Synthesis and domino metathesis of functionalized 7-oxanorbornene analogs toward cis-fused heterocycles

Harnessing the 1,3-azadiene-anhydride reaction for the regioselective and stereocontrolled synthesis of lactam-fused bromotetrahydropyrans by bromoetherification of lactam-tethered trisubstituted tertiary alkenols

Halo-cycloetherification of lactam-tethered alkenols enables the construction of oxygen-heterocycles that are fused to nitrogen heterocycles via intramolecular halonium-induced nucleophilic addition. Specifically, tetrahydropyrans (THPs) that are fused to a nitrogen heterocycle constitute the core of several bioactive molecules, including tachykinin receptor antagonists and alpha-1 adrenergic antagonists. Although the literature is replete with successful examples of the halo-cycloetherification of simple mono- or disubstituted primary alkenols, methods for the modular, efficient, regioselective, and stereocontrolled intramolecular haloetherification of sterically encumbered trisubstituted tertiary alkenols are rare. Here, we describe a simple intramolecular bromoetherification strategy that meets these benchmarks and proceeds with exclusive 6-endo regioselectivity. The transformation employs mild and water-tolerant conditions, which bodes well for late-stage diversification. The hindered ethers contain four contiguous stereocenters as well as one halogen-bearing tetrasubstituted stereocenter.

The crystal structure of 2-[(S)-1-(naphthalen-1-yl)ethyl]-2,3,7,7a- tetrahydro-3a,6-epoxyisoindol-1(6H)-one, C19H20NO2

C19H20NO2, monoclinic, P21 (no. 4), a = 6.685(5) Å, b = 8.878(7) Å, c = 13.239(9) Å, β = 97.720(2)°, V = 778.75(10) Å3, Z = 2, R gt (F) = 0.0423, wR ref (F 2) = 0.0970, T = 296(2) K.

Covalent Ligand Screening Uncovers a RNF4 E3 Ligase Recruiter for Targeted Protein Degradation Applications

Targeted protein degradation has arisen as a powerful strategy for drug discovery allowing the targeting of undruggable proteins for proteasomal degradation. This approach most often employs heterobifunctional degraders consisting of a protein-targeting ligand linked to an E3 ligase recruiter to ubiquitinate and mark proteins of interest for proteasomal degradation. One challenge with this approach, however, is that only a few E3 ligase recruiters currently exist for targeted protein degradation applications, despite the hundreds of known E3 ligases in the human genome. Here, we utilized activity-based protein profiling (ABPP)-based covalent ligand screening approaches to identify cysteine-reactive small-molecules that react with the E3 ubiquitin ligase RNF4 and provide chemical starting points for the design of RNF4-based degraders. The hit covalent ligand from this screen reacted with either of two zinc-coordinating cysteines in the RING domain, C132 and C135, with no effect on RNF4 activity. We further optimized the potency of this hit and incorporated this potential RNF4 recruiter into a bifunctional degrader linked to JQ1, an inhibitor of the BET family of bromodomain proteins. We demonstrate that the resulting compound CCW 28-3 is capable of degrading BRD4 in a proteasome- and RNF4-dependent manner. In this study, we have shown the feasibility of using chemoproteomics-enabled covalent ligand screening platforms to expand the scope of E3 ligase recruiters that can be exploited for targeted protein degradation applications.

Harnessing the anti-cancer natural product nimbolide for targeted protein degradation

Nimbolide, a terpenoid natural product derived from the Neem tree, impairs cancer pathogenicity; however, the direct targets and mechanisms by which nimbolide exerts its effects are poorly understood. Here, we used activity-based protein profiling (ABPP) chemoproteomic platforms to discover that nimbolide reacts with a novel functional cysteine crucial for substrate recognition in the E3 ubiquitin ligase RNF114. Nimbolide impairs breast cancer cell proliferation in-part by disrupting RNF114 substrate recognition, leading to inhibition of ubiquitination and degradation of the tumor-suppressors such as p21, resulting in their rapid stabilization. We further demonstrate that nimbolide can be harnessed to recruit RNF114 as an E3 ligase in targeted protein degradation applications and show that synthetically simpler scaffolds are also capable of accessing this unique reactive site. Our study highlights the utility of ABPP platforms in uncovering unique druggable modalities accessed by natural products for cancer therapy and targeted protein degradation applications.

Ring Rearrangement Metathesis in 7-Oxabicyclo[2.2.1]heptene (7-Oxanorbornene) Derivatives. Some Applications in Natural Product Chemistry

Metathesis reactions is firmly established as a valuable synthetic tool in organic chemistry, clearly comparable with the venerable Diels-Alder and Wittig reactions and, more recently, with the metal-catalyzed cross-coupling reactions. Metathesis reactions can be considered as a fascinating synthetic methodology, allowing different variants regarding substrate (alkene and alkyne metathesis) and type of metathetical reactions. On the other hand, tandem metathesis reactions such Ring Rearrangement Metathesis (RRM) and the coupling of metathesis reaction with other reactions of alkenes such as Diels-Alder or Heck reactions, makes metathesis one of the most powerful and reliable synthetic procedure.

In particular, Ring-Rearrangement Metathesis (RRM) refers to the combination of several metathesis transformations into a domino process such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) and ROM-cross metathesis (CM) in a one-pot operation. RRM delivers complex frameworks that are difficult to assemble by conventional methods constitutingan atom economic process. RRM is applicable to mono- and polycyclic systems of varying ring sizes such as cyclopropene, cyclobutene, cyclopentene, cyclohexene, pyran systems, bicyclo[2.2.1]heptene derivatives, bicyclo[2.2.2]octene derivatives, bicyclo[3.2.1]octene derivatives and bicyclo[3.2.1]octene derivatives.

In this review our attention has focused on the RRM reactions in 7-oxabicyclo[2.2.1]heptene derivatives and on their application in the synthesis of natural products or significant subunits of them.

Theoretical study of the regio- and stereoselectivity of the intramolecular Povarov reactions yielding 5H-chromeno[2,3-c] acridine derivatives

The intramolecular Povarov (IMP) reactions involved in the synthesis of 5H-chromeno[2,3-c] acridine derivatives [Tetrahedron Lett., 2010, 51, 3071–3074] have been studied using density functional theory (DFT) methods.

Recent applications of ring-rearrangement metathesis in organic synthesis

Ring-rearrangement metathesis (RRM) involves multiple metathesis processes such as ring-opening metathesis (ROM)/ring-closing metathesis (RCM) in a one-pot operation to generate complex targets. RRM delivers complex frameworks that are difficult to assemble by conventional methods. The noteworthy point about this type of protocol is multi-bond formation and it is an atom economic process. In this review, we have covered literature that appeared during the last seven years (2008–2014).

Copper-catalyzed direct C-2 difluoromethylation of furans and benzofurans: access to C-2 CF2H derivatives

We report herein the first copper-catalyzed C-2 difluoromethylation of furans and benzofurans. The developed methodology allows the selective introduction of the CF2CO2Et moiety at C-2 using CuI as a catalyst. This process was applied to a broad range of furans and benzofurans, giving the functionalized products in moderate to good yields. The resulting products were then decarboxylated to afford the highly valuable C-2-CF2H-substituted furans and benzofurans in good yields.

Studies on an (S)-2-amino-3-(3-hydroxy-5-methyl-4-isoxazolyl)propionic acid (AMPA) receptor antagonist IKM-159: asymmetric synthesis, neuroactivity, and structural characterization

IKM-159 was developed and identified as a member of a new class of heterotricyclic glutamate analogues that act as AMPA receptor-selective antagonists. However, it was not known which enantiomer of IKM-159 was responsible for its pharmacological activities. Here, we report in vivo and in vitro neuronal activities of both enantiomers of IKM-159 prepared by enantioselective asymmetric synthesis. By employment of (R)-2-amino-2-(4-methoxyphenyl)ethanol as a chiral auxiliary, (2R)-IKM-159 and the (2S)-counterpart were successfully synthesized in 0.70% and 1.5% yields, respectively, over a total of 18 steps. Both behavioral and electrophysiological assays showed that the biological activity observed for the racemic mixture was reproduced only with (2R)-IKM-159, whereas the (2S)-counterpart was inactive in both assays. Racemic IKM-159 was crystallized with the ligand-binding domain of GluA2, and the structure revealed a complex containing (2R)-IKM-159 at the glutamate binding site. (2R)-IKM-159 locks the GluA2 in an open form, consistent with a pharmacological action as competitive antagonist of AMPA receptors.

Ring-rearrangement metathesis of nitroso Diels-Alder cycloadducts

Strained nitroso Diels-Alder bicyclo[2.2.1] or [2.2.2] adducts functionalized with alkene side chains of diverse length undergo a ring-rearrangement metathesis process with external alkenes and Grubbs II or Hoveyda-Grubbs II ruthenium catalysts, under microwave irradiation or classical heating, to deliver cis-fused bicycles of various ring sizes, which contain a N-O bond. These scaffolds are of synthetic relevance for the generation of molecular diversity and to the total synthesis of alkaloids. The observation of unexpected reactions, such as epimerization or one-carbon homologation of the alkene side chain, is also reported.

Regioselective Domino Metathesis of Unsymmetrical 7-Oxanorbornenes with Electron-Rich Vinyl Acetate toward Biologically Active Glutamate Analogues

In this article a regioselective domino metathesis reaction of unsymmetrical 7-oxanorbornenes, readily available by a tandem Ugi/Diels-Alder reaction as a key step, promoted by the Hoveyda-Grubbs second-generation catalyst in the presence of electron-rich vinyl acetate as a cross metathesis (CM) substrate is reported. The mechanism for the unusually high regioselectivity observed in the CM reaction was investigated, and a reaction course where a Fischer-type carbene ["Ru"= CH(OAc)] generates a steric interaction is proposed. The metathesis products were further converted to four artificial glutamate analogues whose structures were inspired by naturally derived excitatory glutamate analogues, dysiherbaine and neodysiherbaine. Interestingly, one of the synthetic analogues (28a) induced a cataleptic state in mice. Further electrophysiological studies suggest that 28a might inhibit excitatory synaptic transmission by a yet unknown indirect pathway.

Regioselective Domino Metathesis of 7-Oxanorbornenes and Its Application to the Synthesis of Biologically Active Glutamate Analogues

A highly regioselective domino metathesis reaction of 7-oxanorbornene was developed that employed an intramolecular association of an amide carbonyl group to a ruthenium metal centre. By using this reaction, twelve glutamate analogues inspired by dysiherbaine were efficiently synthesized over 12-14 steps; one of the analogues exhibited bioactivity consistent with central nervous system depression.