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Heterocyclic Chemistry: The Knorr Synthesis Of Pyrroles

2381 words - 10 pages

The Knorr synthesis of the pyrrole, 2,4-diethoxycarbonyl-3,5-dimethylpyrrole, was achieved using an α-amino ketone, ethyl acetoaminoacetate and reacting it with its predecesso r, ethyl acetoacetate in a double condensation reaction . The product was analysed by 1H NMR, 13C NMR, mass spectrometry and IR spectroscopy giving positive results .
Pyrrole has been a molecule of great interest due to its heterocyclic aromatic properties. The lone pair on the nitrogen is delocalised within the ring, causing the heterocycle to be electron-rich . This causes the ring to become increasingly nucleophilic, and therefore makes it susceptible to attack from electrophiles . The addition of these new substitutions makes more important molecules , which have a huge contribution to not only the chemical industry, but also the biological. 1 Substituted pyrrole molecules are fantastic precursors to make complex molecules, such as medicines, agrochemicals and dyes. Indole, an important benzo-pyrrole, is the basis of the important protein, tryptophan and has functions as a neurotransmitter in the brain. The neurotransmitter, serotonin, is important in making modern drugs such as sumatriptan which treats migraines.2

Pyrrole synthesis can be achieved by a vast number of methods. Some of the most common processes used in today’s laboratory include the Hantzch pyrrole method, Paal-Knoor Knorr synthesis and the Knoorr synthesis; the latter which will be studied in this experiment. Other complex methods explored include Robinson’s utilization of the reaction between an aldehyde and a hydrazine . However, this method requires high temperatures which are not economically favourable to maintain and so the other classic methods are generally used as they can be conducted at room temperature.3 The Knorr synthesis is a typical condensation reaction involving an α-amino ketone and an α-carbonyl compound. This versatile reaction usually takes places in glacial acetic acid, which acts as a catalyst alongside zinc, forming the substituted pyrrole with the required R-groups attached.4

The reaction concerned here uses two equivalents of ethyl acetoacetate 1, the first being converted to the relevant oxime. This oxime is then converted to the α-amino ketone 2 which reacts with the othera second ethyl acetoacetate, producing the pyrrole 3. The ethyl acetoaminoacetate 2 reacts in situ to avoid readily self-condensing, and instead undergoes a condensation reaction with the second equivalent of the α-amino ketone. A summary of this reaction is shown in Scheme 1 .

Scheme 1. General reaction scheme showing the formation of the pyrrole.

A solution of ethyl acetoacetate (7.52 g, 7.5 mL) in glacial acetic acid (18 mL) was prepared and cooled to 5 º C. A solution of sodium nitrite (2.1 g) in water (5 mL) was added drop wise,wise whilst the temperature was monitored ensuring it the temperature did not rise above 7 ºC. The reaction mixture was...

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