De Novo Synthetic Approaches to Hydroporphyrins

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Title: De Novo Synthetic Approaches to Hydroporphyrins
Author: McDowell, Brian Edward
Advisors: Dr. Jonathan Lindsey, Committee Chair
Abstract: Hydroporphyrins are aromatic macrocycles that possess useful photochemical and electronic properties. Some notable naturally occurring hydroporphyrins that play important biological roles in nature include chlorophylls a and b, the tolyporphins, and bacteriochlorophylls. Consequently, these compounds may have application in several biomedical and materials applications such as photodynamic therapy and information storage, respectively. Herein, the synthesis of simple hydroporphyrins (chlorins and bacteriochlorins) is described. The chemistry described should help facilitate the development of more elaborate and complex hydroporphyrins that may be used for the aforementioned applications. Chlorins possessing zero or one meso-substituent were obtained by the acid-catalyzed condensation of a 1,3,3-trimethyltetrahydrodipyrrin and a 1-bromo-9-formyldipyrromethane to give tetrahydrobiladienes that are subsequently cyclized affording zinc-chlorins in 14-42% yield. Aryl substitution at the meso-positions of the chlorin macrocycle afforded slightly red-shifted Qy bands that do not mimic the intense, long-wavelength absorption band characteristic of chlorophyll a. However, the chemistry employed has recently resulted in the synthesis of several chlorins possessing substituents at the 3,13-positions with spectral properties similar to those of chlorophyll. Bacteriochlorins absorb strongly in the near-infrared region and are attractive candidates for use in photodynamic therapy. The synthesis of a stable bacteriochlorin bearing 2,4,6-trimethylphenyl (mesityl) substituents at the 3,13-positions was achieved through the self-condensation of the corresponding 3-aryl substituted dihydrodipyrrin acetal to afford the bacteriochlorin in 6% yield. X-ray crystal analysis verified the 3,13-disubstitution pattern. This 3,13-dimesityl bacteriochlorin exhibits spectral features (Qy = 726 nm, lem = 729 nm) characteristic of naturally occurring bacteriochlorins but is far more stable. Dihydrodipyrrins are important compounds that have been used extensively in chlorin and bacteriochlorin-forming reactions. Recently, enelactones have been shown to be useful synthetic intermediates in the synthesis of dihydrodipyrrins. Consequently, several exploratory routes to enelactones were examined. Among the routes examined, it was found that 2-Iodo-N-phenylsulfonylpyrrole could be efficiently coupled with 4-pentynoic acid to give the N-protected enelactone in >40% yield. The development of this new methodology may provide access to new dihydrodipyrrins. The dihydrodipyrrins may be used to access simple chlorins via a northern-southern approach.
Date: 2007-04-30
Degree: PhD
Discipline: Chemistry

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