PET ligands [18F]LSN3316612 and [11C]LSN3316612 quantify O-linked-ß-N-acetyl-glucosamine hydrolase in the brain.

We aimed to develop effective radioligands for quantifying brain O-linked-ß-N-acetyl-glucosamine (O-GlcNAc) hydrolase (OGA) using positron emission tomography in living subjects as tools for evaluating drug target engagement. Posttranslational modifications of tau, a biomarker of Alzheimer's disease, by O-GlcNAc through the enzyme pair OGA and O-GlcNAc transferase (OGT) are inversely related to the amounts of its insoluble hyperphosphorylated form. Increase in tau O-GlcNAcylation by OGA inhibition is believed to reduce tau aggregation. LSN3316612, a highly selective and potent OGA ligand [half-maximal inhibitory concentration (IC50) = 1.9 nM], emerged as a lead ligand after in silico analysis and in vitro evaluations. [3H]LSN3316612 imaged and quantified OGA in postmortem brains of rat, monkey, and human. The presence of fluorine and carbonyl functionality in LSN3316612 enabled labeling with positron-emitting fluorine-18 or carbon-11. Both [18F]LSN3316612 and [11C]LSN3316612 bound reversibly to OGA in vivo, and such binding was blocked by pharmacological doses of thiamet G, an OGA inhibitor of different chemotype, in monkeys. [18F]LSN3316612 entered healthy human brain avidly (~4 SUV) without radiodefluorination or adverse effect from other radiometabolites, as evidenced by stable brain total volume of distribution (VT) values by 110 min of scanning. Overall, [18F]LSN3316612 is preferred over [11C]LSN3316612 for future human studies, whereas either may be an effective positron emission tomography radioligand for quantifying brain OGA in rodent and monkey.

Rhodium catalysed C-3/5 methylation of pyridines using temporary dearomatisation.

Pyridines are ubiquitous aromatic rings used in organic chemistry and are crucial elements of the drug discovery process. Herein we describe a new catalytic method that directly introduces a methyl group onto the aromatic ring; this new reaction is related to hydrogen borrowing, and is notable for its use of the feedstock chemicals methanol and formaldehyde as the key reagents. Conceptually, the C-3/5 methylation of pyridines was accomplished by exploiting the interface between aromatic and non-aromatic compounds, and this allows an oscillating reactivity pattern to emerge whereby normally electrophilic aromatic compounds become nucleophilic in the reaction after activation by reduction. Thus, a set of C-4 functionalised pyridines can be mono or doubly methylated at the C-3/5 positions.

Secondary Products from Intramolecular Cycloadditions of Azidoalkyl Enol Ethers and Azidoalkyl Vinyl Bromides: 1-Azadienes, Their Reactions with Diphenylketene, and Radical Cyclizations To Form Bi- and Tricyclic Lactams.

Azidoalkyl enol ethers undergo intramolecular 1,3-dipolar cycloaddition to generate stable triazolines; in contrast, the cycloadducts formed by heating analogous azidoalkyl vinyl bromides are unstable with respect to elimination of N2 and HBr, affording 1-azadienes (2-alkenyl cyclic imines). These primary products may be isolated or treated directly with diphenylketene to produce bi- and tricyclic 3,4-dihydropyridin-2(1H)-ones; similar ring systems may also be produced from the azadienes by N-acylation and radical cyclization.

Transition-Metal-Free Reductive Hydroxymethylation of Isoquinolines.

A transition-metal-free reductive hydroxymethylation reaction has been developed, enabling the preparation of tetrahydroisoquinolines bearing C4-quaternary centers from the corresponding isoquinolines. Deuterium labelling studies and control experiments enable a potential mechanism to be elucidated which features a key Cannizzaro-type reduction followed by an Evans-Tishchenko reaction. When isoquinolines featuring a proton at the 4-position are used, a tandem methylation-hydroxymethylation occurs, leading to the formation of 2 new C-C bonds in one pot.

The reductive C3 functionalization of pyridinium and quinolinium salts through iridium-catalysed interrupted transfer hydrogenation.

Aromatic rings are ubiquitous in organic chemistry and form the basis of many commercial products. Despite the numerous routes available for the preparation of aromatic compounds, there remain few methods that allow their conversion into synthetically useful partially saturated derivatives and even fewer that allow new C-C bonds to be formed at the same time. Here we set out to address this problem and uncover a unique catalytic partial reduction reaction that forms partially saturated azaheterocycles from aromatic precursors. In this reaction, methanol and formaldehyde are used for the reductive functionalization of pyridines and quinolines using catalytic iridium; thus, inexpensive and renewable feedstocks are utilized in the formation of complex N-heterocycles. By harnessing the formation of a nucleophilic enamine intermediate, the C-C bond-forming process reverses the normal pattern of reactivity and allows access to the C3 position of the arene. Mechanistic investigations using D-labelling experiments reveal the source of hydride added to the ring and show the reversible nature of the iridium-hydride addition.

A Flexible Strategy for the Regiocontrolled Synthesis of Pyrazolo[1,5-a]pyrazines.

A four-step protocol for the synthesis of pyrazolo[1,5-a]pyrazines has been developed. Commercially available pyrazoles were alkylated and formylated in a regiocontrolled manner to give pyrazole-5-aldehydes bearing 2,2-dialkoxyethyl substitution on N-1. Efficient conditions for the subsequent deprotection and cyclization of these intermediates allowed access to pyrazolo[1,5-a]pyrazines with multiple substitution patterns. The versatility of the pyrazole-5-aldehyde intermediates was further demonstrated through a deprotection and double-reductive amination sequence to give 4,5,6,7-tetrahydropyrazolo[1,5-a]pyrazines.

Two-step cyanomethylation protocol: convenient access to functionalized aryl- and heteroarylacetonitriles.

A two-step protocol has been developed for the introduction of cyanomethylene groups to metalated aromatics through the intermediacy of substituted isoxazoles. A palladium-mediated cross-coupling reaction was used to introduce the isoxazole unit, followed by release of the cyanomethylene function under thermal or microwave-assisted conditions. The intermediate isoxazoles were shown to be amenable to further functionalization prior to deprotection of the sensitive cyanomethylene motif, allowing access to a wide range of aryl- and heteroaryl-substituted acetonitrile building blocks.

New modes for the osmium-catalyzed oxidative cyclization.

The osmium-catalyzed oxidative cyclization of amino alcohol initiators formally derived from 1,4-dienes is an effective method for the construction of pyrrolidines, utilizing a novel reoxidant (4-nitropyridine N-oxide = NPNO). The cyclization of enantiopure syn- and anti-amino alcohols gives rise to enantiopure cis- and trans-2,5-disubstituted pyrrolidines, respectively. Moreover, the cyclization of bis-homoallylic amines bearing an exocyclic chelating group is shown to be a complementary method for trans-pyrrolidine formation.

Osmium-mediated oxidative cyclizations: a study into the range of initiators that facilitate cyclization.

A general route to prepare substituted, saturated five-membered heterocycles has been developed. The application of a wide range of starting materials to the osmium-catalyzed oxidative cyclization reaction is described. Diols, hydroxy-amides, hydroxy-sulfonamides, and carbamates all cyclize in moderate to excellent yields to give cis-tetrahydrofurans and pyrrolidines, depending upon the position of the heteroatoms in the starting materials. These cyclizations all proceed with near total selectivity for the cis-heterocycles, and with stereospecific introduction of a hydroxy group adjacent to the ring. Moreover, routes to enantiopure starting materials are described, which give enantiopure products upon cyclization. Catalyst loadings of as low as one mol percent have been successfully employed for this transformation.