The chemistry and biochemistry of hydroxymethylpyrroles
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
This thesis examines the chemistry and biochemistry of hydroxyrnethylpyrroles of the type 1.9 (X = OH). Pyrrolic compounds play an important role in many areas of chemistry and biology. A large number of biomolecules which are essential for life are derived from simple pyrrole precursors, eg porphyrins and haemoglobin. In addition, pyrroles have been utilised as building blocks and key intermediates in synthesis. Chapter One gives a general introduction to the biological, chemical and medical applications of pyrroles, such as in the biosynthesis of uroporphyrinogen III 1.20, their use as intermediates in organic synthesis, and in the development of latent mechanism-based inhibitors of serine proteases. Chapter Two describes a sequence, based on a Mitsunobu displacement at the hydroxymethyl position of the chiral deuterium-labelled N-substituted hydroxymethylpyrroles 2.26a-d and 2.27a-d, as a means by which to determine the deactivating abilities of various N-substituents on the pyrrole ring. In this reaction, an SN2 mechanism was found to be favoured over reaction via an azafulvenium intermediate 2.20 by employing N-trifluoromethanesulfonyl as the deactivating group. Suppression of this azafulvenium reaction pathway was found to be less effective with other deactivating groups, and the order for deactivation was determined to be triflyl > mesyl > BOC ≈ acetyl. Chapter Three describes the development of a short, convenient and versatile synthetic route to dipyrromethanes which involves coupling of an N-magnesio pyrrole salt (derived from either an oxygen acetal or a thioacetal of pyrrole-2-carboxaldehyde) with a ring-deactivated chloromethylpyrrole. This methodology was extended to the preparation of dipyrromethanes containing a deuterium-label at the interpyrrolic methylene position. An X-ray crystal structure of the N-tosyl chloromethylpyrrole 3.29 showed that the aromaticity of the pyrrole ring was significantly reduced by the introduction of the electron withdrawing group onto the pyrrole nitrogen. Chapter Four describes a ¹H NMR investigation into the mechanism of hydrolysis of N-acylhydroxymethylpyrroles. In this study, the chiral deuterium-labelled N-(Nphthalyl- L-Ieucinyl)hydroxymethylpyrroles 4.8b and 4.8c in CD₃CN were treated with potassium hydroxide in the presence of an equivalent of S-(+)-sec-butylamine. ¹H NMR spectral analysis showed that the hydrolysis proceeds by an initial intramolecular N- to O-acyl transfer with retention of configuration at the labelled centre. Evidence for the subsequent release of an azafulvene 4.3 was gained from the observed scrambling of the deuterium-label at the exo-methylene position of 4.11 and 4.12 on trapping with an external nucleophile, either S-(+)-sec-butylamine or 4.11, respectively. Chapter Five describes the application of N-substituted hydroxymethylpyrroles as mechanism-based inhibitors of serine proteases. As a result, a number of alternate methodologies to incorporate the hydroxymethylpyrrole-moiety into an extended peptide-like sequence were attempted. From these synthetic approaches the pyrrole-based peptidomimetics 5.8a, 5.8b, 5.9a, 5.9b and 5.10b were prepared and subsequently assayed for α-chymotrypsin inhibition activity. These compounds were found to be modest inhibitors of α-chymotrypsin, with none proving to be better inhibitors than previously discussed examples. Chapter Six details the isolation and properties of some unexpected pyrrole-based molecules that were obtained from the attempted N-acylation of the 5-formylpyrrole-2- carboxamides 5.47a-c. Attempts to N-acylate 5.47a-c with hydrocinnamoyl chloride using the DMAP methodology gave the pyrrolizin-3-ones 6.1a-c. These compounds were identified on the basis of one and two dimensional NMR spectral techniques. Conformation of the structure of the pyrrolizin-3-one 6.1a was obtained by a single crystal X-ray analysis. Attempts to N-acylate 5.47a-c with an acid chloride using the sodium hydride methodology gave the azafulvene dimers 6.6a-c. Single crystal X-ray analysis of 6.6b and 6.6c revealed that each of these dimers were only a single diastereomer, and were further dimerised by non-covalent interactions. Spectroscopic evidence is discussed which indicates that the non-covalent dimeric structure was also present in solution. We suggest that the diastereoselective synthesis of 6.6b and 6.6c was controlled by this ability to form the non-covalent dimers, and so represents an example of molecular self-assembly. Chapter Seven describes a general synthesis of 5-acylpyrrole-2-carboxaldehydes which utilises a Stille coupling reaction between a stannylpyrrole and a (fatty) acid chloride. This methodology was used to prepare a series of 5-acyl-2-hydroxymethylpyrroles, including the previously reported natural product mycalazol 11 7.11. These compounds, together with a 5-carboxamido-2-hydroxymethylpyrrole, were assayed for in vitro cytotoxicity against the P388 cell line. From this assay we have found that an increased chain length leads to greater biological activity, and that an acyl side chain has greater activity relative to a carboxamido side chain.