Synthesis of 5-trichloromethyl-Δ4-1,2,4-oxadiazolines and their rearrangement into formamidine derivatives

Synthesis, characterisation and in vitro cytotoxicity of mixed ligand Pt(ii) oxadiazoline complexes with hexamethylenetetramine and 7-nitro-1,3,5-triazaadamantane

trans-Platinum(ii) oxadiazoline complexes with 7-nitro-1,3,5-triazaadamantane (NO₂-TAA) or hexamethylenetetramine (hmta) ligands have been synthesised from trans-[PtCl₂(PhCN)₂] via cycloaddition of nitrones to one of the coordinated nitriles, followed by exchange of the other nitrile by NO₂-TAA or hmta. Stoichiometric control allows for the selective synthesis of mono- and dinuclear complexes where 7-NO₂TAA and hmta act as mono- and bidentate ligands, respectively. Precursors and the target complexes trans-[PtCl₂(hmta)(oxadiazoline)], trans-[PtCl₂(NO₂-TAA)(oxadiazoline)] and trans-[{PtCl₂(oxadiazoline)}₂(hmta)] were characterised by elemental analysis, IR and multinuclear (¹H, ¹³C, ¹⁹⁵Pt) NMR spectroscopy. DFT (B3LYP/6-31G*/LANL08) and AIM calculations suggest a stronger bonding of hmta with the [PtCl₂(oxadiazoline)] fragment, in agreement with the experimentally observed reactivity in the ligand exchange (hmta > 7-NO₂TAA). Replacement of the nitrile by hmta is predicted to be more exothermic than that with 7-NO₂-TAA, although the activation barriers are similar. Protonation of the non-coordinated N atoms is anticipated to weaken the Pt-N bond and lower the activation barrier for ligand exchange. This effect might help activate these compounds in a slightly acidic environment such as some tumour tissues. Ten of the new compounds were tested for their in vitro cytotoxicity in the human cancer cell lines HeLa and A549. Some of the mononuclear complexes are more potent than cisplatin, and their activity is still high in A549 where cisplatin shows little effect. The dinuclear complexes are inactive, presumably due to their lipophilicity and reduced solubility in water.

Copper(I)-Catalyzed 1,3-Dipolar Cycloaddition of Ketonitrones to Dialkylcyanamides: A Step toward Sustainable Generation of 2,3-Dihydro-1,2,4-oxadiazoles

Cu^I-catalyzed cycloaddition (CA) of the ketonitrones, Ph₂C=N⁺(R')O^- (R' = Me, CH₂Ph), to the disubstituted cyanamides, NCNR₂ (R = Me₂, Et₂, (CH₂)₄, (CH₂)₅, (CH₂)₄O, C₉H₁₀, (CH₂Ph)₂, Ph(Me)), gives the corresponding 5-amino-substituted 2,3-dihydro-1,2,4-oxadiazoles (15 examples) in good to moderate yields. The reaction proceeds under mild conditions (CH₂Cl₂, RT or 45 °C) and requires 10 mol % of [Cu(NCMe)₄](BF₄) as the catalyst. The somewhat reduced yields are due to the individual properties of 2,3-dihydro-1,2,4-oxadiazoles, which easily undergo ring opening via N-O bond splitting. Results of density functional theory calculations reveal that the CA of ketonitrones to Cu^I-bound cyanamides is a concerted process, and the copper-catalyzed reaction is controlled by the predominant contribution of the HOMO_dipole-LUMO_{dipolarophile} interaction (group I by Sustmann's classification). The metal-involving process is much more asynchronous and profitable from both kinetic and thermodynamic viewpoints than the hypothetical metal-free reaction.

Zinc(ii)-mediated generation of 5-amino substituted 2,3-dihydro-1,2,4-oxadiazoles and their further ZnII-catalyzed and O2-involving transformations

Zn^II-activated cyanamides NCNR₂ react with the acyclic N-alkyl ketonitrones Ph₂CN⁺(O⁻)R′ achieving uncomplexed 2,3-dihydro-1,2,4-oxadiazoles.

Regio- and stereoselectivity of polar [2+3] cycloaddition reactions between (Z)-C-(3,4,5-trimethoxyphenyl)-N-methylnitrone and selected (E)-2-substituted nitroethenes

[2+3] Cycloaddition reactions of the highly reactive (Z)-C-(3,4,5-trimethoxyphenyl)-N-methylnitrone with (E)-2-R-nitroethenes proceed under mild conditions and yield mixtures of stereoisomeric 2-methyl-3-(3,4,5-trimethoxyphenyl)-4-nitro-5-R-isoxazolidines. The effect of regiospecificity of the cycloadditions may be accounted for by the theory of electrophilicity indexes. Stereoselectivity, however, is determined by a compilation of steric and secondary orbital effects.

Design, synthesis, characterisation and chemical reactivity of mixed-ligand platinum(II) oxadiazoline complexes with potential cytotoxic properties

A series of mixed ligand platinum(II) oxadiazoline complexes bearing 7-nitro-1,3,5-triazaadamantane (7-NO(2)TAA) as a labile and reactive nitrogen ligand has been synthesised from easily accessible starting materials. [2+3] cycloaddition of nitrones R(1)R(2)C-N(+)(Me)O(-) to only one of the nitrile ligands in trans-[PtX(2)(PhCN)(2)] (X = Cl, Br) results in the selective formation of mono-oxadiazoline complexes trans-[PtX(2)(PhCN){N=C(Ph)-O-N(Me)-CR(1)R(2)}] from which the remaining nitrile can be replaced by 7-NO(2)TAA. The resulting complexes trans-[PtX(2)(7-NO(2)TAA) {N=C(Ph)-O-N(Me)-CR(1)R(2)}] and their precursors were characterised by elemental analysis, IR and multinuclear NMR spectroscopy.The suitability of the target complexes as anticancer agents was extrapolated from their general chemical reactivity. They are stable in DMSO, but react with thiols and undergo aquation of a chloro ligand. In the absence of a competing ligand, the coordinated 7-NO(2)TAA ligand slowly hydrolyses in an aqueous medium under release of formaldehyde, and this could induce bioactivity independent of the one typically found with platinum compounds. With nitrogen heterocycles such as pyridine a slow exchange of the 7-NO(2)TAA ligand occurs. A combined DFT/AIM study confirms the reaction observed in the experiment and predicts that other nitrogen heterocycles such as DNA nucleobases should react in the same way. Moreover, the 7-NO(2)TAA should be even more labile in an aqueous medium where protonation of the remaining amines can occur. A PM6 molecular modelling study suggests that the PtCl(oxadiazoline) fragment formed after release of one chloro and the labile 7-NO(2)TAA ligand fits well into the DNA groove and is able to form d(GpG) intrastrand crosslinks similar to the ones observed with cisplatin.