Its reactivity is similar to that of benzene and aniline, in that it is easy to alkylate and acylate. This intermediate is then modified by methylation (which incorporates a methyl group from L-methionine onto the alcohol at the 6 position) and oxidation of the primary alcohol to the aldehyde to yield the core A-B ring structures.ĭue to its aromatic character, pyrrole is difficult to hydrogenate, does not easily react as a diene in Diels–Alder reactions, and does not undergo usual olefin reactions. This fragment is then able to react with the masked carbanion formed from the PLP mediated decarboxylation of L-Serine, which cyclizes in a dehydration reaction to yield the second pyrrole ring. Ring A fragment is transferred from the peptidyl carrier protein (PCP) to the Acyl Carrier Protein (ACP) by a KS domain, followed by transfer to malonyl-ACP via decarboxylative Claisen condensation.
Ring A is then expanded via the polyketide synthase pathway to incorporate L-serine into ring B (figure 3). Ring A is synthesized from L-proline through the nonribosomal peptide synthase (NRPS) pathway (figure 2), wherein the pyrrolidine ring of proline is oxidized twice through FAD+ to yield pyrrole ring A. The biosynthesis of Prodigiosin involves the convergent coupling of three pyrrole type rings (labeled A, B, and C in figure 1) from L-proline, L-Serine, L-methionine, pyruvate, and 2-octenal. Occurrence in nature įigure 1: Structure of Prodigiosin 1 highlighting the A, B, and C pyrrole rings Its name comes from the Greek pyrrhos ( πυρρός, “reddish, fiery”), from the reaction used to detect it-the red color that it imparts to wood when moistened with hydrochloric acid. In 1857, it was isolated from the pyrolysate of bone. Runge in 1834, as a constituent of coal tar. Pyrrole is also weakly acidic at the N–H position, with a p K a of 16.5.Īs a hydrogen bonding Lewis acid it is classified as a hard acid and the ECW model lists its acid parameters as E A = 1.38 and C A = 0.68. Substitution of pyrrole with alkyl substituents provides a more basic molecule-for example, tetramethylpyrrole has a conjugate acid p K a of +3.7. The most thermodynamically stable pyrrolium cation (C 4H 6N +) is formed by protonation at the 2 position. Pyrrole is weakly basic, with a conjugate acid p K a of −3.8. In CDCl 3, it has chemical shifts at 6.68 (H2, H5) and 6.22 (H3, H4). Unlike furan and thiophene, it has a dipole in which the positive end lies on the side of the heteroatom, with a dipole moment of 1.58 D. Pyrrole is a 5-membered aromatic heterocycle, like furan and thiophene. Pyrrole is a colorless volatile liquid that darkens readily upon exposure to air, and is usually purified by distillation immediately before use. 5.1 Reaction of pyrrole with electrophiles.4.1.6 Piloty–Robinson pyrrole synthesis.
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