Novel Foscan®-derived ring-fused chlorins for photodynamic therapy of cancer

Photodynamic therapy (PDT) is an established anticancer treatment that combines the use of a photosensitiser (PS) and a light source of a specific wavelength for the generation of reactive oxygen species (ROS) that are toxic to the tumour cells. Foscan® (mTHPC) is a clinically-approved chlorin used for the PDT treatment of advanced head and neck, prostate and pancreatic cancers but is characterized by being photochemically unstable and associated with prolonged skin photosensitivity. Herein, we report the synthesis of new 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused chlorins, having the meso-tetra(3-hydroxyphenyl)macrocycle core of mTHPC, by exploring the [8π + 2π] cycloaddition of a meso-tetra(3-hydroxyphenyl)porphyrin derivative with diazafulvenium methides. These chlorins have photochemical properties similar to Foscan® but are much more photostable. Among the novel compounds, two chlorins with a hydroxymethyl group and its azide derivative present in the 4,5,6,7-tetrahydropyrazolo[1,5-a]pyridine-fused system, are promising photodynamic agents with activity in the 100 nM range against triple-negative breast cancer cells and, in the case of azidomethyl chlorin, a safer phototherapeutic index compared to Foscan®.

Classic highlights in porphyrin and porphyrinoid total synthesis and biosynthesis

Porphyrins feature prominently in nature, be it as enzymatic cofactors, electron and exciton shuffles, as photoactive dyes, or as signaling substances. Their involvement in the generation, storage and use of oxygen is pivotal to life, while their photochemical properties are central to the biochemical functioning of plants. When complexed to metals, porphyrins can engage in a multitude of contemporary applications ranging from solar energy generation to serving as catalysts for important chemical reactions. They are also able to function as useful theranostic agents, and as novel materials for a wide range of applications. As such, they are widely considered to be highly valuable molecules, and it almost goes without saying that synthetic organic chemistry has dramatically underpinned all the key advances made, by providing reliable access to them. In fact, strategies for the synthesis of functionalized porphyrins have now reached a state of refinement where pretty well any desired porphyrin can successfully be synthesized with the approaches that are available, including a cornucopia of related macrocycle-modified porphyrinoids. In this review, we are going to illustrate the development of this exciting field by discussing a number of classic syntheses of porphyrins. Our coverage will encompass the natural protoporphyrins and chlorophylls, while also covering general strategies for the synthesis of unsymmetrical porphyrins and chlorins. Various industrial syntheses of porphyrins will also be discussed, as will other routes of great practical importance, and avenues to key porphyrinoids with modified macrocycles. A range of selected examples of contemporary functionalization reactions will be highlighted. The various key syntheses will be described and analyzed from a traditional mechanistic organic chemistry perspective to help student readers, and those who are new to this area. The aim will be to allow readers to mechanistically appreciate and understand how many of these fascinating ring-systems are built and further functionalized.

Symmetry Effects in Photoinduced Electron Transfer in Chlorin‐Quinone Dyads: Adiabatic Suppression in the Marcus Inverted Region

In donor–acceptor dyads undergoing photoinduced electron transfer (PET), a direction or pathway for electron movement is usually dictated by the redox properties and the separation distance between the donor and acceptor subunits, while the effect of symmetry is less recognized. We have designed and synthesized two isomeric donor–acceptor assemblies in which electronic coupling between donor and acceptor is altered by the orbital symmetry control with the reorganization energy and charge transfer exothermicity being kept unchanged. Analysis of the optical absorption and luminescence spectra, supported by the DFT and TD‐DFT calculations, showed that PET in these assemblies corresponds to the Marcus inverted region (MIR) and has larger rate for isomer with weaker electronic coupling. This surprising observation provides the first experimental evidence for theoretically predicted adiabatic suppression of PET in MIR, which unambiguously controlled solely by symmetry.

meso-Aryl substituted free-base tripyrrins: preparation and electrochemically induced protonation/deprotonation reactions. Single crystal X-ray analysis of (2,6-diFPh)2TriPyH

Three meso-substituted free-base tripyrrins were successfully isolated as side-products from the synthesis of triarylcorroles and characterized as to their electrochemical properties in nonaqueous media.

Intramolecular Hydrogen Bonding and Linear Pentapyrrole Conformation

Three new linear pentapyrrole rubinoid analogs: 2,3,7,8,17,18,22,23-octamethyl-12,13-bis-(2-carboxyethyl)-1,10,15,24,25,27,28,29-octahydro-27H-pentapyrrin-1,24-dione and 2,3,8,12,13,17,22,23-octamethyl-7,18-bis-(2-carboxyethyl)-1,10,15,24,25,26,27,28,29-octahydro-27H-pentapyrrin-1,24-dione, and its 7,18-dihexanoic acid analog were synthesized, respectively, from 2,3,7,8-tetramethyl-(10H)-dipyrrin-2-one, from 2,3,8-trimethyl-7-[2-(methoxycarbonyl)ethyl]-(10H)-dipyrrinone, and 2,3,8-trimethyl-7-[5-(methoxycarbonyl)pentyl]-(10H)-dipyrrinone. ¹³C NMR and ¹H NMR measurements in (CD₃)₂SO confirmed the pentapyrrole structures, while ¹H NMR data indicate intramolecular hydrogen bonding between the CO₂H and dipyrrinone groups. Molecular mechanics modeling studies suggest stable U-shape conformations capable of encapsulating small planar aromatic molecules.

The role of the chromophore in the biological photoreceptor phytochrome: an approach using chemically synthesized tetrapyrroles

In plants and bacteria, phytochromes serve as light-inducible, red-/far-red light sensitive photoreceptors that control a wide range of photomorphogenetic processes. Phytochromes comprise a protein moiety and a covalently bound bilin chromophore. Bilins are open-chain tetrapyrrole compounds that derive biosynthetically from ubiquitous porphyrins. The investigations of phytochromes reveal that precise interactions between the protein moiety and its bilin chromophore are essential for the proper functioning of this photoreceptor; accordingly, synthetic manipulation of the parts is an important method for studying the whole. Although variations in the protein structure are readily accomplished by routine mutagenesis protocols, the generation of structurally modified bilins is a laborious, multistep process. Recent improvement in the synthesis of open-chain tetrapyrroles now permits the generation of novel, structurally modified (and even selectively isotope-labeled) chromophores. Furthermore, by using the capability of recombinant apo-phytochrome to bind the chromophore autocatalytically, researchers can now generate novel chromoproteins with modified functions. In the protein-bound state, the phytochrome chromophore is photoisomerized at one double bond, in the bridge between the last two of the four pyrrole rings (the C and D rings), generating the thermally stable, physiologically active P(fr) form. This conversion--photoisomerization from the form absorbing red light (P(r)) to the form absorbing far-red light (P(fr))--covers 12 orders of magnitude, from subpicoseconds to seconds. Such spectroscopic and kinetic studies yield a wealth of time-resolved spectral data, even more so, if proteins with changed sequence or chromophore structure are utilized. In particular, bilins with a changed substitution pattern at the photoisomerizing ring D have shed light on the chromophore-protein interactions during the photoisomerization. The mechanisms generating and stabilizing the light-induced P(fr) form of phytochromes are now seen in greater detail. On the other hand, the use of bilins with selective incorporation of stable isotopes identify light-induced conformational motions when studied by vibrational (FTIR and Raman) and NMR spectroscopy. In this Account, we present spectroscopic investigations that provide structural details in these biological photoreceptors with great precision and document the dynamics elicited by light excitation. This approach yields important information that complements the data deduced from crystal structure.

Synthesis of a peptide-linked chlorin dyad as a model compound for the photosynthetic reaction centre

The enantiomerically pure chlorins 19 and 21 were synthesised from tripyrrolic nickel complex rac-17 and pyrrole building blocks 12 and 16. The pyrroles are annelated with norbornane moieties which contain the chiral information as well as two different functionalities. The functional groups, namely a carboxylic acid ester and a carbonitrile group, of chlorin 21 finally allowed the formation of an amino acid functionality at the periphery of the macrotetracycle. By using principles of peptide chemistry, the two chlorin subunits were joined to form the peptide-linked chlorin-chlorin dyad 24, which mimics the molecular parts of the natural photosynthetic reaction centre.

Discovery of pyrrole-based hepatoselective ligands as potent inhibitors of HMG-CoA reductase

In an effort to identify hepatoselective inhibitors of HMG-CoA reductase, two series of pyrroles were synthesized and evaluated. Efforts were made to modify (3R,5R)-7-[3-(4-fluorophenyl)-1-isopropyl-4-phenyl-5-phenylcarbamoyl-1H-pyrrol-2-yl]-3,5-dihydroxy-heptanoic acid sodium salt 30 in order to reduce its lipophilicity and therefore increase hepatoselectivity. Two strategies that were explored were replacement of the lipophilic 3-phenyl substituent with either a polar function (pyridyl series) or with lower alkyl substituents (lower alkyl series) and attachment of additional polar moieties at the 2-position of the pyrrole ring. One compound was identified to be both highly hepatoselective and active in vivo. We report the discovery, synthesis, and optimization of substituted pyrrole-based hepatoselective ligands as potent inhibitors of HMG-CoA reductase for reducing low density lipoprotein cholesterol (LDL-c) in the treatment of hypercholesterolemia.

Pillars, Layers, Pores and Networks from Nickeltripyrrins: A Porphyrin Fragment as a Versatile Building Block for the Construction of Supramolecular Assemblies

Sterically hindered nickel- tripyrrins [Ni(trpy)X] with different di-, tri- and tetradentate anions X have been prepared with the aim of finding coordination polymers formed by self-association. The syntheses were performed by simple ligand-exchange reactions and proceeded successfully with the pseudohalides CN(-), OCN(-), SCN(-), SeCN(-), N(CN)(2) (-) (dicyanoamido, dca) and C(CN)(3) (-) (tricyanomethanido, tcm), the cyanidometallates [Ag(CN)(2)](-) and [Ni(CN)(4)](2-) and the salicylate anion (sal(-)). X-ray crystallographic analyses revealed that the complexes with cyanido and isocyanato ligands, as well as the compound with a salicylato ligand, are prototypes for structurally distinct monomeric species in the solid state, whereas one-dimensional coordination polymers or supramolecular three-dimensional networks are formed from all other combinations. The polymeric compounds display a variety of individual pillar and network architectures with functionalised pores and clefts and with the Ni(trpy) fragments in different relative orientations. Hydrogen bonding and pi stacking were found to be additional structure-directing effects, which increased the structural complexity of the system. The Ni(trpy) subunit has thus been proven to be a versatile building block for the construction of supramolecular assemblies and metal organic frameworks (MOFs) from pentacoordinate Ni(II) ions.

Free-base tripyrrins

The hitherto unavailable free-base tripyrrins HTrpy have been prepared by cyanide promoted demetallation of a nickel(ii) tripyrrin and by acid-induced condensation of a diformylpyrrole and an alkylpyrrole with a sterically shielded alpha-position.

Monomeric and polymeric copper and zinc tripyrrins

Neutral transition metal complexes of different alpha,omega-dimethyltripyrrins TrpyMX with M = Cu(II) and Zn(II) have been prepared with a variety of anionic halogeno and pseudohalogeno ligands X, and have been studied with respect to coordination modes and structural distortion. Only four- and five-coordinate species have been observed throughout the series. All four-coordinate species display unstrained, but distorted tetrahedral or strained and distorted square-planar coordination environments for zinc(II) and copper(II) species, respectively, thus following the expectations from simple ligand field arguments. Five-coordinate species do not form easily and were observed either in donor solvents or in the solid as 1D coordination polymers with distorted trigonal-bipyramidal coordination and different topologies.

Cyanide-Promoted Demetalation of TrpyNiNCO: A Route to Sensitive Metallotripyrrins of CoII, FeII, and MnII

Etheral solutions of free base tripyrrins (HTrpy) were prepared by treatment of nickel isocyanate complexes (TrpyNiNCO) with excess cyanide. From these solutions sensitive metallotripyrrins with cobalt(II), iron(II), and manganese(II) ions (TrpyMX) and with a choice of external ligands X could be obtained in pure, crystalline form. Four cobalt and one iron chelate were characterized by X-ray crystallography. Tetracoordinate cobalt(II) species with X = I, NCO, and NCS displayed unstrained tetrahedral coordination geometries, whereas the pentacoordinate TrpyCoNO3 with the O,O-nitrato ligand narrows a trigonal bipyramidal coordination. TrpyFeNCO undergoes a redox-transformation to (TrpyFeNCO)2O upon crystallization and was structurally characterized as this with an almost linear Fe-O-Fe subunit. Donor association was studied by UV-vis spectroscopy employing different solvents and showed that TrpyMnX and TrpyFeX species are very prone to the formation of pentacoordinate species, whereas TrpyCoX compounds have an intermediate tendency to do so. Nevertheless, complex fragments of all three metal ions form 1D coordination polymers with dicyanamido ligands, which were investigated by means of IR and SQUID measurements.

Cadmium(II), Nickel(II), and Zinc(II) Complexes of Vacataporphyrin: A Variable Annulene Conformation inside a Standard Porphyrin Frame

5,10,15,20-Tetraaryl-21-vacataporphyrin, 1 (butadieneporphyrin, annulene-porphyrin hybrid), which contains a vacant space instead of heteroatomic bridge, gives diamagnetic zinc(II) 1-ZnCl and cadmium(II) 1-CdCl and paramagnetic nickel(II) 1-NiCl complexes. A metal ion is bound in the macrocyclic cavity by three pyrrolic nitrogens. Coordination imposes a steric constraint on the geometry of the ligand and leads to two stereoisomers with a butadiene fragment oriented toward 1-MCl-i or outward 1-MCl-o of the macrocyclic center. 1-CdCl-o, 1-ZnCl-o, and the free base share a common 1H NMR spectral pattern as the basic structural features of 1 are preserved after metal ion insertion. The 1H NMR spectra of 1-CdCl-i and 1-ZnCl-i reflect a decrease of aromaticity accounted for by the inverted butadiene geometry. The proximity of the butadiene fragment to the metal ion induces direct couplings between the spin-active nucleus of the metal ((111/113)Cd) and the adjacent 1H nuclei of butadiene. The pattern of chemical shifts detected for isomeric 1-NiCl-i and 1-NiCl-o is typical for high-spin nickel(II) complexes of porphyrin analogues. Resonances 2,3-H of 1-NiCl-o or 1-NiCl-i present the chemical shift typical for the beta-H pyrrolic position despite the vacancy in the location of nitrogen-21. Coordination of imidazole, methanol-d4, acetonitrile-d3, or chloride converts 1-NiCl-i and 1-NiCl-o into distinct species which contain two axial ligands: 1-Ni(Im)2+; 1-Ni(CD3OD)2+; 1-Ni(CD3CN)2+; 1-Ni(Cl)2-. The density functional theory (DFT) has been applied to model the molecular and electronic structure of feasible 1-CdCl stereoisomers. The total energies calculated using the B3LYP/LANL2DZ approach demonstrate a very small energy difference (2.3 kcal/mol) between 1-CdCl-o and 1-CdCl-i stereoisomers consistent with their concurrent formation.