Vânia Maria Teixeira Carneiro

The indiscriminate use of herbicides makes weeds resistant, increasing the demand for more efficient herbicides. Novel potential herbicides were prepared from sorbic acid, being 8 dienamides and 6 epoxides. Subsequently, phytotoxic activities of sorbic acid and 14 synthesized compounds were evaluated against sorghum, onion, cucumber, lettuce, and beggartick seeds. Furthermore, the protein target of the most active substance in plants was identified in silico. Among the tested compounds, (2E,4E)-N-(p-fluorophenyl)hexa-2,4-dienamide (2c), (2E,4E)-N-(o-naphthyl) hexa-2,4-dienamide (2d), (E)-N-methylphenyl-3-((2R,3R)-3-methyloxiran-2-yl)acrylamide (3b), (E)-N-bromophenyl-3-((2R,3R)-3-methyloxiran-2-yl)acrylamide (3e), and (E)-N-methoxyphenyl3-((2R,3R)-3-methyloxiran-2-yl)acrylamide (3f) showed pronounced inhibitory activities, both of shoots and roots, making them candidates for promising herbicides. All compounds altered seed development, and compounds 2c, 2d, 3b, 3e, and 3f presented equal or superior results to the commercial control herbicide S-metolachlor. Docking studies suggest that enzyme mitogenactivated protein kinase can be the target of those compounds.

Fluorescence spectra and lifetimes were determined for 16 synthetic flavylium cation analogues of anthocyanin plant pigments in dry acetonitrile acidified with trifluoroacetic acid (TFA). Phosphorescence was also observed from the lowest excited triplet state for all of the flavylium cations at 77 K in a rigid TFA-acidified isopropanol glass. The fluorescence quantum yields and lifetimes depend in a systematic manner on the nature and position of the substituents on the flavylium chromophore and three specific substitution patterns associated with significant decreases in the fluorescence quantum yield were identified. A 4′-bromo or 4′-iodo substituent in the B-ring of the flavylium cation produced a small but normal heavy-atom effect, reducing the fluorescence quantum yield and the phosphorescence lifetime relative to analogues without the halogen atom. In contrast, three flavylium cations with a 3′-bromo substituent exhibited an “inverse” heavy atom effect, i.e., an increase in the fluorescence quantum yield rather than a decrease, which was rationalized on the basis of the nodal properties of the natural transition orbitals (NTOs) involved in the S₀→S₁ radiative transition.

Anthocyanins, which are responsible for most of the red, blue and purple colors of fruits and flowers, are very efficient at absorbing and dissipating light energy via excited state proton transfer or charge-transfer mediated internal conversion without appreciable excited triplet state formation. During the maturation of red wines, grape anthocyanins are slowly transformed into pyranoanthocyanins, which have a much more chemically stable pyranoflavylium cation chromophore. Development of straightforward synthetic routes to mono- and disubstituted derivatives of the pyranoflavylium cation chromophore has stimulated theoretical and experimental studies that highlight the interesting absorption and emission properties and redox properties of pyranoflavylium cations. Thus, p-methoxyphenyl substitution enhances the fluorescence quantum yield, while a p-dimethylaminophenyl substituent results in fast decay via a twisted intramolecular charge-transfer (TICT) state. Unlike anthocyanins and their synthetic analogs (flavylium cations), a variety of pyranoflavylium cations form readily detectable excited triplet states that sensitize singlet oxygen formation in solution and exhibit appreciable two-photon absorption cross sections for near-infrared light, suggesting a potential for applications in photodynamic therapy. These excited triplet states have microsecond lifetimes in solution and excited state reduction potentials of at least 1.3 V vs. SCE, features that are clearly desirable in a triplet photoredox catalyst.

An improved procedure is described for the preparation of pyranoflavylium cations from the reaction of 5,7-dihydroxy-4-methylflavylium cation with aromatic aldehydes. Modifications of the procedure of Chassaing et al. (Tetrahedron Lett. 2008, 49, 6999–7004; Tetrahedron 2015, 71, 3066–3078) circumvent the reported restriction to electron-rich benzaldehydes and provide access to a wide variety of substituted pyranoflavylium cations, including those with electron-withdrawing substituents or an attached heterocyclic or polycyclic aromatic ring. This opens the way for studies of substituent and structural effects on the ground and excited states of these pyranoanthocyanin analogues, the behavior of which should mirror fundamental aspects of the chemistry and photophysics of the pyranoanthocyanin chromophores present in mature red wines.

Cryptomoscatones D1 and D2, dihydropyranones isolated from C. mandiocanna, had their stereochemistry solved after total syntheses and comparison of spectroscopic data of natural and synthetic samples. Natural dihydropyranones coibacins A and B, metabolites isolated from marine cyanobacterium cf. Oscillatoria sp, which display antileishmanial activity and anti-inflammatory properties, were synthesized and had their absolute configuration determined. Our efforts towards the synthesis of cryptomoscatone E3, natural dihydropyranone with unknown absolute configuration, are also discussed.