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.

Photosensitized direct C-H fluorination and trifluoromethylation in organic synthesis

The importance of fluorinated products in pharmaceutical and medicinal chemistry has necessitated the development of synthetic fluorination methods, of which direct C-H fluorination is among the most powerful. Despite the challenges and limitations associated with the direct fluorination of unactivated C-H bonds, appreciable advancements in manipulating the selectivity and reactivity have been made, especially via transition metal catalysis and photochemistry. Where transition metal catalysis provides one strategy for C-H bond activation, transition-metal-free photochemical C-H fluorination can provide a complementary selectivity via a radical mechanism that proceeds under milder conditions than thermal radical activation methods. One exciting development in C-F bond formation is the use of small-molecule photosensitizers, allowing the reactions i) to proceed under mild conditions, ii) to be user-friendly, iii) to be cost-effective and iv) to be more amenable to scalability than typical photoredox-catalyzed methods. In this review, we highlight photosensitized C-H fluorination as a recent strategy for the direct and remote activation of C-H (especially C(sp3)-H) bonds. To guide the readers, we present the developing mechanistic understandings of these reactions and exemplify concepts to assist the future planning of reactions.

[3 + 2] Cycloaddition with photogenerated azomethine ylides in β-cyclodextrin

Stability constants for the inclusion complexes of cyclohexylphthalimide 2 and adamantylphthalimide 3 with β-cyclodextrin (β-CD) were determined by 1H NMR titration, K = 190 ± 50 M-1, and K = 2600 ± 600 M-1, respectively. Photochemical reactivity of the inclusion complexes 2@β-CD and 3@β-CD was investigated, and we found out that β-CD does not affect the decarboxylation efficiency, while it affects the subsequent photochemical H-abstraction, resulting in different product distribution upon irradiation in the presence of β-CD. The formation of ternary complexes with acrylonitrile (AN) and 2@β-CD or 3@β-CD was also essayed by 1H NMR. Although the formation of such complexes was suggested, stability constants could not be determined. Irradiation of 2@β-CD in the presence of AN in aqueous solution where cycloadduct 7 was formed highly suggests that decarboxylation and [3 + 2] cycloaddition take place in the ternary complex, whereas such a reactivity from bulky adamantane 3 is less likely. This proof of principle that decarboxylation and cycloaddition can be performed in the β-CD cavity has a significant importance for the design of new supramolecular systems for the control of photoreactivity.

Fluorofunctionalizations of C-C Multiple Bonds and C-H Bonds

In spite of only a few naturally occurring products having one or more fluorine atoms, organofluorine compounds have been widely utilized in pharmaceutical, agrochemical, and functional material science fields due to the characteristic properties of the fluorine atom. Therefore, the development of new methods for the introduction of fluorine-containing functional groups has been a long-standing research topic. This article discusses our contributions to this area. The first topic is on the trifluoromethylations of C-C multiple bonds using Togni reagent based on our working hypothesis that hypervalent iodine could be activated by coordination of the carbonyl moiety to the Lewis acid catalyst. The second topic relates to asymmetric fluorofunctionalization of alkenes. A newly designed phase-transfer catalyst consisting of a carboxylate anion functioning as a phase-transfer agent and a primary hydroxyl group as a site that captures the anionic substrate was revealed to be an effective catalyst for asymmetric fluorolactonization. Inspired by the mechanistic studies of fluorolactonization, we produced a linked binaphthyl dicarboxylate catalyst, which catalyzes the 6-endo-fluorocyclization and the deprotonative fluorination of allylic amides in a highly enantioselective manner. The third topic is on C-H fluorofunctionalizations using either catalysis or photoactivation. Benzylic trifluoromethylation, which is still a rare reaction, using Togni reagent and aromatic C-H trifluoromethylation using Umemoto reagent under simple photoirradiation conditions were achieved. In addition, the Csp³-H fluorination of alkyl phthalimide derivatives is demonstrated.

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.

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.

Visible-Light Photoredox-Catalyzed Giese Reaction: Decarboxylative Addition of Amino Acid Derived α-Amino Radicals to Electron-Deficient Olefins

A tin- and halide-free, visible-light photoredox-catalyzed Giese reaction was developed. Primary and secondary α-amino radicals were generated readily from amino acids in the presence of catalytic amounts of an iridium photocatalyst. The reactivity of the α-amino radicals has been evaluated for the functionalization of a variety of activated olefins.

Catalyst-free photoredox addition-cyclisations: exploitation of natural synergy between aryl acetic acids and maleimide

Suitably functionalised carboxylic acids undergo a previously unknown photoredox reaction when irradiated with UVA in the presence of maleimide. Maleimide was found to synergistically act as a radical generating photoxidant and as a radical acceptor, negating the need for an extrinsic photoredox catalyst. Modest to excellent yields of the product chromenopyrroledione, thiochromenopyrroledione and pyrroloquinolinedione derivatives were obtained in thirteen preparative photolyses. In situ NMR spectroscopy was used to study each reaction. Reactant decay and product build-up were monitored, enabling reaction profiles to be plotted. A plausible mechanism, whereby photo-excited maleimide acts as an oxidant to generate a radical ion pair, has been postulated and is supported by UV/Vis. spectroscopy and DFT computations. The radical-cation reactive intermediates were also characterised in solution by EPR spectroscopy.

Antiproliferative and antiviral activity of three libraries of adamantane derivatives

Three libraries of adamantane derivatives were synthesized and evaluated for antiviral and antiproliferative activities against a broad variety of DNA and RNA viruses. Whereas none of the compounds exhibit antiviral activity at subtoxic concentrations, antiproliferative activity was found against murine leukemia cells (L1210), human T-lymphocyte cells (CEM), and cervix carcinoma cells (HeLa) for 4, 8, and 10.

N-Phthaloyltranexamic acid ammonium salt derivatives as photocaged superbase for redox free radical photopolymerization

N-Phthaloyltranexamic acid ammonium salt derivatives integrating with peroxides can invoke redox photopolymerization and propagating polymerization.

Organic aspects. Oxygen-containing functions

In this chapter, most of the reported work deals with the photochemistry of carbonyl compounds; however, the photoreactions of other functions, such as the photo-Claisen rearrangement or the photocleavage of cyclic ethers, are also included. In the present volume, time coverage is 2010–2011, and only original research articles are quoted. In general, reviews or purely theoretical calculations are not systematically included. As usually, the material is organized according to established types of reactions (e.g., Norrish I/II, hydrogen abstraction, Paternò-Büchi, photoelimination, photo-Fries/photo-Claisen, etc.). After presenting the basic photochemical aspects, more specific findings are reported. They include synthetic applications, stereoselectivity, and biological or technological implications. Next, the attention is focused on photochemical reactions in anisotropic media, including (micro)heterogeneous or supramolecular systems, solid matrixes or fully organized crystals. Finally, mechanistic studies based on direct experimental evidence are highlighted, especially when transient absorption spectroscopy or related ultrafast detection are employed.