A synthetic and spectroscopic investigation of the asymmetric α-lithiation-trapping of six-membered N-Boc heterocycles using Alexakis diamines

The asymmetric lithiation-trapping of six-membered N-Boc heterocycles using s-BuLi and two different Alexakis diamines is reported. These readily available ligands outperform the current ‘best-in-class’ sparteine-type diamines in the lithiation and benzophenone trapping of N-Boc piperazines and the lithiation-cyclisation-trapping of N-Boc-4-chloropiperidine. In situ IR spectroscopy has been used to optimise lithiation times and to discover previously unknown subtleties regarding the lithiation step

A Simple and Cost-efficient Technique to Generate Hyperpolarized Long-lived 15N-15N Nuclear Spin Order in a Diazine by Signal Amplification by Reversible Exchange

Signal Amplification by Reversible Exchange (SABRE) is an inexpensive and simple hyperpolarization technique and is capable of boosting Nuclear Magnetic Resonance (NMR) sensitivity by several orders of magnitude. It utilizes the reversible binding of para-hydrogen as hydride ligands and a substrate of interest to a metal catalyst to allow polarization transfer from para-hydrogen to the substrate nuclear spins. The nuclear spin lifetime of the created magnetization sets a strict upper limit on experimental timeframe. Short nuclear spin lifetimes are therefore a challenge for hyperpolarized metabolic imaging prospects. In this report we demonstrate how hyperpolarization and long nuclear spin lifetime can simultaneously be achieved in nitrogen-15 containing pyridazine and phthalazine derivatives by SABRE. These reflect two distinct classes of 15N2-coupled species with respect to their chemical symmetry and thus show different nuclear spin lifetime with the pyridazine derivative having a singlet state lifetime of ca. 2.5 minutes, produced with a signal enhancement of ca. 2,700. In contrast the phthalazine derivative yields a superior 15,000-fold enhancement at 11.7 T but has a much shorter singlet lifetime

The Detection and Reactivity of Silanols and Silanes Using Hyperpolarized 29Si Nuclear Magnetic Resonance

Silanols and silanes are key precursors and intermediates for the synthesis of silicon-based materials. While their characterization and quantification using 29Si NMR has received significant attention, it is a technique that is limited by the low natural abundance of 29Si and its low sensitivity. Here, we describe a method using p-H2 to hyperpolarize 29Si. The observed signal enhancements, approaching 3000-fold at 11.7 T, would take many days of measurement for comparable results under Boltzmann conditions. The resulting signals are exploited to monitor the rapid reaction of tris(tert-butoxy)silanol with triflic anhydride in a T1 corrected process that allows for rapid quantification. These results demonstrate a novel route to quantify dynamic processes and intermediates in the synthesis of silicon materials

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. It 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 rational 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]

Steric and Electronic Effects on the 1H Hyperpolarisation of Substituted Pyridazines by Signal Amplification by Reversible Exchange

Utility of the pyridazine motif is growing in popularity as pharmaceutical and agrochemical agents. The detection and structural characterisation of such materials is therefore imperative for the successful development of new products. Signal Amplification by Reversible Exchange (SABRE) offers a route to dramatically improve the sensitivity of magnetic resonance methods and we apply it here to the rapid and cost effective hyperpolarisation of substituted pyridazines. The 33 substrates investigated cover a range of steric and electronic properties and their capacity to perform highly effective SABRE to be assessed. We find the method to be tolerant to a broad range of electron donating and withdrawing groups, however, sensitivity is evident when steric bulk is added to the 3 and 6-positions of the pyridazine ring. We optimise the method by reference to a disubstituted ester that yields signal gains of >9 000-fold at 9.4 T (>28% spin polarisation)

Hyperpolarisation of weakly binding N-heterocycles using Signal Amplification by Reversible Exchange

Signal Amplification by Reversible Exchange (SABRE) is a catalytic method for improving the detection of molecules by magnetic resonance spectroscopy. It achieves this by simultaneously binding the target substrate (sub) and para-hydrogen to a metal centre. To date, sterically large substrates are relatively inacessable to SABRE due to their weak binding leading to catalyst destabilisation. We overcome this problem here through a simple co-ligand strategy that allows the hyperpolarisation of a range of weakly binding and sterically encumbered N-heterocycles. The resulting 1H NMR signal size is increased by up to 1400 times relative to their more usual Boltzmann controlled levels at 400 MHz. Hence, a significant reduction in scan time is achieved. The SABRE catalyst in these systems takes the form [IrX(H)2(NHC)(sulfoxide)(sub)] where X = Cl, Br or I. These complexes are shown to undergo very rapid ligand exchange and lower temperatures dramatically improves the efficiency of these SABRE catalysts

Quantification of hyperpolarisation efficiency in SABRE and SABRE-Relay enhanced NMR spectroscopy

para-Hydrogen (p-H 2) induced polarisation (PHIP) is an increasingly popular method for sensitivity enhancement in NMR spectroscopy. Its growing popularity is due in part to the introduction of the signal amplification by reversible exchange (SABRE) method that generates renewable hyperpolarisation in target analytes in seconds. A key benefit of PHIP and SABRE is that p-H 2 can be relatively easily and cheaply produced, with costs increasing with the desired level of p-H 2 purity. In this work, the efficiency of the SABRE polarisation transfer is explored by measuring the level of analyte hyperpolarisation as a function of the level of p-H 2 enrichment. A linear relationship was found between p-H 2 enrichment and analyte 1H hyperpolarisation for a range of molecules, polarisation transfer catalysts, NMR detection fields and for both the SABRE and SABRE-Relay transfer mechanisms over the range 29-99% p-H 2 purity. The gradient of these linear relationships were related to a simple theoretical model to define an overall efficiency parameter, E, that quantifies the net fraction of the available p-H 2 polarisation that is transferred to the target analyte. We find that the efficiency of SABRE is independent of the NMR detection field and exceeds E = 20% for methyl-4,6-d 2-nicotinate when using a previously optimised catalyst system. For the SABRE-Relay transfer mechanism, efficiencies of up to E = 1% were found for 1H polarisation of 1-propanol, when ammonia was used as the polarisation carrier

Pharmacokinetics of the SABRE agent 4,6-d2-Nicotinamide and also Nicotinamide in rats following oral and intravenous administration

To prepare the way for using the isotopically labelled SABRE hyperpolarised 4,6-d2-nicotinamide as an MRI agent in humans we have performed an in-vivo study to measure its pharmacokinetics in the plasma of healthy rats after intravenous and oral administration. Male Han Wistar rats were dosed with either 4,6-d2-nicotinamide or the corresponding control, non-labelled nicotinamide, and plasma samples were obtained at eight time points for up to 24 hours after administration. Pharmacokinetic parameters were determined from agent concentration-versus-time data for both 4,6-d2-nicotinamide and nicotinamide. 4,6-d2-nicotinamide proved to be well tolerated regardless of route of administration at the concentrations used (20, 80 and 120 mg/kg). Pharmacokinetic parameters were similar after oral and intravenous administration and similar to those obtained for nicotinamide. Analysis of nicotinamide plasma concentrations after dosing 4,6-d2-nicotinamide intravenously demonstrates a reversible exchange of endogenous nicotinamide by this labelled agent over the time-course of our assays. Supported by a large body of evidence for the safety of nicotinamide when dosed orally in humans, we conclude that 4,6-d2-nicotinamide can also be safely administered intravenously, which will provide significant benefit when using this agent for planned imaging studies in humans

Prion propagation is dependent on key amino acids in Charge cluster 2 within the prion protein

To dissect the N-terminal residues within the cellular prion protein (PrPC) that are critical for efficient prion propagation, we generated a library of point, double, or triple alanine replacements within residues 23-111 of PrP, stably expressed them in cells silenced for endogenous mouse PrPC and challenged the reconstituted cells with four common but biologically diverse mouse prion strains. Amino acids (aa) 105-111 of Charge Cluster 2 (CC2), which is disordered in PrPC, were found to be required for propagation of all four prion strains; other residues had no effect or exhibited strain-specific effects. Replacements in CC2, including aa105-111, dominantly inhibited prion propagation in the presence of endogenous wild type PrPC whilst other changes were not inhibitory. Single alanine replacements within aa105-111 identified leucine 108 and valine 111 or the cluster of lysine 105, threonine 106 and asparagine 107 as critical for prion propagation. These residues mediate specific ordering of unstructured CC2 into β-sheets in the infectious prion fibrils from Rocky Mountain Laboratory (RML) and ME7 mouse prion strains