Modified Synthesis of 2-Ethoxycarbonyl-3,4-Diethyl-Pyrrole and the Corresponding Octaethylporphyrin

Abstract

In the synthesis of 2-ethoxycarbonyl-3, 4-diethyl-pyrrole (1) by the Barton-Zard reaction of 4-acetoxy-3-nitrohexane (1b) with ethyl isocyanoacetate, the organic base 1, 8-diazabicyclo [5.4.0] undec-7-ene (DBU) and the solvent tetrahydrofuran (THF) were replaced with K2CO3 and ethanol, respectively. Moreover, the preparation of the intermediate 1b from 1a by acetylation reaction was carried out by employing acetyl chloride in boiling toluene solution instead of acetic anhydride and the catalyst concentrated sulfuric acid. After treatment of 1 with excess LiAlH4 at low temperature, the crude product 1c (α-hydroxymethyl-3, 4-diethyl-pyrrole) was immediately reacted via a tetramerization in the presence of the catalyst BF3.OEt2 in crude CHCl3,followed by oxidation of 2, 3-dichloro-5, 6-dicyano-1, 4-benzoquinone (DDQ) to provide the corresponding octaethylporphyrin 2. The results indicate that using BF3.OEt2 as catalyst instead of p-toluenesulphonic acid (p-TsOH)in the synthesis of 2 from 1c can obviously increase the yield of product 2 up to 80% from 69%.