π-Extended Donor-Acceptor Porphyrins and Metalloporphyrins for Antimicrobial Photodynamic Inactivation.

Free base, zinc and palladium π-extended porphyrins containing fused naphthalenediamide units were employed as photosensitizers in antimicrobial photodynamic therapy (aPDT). Their efficacy, assessed by photophysical and in vitro photobiological studies on Gram-positive bacteria, was found to depend on metal coordination, showing a dramatic enhancement of photosensitizing activity for the palladium complex.

Electrophilic Aromatic Coupling of Hexapyrrolylbenzenes. A Mechanistic Analysis.

Oxidative transformation of hexapyrrolylbenzenes into azacoronenes using bromine electrophiles as alternative coupling agents is shown to occur according to a distinct mechanism, different from those proposed for typical high-potential oxidants. It is shown that consecutive cyclizations do not involve brominated intermediates and can be rationalized by assuming a relayed arenium cation attack followed by deprotonation and conjugate elimination. The final cyclization is incompatible with this mechanism and is found to involve double-electrophilic bromination followed by thermal elimination of dibromine. These findings provide insight into the reactivity of sterically congested aromatic systems and may help in designing new methods of C-C bond formation.

Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps.

Chiral heteroaromatic propellers based on radially π-extended hexapyrrolohexaazacoronenes were obtained in a concise synthesis from suitably functionalized donor-acceptor monopyrroles. To overcome steric hindrance, a new cyclodehydrogenation method was developed, and it uses bromine electrophiles as oxidative coupling agents instead of the commonly employed high-potential oxidants. The new reaction offers high yields of propeller-shaped targets, even for electron-deficient precursors, and shows electrophile-dependent stereoselectivity, with N-bromosuccinimide and dibromine yielding, respectively D6 - and C2 -symmetric products. The propeller azacoronenes are chiral and can be separated into configurationally stable enantiomers. In addition to providing steric bulk, peripheral functionalization considerably affects the electronic properties of the propellers, which exhibit reduced optical and electrochemical band gaps, and a more clearly defined electroreduction behavior.

Pd-Induced Double C-H Bond Activation in Annulative Syntheses of Bipyrrole Boomerangs: Mechanistic Insights from NMR Spectroscopy and Computation.

1, n-Dipyrrolylalkanes can be efficiently converted into extensively π-conjugated bipyrroles by PdII-mediated annulative double C-H activation, and this approach might be further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. Herein, the mechanism of these transformations is explored using NMR spectroscopy and DFT calculations. The kinetics of the annulation are found to depend on the conjugation extent and donor-acceptor character of the pyrroles, as well as on substitution and the linker length. Combined experimental and theoretical evidence indicates that a change of the rate-determining step occurs for the most electron-deficient substrates. The unprecedented double α-oxygenation of bipyrroles is found to be a stepwise process, involving α-acetoxylated intermediates.

Electron-Deficient Bipyrrole Boomerangs: Bright Fluorophores Obtained via Double C-H Bond Activation.

PdII -mediated annulative double C-H activation is shown to efficiently convert 1,n-dipyrrolylalkanes into extensively π-conjugated bipyrroles not accessible by conventional oxidative coupling protocols. This approach is applicable to both electron-rich and electron-deficient systems, and has been further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. The new bipyrrole intermediates show enhanced fluorescence as well as tunable optical properties controlled by the alignment of chromophore subunits. Photophysical data, including femtosecond transient absorptions, reveal solvent-induced intramolecular charge transfer in their excited states, dependent on the polarity of the medium.

Synthesis of sulfated brassinosteroids.

A number of water soluble sulfates of 24-epibrassinolide including the 2α,3α-disulfate and all possible monosulfates were synthesized. The target compounds were isolated in crystalline form as the corresponding sodium salts. Pyridine-sulfur trioxide complex was used as sulfating agent followed by transformation of the resulting pyridinium salts into the sodium sulfates by treatment with NaOH. The control of the regioselectivity was achieved by an appropriate use of acetyl and benzyl protecting groups.

An Electron-Deficient Azacoronene Obtained by Radial π†Extension.

A hexapyrrolohexaazacoronene derivative containing 37 fused rings, the largest such system to date, was obtained from a naphthalenomonoimide-pyrrole hybrid in a concise and efficient synthesis. This large heterocycle is electron-deficient and shows extended redox activity, spanning at least 13 oxidation levels, but is otherwise chemically stable. Radial expansion of the π system creates a chromophore characterized by strong fluorescence and solvatochromism in the neutral state, and strong near-infrared absorption in the charged states. Additionally, the enlarged and ruffled aromatic surface supports a unique self-assembly mode in the crystal, leading to the formation of highly solvated organic clathrates.

Bandgap Engineering in π-Extended Pyrroles. A Modular Approach to Electron-Deficient Chromophores with Multi-Redox Activity.

A family of bandgap-tunable pyrroles structurally related to rylene dyes was computationally designed and prepared using robust, easily scalable chemistry. These pyrroles show highly variable fluorescence properties and can be used as building blocks for the synthesis of electron-deficient oligopyrroles. The latter application is demonstrated through the development of π-extended porphyrins containing naphthalenediamide or naphthalenediimide units. These new macrocycles exhibit simultaneously tunable visible and near-IR absorptions, an ability to accept up to 8 electrons via electrochemical reduction, and high internal molecular free volumes. When chemically reduced under inert conditions, the most electron-deficient of these macrocycles revealed reversible formation of eight charged states, characterized by remarkably red-shifted optical absorptions, extending beyond 2200 nm. Such features make these oligopyrroles of interest as functional chromophores, charge-storage materials, and tectons for crystal engineering.

Comprehensive physiology and toxicology of ecdysogens--The metabolically activated porphyrin-ecdysteroid complexes in insects.

The polyhydroxylated derivatives of 6-keto,7-dehydrocholesterol (ecdysone, ecdysteroids, Ecd) are natural compounds widely distributed in plants. They exhibit strong anabolic, vitamin D-like, pharmacological effects in vertebrate animals and in the human body. In the larval stages of insects, injections of pure Ecd cause serious pathophysiological, "hyperecdysonic" syndromes associated with neuromuscular paralysis, premature cuticular apolysis and complete inhibition of ecdysis. Ecds do not penetrate insect cuticle. For this reason, all previous attempts to induce ecdysone responses by topical applications of Ecd failed. In this work, we tried to induce the topical effects of Ecd by preparation of more lipophilic complexes, with 2 or 4 molecules of 20-hydroxyecdysone (E20) attached to a relatively large nucleus of the porphyrin. The resulting porphyrin-E20 complexes (ecdysogens) have been subjected to standardised assays for ecdysone activity in the ligatured larvae ("dauerlarvae") of the greater waxmoth (Galleria mellonella). Similarly like the free E20 alone, porphyrin-E20 complexes had no effect when applied on the body surface or administered in the larval diet. When injected, however, they exhibited delayed effects, but the adverse ("hyperecdysonic") pathophysiological syndromes were reduced or abolished. It is concluded, therefore, that the replacement of pathophysiological, precocious or "hyperecdysonic" moults by the larval-pupal transformation, was due to successive metabolic liberation of the biologically active, free E20 from the porphyrin-E20 complex. The biological status of Ecd does not agree with their definition as the prothoracic gland (PG) hormone of insects, nor with the assumptions about a growth hormone of plants. A possibility that the most important status of Ecd may depend on the pharmacological properties of a sterolic D6 vitamin has been discussed.

Regio- and stereocontrolled synthesis of novel steroidal isoxazolines: a new route to the formation of selectively modified steroid side chains.

1,3-Dipolar cycloaddition reaction of acetonitrile oxide to cis- and trans-22-hydroxy-24-alkoxycarbonyl-Δ(23)-steroids is investigated. An unusual stereochemical course of the cycloaddition, leading to the same set of the isoxazoline adducts for both (Z)- and (E)-disubstituted olefins is revealed. It is shown, the reaction is regioselective and all possible 4',5'-diastereoisomers of resulting isoxazolines can be prepared as major products by cycloaddition to the suitable alkene. The structure of the key compounds is confirmed by X-ray and 2D NMR analysis.

Design and studies of novel polyoxysterol-based porphyrin conjugates.

New types of steroid-porphyrin conjugates derived from 20-hydroxyecdysone (20E) and 24-epibrassinolide (EBl) were synthesized. An exceptional regioselectivity in the reaction of both steroids with porphyrin boronic acids was found to give side-chain-conjugated boronic esters as sole products. UV-Vis-, fluorescence and NMR spectroscopy yielded similar data for all the studied compounds confirming the solvent driven supramolecular assembly with formation of J-aggregates. CD measurements of water diluted solutions showed a clear difference between 20E and EBl conjugates. The latter showed a strong supramolecular chirality, whereas 20E J-aggregates did not.

Stereoselective synthesis of 9alpha-hydroxylated ecdysteroids.

Treatment of ecdysteroids with excess of TBAF in THF was shown to proceed with stereoselective oxidation at the 9alpha position of the carbon skeleton. The stereochemistry of the products was confirmed by X-ray analysis. Using this method, 9alpha-hydroxyecdysteroids were obtained in good yield. The results open a route to novel type of natural and modified steroids that are difficult to access otherwise.