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Iminium-functionalized 2,3-dihydroisoxazoles (4-isoxazolines) were prepared in high yields by [3+2] cycloaddition of N-methyl-C-phenylnitrone and acetylenic iminium salts. Conversion of the very hygroscopic cycloadducts into neutral compounds was achieved by hydrolysis or hydride reduction of the iminium functional group. By a thermal rearrangement, the cycloadducts were converted into 2,3-dihydro-1H-imidazolium salts. With C,N-diphenylnitrone as the reaction partner, the expected cycloadduct, a [(2,3-dihydroisoxazol-4-yl)-1-(phenyl)methylene]-N,N-dimethylammonium triflate, could be prepared at −75°C and was characterized by NMR. This product was found to be thermally unstable above −30°C, being converted into an inseparable product mixture. Basic hydrolysis of this mixture afforded a structurally interesting acyclic enaminone/half-aminal, which is likely derived from a 2,3-dihydro-1,3-oxazole.

Carbon-13 hyperpolarized pyruvate is about to become the next-generation contrast agent for molecular magnetic resonance imaging of cancer and other diseases. Here, efficient and rapid pyruvate hyperpolarization is achieved via Signal Amplification by Reversible Exchange (SABRE) with parahydrogen through synergistic use of substrate deuteration, alternating, and static microtesla magnetic fields. Up to 22% and 6% long-lasting 13C polarization (T1=3.7±0.25min and T1=1.7±0.1min) is demonstrated for the C1 and C2 nuclear sites, respectively. The remarkable polarization levels become possible due to favorable relaxation dynamics at the microtesla fields. The ultra-long polarization lifetimes will be conducive to yielding high polarization after purification, quality assurance, and injection of the hyperpolarized molecular imaging probes. These results pave the way to future in vivo translation of carbon-13 hyperpolarized molecular imaging probes prepared by this approach.

1,3-Bis(trifluoromethyl)prop-2-ene 1-iminium triflate salts were prepared for the first time and some synthetic applications as 1,3-biselectrophilic building blocks were established. They were found to react with dimethoxybenzenes or methylene-1,2-dioxybenzenes to furnish vinylogous trifluoroacetylation products (4-aryl-1,1,1,5,5,5-hexafluoropent-3-en-2-ones) and 1-dialkylamino-1,3-bis(trifluoromethyl)-1H-indenes. With aniline and ring-substituted anilines, 2,4-bis(trifluoromethyl)quinolines were formed. An unusual 4H-pyran, formally a condensation product of the N,N-dimethyl-1,3-bis(trifluoromethyl)prop-2-en-1-iminium ion and its enaminone precursor, is also reported.

3-(Trifluoromethyl)prop-2-yne 1-iminium triflate salts were generated from 3-trifloxy-3-(trifluoromethyl)prop-2-ene 1-iminium salts by triethylamine-assisted elimination of triflic acid and trapped in situ in various cycloaddition reactions. 3-(Trifluoromethyl)prop-2-yne 1-iminium triflate salts were generated from 3-trifloxy-3-(trifluoromethyl)prop-2-ene 1-iminium salts by triethylamine-assisted elimination of triflic acid and trapped in situ in various cycloaddition reactions. These included Diels-Alder reactions with 1,3-dienes, [2+2] cycloaddition with 1,4-diphenylbuta-1,3-diene, and [3+2] cycloaddition with organoazides. All these reactions revealed the excellent dienophilic and dipolarophilic reactivity of the electrophilic C,C triple bond of the novel 3-CF3-propyniminium salts., publishedVersion

Magnetic resonance imaging of $^{13}$C-labeled metabolites enhanced by parahydrogen-induced polarization (PHIP) can enable real-time monitoring of processes within the body. We introduce a robust, easily implementable technique for transferring parahydrogen-derived singlet order into 13C magnetization using adiabatic radio-frequency sweeps at $\mu$T fields. We experimentally demonstrate the applicability of this technique to several molecules, including some molecules relevant for metabolic imaging, where we show significant improvements in the achievable polarization, in some cases reaching above 60%. Furthermore, we introduce a site-selective deuteration scheme, where deuterium is included in the coupling network of a pyruvate ester to enhance the efficiency of the polarization transfer. These improvements are enabled by the fact that the transfer protocol avoids relaxation induced by strongly coupled quadrupolar nuclei.

3-Aryl-1-(trifluoromethyl)prop-2-yn-1-iminium triflate salts represent a novel, highly reactive class of acetylenic iminium salts. Herein we present several reactions which are based on the electron-poor acetylenic bond and on the high electrophilicity of the CF3-substituted iminium group. These salts were found to be highly reactive dienophiles in Diels–Alder reactions with cyclopentadiene, 2,3-dimethylbutadiene and even anthracene. At higher temperature, the cycloadducts undergo an intramolecular SE(Ar) reaction leading to condensed carbocycles incorporating a 1-(trifluoromethyl)-1-(dimethylamine)indene ring system. With styrenes and some substituted styrenes, cascade reactions take place, which likely include cyclobutene and several cationic intermediates and mainly yield 2-(1-phenylvinyl)indenes. In a similar reaction cascade, a fulvene derivative was obtained with 1,4-diphenylbutadiene as the substrate.

N-Phenyl-N-triflylpropiolamides react with iodine chloride or iodine bromide by an intramolecular electrophilic ipso-halocyclization/nucleophilic halide addition sequence to furnish cyclohexadiene-spiro-γ-lactams. These products can undergo cleavage of the amide bond with primary amines and of the N–Cspiro bond with secondary amines, leading to N-alkyl-2-iodo-3-phenylacrylamides and N-(4-halophenyl)-2-iodo-3-(2-triflylamino)phenylacrylamides, respectively.

Nuclear spin hyperpolarization provides a promising route to overcome the challenges imposed by the limited sensitivity of nuclear magnetic resonance. Here we demonstrate that dissolution of spin-polarized pentacene-doped naphthalene crystals enables transfer of polarization to target molecules via intermolecular cross relaxation at room temperature and moderate magnetic fields (1.45$\,$T). This makes it possible to exploit the high spin polarization of optically polarized crystals while mitigating the challenges of its transfer to external nuclei, particularly of the large distances and prohibitively weak coupling between source and target nuclei across solid-solid or solid-liquid interfaces. With this method, here we inject the highly polarized mixture into a benchtop NMR spectrometer and observe the polarization dynamics for target $^1$H nuclei. Although the spectra are radiation damped due to the high naphthalene magnetization, we describe a procedure to process the data in order to obtain more conventional NMR spectra, and extract the target nuclei polarization. With the entire process occurring on a timescale of one minute, we observe NMR signals enhanced by factors between -200 and -1730 at 1.45$\,$T for a range of small molecules., 8 pages, 4 figures

Acetylenic iminium triflates with the general formula [R–C≡ C–C(Ar)=N+R2 TfO−] were found to be excellent dipolarophiles in [3+ 2] cycloaddition reactions with diazoacetates leading to (1H-pyrazol-3(5)-yl)methanaminium triflates in high yields. The terminal acetylenic iminium salt (propyne iminium salt) [HC≡C–C(Ph)=N+Me2 TfO−] reacted with an equimolar amount of methyl diazoacetate instantaneously at 20°C to form the expected pyrazole in almost quantitative yield. When a 2:1 stoichiometry was applied, subsequent Michael addition of the pyrazole at the alkyne occurred and the bis(iminium) ditriflate 4 was obtained in high yield. By hydride reduction or hydrolysis of the iminium group, some of the highly hygroscopic pyrazole iminium salts were converted into neutral, twofold functionalized, di- and tri-C-substitued 1H-pyrazoles.