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Two-Step, One-Flask Synthesis of a Meso-Substituted Phlorin

Kim, Dongjoon, Chun, Hao-Jung, Donnelly, Christopher C., Geier, G. Richard
Journal of organic chemistry 2016 v.81 no.12 pp. 5021-5031
acetone, aldehydes, benzophenones, catalysts, organic chemistry, oxidants, porphyrins
A two-step, one-flask reaction of pyrrole with pentafluorobenzaldehyde and acetone was investigated to determine the potential for a streamlined synthesis of a phlorin and/or 5-isocorrole as an alternative to stepwise, dipyrromethanecarbinol routes. Analytical-scale reactions were performed examining the effect of reactant concentration, reactant ratio, acid catalyst (TFA or BF₃·OEt₂), concentration of acid catalyst, oxidant quantity, and reaction time on the distribution of phlorin and 5-isocorrole as well as three additional porphyrinoids (porphodimethene, porphyrin, and corrole). Phlorin was observed ubiquitously in yields up to 20–26%, whereas 5-isocorrole was not detected. Promising reaction conditions for the one-flask synthesis of the phlorin were performed on a preparative scale. The best reaction condition afforded the phlorin in an isolated yield of 20–21% (249–268 mg). Preliminary attempts to extend the methodology to the preparation of phlorins derived from other ketones resulted in a low yield of phlorin from acetophenone (5%) and no detectable phlorin from benzophenone. The discovery of reaction conditions for the two-step, one-flask synthesis of a phlorin provides easier access to this interesting compound, and provides encouragement for the further study of reactions of pyrrole with an aldehyde and a ketone.