The chiral 5,6-cyclohexane-fused uracil ring-system: A molecular platform with promising activity against SARS-CoV-2

The recurrent global exposure to highly challenging viral epidemics, and the still limited spectrum of effective pharmacological options step on the accelerator towards the development of new antiviral medicines. In this work we explored the anti-SARS-CoV-2 potential of a recently launched chiral ring system based on the uracil scaffold fused to carbocycle rings. The asymmetric synthesis of two generations of chiral uracil-based compounds (overall 31 different products), and their in vitro cytotoxicity and antiviral screening against wild-type SARS-CoV-2 in U87.ACE cells allowed us to identify seven non-cytotoxic enantioenriched derivatives exhibiting in vitro EC50 in the 6-37 μM range. Among these compounds, bicyclic uracil 10 showed the best antiviral potency against SARS-CoV-2 (EC50 20A.EU2 = 7.41 μM and EC50 Omicron = 19.4 μM), combined with a favourable selectivity index. Additionally, it exhibited single-digit micromolar inhibition of the isolated SARS-CoV-2 RNA-dependent RNA polymerase (IC50 = 2.1 μM). Starting from a reported cryo-EM structure of RdRp, docking and molecular dynamics simulations were performed to rationalize possible binding modes of the active compounds. Interestingly, no inhibition of viral replication in cells was observed against a wide spectrum of human viruses, while some derivatives, and especially hit compound 10, exhibited specific low micromolar antiviral effect against β-coronavirus OC43. Collectively, these data indicate that this novel uracil-based ring system represents a valid starting point for further development of a new class of RdRp inhibitors to treat SARS-CoV-2 and, potentially, other β-coronavirus infections.

Expeditious synthesis of CF3-phenanthridones through a base-mediated cross-conjugated vinylogous benzannulation (VBA)

Herein, we report a mild, efficient, and rapid approach for the preparation of CF3-phenanthridones through a cross-conjugated vinylogous [4 + 2] benzannulation of easily accessible 4-methyl-3-trifluoroacetylquinolones and nitro-olefins. The present transformation is superior to previous approaches for obtaining CF3-phenanthridones, in that it proceeds exclusively with the assistance of a simple base, eliminating the need for transition metal catalysts or oxidants. The strong electron-withdrawing nature of the CF3-group present in the quinolone moiety promotes the formation of a reactive cross-conjugated vinylogous enolate.

KOtBu-Promoted [4+2] Annulation-Dehydration Cascade Enabling the Construction of Diverse 2-Pyridone-Fused Uracils

A KOtBu-promoted [4+2] annulation-dehydration cascade reaction has been developed, enabling the efficient synthesis of diverse 2-pyridone-fused uracils through a vinylogous enolization strategy involving o-quinodimethane (oQDM) dienolate intermediates. This method provides a simple yet robust approach for constructing structurally interesting fused N-heterocycles that incorporate two privileged scaffolds, both of which are widely recognized in drug discovery. Consequently, these compounds hold significant potential for biological and pharmacological applications. Moreover, further transformations of the products obtained from this process allow access to highly functionalized and structurally diverse uracil derivatives, expanding the scope of accessible chemical diversity.

Shaping Chirality via Stereoselective, Organocatalytic [4+2] Cycloadditions involving Heterocyclic ortho-Quinodimethanes

Polycyclic compounds bearing a complex heterocyclic core such as an aromatic heterocycle "fused" with one or more functionalized rings, are widespread leading molecules in the domain of synthetic organic chemistry and pharmaceuticals. Although many synthetic methodologies have been devised to access achiral, fused heteroaromatic scaffolds, or related chiral variants adorned with out-of-cycle stereogenic elements, equally efficient strategies to afford chiral heterocycles featuring in-cycle stereocenters, exist to a lesser extent and presently represent a growing field of investigation. The mild, organocatalytic generation of elusive ortho-quinodimethane intermediates (oQDMs), derived from suitable heteroaromatic carbonyl- or carbonyl-like pronucleophiles has recently proved successful in the synthesis of such peculiar chiral architectures via stereoselective [4+2] cycloadditions. This review provides an overview of the most important advances attained in this field over the last decade.

Unusual Reactivities of ortho-Hydroxy-β-nitrostyrene

Nitrostyrene derivatives are widely used in organic syntheses as a substrate for Michael addition, photoisomerization and cycloaddition. In contrast, ortho-hydroxy derivatives exhibit unusual behaviors in these reactions. Conjugate addition proceeded upon treatment of the ortho-hydroxy-β-nitrostyrene with an amine; however, subsequent C–C bond cleavage readily occurred to afford the corresponding imine. Moreover, conversion of the trans-isomer to a cis-isomer did not occur efficiently, even when UV light was irradiated. We studied these unusual behaviors of β-nitrostyrene, focusing on the role of the ortho-hydroxy group.

N-Heterocyclic Carbene-Catalyzed Enantioselective Synthesis of Carbocycle-Fused Uracils via Formal [4 + 2] Annulation/Lactone Formation/Decarboxylation Cascade

An unprecedented organocatalytic asymmetric construction of novel six-membered carbocycle fused uracils has been demonstrated by the reaction of the remotely enolizable 6-methyluracil-5-carbaldehydes with 2-bromoenals. The reaction involves an N-heterocyclic carbene-catalyzed formal [4 + 2] annulation of o-quinodimethane (oQDM) dienolates with α,β-unsaturated acylazoliums, followed by a cascade lactone formation/decarboxylation process. This protocol unlocks a new platform to access enantioenriched carbocycle-fused uracils.

Regiodivergent Organocatalytic Reactions

Organocatalysts are abundantly used for various transformations, particularly to obtain highly enantio- and diastereomeric pure products by controlling the stereochemistry. These applications of organocatalysts have been the topic of several reviews. Organocatalysts have emerged as one of the very essential areas of research due to their mild reaction conditions, cost-effective nature, non-toxicity, and environmentally benign approach that obviates the need for transition metal catalysts and other toxic reagents. Various types of organocatalysts including amine catalysts, Brønsted acids, and Lewis bases such as N-heterocyclic carbene (NHC) catalysts, cinchona alkaloids, 4-dimethylaminopyridine (DMAP), and hydrogen bond-donating catalysts, have gained renewed interest because of their regioselectivity. In this review, we present recent advances in regiodivergent reactions that are governed by organocatalysts. Additionally, we briefly discuss the reaction pathways of achieving regiodivergent products by changes in conditions such as solvents, additives, or the temperature.