Synthesis of Substituted Pyridines via Formal (3+3) Cycloaddition of Enamines with Unsaturated Aldehydes and Ketones.

An organocatalyzed, formal (3+3) cycloaddition reaction is described for the practical synthesis of substituted pyridines. Starting from readily available enamines and enal/ynal/enone substrates, the protocol affords tri- or tetrasubstituted pyridine scaffolds bearing various functional groups. This method was demonstrated on a 50 g scale, enabling the synthesis of 2-isopropyl-4-methylpyridin-3-amine, a raw material used for the manufacture of sotorasib. Mechanistic analysis using two-dimensional nuclear magnetic resonance (NMR) spectrometry revealed the transformation proceeds through the reversible formation of a stable reaction off-cycle species that precedes pyridine formation. In situ reaction progress kinetic analysis and control NMR studies were employed to better understand the role of FeCl3 and pyrrolidine hydrochloride in promoting the reaction.

Telescoped Process to Manufacture 6,6,6-Trifluorofucose via Diastereoselective Transfer Hydrogenation: Scalable Access to an Inhibitor of Fucosylation Utilized in Monoclonal Antibody Production.

IgG1 monoclonal antibodies with reduced glycan fucosylation have been shown to improve antibody-dependent cellular cytotoxicity (ADCC) by allowing more effective binding of the Fc region of these proteins to T cells receptors. Increased in vivo efficacy in animal models and oncology clinical trials has been associated with the enhanced ADCC provided by these engineered mAbs. 6,6,6-Trifluorofucose (1) is a new inhibitor of fucosylation that has been demonstrated to allow the preparation of IgG1 monoclonal antibodies with lower fucosylation levels and thus improve the ADCC of these proteins. A new process has been developed to support the preparation of 1 on large-scale for wide mAb manufacture applications. The target fucosylation inhibitor (1) was synthesized from readily available d-arabinose in 11% overall yield and >99.5/0.5 dr (diastereomeric ratio). The heavily telescoped process includes seven steps, two crystallizations as purification handles, and no chromatography. The key transformation of the sequence involves the diastereoselective preparation of the desired trifluoromethyl-bearing alcohol in >9/1 dr from a trimethylsilylketal intermediate via a ruthenium-catalyzed tandem ketal hydrolysis-transfer hydrogenation process.

Pyridine N-Oxide vs Pyridine Substrates for Rh(III)-Catalyzed Oxidative C-H Bond Functionalization.

The origin of the high reactivity and site selectivity of pyridine N-oxide substrates in O-pivaloyl hydroxamic acid-directed Rh(III)-catalyzed (4+2) annulation reactions with alkynes was investigated computationally. The reactions of the analogous pyridine derivatives were previously reported to be slower and to display poor site selectivity for functionalization of the C(2)-H vs the C(4)-H bonds of the pyridine ring. The N-oxide substrates are found to be more reactive overall because the directing group interacts more strongly with Rh. For N-oxide substrates, alkyne insertion is rate-limiting and selectivity-determining in the reaction with a dialkyl alkyne, but C-H activation can be selectivity-determining with other coupling partners such as terminal alkynes. The rates of reaction with a dialkyl alkyne at the two sites of a pyridine substrate are limited by two different steps: C-H activation is limiting for C(2)-functionalization, while alkyne insertion is limiting for C(4)-functionalization. Consistent with the observed poor site selectivity in the reaction of a pyridine substrate, the overall energy barriers for functionalization of the two positions are nearly identical. High C(2)-selectivity in the C-H activation step of the reaction of the N-oxide is due to a cooperative effect of the C-H Brønsted acidity, the strength of the forming C-Rh bond, and intramolecular electrostatic interactions between the [Rh]Cp* and the heteroaryl moieties. On the other hand, some of these forces are in opposition in the case of the pyridine substrate, and C(4)-H activation is moderately favored overall. The alkyne insertion step is favored at C(2) over C(4) for both substrates, and this preference is largely influenced by electrostatic interactions between the alkyne and the heteroarene. Experimental results that support these calculations, including kinetic isotope effect studies, H/D exchange studies, and results using a substituted pyridine, are also described.

AMG 580: a novel small molecule phosphodiesterase 10A (PDE10A) positron emission tomography tracer.

Phosphodiesterase 10A (PDE10A) inhibitors have therapeutic potential for the treatment of psychiatric and neurologic disorders, such as schizophrenia and Huntington's disease. One of the key requirements for successful central nervous system drug development is to demonstrate target coverage of therapeutic candidates in brain for lead optimization in the drug discovery phase and for assisting dose selection in clinical development. Therefore, we identified AMG 580 [1-(4-(3-(4-(1H-benzo[d]imidazole-2-carbonyl)phenoxy)pyrazin-2-yl)piperidin-1-yl)-2-fluoropropan-1-one], a novel, selective small-molecule antagonist with subnanomolar affinity for rat, primate, and human PDE10A. We showed that AMG 580 is suitable as a tracer for lead optimization to determine target coverage by novel PDE10A inhibitors using triple-stage quadrupole liquid chromatography-tandem mass spectrometry technology. [(3)H]AMG 580 bound with high affinity in a specific and saturable manner to both striatal homogenates and brain slices from rats, baboons, and human in vitro. Moreover, [(18)F]AMG 580 demonstrated prominent uptake by positron emission tomography in rats, suggesting that radiolabeled AMG 580 may be suitable for further development as a noninvasive radiotracer for target coverage measurements in clinical studies. These results indicate that AMG 580 is a potential imaging biomarker for mapping PDE10A distribution and ensuring target coverage by therapeutic PDE10A inhibitors in clinical studies.

Rh(III)-catalyzed C-H activation and double directing group strategy for the regioselective synthesis of naphthyridinones.

A general Rh(III)-catalyzed synthesis of naphthyridinone derivatives is described. It relies on a double-activation and directing approach leveraging nicotinamide N-oxides as substrates. In general, high yields and selectivities can be achieved using low catalyst loadings and mild conditions (room temperature) in the couplings with alkynes, while alkenes require slightly more elevated temperatures.

Total synthesis of bryostatins: the development of methodology for the atom-economic and stereoselective synthesis of the ring C subunit.

Bryostatins, a family of structurally complicated macrolides, exhibit an exceptional range of biological activities. The limited availability and structural complexity of these molecules makes development of an efficient total synthesis particularly important. This article describes our initial efforts towards the total synthesis of bryostatins, in which chemoselective and atom-economical methods for the stereoselective assembly of the ring C subunit were developed. A Pd-catalyzed tandem alkyne-alkyne coupling/6-endo-dig cyclization sequence was explored and successfully pursued in the synthesis of a dihydropyran ring system. Elaboration of this methodology ultimately led to a concise synthesis of the ring C subunit of bryostatins.

Iodide-catalyzed reductions: development of a synthesis of phenylacetic acids.

A new convenient and scalable synthesis of phenylacetic acids has been developed via the iodide catalyzed reduction of mandelic acids. The procedure relies on in situ generation of hydroiodic acid from catalytic sodium iodide, employing phosphorus acid as the stoichiometric reductant.

An efficient and scalable Ritter reaction for the synthesis of tert-butyl amides.

A scalable procedure for the conversion of nitriles to N-tert-butyl amides via the Ritter reaction was optimized employing tert-butyl acetate and acetic acid. The reaction has a broad scope for aromatic, alkyl, and alpha,beta-unsaturated nitriles.

Practical synthesis of a p38 MAP kinase inhibitor.

p38 MAP kinase inhibitors have attracted considerable interest as potential agents for the treatment of inflammatory diseases. Herein, we describe a concise and efficient synthesis of inhibitor 1 that is based on a phthalazine scaffold. Highlights of our approach include a practical synthesis of a 1,6-disubstituted phthalazine building block 24 as well as the one-pot formation of boronic acid 27. Significant synthetic work to understand the reactivity principles of the intermediates helped in selection of the final synthetic route. Subsequent optimization of the individual steps of the final sequence led to a practical synthesis of 1.

Selective ortho methylation of nitroheteroaryls by vicarious nucleophilic substitution.

An efficient and scalable three-step one-pot approach to 6-methyl-5-nitroisoquinoline (1) from inexpensive 5-nitroisoquinoline, utilizing the vicarious nucleophilic substitution (VNS) as a key step, is described. The optimized reaction conditions can be applied to a limited number of other aromatic and heteroaromatic nitro compounds. Attempts to understand the observed selectivity in the VNS step led to the discovery of two new reaction pathways under VNS conditions, one leading to an isoxazole and the other resulting in the formal cyclopropanation of an aromatic nitro compound.

Co/NHPI-mediated aerobic oxygenation of benzylic C-H bonds in pharmaceutically relevant molecules.

A simple cobalt(ii)/N-hydroxyphthalimide catalyst system has been identified for selective conversion of benzylic methylene groups in pharmaceutically relevant (hetero)arenes to the corresponding (hetero)aryl ketones. The radical reaction pathway tolerates electronically diverse benzylic C-H bonds, contrasting recent oxygenation reactions that are initiated by deprotonation of a benzylic C-H bond. The reactions proceed under practical reaction conditions (1 M substrate in BuOAc or EtOAc solvent, 12 h, 90-100 °C), and they tolerate common heterocycles, such as pyridines and imidazoles. A cobalt-free, electrochemical, NHPI-catalyzed oxygenation method overcomes challenges encountered with chelating substrates that inhibit the chemical reaction. The utility of the aerobic oxidation method is showcased in the multigram synthesis of a key intermediate towards a drug candidate (AMG 579) under process-relevant reaction conditions.

Oxopyrido[2,3-d]pyrimidines as Covalent L858R/T790M Mutant Selective Epidermal Growth Factor Receptor (EGFR) Inhibitors.

In nonsmall cell lung cancer (NSCLC), the threonine(790)-methionine(790) (T790M) point mutation of EGFR kinase is one of the leading causes of acquired resistance to the first generation tyrosine kinase inhibitors (TKIs), such as gefitinib and erlotinib. Herein, we describe the optimization of a series of 7-oxopyrido[2,3-d]pyrimidinyl-derived irreversible inhibitors of EGFR kinase. This led to the discovery of compound 24 which potently inhibits gefitinib-resistant EGFR(L858R,T790M) with 100-fold selectivity over wild-type EGFR. Compound 24 displays strong antiproliferative activity against the H1975 nonsmall cell lung cancer cell line, the first line mutant HCC827 cell line, and promising antitumor activity in an EGFR(L858R,T790M) driven H1975 xenograft model sparing the side effects associated with the inhibition of wild-type EGFR.

Palladium-catalyzed chemoselective monoarylation of hydrazides for the synthesis of [1,2,4]triazolo[4,3-a]pyridines.

An efficient and convenient method for the synthesis of [1,2,4]triazolo[4,3-a]pyridines was exemplified by the synthesis of 20 analogues bearing a variety of substituents at the 3-position. The methodology involves a palladium-catalyzed addition of hydrazides to 2-chloropyridine, which occurs chemoselectively at the terminal nitrogen atom of the hydrazide, followed by dehydration in acetic acid under microwave irradiation.

Practical synthesis of a vanilloid receptor-1 antagonist.

Small molecule TRPV1 antagonists have been a recent focus in the search for pain treatment agents. We herein describe a practical and scalable synthesis of AMG 628 (1), a bis-substituted pyrimidine derivative that was identified as a highly efficacious agent, suitable for clinical development. Highlights of our approach include a practical route to a substituted benzothiazole, a scalable synthesis of an enantiopure piperazine fragment, and identification of conditions for selective coupling reactions on 2,6-dichloropyrimidine, to access the active pharmaceutical ingredient in high purity and overall yield.

DYKAT of Baylis-Hillman adducts: concise total synthesis of furaquinocin E.

Baylis-Hillman adducts are easily accessible building blocks; the lack of asymmetric versions of the Baylis-Hillman reaction has however precluded their widespread use in asymmetric synthesis. A Pd-catalyzed DYKAT on carbonates derived from Baylis-Hillman adducts, followed by a reductive Heck reaction, allows the enantio- and diastereoselective construction of dihydrobenzofurans in a very efficient manner. These synthons represent the core structure of the furaquinocins. Introduction of different side chains and use of different squaric acid derivatives for the construction of the naphthoquinone allow the flexible synthesis of this class of natural products. This new approach is successfully applied to the synthesis of furaquinocin E and an analogue.

Total syntheses of furaquinocin A, B, and E.

A modular approach to the total synthesis of furaquinocins culminated in the total syntheses of furaquinocin A, B, and E. A Pd-catalyzed dynamic kinetic asymmetric transformation (DYKAT) on carbonates derived from Baylis-Hillman adducts, followed by a reductive Heck cyclization allows the enantio- and diastereoselective construction of dihydrobenzofuran 32. Introduction of a double unsatured side chain via Horner-Wadsworth-Emmons reaction and assembly of the naphthoquinone with squaric acid based methodology leads to furaquinocin E. The use of differentially substituted squaric acid derivatives allows the synthesis of three analogues of furaquinocin E. The additional stereocenters in furaquinocin A and B can be introduced with a diastereoselective Sakurai allylation. The stereoselective elongation of the side chain is possible using cross metathesis or ring closing metathesis. The obtained late-stage intermediates were successfully transformed to furaquinocin A and B.