Estudos sobre síntese de novos fotossensibilizadores a partir de produtos naturais porfirinóides
Santos, Luna Schlittler dos
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In this work, we studied the structural transformation of methylpheophorbide a (2), a derivative of chlorophyll a (1) that was extracted from seaweed Spirulina maxima. The main propose of the study was to obtain compounds with low aggregation and appropriate photophysical characteristics for applications in Photodynamic Therapy (PDT). In this way, we prompted to transform chlorin type derivatives into the corresponding bacteriochlorins using cycloaddition reaction approaches. The use of chlorophyll a (1) was mainly motivated by its availability and low cost. In addition, this compound has been a focus of study in our research group. Therefore, it was initially developed a method of extracting/processing chlorophyll a which directly provided compound 2 (Scheme 1). In this process we used an extraction/transesterification one-pot method using an acid solution of methanol and chlorin 2 was obtained in 0.8% yield from dried seaweed. Compound 3 was obtained from 2 (77%) and then hydrogenated to give 4 (80%). The chlorophyll derivative 4 was used as starting material in order to prepare the desired bacteriochlorin via cycloaddition methodology (Scheme 2). Scheme 1. Functionalization reactions of chlorophyll a. i, ii, iii, iv, v, vi) extracting/processing one-pot reaction of compound 2 from the seaweed Spirulina maxima dehydrated in the presence of MeOH/H2SO4 5% (v/v) for 24h 0.8% . vii) MeONa / MeOH, r.t., 8h, 77%; viii) H2, 1 atm, Pd/C, 80%. Several azomethine and nitrile oxide dipoles were tested in the 1,3- dipolar cycloaddition reactions. Nevertheless, none of the studied conditions provided the desired cycloaddition adduct (bacteriochlorin). In some cases, decarboxylation by-products or total/partial substrate 4 degradation was observed. Scheme 2. Synthesis of new bacteriochlorins. An alternative method for the synthesis of bacteriochlorins using a cycloaddition approach was designed starting from the vinyl-chlorin 13 and Nphenylmaleimide (15) (Scheme 3). The synthesis of compound 13 was accomplished starting from compound 4 by dihydroxylation reaction with OsO4 (67%) followed by double dehydration in 1,2-dichlorobenzene (30%). The most promising results from cycloaddition reaction were obtained with 13 and 15 with yields around 47% (75:25 ratio of diastereoisomers determined by 1H-NMR spectroscopy). Scheme 3. Synthesis of new bacteriochlorins. The synthesized compounds were characterized by 1D and 2D NMR and UV-vis spectroscopy, infra-red and mass spectrometry. Some preliminary photophysical studies, such as photobleaching, were also conducted in order to qualify bacteriochlorin compound 14 as a potential photosensitizer. Moreover, aggregation studies by UV-vis were performed, which confirmed the low aggregation of the obtained bacteriochlorins.