Dihydroporphyrin synthesis: New methodology

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Selective formation of trans-nitrochlorins 16-19, cyclopropylchlorins 14, 15, and 20-23, or functionalized trans-chlorins 5-13 by reaction of 2- nitro-5,10,15,20-tetraphenylporphyrin 1 with 'active' methylene compounds such as malonates or malononitrile in the presence of base has been achieved. Reaction control is accomplished via sequential Michael additions, followed by intra-molecular nucleophilic displacement of a secondary nitro group. Steric as well as thermodynamic effects have been found to govern the selectivity of product formation. Ambient temperature or bulky carbanion substituents lead to nitrochlorins and/or cyclopropylchlorins. Increased reaction temperatures, combined with sterically less encumbered carbanion substituents, favor the formation of disubstituted trans-chlorins. Nucleophilic ring-opening reactions of cyclopropyl-derivative 14 afford disubstituted trans-chlorin products 5 and 25 and provide additional mechanistic evidence for the intermediacy of the cyclopropylchlorin. Use of porphyrins with modified meso-phenyl positions illustrates the generality of this methodology and allows a novel method for the preparation of a wide range of reduced porphyrins, which may find application in fields such as the photodynamic therapy (PDT) of cancer.

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Journal of Organic Chemistry

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