Using 2 H labelling to improve the NMR detectability of pyridine and its derivatives by SABRE

Designing chemical systems for precision deuteration of medicinal building blocks

Methods are lacking that can prepare deuterium-enriched building blocks, in the full range of deuterium substitution patterns at the isotopic purity levels demanded by pharmaceutical use. To that end, this work explores the regio- and stereoselective deuteration of tetrahydropyridine (THP), which is an attractive target for study due to the wide prevalence of piperidines in drugs. A series of d0-d8 tetrahydropyridine isotopomers were synthesized by the stepwise treatment of a tungsten-complexed pyridinium salt with H-/D- and H+/D+. The resulting decomplexed THP isotopomers and isotopologues were analyzed via molecular rotational resonance (MRR) spectroscopy, a highly sensitive technique that distinguishes isotopomers and isotopologues by their unique moments of inertia. In order to demonstrate the medicinal relevance of this approach, eight unique deuterated isotopologues of erythro-methylphenidate were also prepared.

Real-Time High-Sensitivity Reaction Monitoring of Important Nitrogen-Cycle Synthons by 15N Hyperpolarized Nuclear Magnetic Resonance

Here, we show how signal amplification by reversible exchange hyperpolarization of a range of ¹⁵N-containing synthons can be used to enable studies of their reactivity by ¹⁵N nuclear magnetic resonance (NO₂^– (28% polarization), ND₃ (3%), PhCH₂NH₂ (5%), NaN₃ (3%), and NO₃^– (0.1%)). A range of iridium-based spin-polarization transfer catalysts are used, which for NO₂^– work optimally as an amino-derived carbene-containing complex with a DMAP-d₂ coligand. We harness long ¹⁵N spin-order lifetimes to probe in situ reactivity out to 3 × T₁. In the case of NO₂^– (T₁ 17.7 s at 9.4 T), we monitor PhNH₂ diazotization in acidic solution. The resulting diazonium salt (¹⁵N-T₁ 38 s) forms within 30 s, and its subsequent reaction with NaN₃ leads to the detection of hyperpolarized PhN₃ (T₁ 192 s) in a second step via the formation of an identified cyclic pentazole intermediate. The role of PhN₃ and NaN₃ in copper-free click chemistry is exemplified for hyperpolarized triazole (T₁ < 10 s) formation when they react with a strained alkyne. We also demonstrate simple routes to hyperpolarized N₂ in addition to showing how utilization of ¹⁵N-polarized PhCH₂NH₂ enables the probing of amidation, sulfonamidation, and imine formation. Hyperpolarized ND₃ is used to probe imine and ND₄⁺ (T₁ 33.6 s) formation. Furthermore, for NO₂^–, we also demonstrate how the ¹⁵N-magnetic resonance imaging monitoring of biphasic catalysis confirms the successful preparation of an aqueous bolus of hyperpolarized ¹⁵NO₂^– in seconds with 8% polarization. Hence, we create a versatile tool to probe organic transformations that has significant relevance for the synthesis of future hyperpolarized pharmaceuticals.

Rapid SABRE Catalyst Scavenging Using Functionalized Silicas

In recent years the NMR hyperpolarisation method signal amplification by reversible exchange (SABRE) has been applied to multiple substrates of potential interest for in vivo investigation. Unfortunately, SABRE commonly requires an iridium-containing catalyst that is unsuitable for biomedical applications. This report utilizes inductively coupled plasma-optical emission spectroscopy (ICP-OES) to investigate the potential use of metal scavengers to remove the iridium catalytic species from the solution. The most sensitive iridium emission line at 224.268 nm was used in the analysis. We report the effects of varying functionality, chain length, and scavenger support identity on iridium scavenging efficiency. The impact of varying the quantity of scavenger utilized is reported for the three scavengers with the highest iridium removed from initial investigations: 3-aminopropyl (S₁), 3-(imidazole-1-yl)propyl (S₄), and 2-(2-pyridyl) (S₅) functionalized silica gels. Exposure of an activated SABRE sample (1.6 mg mL^-1 of iridium catalyst) to 10 mg of the most promising scavenger (S₅) resulted in <1 ppm of iridium being detectable by ICP-OES after 2 min of exposure. We propose that combining the approach described herein with other recently reported approaches, such as catalyst separated-SABRE (CASH-SABRE), would enable the rapid preparation of a biocompatible SABRE hyperpolarized bolus.

Selective NMR detection of individual reaction components hyperpolarised by reversible exchange with para-hydrogen

NMR spectroscopy can sometimes be hampered by two inherent weaknesses: low sensitivity and overlap of signals in complex mixtures. Hyperpolarisation techniques using para-hydrogen (including the method known as SABRE) can...

Optimisation of pyruvate hyperpolarisation using SABRE by tuning the active magnetisation transfer catalyst

Hyperpolarisation techniques such as signal amplification by reversible exchange (SABRE) can deliver NMR signals several orders of magnitude larger than those derived under Boltzmann conditions. SABRE is able to catalytically transfer latent magnetisation from para-hydrogen to a substrate in reversible exchange via temporary associations with an iridium complex. SABRE has recently been applied to the hyperpolarisation of pyruvate, a substrate often used in many in vivo MRI studies. In this work, we seek to optimise the pyruvate-13C2 signal gains delivered through SABRE by fine tuning the properties of the active polarisation transfer catalyst. We present a detailed study of the effects of varying the carbene and sulfoxide ligands on the formation and behaviour of the active [Ir(H)2(η2-pyruvate)(sulfoxide)(NHC)] catalyst to produce a rationale for achieving high pyruvate signal gains in a cheap and refreshable manner. This optimisation approach allows us to achieve signal enhancements of 2140 and 2125-fold for the 1-13C and 2-13C sites respectively of sodium pyruvate-1,2-[13C2].

Hyperpolarising Pyruvate through Signal Amplification by Reversible Exchange (SABRE)

Hyperpolarisation methods that premagnetise agents such as pyruvate are currently receiving significant attention because they produce sensitivity gains that allow disease tracking and interrogation of cellular metabolism by magnetic resonance. Here, we communicate how signal amplification by reversible exchange (SABRE) can provide strong 13 C pyruvate signal enhancements in seconds through the formation of the novel polarisation transfer catalyst [Ir(H)2 (η2 -pyruvate)(DMSO)(IMes)]. By harnessing SABRE, strong signals for [1-13 C]- and [2-13 C]pyruvate in addition to a long-lived singlet state in the [1,2-13 C2 ] form are readily created; the latter can be observed five minutes after the initial hyperpolarisation step. We also demonstrate how this development may help with future studies of chemical reactivity.

Simulating Non-linear Chemical and Physical (CAP) Dynamics of Signal Amplification By Reversible Exchange (SABRE)

The hyperpolarization of nuclear spins by using parahydrogen (pH₂ ) is a fascinating technique that allows spin polarization and thus the magnetic resonance signal to be increased by several orders of magnitude. Entirely new applications have become available. Signal amplification by reversible exchange (SABRE) is a relatively new method that is based on the reversible exchange of a substrate, catalyst and parahydrogen. SABRE is particularly interesting for in vivo medical and industrial applications, such as fast and low-cost trace analysis or continuous signal enhancement. Ever since its discovery, many attempts have been made to model and understand SABRE, with various degrees of simplifications. In this work, we reduced the simplifications further, taking into account non-linear chemical and physical (CAP) dynamics of several multi-spin systems. A master equation was derived and realized using the MOIN open-source software. The effects of different parameters (exchange rates, concentrations, spin-spin couplings) on relaxation and the polarization level have been evaluated and the results provide interesting insights into the mechanism of SABRE.

Effects of Deuteration of 13C-Enriched Phospholactate on Efficiency of Parahydrogen-Induced Polarization by Magnetic Field Cycling

We report herein a large-scale (>10 g) synthesis of isotopically enriched 1-¹³C-phosphoenolpyruvate and 1-¹³C-phosphoenolpyruvate-d₂ for application in hyperpolarized imaging technology. The 1-¹³C-phosphoenolpyruvate-d₂ was synthesized with 57% overall yield (over two steps), and >98% ²H isotopic purity, representing an improvement over the previous report. The same outcome was achieved for 1-¹³C-phosphoenolpyruvate. These two unsaturated compounds with C=C bonds were employed for parahydrogen-induced polarization via pairwise parahydrogen addition in aqueous medium. We find that deuteration of 1-¹³C-phosphoenolpyruvate resulted in overall increase of ¹H T₁ of nascent hyperpolarized protons (4.30 ± 0.04 s versus 2.06 ± 0.01 s) and ¹H polarization (~2.5% versus ~0.7%) of the resulting hyperpolarized 1-¹³C-phospholactate. The nuclear spin polarization of nascent parahydrogen-derived protons was transferred to 1-¹³C nucleus via magnetic field cycling procedure. The proton T₁ increase in hyperpolarized deuterated 1-¹³C-phospholactate yielded approximately 30% better ¹³C polarization compared to non-deuterated hyperpolarized 1-¹³C-phospholactate. Analysis of T₁ relaxation revealed that deuteration of 1-¹³C-phospholactate may have resulted in approximately 3-fold worse H→¹³C polarization transfer efficiency via magnetic field cycling. Since magnetic field cycling is a key polarization transfer step in the Side-Arm Hydrogenation approach, the presented findings may guide more rationale design of contrast agents using parahydrogen polarization of a broad range of ¹³C hyperpolarized contrast agents for molecular imaging employing ¹³C MRI. The hyperpolarized 1-¹³C-phospholactate-d₂ is of biomedical imaging relevance because it undergoes in vivo dephosphorylation and becomes ¹³C hyperpolarized lactate, which as we show can be detected in the brain using ¹³C hyperpolarized MRI; an implication for future imaging of neurodegenerative diseases and dementia.

Fine-tuning the efficiency of para-hydrogen-induced hyperpolarization by rational N-heterocyclic carbene design

Iridium N-heterocyclic carbene (NHC) complexes catalyse the para-hydrogen-induced hyperpolarization process, Signal Amplification by Reversible Exchange (SABRE). This process transfers the latent magnetism of para-hydrogen into a substrate, without changing its chemical identity, to dramatically improve its nuclear magnetic resonance (NMR) detectability. By synthesizing and examining over 30 NHC containing complexes, here we rationalize the key characteristics of efficient SABRE catalysis prior to using appropriate catalyst-substrate combinations to quantify the substrate's NMR detectability. These optimizations deliver polarizations of 63% for 1H nuclei in methyl 4,6-d2-nicotinate, 25% for 13C nuclei in a 13C2-diphenylpyridazine and 43% for the 15N nucleus of pyridine-15N. These high detectability levels compare favourably with the 0.0005% 1H value harnessed by a routine 1.5 T clinical MRI system. As signal strength scales with the square of the number of observations, these low cost innovations offer remarkable improvements in detectability threshold that offer routes to significantly reduce measurement time.