Diastereo- and Enantioselective Synthesis of Pyrrolo[1,4]benzodiazepines through Decarboxylative Photocyclization

Photochemistry of phthalimidoadamantane dipeptides: effect of amino acid side chain on photocyclization

A series of dipeptides 1 was synthesized that at the N-site contained 3-(N-phthalimidoadamantane-1-carboxylic acid and at the C-site different aliphatic or aromatic L- or D-amino acids. The photochemical reaction of dipeptides 1 under acetone-sensitized conditions gave simple decarboxylation products 6, and decarboxylation-induced cyclization products 7, as well as some secondary products 8 and 9 formed by elimination of H2O or ring enlargement, respectively. Molecules 9 undergo secondary photoinduced H-abstractions by the phthalimide chromophore, delivering more complex polycycles 11. The photodecarboxylation-induced cyclization to 7 was observed only with phenylalanine (Phe), proline (Pro), leucine (Leu) and isoleucine (Ile). Contrary to dipeptides with Phe, the cyclization takes place with almost complete racemization at the amino acid chiral center, but diastereoselectively giving only one pair of enantiomers. The conducted investigation is important as it provides the breath and the scope of dipeptide cyclizations activated by phthalimides.

Awakening a Molecular Mummy: The Inter-and Intramolecular Photochemistry of Pyromellitic Diimides with Alkyl Carboxylates

Pyromellitic acid diimides are not as chemically unreactive as conjecturable (and presupposed) from their numerous applications as electron acceptor units or electron carriers in molecular donor-acceptor dyads or triads. Similar to the corresponding phthalimides, electronically excited pyromellitic diimides oxidize alkyl carboxylates in aqueous solution via intermolecular electron transfer (PET) processes, which eventually results in radical-radical combination products, e.g., the benzylation product 6 from N,N'-dimethyl pyromellitic diimide 5. The analogous product 7 was formed with pivalic acid as tert-butyl radical source. One additional product 8 was isolated from alkylation/dearomatization and multiple radical additions, respectively, after prolonged irradiation. In intramolecular versions, from N-carboxyalkylated pyromellitic diimides 9a-e (C1 to C5-spaced), degradation processes were detected, e.g., the cyclization products 10 from the GABA substrate 9c. In sharp contrast to phthalimide photochemistry, the green pyromellitic diimide radical anion was detected here by UV-vis absorption (λabs = 720 nm), EPR (from 9d), and NMR spectroscopy for several intramolecular electron transfer examples. Only the yellow 1,4-quinodial structure is formed from intermolecular PET, which was deduced from the absorption spectra (λabs = 440 nm) and the subsequent chemistry. The pyromellitimide radical anion lives for hours at room temperature in the dark, but is further degraded under photochemical reaction conditions.

Enantioselective synthesis of heterocyclic compounds using photochemical reactions

Different methods for the direct enantioselective photochemical synthesis of heterocycles are presented. Currently, asymmetric catalysis with templates involving hydrogen bonds or metal complexes is intensively investigated. Enzyme catalysis can be simplified under photochemical conditions. For example, in multi enzyme systems, one or more enzyme catalytic steps can be replaced by simple photochemical reactions. Chiral induction in photochemical reactions performed with homochiral crystals is highly efficient. Such reactions can also be carried out with crystalline inclusion complexes. Inclusion of a photochemical substrate and an enantiopure compound in zeolites also leads to enantioselective compounds. In all these methods, the conformational mobility of the photochemical substrates is reduced or controlled. Memory of chirality is a particular case in which a chiral information is temporally lost but the rigid conformations stabilize the molecular structure which leads to the formation of enantiopure compounds. Such studies allows a profound understanding on how particular conformations determine the configuration of the final products.Graphical abstract.

Photochemical Methods for Peptide Macrocyclisation

Photochemical reactions have been the subject of renewed interest over the last two decades, leading to the development of many new, diverse and powerful chemical transformations. More recently, these developments have been expanded to enable the photochemical macrocyclisation of peptides and small proteins. These constructs benefit from increased stability, structural rigidity and biological potency over their linear counterparts, providing opportunities for improved therapeutic agents. In this review, an overview of both the established and emerging methods for photochemical peptide macrocyclisation is presented, highlighting both the limitations and opportunities for further innovation in the field.

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

Organic compounds that show Thermally Activated Delayed Fluorescence (TADF) have become wildly popular as next-generation emitters in organic light emitting diodes (OLEDs). Since 2016, a subset of these have found increasing use as photocatalysts. This review comprehensively highlights their potential by documenting the diversity of the reactions where an organic TADF photocatalyst can be used in lieu of a noble metal complex photocatalyst. Beyond the small number of TADF photocatalysts that have been used to date, the analysis conducted within this review reveals the wider potential of organic donor-acceptor TADF compounds as photocatalysts. A discussion of the benefits of compounds showing TADF for photocatalysis is presented, which paints a picture of a very promising future for organic photocatalyst development.

One-Pot Synthesis of Seven-Membered Heterocyclic Derivatives of Diazepines Involving Copper-Catalyzed Rearrangement Cascade Allyl-Amination

A novel and efficient method has been proposed for the synthesis of 1,4-benzodiazepine-5-ones from o-nitrobenzoic N-allylamides by using molybdenyl acetylacetonate and copper(II) trifluoromethanesulfonate as catalysts in the presence of triphenylphosphine. This synthesis process involves nitrene formation, C-H bond insertion, C═C bond rearrangement, and C-N bond formation cascade reactions via copper- and molybdenum-catalyzed mediation. The method features a wide substrate scope and a moderate to high yield (up to 90%), exhibiting the possibility for practical applications.

Photophysical properties and electron transfer photochemical reactivity of substituted phthalimides

Substituents on phthalimide affect its photophysics and photochemical reactivity. Electron donors generally result in low quantum yields of intersystem crossing and reactivity from singlet excited states.

Photocyclization of Tetra- and Pentapeptides Containing Adamantylphthalimide and Phenylalanines: Reaction Efficiency and Diastereoselectivity

A series of tetrapeptides and pentapeptides was synthesized bearing a phthalimide chromophore at the N-terminus. The C-terminus of the peptides was strategically substituted with an amino acid, Phe, Phe(OMe), or Phe(OMe)₂ characterized by different oxidation potentials. The photochemical reactivity of the peptides was investigated by preparative irradiation and isolation of photoproducts, as well as with laser flash photolysis. Upon photoexcitation, the peptides undergo photoinduced electron transfer (PET) and decarboxylation, followed by diastereoselective cyclization with the retention of configuration for tetrapeptides or inversion of configuration for pentapeptides. Molecular dynamics (MD) simulations and NOE experiments enabled assignment of the stereochemistry of the cyclic peptides. MD simulations of the linear peptides disclosed conformational reasons for the observed diastereoselectivity, being due to the peptide backbone spatial orientation imposed by the Phe amino acids. The photochemical efficiency for the decarboxylation and cyclization is not dependent on the peptide length, but it depends on the oxidation potential of the amino acid at the C-terminus. The results described herein are particularly important for the rational design of efficient photochemical reactions for the preparation of cyclic peptides with the desired selectivity.

Oxidative coupling of enolates using memory of chirality: an original enantioselective synthesis of quaternary α-amino acid derivatives

We describe here the first enantioselective oxidative heterocoupling of enolates. Our strategy relies on the memory of chirality concept and allows the stereocontrolled formation of quaternary centres on α-amino acid derivatives with an enantiomeric excess of up to 94%.

Silver-Mediated Oxidative Decarboxylative Intramolecular Asymmetric Radical Cyclization (Csp3-Csp2) via Memory of Chirality: Access to Circumdatin Alkaloids

A novel silver-mediated oxidative decarboxylative intramolecular asymmetric radical cyclization has been developed to form a C_sp3-C_sp2 bond via memory of chirality. The application of the process has been demonstrated for the synthesis of the circumdatin class of alkaloids in high enantiopurity with retention of the configuration. The developed protocol is mild and works with an inexpensive silver catalyst in the absence of ligand, base, or additives. The involvement of a monoradical in the reaction has been established by trapping the radical intermediate.

Recent Advances in Photodecarboxylations Involving Phthalimides

Owing to their favourable photophysical and electrochemical properties, phthalimides undergo a variety of highly efficient photodecarboxylation reactions. These transformations have been applied to the synthesis of macrocyclic compounds as well as bioactive addition adducts. N-Acetoxyphthalimides are versatile precursors to imidyl and alkyl radicals through photodecarboxylation and have subsequently been used for a variety of coupling reactions. The generally mild reaction conditions make these reactions attractive for green chemical applications. The process protocols were successfully transferred to novel photoreactor devices, among these falling film or continuous flow reactors.

Studies in organic and physical photochemistry - an interdisciplinary approach

Traditionally, organic photochemistry when applied to synthesis strongly interacts with physical chemistry. The aim of this review is to illustrate this very fruitful interdisciplinary approach and cooperation. A profound understanding of the photochemical reactivity and reaction mechanisms is particularly helpful for optimization and application of these reactions. Some typical reactions and particular aspects are reported such as the Norrish-Type II reaction and the Yang cyclization and related transformations, the [2 + 2] photocycloadditions, particularly the Paternò-Büchi reaction, photochemical electron transfer induced transformations, different kinds of catalytic reactions such as photoredox catalysis for organic synthesis and photooxygenation are discussed. Particular aspects such as the structure and reactivity of aryl cations, photochemical reactions in the crystalline state, chiral memory, different mechanisms of hydrogen transfer in photochemical reactions or fundamental aspects of stereoselectivity are discussed. Photochemical reactions are also investigated in the context of chemical engineering. Particularly, continuous flow reactors are of interest. Novel reactor systems are developed and modeling of photochemical transformations and different reactors play a key role in such studies. This research domain builds a bridge between fundamental studies of organic photochemical reactions and their industrial application.

Transition metal-free decarboxylative alkylation reactions

This review summarizes recent advances in the transition metal-free decarboxylative alkylation of carboxylic acids and their derivatives.

Memory of chirality in cascade rearrangements of enediynes

The cascade rearrangement of chiral enediynes 1c-e, involving successively 1,3-proton shift, Saito-Myers cyclization, 1,5-hydrogen atom transfer, and intramolecular coupling of the resulting biradical, proceeded at 80 °C to form tri- and tetracyclic heterocycles possessing a quaternary stereogenic center with a very high level of memory of chirality.

Decoding stereocontrol during the photooxygenation of oxazolidinone-functionalized enecarbamates

Systematically designed oxazolidinone-derived enecarbamates reveal that solvent and temperature effects on the stereoselectivity during photooxygenation are likely due to the conformational flexibility of the chiral phenethyl side chain (entropy factors); the extent of enantiomeric excess in the photoproduct is dictated by the alkene geometry.

Photochemically induced radical addition of tertiary amines to C=C and C=O double bonds: A green chemistry contribution to organic synthesis

Photochemically induced electron transfer considerably enriches the redox chemistry of organic molecules. This primary step has been used to produce α-amino alkyl radicals that can be added to various double bonds. The addition to olefinic and carbonyl bonds is discussed. Homogeneous and heterogeneous photocatalysis methods with various electron-transfer sensitizers are described.

Photochemical asymmetric synthesis of phenyl-bearing quaternary chiral carbons using chiral-memory effect on β-hydrogen abstraction by thiocarbonyl group

Quaternary chiral carbons were effectively generated from tertiary chiral carbons via photochemical intramolecular beta-hydrogen abstraction reaction of thioimides involving the highly-controlled chiral-memory effect.

Sustainable chemistry: starting points and prospects

We review here the concept of sustainable chemistry (SC), which is still in its early development. One important element of SC is commonly defined as chemical research aiming at the optimization of chemical processes and products with respect to energy and material consumption, inherent safety, toxicity, environmental degradability, and so on. An increasing number of assessment systems containing quantitative indicators for these aspects are currently being developed. In addition, however, SC should also address the societal aspect of sustainability. With respect to scientific research, the societal aspect is defined here by two requirements: (1) the assumptions, objectives and implications of chemical research and its technical application should be made more transparent to various societal actors; (2) uncertainty and ignorance should be treated more explicitly in the course of scientific research. Meeting these requirements is necessary in order to lift the division between the allegedly disinterested and non-normative scientific research and the value-laden sphere of societal needs, preferences and decision-making situations. This, in turn, is understood here as a contribution to a more sustainable scientific practice. We illustrate the two elements of SC-optimization of products and processes as well as including the societal aspect-with the examples of environmental chemistry, green chemistry and the environmental assessment of chemical products. While considerable progress has been made in these fields, the societal aspect of SC remains to be recognized more fully in all branches of chemical research. One prerequisite for this is the inclusion of SC into chemical education from the very beginning.

[Innovative molecular transformation using optically active alpha-amino acids]

Some methods for innovative molecular transformations using optically active alpha-amino acids have been exploited. 1) The non-Kolbe reaction of the N-benzoyloxazoline derivative 1 derived from L-serine gave the optically active N,O-acetal 2 when graphite was used an anode material. This reaction represents the first example of "memory of chirality" in the carbenium ion chemistry. 2) The optically active pipecolic acid derivative 13, prepared from L-lysine by using electrochemical oxidation, was cyclopropanated with high diastereoselectivity (96.6%de), and the product 14 was transformed into (2S,3R)-metanopipecolic acid (7). 3) An enantiomerically pure 1,2-dihydropyridine 23 was prepared from L-lysine using electrochemical oxidation as a key step and was utilized as a chiral diene synthon in the Diels-Alder reaction. That is, in the presence of AlCl3, the Diels-Alder reaction between 23 and N-acryloyloxazolidinone 24 gave a cycloadduct with high stereoselectivity, which was converted to an optically active isoquinuclidine derivative 26 (96.8% ee). 4) The Hofmann rearrangement of the L-glutamine derivative 27 to the enantiomerically pure 2-aminobutyric acid derivative 28 was successfully achieved with an electrochemical method using a trifluoroethanol-MeCN solvent system. 5) Some types of N-formyl cyclic amine derivatives were found to be effective activators of trichlorosilane to reduce ketones and imines. Namely, the reduction of ketones and imines by trichlorosilane with a catalytic amount of L-proline derivatives 30 and 32 gave enantiomerically enriched sec-alcohols and amines, respectively, to some extent of optical yields.

Asymmetric photochemistry and photochirogenesis

One of the most interesting phenomena on Earth is the chirality of biomolecules, the origin of which remains unknown. A challenge arising from this phenomenon is the selective, atom-economic synthesis of enantiomerically pure target molecules from nonchiral starting materials. Herein, new developments in the field of asymmetric photochemistry and photochirogenesis are described with special emphasis on absolute asymmetric synthesis. In this context, the elucidation of the ultimate cause of homochirality phenomena on earth and the possible correlation with physicochemical parameters are also presented.