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.

NBS-assisted palladium-catalyzed bromination/cross-coupling reaction of 2-alkynyl arylazides with KSCN: an efficient method to synthesize 3-thiocyanindoles

An efficient NBS-assisted palladium-catalyzed bromination/cross-coupling synthesis of 3-thiocyanindoles from 2-alkynyl arylazides with KSCN has been described.