Introduction

This chapter addresses basic methodology that can be used to obtain tetrapyrrole macrocycles in the porphyrin and chlorin series from natural materials and some simple methods for the total chemical synthesis of typical pyrroles and porphyrins. The aim is to provide investigators with enough information to decide whether to take on the task of preparing samples of useful porphyrin and chlorophyll derivatives or whether to simply purchase them or collaborate with other individuals more expert in the established synthetic procedures. The procedures reported herein are usually those which are easiest for the nonexpert to perform, while at the same time being sufficient to provide pure samples of the required product.

The porphyrin field has a very rich history; Hans Fischer's books present a laboratory approach to synthesis of porphyrin compounds dating back from the 1930s (20,22,24). In 1975, Porphyrins and Metal-loporphyrins was published (64); this contained a fairly detailed laboratory methods section, which was useful at that time and is probably still useful to many investiga-

Heme, Chlorophyll, and Bilins: Methods and Protocols Edited by A.G. Smith and M. Witty ©2002 Humana Press, Totowa, NJ

tors. An up-to-date and highly detailed description of the synthetic art of porphyrin chemistry can be found in The Porphyrin Handbook (39).

At the outset it must be mentioned that a certain degree of expertise in experimental organic chemistry is essential for success in the endeavors described herein; also essential are the appropriate laboratory equipment (fume hoods, rotary evaporators, temperature controlled reaction monitors, chromatographic equipment, etc.) and glassware. Since hazardous waste chemicals and solvents will also need to be disposed of, approved facilities for these responsibilities must also be available.

In terms of chemical technique and procedures, pyrrole and porphyrin derivatives tend to be easy to work with. With the exception of porphyrinogens, they usually do not require stringent exclusion of oxygen and water vapor (as is the case with much of the rest of organometallic chemistry), they are stable at room temperature and higher temperatures, and they can be purified by recrystallization and chro-matography in the air at room temperature. As might be expected with any colored compound (which will be absorbing light of various wavelengths and therefore will be accessing excited electronic states—porphyrins fluoresce strongly), attempts should be made routinely to keep porphyrin and chlorin compounds out of the light; this is not difficult, and aluminum foil wrapped around a sample flask or around a chromatography column usually suffices. In the particular case of protoporphyrin IX [1] or its dimethyl ester [2], a well-characterized so-called Diels-Alder reaction is known to take place in the presence of oxygen and light to afford a mixture of photoprotoporphyrin and isopho-toprotoporphyrin IX dimethyl ester [3 and 4, respectively] (7,34); this represents the extreme of normal porphyrin photolability and is caused by the presence of the 3- or 8-vinyl groups. If you can successfully handle protoporphyrin IX without continually

generating two polar green bands upon chromatography, you should do just fine. Further advice on the specific requirements for handling these molecules can be found in Chapter 3 by Bommer and Hambright.

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