and ex vivo imaging studies were performed using LNCaP and PC‐3 tumor‐bearing murine models. Surgical removal of tumor nodules in murine models with peritoneal carcinomatosis was performed under a fluorescence imaging system. After radiolabeling procedures with Tc‐99m, Tc‐99m GRFLT‐ECG‐ROX complexes were prepared in high yield (>96%). The binding affinity value (K_d) of Tc‐99m GRFLT‐ECG‐ROX for LNCaP cells was estimated to be 9.5 ± 1.3 nM. In gamma camera imaging, the tumor to normal muscle uptake ratios of Tc‐99m GRFLT‐ECG‐ROX increased with time (3.1 ± 0.2, 4.0 ± 0.4, and 6.3 ± 0.9 at 1, 2, and 3 h, respectively). Under real‐time optical imaging, the removal of visible nodules was successfully performed. Thus, Tc‐99m GRFLT‐ECG‐ROX could provide both preoperative molecular imaging and fluorescence imaging guidance for tumor removal.     

A side reaction in solid phase synthesis Insertion of glycine residues into peptide chains via Nim→ Nα transfer

In solid-phase peptide synthesis using N^α-Boc-N^im-tosyl-histidine (Boc-His(Tos)), byproducts having extra Gly residues in the peptide chain were observed at a high rate. When a Boc-amino acid such as Asn was incorporated after assembly of Boc-His(Tos), the N^im-tosyl group was partially or fully cleaved by an activating agent, 1-hydroxybenzotriazole. In the successive coupling reactions, Boc-Gly was incorporated into the free N^im ring as well as the α-amino function, and the N^im-Gly was then transferred to the α-amino group of Gly of the peptide chain after removal of these Boc groups to give extra Gly residues at the position of Gly. This was observed in only the coupling reaction with Boc-Gly and could be circumvented using a more stable N^im protecting group for His, such as a dinitrophenyl group.     

Automated radiosynthesis of N‐(4‐[18F]fluorobenzyl)‐2‐bromoacetamide: an F‐18‐labeled reagent for the prosthetic radiolabeling of oligonucleotides

The potential for radiolabeled antisense oligonucleotides to image gene expression combined with the enhanced resolution of positron‐emission tomography justifies the continued interest in the development of oligonucleotides tagged with positron‐emitting radionuclides. The radiolabeling of oligonucleotides is a multi‐step process and may require handling large amounts of radioactivity initially. A previously reported method for radiolabeling oligonucleotides with N‐(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide was adapted for use in a commercially available automated synthesis unit by linking two reaction trains. The yield of N‐(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide ranged from 3 to 18% and the synthesis was completed within 1 h. Challenges in using this unit included the maintenance of anhydrous conditions for the effective reduction of 4‐[¹⁸F]fluorobenzonitrile. Preliminary results indicated that a mean yield of 36% could be obtained upon incubation of an oligonucleotide with N–(4‐[¹⁸F]fluorobenzyl)‐2‐bromoacetamide. The entire synthesis could be performed within 3 h. Copyright © 2008 John Wiley & Sons, Ltd.