Anthony C. Willis

The reaction of indigo with two equivalents of the electrophile ethyl bromoacetate with caesium carbonate as a base result in the formation of structurally complex polyheterocyclics, including a fused spiroimidazole and a spiro[1,3]oxazino derivative, together with a biindigoid-type derivative, through a convenient one-pot reaction. Further assessment of the reaction using five equivalents of the electrophile gave rise to other molecules incorporating the 2-(7,13,14-trioxo-6,7,13,14-tetrahydropyrazino[1,2-a:4,3-a′]diindol-6-yl) scaffold. The reaction of ethyl bromoacetate with the less reactive indirubin resulted in the synthesis of three derivatives of a new class of polyheterocyclic system via a cascade process, although yields were low. These compounds were derived from the parent indolo[1,2-b]pyrrolo[4,3,2-de]isoquinoline skeleton. Despite the modest yields of the reactions, they represent quick cascade routes to a variety of heterocycles from cheap starting materials, with these structures otherwise being difficult to synthesise in a traditional stepwise manner. These outcomes also contribute significantly to the detailed understanding of the indigo/indirubin cascade reaction pathways initiated by base-catalysed N-alkylation.

In a continuation of the exploration of indigo cascade reactions, a series of –OMe, –Ph, –Br and –NO₂ substituted indigos 1a–i were synthesised to probe electronic effects upon the outcome of allylation cascade reactions.

The reactions of α,β-unsaturated N-acyliminium ions, generated in situ from 4(S)-O-substitutedhydroxy-5-hydroxy-5-vinyl-N-alkylpyrrolidin-2-ones, with allylsilanes and indoles leading to the formation of spirocyclic heterocycles, are reported. Six single crystal X-ray structures and extensive 2D NMR experiments confirmed the structures and stereochemistries of these products. In addition, computational studies provided mechanistic insights and an understanding of the stereochemical outcomes of these reactions.

The synthesis of structurally diverse heterocycles for chemical space exploration was achieved via the cascade reactions of indigo with propargylic electrophiles. New pyrazinodiindolodione, naphthyridinedione, azepinodiindolone, oxazinoindolone and pyrrolodione products were prepared in one pot reactions by varying the leaving group (‐Cl, ‐Br, ‐OMs, ‐OTs) or propargyl terminal functionality (‐H, ‐Me, ‐Ph, ‐Ar). Mechanistic and density functional theory studies revealed that the unsaturated propargyl moiety can behave as an electrophile when aromatic terminal substitutions are made, and therefore competes with leaving group substitution for new outcomes. Selected products from the cascade reactions were investigated for their absorption and fluorescence properties, including transient absorption spectroscopy. This revealed polarity dependent excited state relaxation pathways, fluorescence, and triplet formation, thus highlighting these reactions as a means to access diverse functional materials rapidly.

The mononuclear Ni(II) complex [Ni(Lp)2(CH3OH)2]Cl2 has been synthesized by reacting 1-(5-hydroxy-3-methyl-5-phenyl-4,5-dihydro-1H-pyrazol-1-yl)ethan-1-one ligand (HL) with NiCl2·6H2O in methanol solution. In the reaction, the tridentate ligand, HL, was converted in situ into 4-hydroxy-4-phenylbut-3-en-2-ylidene)acetohydrazid ligand, (pyrazole, Lp). The pyrazole ligand acts as bidentate neutral ligand and the hydroxyl group is left uncoordinated. The structure of the Ni(II) complex has been established by X-ray crystallography. The Ni(II) is six-coordinate and has a distorted octahedral geometry. It is bonded by two nitrogen and by two oxygen atoms of the two pyrazole ligands and two oxygen atoms of methanol molecules. The Hirshfeld surface analysis and the 2D the fingerprint plot are used to analyses all of the intermolecular contacts in the crystal structures. The main intermolecular contacts are H/H and Cl/H interactions.

A series of novel isomeric nickel Schiff base complexes, as well as nickel complexes of related ligands having asymmetric structures have been prepared and characterised using microanalysis, ¹H and ¹³C NMR spectroscopy and ESI-MS. The Schiff base ligands were prepared by condensation reactions involving ethylenediamine and different derivatives of benzophenone. The solid-state structures of eight of the complexes were also determined and revealed that each possessed a regular square planar coordination geometry around the metal ion. Many of the new complexes featured at least one, and in many instances two, protonatable pendant groups that enhance aqueous solubility. This enabled the DNA binding properties of the latter complexes to be explored using a variety of instrumental approaches, including ESI-MS, circular dichroism (CD) spectroscopy, FRET melting assays and FID assays, as well as molecular docking studies. The results of experiments performed using ESI-MS suggested that none of the nickel complexes exhibit a high affinity towards either a double stranded DNA (dsDNA) molecule D2, or the parallel unimolecular quadruplex DNA (qDNA) molecule Q1. In contrast, complexes (8) and (12) both gave spectra which reflected a significant level of binding to the parallel tetramolecular qDNA Q4. The results of binding experiments performed using CD spectroscopy suggested that (12) exhibits a significant level of affinity towards most types of DNA, while (4) shows a preference for interacting with parallel, unimolecular qDNA molecules. Complex (4) produced the lowest values of DC₅₀ in FID assays performed using parallel Q1 or Q4, confirming its affinity for these qDNA molecules. The results of FRET melting experiments provided further evidence that (12), along with (8), can interact extensively with anti-parallel unimolecular qDNA. Experiments which monitored the effect of the nickel complexes on the melting temperature of D2 showed that none had a stabilising effect on this dsDNA molecule.

The total syntheses of the title compounds (I)‐(III) from the microbially derived enantiomerically pure catechol (IV) are presented along with the confirmation of the structures of ribisins A, B and D.