Selective C8-Metalation of Purine Nucleosides via Oxidative Addition

Bond Formation at C8 in the Nucleoside and Nucleotide Purine Scaffold: An Informative Selection

This paper presents methods for the introduction and exchange of substituents in a nucleobase and its nucleosides and nucleotides with emphasis on the C8-position in the purine skeleton. The nucleobase is open for electrophilic and nucleophilic chemistry. The nucleophilic chemistry consists mainly of displacement reactions when the C8-substituent is a good leaving group such as a halogen atom. The heteroatom in amines, sulfides, or oxides is a good nucleophile. Halides are good reaction partners. Metal-promoted cross-coupling reactions are important for carbylations. Direct oxidative metalation reactions using sterically hindered metal amides offer chemo- and regio-selectivity besides functional tolerance and simplicity. The carbon site is highly nucleophilic after metalation and adds electrophiles resulting in chemical bond formation. Conditions for metal-assisted reactions are described for nucleobases and their glycosides.

Synthesis, Base Pairing Properties, and Biological Activity Studies of Platinum(II) Complexes Based on Uracil Nucleosides

The synthesis and base pairing properties of platinum complexes based on uridine and deoxyuridine nucleosides and preliminary studies of their antiproliferative activity are described. Platinum(II) uridine and deoxyuridine complexes were synthesized by C-I oxidative addition to Pt(0)(PPh3)4. First, the synthesis was performed with protected nucleosides to generate complexes 1 and 2, which were deprotected under basic conditions, affording complexes 3 and 4 in good yields. The synthesis with the unprotected nucleosides was also performed and provided complexes 3 and 4 effectively. Base pairing interactions were measured for complex 1, either for self-base pairing or for the Watson-Crick base pair. Complex 1 undergoes self-base pairing in CDCl3, and this aggregation was found not to be dependent on metalation. Contrastingly, for the Watson-Crick base pair with adenine, base pairing was also observed, but metalation was found to affect hydrogen bonding considerably. Complexes 3 and 4 and the corresponding ligand precursors were evaluated for their antiproliferative activity against human glioblastoma cell line U-251. The compounds showed IC50 values of 3.30 (3) and 1.84 (4) μM but are also toxic for nontumorous cell lines.

Palladium and Platinum Complexes of the Antimetabolite Fludarabine with Vastly Enhanced Selectivity for Tumour over Non-Malignant Cells

The purine derivative fludarabine is part of frontline therapy for chronic lymphocytic leukaemia (CLL). It has shown positive effects on solid tumours such as melanoma, breast, and colon carcinoma in clinical phase I studies. As the treatment of CLL cells with combinations of fludarabine and metal complexes of antitumoural natural products, e.g., illudin M ferrocene, has led to synergistically enhanced apoptosis, in this research study different complexes of fludarabine itself. Four complexes bearing a trans-[Br(PPh₃)₂]Pt/Pd fragment attached to atom C-8 via formal η¹-sigma or η²-carbene bonds were synthesised in two or three steps without protecting polar groups on the arabinose or adenine. The platinum complexes were more cytotoxic than their palladium analogues, with low single-digit micromolar IC₅₀ values against cells of various solid tumour entities, including cisplatin-resistant ones and certain B-cell lymphoma and CLL, presumably due to the ten-fold higher cellular uptake of the platinum complexes. However, the palladium complexes interacted more readily with isolated Calf thymus DNA. Interestingly, the platinum complexes showed vastly greater selectivity for cancer over non-malignant cells when compared with fludarabine.

Diversely C8-functionalized adenine nucleosides via their underexplored carboxaldehydes

The potentially versatile N-unprotected 8-formyl derivatives of adenosine and 2'-deoxyadenosine are highly underexploited for C8 modifications of these nucleosides. Only in situ formation of 8-formyladenosine is known and a single application of an N-benzoyl derivative has been reported. On the other hand, 8-formyl-2'-deoxyadenosine and its applications remain unknown. Herein, we report straightforward, scalable syntheses of both N-unprotected 8-formyladenine nucleoside derivatives, and demonstrate broad diversification at the C8 position by hydroxymethylation, azidation, CuAAC ligation, reductive amination, as well as olefination and fluoroolefination with modified Julia and a Horner-Wadsworth-Emmons reagents.

Synthesis of Platinum(II) N-Heterocyclic Carbenes Based on Adenosine

Organometallic derivatization of nucleosides is a highly promising strategy for the improvement of the therapeutic profile of nucleosides. Herein, a methodology for the synthesis of metalated adenosine with a deprotected ribose moiety is described. Platinum(II) N-heterocyclic carbene complexes based on adenosine were synthesized, namely N-heterocyclic carbenes bearing a protected and unprotected ribose ring. Reaction of the 8-bromo-2′,3′,5′-tri-O-acetyladenosine with Pt(PPh₃)₄ by C8−Br oxidative addition yielded complex 1, with a Pt^II centre bonded to C-8 and an unprotonated N7. Complex 1 reacted at N7 with HBF₄ or methyl iodide, yielding protic carbene 2 or methyl carbene 3, respectively. Deprotection of 1 to yield 4 was achieved with NH₄OH. Deprotected compound 4 reacted at N7 with HCl solutions to yield protic NHC 5 or with methyl iodide yielding methyl carbene 6. Protic N-heterocyclic carbene 5 is not stable in DMSO solutions leading to the formation of compound 7, in which a bromide was replaced by chloride. The cis-influence of complexes 1–7 was examined by ³¹P{¹H} and ¹⁹⁵Pt NMR. Complexes 2, 3, 5, 6 and 7 induce a decrease of ¹J_Pt,P of more than 300 Hz, as result of the higher cis-influence of the N-heterocyclic carbene when compared to the azolato ligand in 1 and 4.

Oxidative addition of a 8-bromotheobromine derivative to d 10 metals

Reaction of 8-bromo-7-ethyl-3-methylxanthine 1 with zerovalent group 10 metal complexes gave via an oxidative addition of the C–Br bond the neutral complexes trans-[2] (M = Pd) and trans-[3] (M = Pt) bearing a theobromine-derived azolato ligand. While the oxidative addition to [Pd0(PPh3)4] gave exclusively trans-[2], the reaction with the more substitution-inert [Pt0(PPh3)4] yielded after 1 day the kinetic product cis-[3], which was converted under heating for a total of 3 days completely into the thermodynamically more stable complex trans-[3]. Treatment of trans-[2], trans-[3] or the mixture of cis-/trans-[3] with the proton source HBF4⋅Et2O led to complexes trans-[4]BF4, trans-[5]BF4 and a mixture of cis/trans-[5]BF4 with retention of the original ratio of cis to trans, respectively. The molecular structures of the azolato complexes trans-[2] and trans-[3] and of the theobromine derived pNHC complexes trans-[4]BF4 and trans-[5]BF4 have been determined by X-ray diffraction studies.

Mononuclear and dinuclear gold(i) complexes with a caffeine-based di(N-heterocyclic carbene) ligand: synthesis, reactivity and structural DFT analysis

Two gold(i) complexes with caffeine-based di(N-heterocyclic carbene) ligands were synthesised and fully characterised.

Synthesis of IrIII Hydrido Complexes by Oxidative Addition of Halogenated Theophylline and Adenine Derivatives

The Ir^III hydrido complexes [1] and [2] have been synthesized by the regioselective oxidative addition of the N7-H bond of 8-halogenotheophyllines to [IrCl(coe)₂]₂ in the presence of PPh₃. The use of dppf in this reaction yielded the bimetallic Ir^III/Fe^II hydrido complexes [3] and [4]. X-ray diffraction studies confirmed that complexes [1]-[4] feature a theophyllinato ligand coordinated to the metal center in the rarely observed, chelating fashion via the N7 and O1 atoms. In addition, 8-bromoadenine reacts with [IrCl(coe)₂]₂ in the presence of PPh₃ to form the Ir^III hydrido complex [5] which features one anionic 8-bromoadeninato and one neutral 8-bromoadenine ligand linked by an intramolecular hydrogen bond. Complex [5] was characterized by high-resolution mass spectrometry and an X-ray diffraction analysis but could not be analyzed by nuclear magnetic resonance spectroscopy because of its low solubility.