From norbornane-based nucleotide analogs locked in South conformation to novel inhibitors of feline herpes virus

Bicyclo[2.1.1]hexanes via Intramolecular Formal (3+2)-Cycloaddition

We report a synthesis of bicyclo[2.1.1]hexanes via an intramolecular formal (3+2) cycloaddition of allylated cyclopropanes bearing a 4-nitrobenzimine substituent. Both activated and unactivated alkenes are tolerated in the transformation. The bicyclic imine products are prone to photo-induced ring opening, allowing for the epimerization of C5-stereogenic compounds.

Unveiling Mechanistic Insights and Photocatalytic Advancements in Intramolecular Photo-(3 + 2)-Cycloaddition: A Comparative Assessment of Two Paradigmatic Single-Electron-Transfer Models

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

Conformation Locking of the Pentose Ring in Nucleotide Monophosphate Coordination Polymers via π-π Stacking and Metal-Ion Coordination

The conformation of the pentose ring in nucleotides is extremely important and a basic problem in biochemistry and pharmaceutical chemistry. In this study, we used a strategy to regulate the conformation of pentose rings of nucleotides via the synergistic effect of metal-ion coordination and π-π stacking. Seven types of coordination complexes were developed and characterized using Fourier transform infrared spectroscopy, elemental analysis, thermogravimetric analysis, powder X-ray diffraction, ultraviolet-visible spectroscopy, 1H nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction. On the basis of two conformational parameters obtained from single-crystal structure analysis, i.e., the pseudorotation phase angle and degree of puckering, the exact conformation of the furanose ring in these coordination polymers was unequivocally determined. Crystallographic studies demonstrate that a short bridging ligand (4,4'-bipyridine) is conducive to the formation of a twist form, and long auxiliary ligands [1,2-bis(4-pyridyl)ethene and 4,4'-azopyridine] induce the formation of an envelope conformation. However, the longest auxiliary ligands [1,4-bis(4-pyridyl)-2,3-diaza-1,3-butadiene] cannot limit the flexibility of a nucleotide. Our results demonstrated that the proposed strategy is universal and controllable. Moreover, the chirality of these coordination polymers was examined by combining the explanation of their crystal structures with solid-state circular dichroism spectroscopy measurements.

Efficient Synthesis and Docking Studies of Novel Benzothiazole-Based Pyrimidinesulfonamide Scaffolds as New Antiviral Agents and Hsp90α Inhibitors

A series of novel substituted 2-pyrimidylbenzothiazoles incorporating either sulfonamide moieties or the amino group at C2 of the pyrimidine ring were synthesized and evaluated for its antiviral potency. The novel synthesis of the ring system was carried out by reacting guanidine or N-arylsulfonated guanidine with different derivatives of ylidene benzothiazole based on Michael addition pathways. The antiviral activity of the newly synthesized compounds was examined by a plaque reduction assay against HSV-1, CBV4, HAV HM 175, HCVcc genotype 4 viruses, and HAdV7. In the case of HSV-1, it was determined that 5 out of the 21 synthesized compounds exhibited superior viral reduction in the range of 70-90% with significant IC₅₀, CC₅₀, and SI values as compared with acyclovir. In the case of CBV4, nine compounds have shown more than 50% reduction. Comparable results were obtained for seven of these synthesized compounds when evaluated against HAV with only a couple of them showing 50% reduction or more against HCVcc genotype 4. Remarkably, one compound, 9a, has shown broad action against all five examined viruses, rendering it as potentially an effective antiviral agent. The five potent compounds 9a, 9b, 14b, 14g, and 14h against HSV-1 have also presented inhibitory activity against the Hsp90α protein with IC₅₀ in the range of 4.87-10.47 μg/mL. Interestingly, a combination of the potent synthesized compounds with acyclovir led to IC₅₀ values lower than that of acyclovir alone. The potent compounds 9a, 9b, 14b, 14g, and 14h were also docked inside the active site of Hsp90α to assess the interaction pattern between the tested compounds and the active site of the protein.

Exploiting Imine Photochemistry for Masked N-Centered Radical Reactivity

This report details the development of a masked N-centered radical strategy that harvests the energy of light to drive the conversion of cyclopropylimines to 1-aminonorbornanes. This process employs the N-centered radical character of a photoexcited imine to facilitate the homolytic fragmentation of the cyclopropane ring and the subsequent radical cyclization sequence that forms two new C-C bonds en route to the norbornane core. Achieving bond-forming reactivity as a function of the N-centered radical character of an excited state Schiff base is unique, requiring only violet light in this instance. This methodology operates in continuous flow, enhancing the potential to translate beyond the academic sector. The operational simplicity of this photochemical process and the structural novelty of the (hetero)aryl-fused 1-aminonorbornane products are anticipated to provide a valuable addition to discovery efforts in pharmaceutical and agrochemical industries.

Providing a New Aniline Bioisostere through the Photochemical Production of 1-Aminonorbornanes

This report describes the photochemical conversion of aminocyclopropanes into 1-aminonorbornanes via formal [3+2] cycloadditions initiated by homolytic fragmentation of amine radical cation intermediates. Aligning with the modern movement toward sp ³ -rich motifs in drug discovery, this strategy provides access to a diverse array of substitution patterns on this saturated carbocyclic framework while offering the robust functional group tolerance (e.g. -OH, -NHBoc) necessary for further derivatization. Evaluating the metabolic stability of selected morpholine-based 1-aminonorbornanes demonstrated a low propensity for oxidative processing and no proclivity toward reactive metabolite formation, suggesting a potential bioisosteric role for 1-aminonorbornanes. Continuous flow processing allowed for efficient operation on gram-scale, providing promise for translation to industrially-relevant scales. This methodology only requires low loadings of a commercially-available, visible light-active photocatalyst and a simple salt, thus it stays true to sustainability goals while readily delivering saturated building blocks that can reduce metabolic susceptibility within drug development programs.

Control of α/β Anomer Formation by a 2',5' Bridge: Toward Nucleoside Derivatives Locked in the South Conformation

We describe a novel stereoselective synthesis of nucleoside derivatives with the ribose ring locked in the South conformation by a bridge between C2' and C5'. Despite the intrinsic constraints of the bicyclic structure, we demonstrate that their synthesis can be achieved by ring closing metathesis of readily accessible precursors. The obtained ribose derivatives are, however, very poor substrates for further installation of the nucleobases, and even simple nucleophiles, such as azido or cyano anions, react with unexpected stereo- or regioselectivity under standard glycosylation conditions. Here we explain this behavior by employing density functional theory (DFT) computations and devise an alternative approach resulting in isomers with the desired orientation of the nucleobase.

New carbocyclic N(6)-substituted adenine and pyrimidine nucleoside analogues with a bicyclo[2.2.1]heptane fragment as sugar moiety; synthesis, antiviral, anticancer activity and X-ray crystallography

New nucleoside analogues with an optically active bicyclo[2.2.1]heptane skeleton as sugar moiety and 6-substituted adenine were synthesized by alkylation of 6-chloropurine intermediate. Thymine and uracil analogs were synthesized by building the pyrimidine ring on amine 1. X-ray crystallography confirmed an exo-coupling of the thymine to the ring and an L configuration of the nucleoside analogue. The library of compounds was tested for their inhibitory activity against influenza virus A∖California/07/09 (H1N1)pdm09 and coxsackievirus B4 in cell culture. Compounds 13a and 13d are the most promising for their antiviral activity against influenza, and compound 3c against coxsackievirus B4. Compounds 3b and 3g were tested for anticancer activity.

Synthesis and biological evaluation of conformationally restricted adenine bicycloribonucleosides

We prepared a novel series of conformationally restricted bicyclonucleosides and nucleotides. The synthetic approach employed a ring closing metathesis to provide access to both 6 and 7 membered saturated and unsaturated rings linking the 3' to 5' methylene groups of the sugar. The bicyclonucleosides were also transformed to the corresponding phosphoramidate prodrugs by an innovative one-pot protocol of boronate ester protection, coupling of the phosphoryl chloridate and deprotection of the boronate. A similar strategy was also employed for the synthesis of the corresponding monophosphates as crucial intermediates for the synthesis of selected triphosphates. The biological properties of the nucleosides and monophosphate prodrugs were assessed for antiviral and cytostatic activities in cell based assays whilst the triphosphates were evaluated in enzymatic assays. The lack of significant effects suggests that the linkage of the 3' to 5'via a ring system and the subsequent conformational restriction of the ribose ring to the South conformation are incompatible with the kinases and polymerases that recognize nucleosides and their metabolites.

Synthesis, conformational studies, and biological properties of phosphonomethoxyethyl derivatives of nucleobases with a locked conformation via a pyrrolidine ring

Systematic structure-activity studies on a diverse family of nucleoside phosphonic acids has led to the development of potent antiviral drugs such as HPMPC (CidofovirTM), PMEA (AdefovirTM), and PMPA (TenofovirTM), which are used in the treatment of CMV-induced retinitis, hepatitis B, and HIV, respectively. Here, we present the synthesis of a novel class of acyclic phosphonate nucleotides that have a locked conformation via a pyrrolidine ring. NMR analysis of these compounds revealed that the pyrrolidine ring has a constrained conformation when in the cis-form at pD < 10 via hydrogen bonding. Four of these compounds were tested as inhibitors of the human and Plasmodium falciparum 6-oxopurine phosphoribosyltransferases. The most potent has a Ki of 0.6 μM for Plasmodium falciparum HGXPRT.

Norbornane-based nucleoside and nucleotide analogues locked in North conformation

We report on the synthesis of novel conformationally locked nucleoside and nucleotide derivatives, which are structurally closely related to clinically used antivirals such as didanosine and abacavir. As a suitable conformationally rigid substitute of the sugar/pseudosugar ring allowing a permanent stabilization of the nucleoside in North conformation we employed bicyclo[2.2.1]heptane (norbornane) substituted in the bridgehead position with a hydroxymethyl group and in the C-3 position with a nucleobase. Prepared nucleoside derivatives were also converted into appropriate phosphoramidate prodrugs (ProTides) in order to increase delivery of the compounds in the cells. All target compounds were evaluated in a broad antiviral and cytostatic assay panel.