Aiming to improve their effectiveness, three modifications were introduced into the structure of the natural phytotoxins nostoclides, leading to the synthesis of novel 3-benzyl-4-isopropyl-5-(arylmethylene)furan-2(5H)-ones, 3-benzyl-5-(furan-2-ylmethylene)furan-2(5H)-ones, and 3,4-dihalo-5-arylidenefuran-2(5H)-ones. All compounds were characterized by IR, 1 H and 13 C NMR, NOEDIF, COSY, HETCOR and MS spectrometry. Increasing the length of the molecule was found to reduce the ability to interfere with ferricyanide reduction by isolated chloroplasts. The addition of an isopropyl moiety into the heterocyclic ring, as in naturally-occurring nostoclides, did not influence the inhibitory potential. Also the replacement of the electron-drawing phenyl substituent with two halogen residues did not improve the resulting activity. However, both latter modifications influenced the interaction with the photosynthetic machinery. These analogues could therefore represent novel leads to be explored toward the development of new herbicides targeting the chloroplastic electron transport chain.

Julolidines constitute a class of N‐heterocycle compounds which have in common the 2,3,6,7‐tetrahydro‐1H,5H‐benzo[1,2]quinolizine ring. Due mainly to its fluorescence properties, such structures have shown potential application in a range of scientific and technological areas and have attracted attention of a great variety of research groups. For example, julolidines have been used for detection of ions and volatile compounds in environmental and biological samples, to the construction of dye‐sensitized solar cells, photoconductive materials and as fluorescent sensors for bioimaging. This review summarizes the strategies reported to the synthesis of the julolidine ring and the principal modifications for obtaining more complex julolidine derivatives with improved fluorescence properties of these compounds. In this context, aldol condensation, olefination, imine synthesis, and cross‐coupling reactions have been extensively explored and discussed. Examples of applications, which illustrate the great potential of such structures, will also be briefly presented.

A new calixarene-catalyzed cascade process, via the Povarov reaction, for the synthesis of tetrahydroquinolines is described, and the proposed reaction mechanism was validated.

In a recent paper, we reported the synthesis and photosynthesis-inhibitory activity of a series of analogues of rubrolides. From quantitative structure-activity relationship (QSAR) studies, we found that the most efficient compounds are those having higher ability to accept electrons. On the basis of those findings, we directed our effort to synthesize new analogues bearing a strong electron-withdrawing group (nitro) in the benzylidene ring and evaluate their effects on photosynthesis. However, the employed synthetic approach led to novel cyclopent-4-ene-1,3-diones as major products. Here, we report the synthesis and mechanism of action of such cyclopent-4-ene-1,3-diones as a new class of photosynthesis inhibitors. These compounds block the electron transport at the QB level by interacting at the D1 protein at the reducing side of Photosystem II and act as Hill reaction inhibitors, with higher activity than the corresponding rubrolides. To the best of our knowledge, this is the first report on the photosynthesis inhibitory activity of cyclopentenediones.

No abstract

Natural products called rubrolides have been investigated as a model for the development of new herbicides that act on the photosynthesis apparatus. This study comprises a comprehensive analysis of the photosynthesis inhibitory ability of 27 new structurally diverse rubrolide analogues. In general, the results revealed that the compounds exhibited efficient inhibition of the photosynthetic process, but in some cases low water solubility may be a limiting factor. To elucidate their mode of action, the effects of the compounds on PSII and PSI, as well as their partial reaction on chloroplasts and the chlorophyll a fluorescence transients were measured. Our results showed that some of the most active rubrolide analogues act as a Hill reaction inhibitors at the QB level by interacting with the D1 protein at the reducing side of PSII. All of the active analogues follow Tice's rule of 5, which indicates that these compounds present physicochemical properties suitable for herbicides.

No abstract

To investigate the herbicidal potential of 2,5-diketopiperazines (2,5-DKPs), we applied a known protocol to produce a series of 2,5-DKPs through intramolecular N-alkylation of Ugi adducts. However, the method was not successful for the cyclization of adducts presenting aromatic rings with some substituents at the ortho position. Results from DFT calculations showed that the presence of voluminous groups at the ortho position of a benzene ring results in destabilization of the transition structure. Lower activation enthalpies for the SN2-type cyclization of Ugi adducts were obtained when bromine, instead of a chlorine anion, is the leaving group, indicating that the activation enthalpy for the cyclization step controls the formation of the 2,5-DKP. Some Ugi adducts and 2,5-DKPs formed crystals with suitable qualities for single-crystal X-ray diffraction data collection. Phytotoxic damage of some 2,5-DKPs on leaves of the weed Euphorbia heterophylla did not differ from those caused by the commercial herbicide diquat.