Browsing by Subject "Cyclobutane"

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Open Access
Template-directed photochemical [2+2] cycloaddition reactions of naphthalene acrylic acid derivatives and an investigation of single-crystal-to-single-crystal transformation
(2024-08) Temel, Merve
The first part of the thesis investigated the methodology study on the use of 1,8- dihydroxynaphthalene as a covalent template in the photochemical [2+2] cycloaddition reactions of naphthalene acrylic acid derivatives. This study showed that both in solution and solid state reactions, from the unsymmetrical template-bound diesters, the [2+2] cycloadducts can be formed. Importantly, the removal of the template molecule provided the products with high diastereoselectivities, whereas reactions in solution phase provided higher yields compared to solid state reactions. In the second part of the thesis, the same strategy was applied to the template-bound symmetrical diester. Photochemical [2+2] cycloaddition reactions were investigated in three different phases: (i) in powder form, (ii) as a single crystal, (iii) in solution, under 365 nm UV light. In addition to that, the effect of daylight on the cyclization of symmetrical diester was examined. For the single crystal, single-crystal-to-single-crystal (SCSC) transformation was studied. In all cases, the [2+2] cycloadduct was synthesized and isolated in good yields and with high diastereoselectivity.
Open Access
Thymidine dinucleotides induce S phase cell cycle arrest in addition to increased melanogenesis in human melanocytes
(1998) Pedeux, R.; Al-Irani, N.; Marteau, C.; Pellicier, F.; Branche, R.; Ozturk, M.; Franchi, J.; Doré, J. F.
Although the induction of pigmentation following exposure of melanocytes to ultraviolet light in vivo and in vitro is well documented, the intracellular mechanisms involved in this response are not yet fully understood. Exposure to UV-B radiation leads to the production of DNA damage, mainly cyclobutane pyrimidine dimers, and it was recently suggested that the thymidine dinucleotide pTpT, mimicking small DNA fragments released in the course of excision repair mechanisms, could trigger melanin synthesis. We now report that the thymidine dinucleotide pTpT induces melanogenesis both in human normal adult melanocytes and in human melanoma cells. Thus, the SOS- like response suggested by Gilchrest's work to be evolutionary conserved, based primarily on work in murine cells and guinea pigs, is also apparently present in the human. Thymidine dinucleotide is non toxic to melanoma cells and does not induce apoptosis in these cells, but induces S phase cell cycle arrest and a proliferation slow down. Because thymidine excess in culture medium leads to the synchronization of cells in S phase, we investigated whether this phenomenon was involved in the increase in melanin synthesis. We show that melanin synthesis is specifically triggered by the dimeric form of the thymidine and not by the monomeric form pT. Thus, our data strongly support that thymidine dinucleotides pTpT mimic at least part of the effects of ultraviolet irradiation, and may hence represent an invaluable model in the study of the molecular events involved in melanogenesis induction triggered through DNA damage.