Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure.

Invited for the cover of this issue are the groups of Kazuteru Usui and Satoru Karasawa at Showa Pharmaceutical University and Yasuhiro Kobori of Kobe University. The image depicts chirality control of helical compounds through cycles of photocleavage and recombination under sunlight with a "Jack and the Beanstalk" motif. Read the full text of the article at 10.1002/chem.202302413.

Quantitative and Nondestructive Colorimetric Amine Detection Method for the Solid-Phase Peptide Synthesis as an Alternative to the Kaiser Test.

Solid-phase peptide synthesis (SPPS) is an essential technique for the synthesis of peptide. For half a century, many amine detection methods have been developed to monitor coupling reactions during SPPS. Despite such efforts, to the best of our knowledge, a nondestructive and quantitative colorimetric method has not been developed. Here, we developed the first quantitative and nondestructive colorimetric amine detection method based on an acid-base reaction between HCl salt of electron donor-acceptor type dyes and amino groups on the resin. In this method, a noncolored solution of HCl salt consisting of dyes, whose pKBH+ value was carefully tuned, was deprotonated by amines, allowing the appearance of a yellow color. A good linear relationship (R2 = 0.999) between the absorption of the colored solution and the amine group quantities was confirmed. For all tested proteinogenic and nonproteinogenic amino acids, we achieved quantitative colorimetric analysis with a small relative standard deviation (RSD < 3.6%). Furthermore, during the practical synthesis of an octapeptide containing undetectable amino acids with the Kaiser test, our amine detection allowed for detailed monitoring of the coupling reaction, resulting in a significantly purer peptide in the crude form than that obtained using the Kaiser test.

Photoisomerization of "Partially Embedded Dihydropyridazine" with a Helical Structure.

Herein, we report the synthesis of two "partially embedded fused-dihydropyridazine N-aryl aza[5]helicene derivatives" (PDHs) and the demonstration of their intrinsic photo-triggered multi-functional properties based on a Kekulé biradical structure. Introducing bulky electron-withdrawing trifluoromethyl or pentafluoroethyl groups into the aza[5]helicene framework (PDH-CF3 and -C2 F5 ) gives PDH axial chirality based on the helicity of the P and M forms, even at room temperature. Upon photo-irradiation of PDH-CF3 in a frozen solution, an ESR signal from the triplet biradical with zero-field splitting values, generated by N-N bond dissociation, was observed. However, when the irradiation was turned off, the ESR signal became silent, thus indicating the existence of two equilibria: between the biradical and quinoidal forms based on the Kekulé structure, and between N-N bond cleavage and recombination. The observed photo- and thermally induced behaviors indicate that T-type photochromic molecules are involved in the photoisomerization mechanism involving the two equilibria. Inspired by the photoisomerization, chirality control of PDH by photoracemization was achieved. Multiple functionalities, such as T-type photochromism, photo-excitation-mediated triplet biradical formation, and photoracemization, which are attributed to the "partially embedded dihydropyridazine" structure, are demonstrated.

Oxidation of an Internal-Edge-Substituted [5]Helicene-Derived Phosphine Synchronously Enhances Circularly Polarized Luminescence.

Invited for the cover of this issue are the groups of Kazuteru Usui and Satoru Karasawa at Showa Pharmaceutical University, and Yoshitane Imai at Kindai University. The image depicts how a phosphine-oxide-bearing helicene exhibits markedly enhanced CPL response in the excited state compared with that of one with a corresponding phosphine. Read the full text of the article at 10.1002/chem.202202922.

Oxidation of an Internal-Edge-Substituted [5]Helicene-Derived Phosphine Synchronously Enhances Circularly Polarized Luminescence.

Small chiral organic molecules with CD properties are in high demanded due to their potential use in promising electronic and biological applications. Herein, we reveal a system in which the oxidation of a phosphino group to the corresponding phosphine oxide on the inner rim of a helicene derivative induces a CPL response. Laterally π-extended 7,8-dihydro[5]helicenes bearing phosphine and phosphine oxide groups on their inner helical rims (i. e., the C1 position) were synthesized, and their helical structures were unambiguously determined by X-ray crystallography. The photophysical (UV/visible and emission) and chiroptical properties of these compounds were investigated in various solvents. Despite their structural similarities, phosphine oxide showed a significantly better CPL response than phosphine, with a high dissymmetry factor for emission (|glum |=(1.3-1.9)×10-3 ) that can be attributed to structural changes in the interior of the helicene helix.

Effects of Substituents on the Properties of Metal-Free MRI Contrast Agents.

Materials possessing electron spin can shorten the T 1 relaxation times in magnetic resonance imaging (MRI). For example, gadolinium (Gd) complexes with seven f-orbital electrons are widely used as contrast agents in clinical applications. However, Gd has severe potential side effects, and thus metal-free alternatives are needed. Toward this end, we synthesized seven NO radicals consisting of a dioxa-azaspiro[4.5]decane framework having various substituents, DAD-X (X = methyl, ethyl, n-propyl, c-propyl, vinyl, phenyl, and 2-pyridyl), that functioned as metal-free MRI contrast agents. The relationship between (i) water-proton relaxivity and log P and (ii) reactivity for ascorbic acid and the spin density of the NO oxygen atom were established, which provided a basis for the rational design of practical metal-free contrast agents.