This thesis examines ways in which the chemical reactivity of benzimidazole-, pyrrole- or pyrrolizinone-based heterocycles can be modulated. The latent reactivity of these systems has been used to develop enzyme inhibitors, antitumour compounds and a novel amino acid derivatisation technique. Chapter One introduces the concept of modulated chemical reactivity, with illustrative examples from biochemistry, synthesis and medicinal chemistry. The application of latent reactivity towards the inhibition of serine proteases, a class of proteolytic enzymes, is then reviewed. Two inhibitor classes are discussed, the first of which require an inorganic cofactor, e.g. a metal ion, for enhanced activity. Secondly, mechanism-based inhibitors, which rely on activation by the protease to covalently derivatise the enzyme, are discussed. The application of latent reactivity to effect DNA alkylation, and hence inhibit tumour cell growth, is also reviewed. This section places particular emphasis on alkylators that arc attached to an intercalator to enhance their affinity for DNA. Chapter Two describes the inhibition of -chymotrypsin, a representative serine protease, by compounds that were proposed to exhibit enhanced activity upon the coordination of Zn2+. The attempted synthesis of the putative inhibitors 2.1a-b was hindered by the instability of the diamine precursors 2.2a-b. The model compound 2.1c was prepared, however this compound was prone to deuteration and oxidation at its methylene bridge. Benzimidazoles 2.14a-c were subsequently prepared, in addition to the dihydrochloride salt of 2.14a. These compounds were moderate to weak inhibitors of a-chymotrypsin, and no Zn2+-mediated enhancement of inhibition was observed. Benzimidazole 2.14b was shown to coordinate Zn(OAc)2 by 1H NMR spectroscopy, which illustrated that the lack of Zn2+-enhanced inhibition of a-chymotrypsin by compounds of type 2.14 did not result from an inability to complex metal ions. Chapter Three describes the inhibition of a-chymotrypsin by a new series of C2-acyl-C5- (hydroxyalkyl)pyrroles. These compounds are postulated to be mechanism-based inhibitors. The pyrrole-based peptidomimetics 3.13a and 3.14a were prepared and assayed against a-chymotrypsin. Minimal difference in activity was observed between these compounds, despite the bulky hydrophobic moiety at C5 of 3.14a, which was proposed to bind more tightly to the S1 subsite than the 5-(hydroxymethyl) group of 3.13a. Mass spectrometry of 3.13a incubated with a-chymotrypsin suggested that inhibition was noncovalent in nature. Modifications at the C2 position of 3.13a led to the preparation of 3.13b, 3.24 and 3.25, of which the latter was the optimal inhibitor, and an IC50 value of 108 M was determined for this compound. The attempted solid phase synthcsis of 3.13a was ineffectual due to the instability of the 5-(hydroxymethyl)pyrrole moiety to TFA. Solid phase synthesis of a 5-formylpyrrole precursor to 3.13a gave the desired product 3.36 as an inseparable mixture with pyrrole acid 3.16b. Solution phase synthesis of the 5- (hydroxymethyl)pyrrole 3.35, which possessed a carboxylic acid moiety, afforded an unstable compound. Chapter Four details a novel derivatisation method for amino acids, whereby the amine is first "capped" by pyrrole acid 3.16b. Subsequent reaction with hydrocinnamoyl chloride releases the latent spectrophotometric properties of the pyrrole-amino acid adduct. A previous study afforded pyrrolizinones 4.11a-c, and this study expanded the range of pyrrolizinones available for analysis to include 4.11d-i and 4.17a-b. Initial UV-vis spectroscopic analysis revealed that methanol degraded pyrrolizinone 4.11e. UV-vis spectroscopy of 4.11a-i/4.17a-b in acetonitrile revealed that the variable R1 and R2 groups did not affect the Amax and 8 values in a systematic manner. Analysis of the LRMS fragmentation patterns for 4.11a i/4.17a-b, and their pyrrole precursors 3.18a-i/3.29a-b, revealed that a production was generated whose mass was dependent on the R1 group. The abundance of the diagnostic product ion was dependent on the nature of R1 and R2. Fragmentation was increased by the technique of electron impact, compared to electro spray ionisation. Chapter Five describes the application of 5-(hydroxymethyl)pyrroles towards the development of DNA alkylators with latent reactivity. Attachment of an intercalator to the pyrrole moiety via an alkyl linker was proposed to increase cytotoxicity. Model studies afforded 5.21 (naphthyl-based) and 5.31 (quinolyl-based), however the synthetic methodology was not amenable to compounds that incorporated anthryl or acridine moieties, i.e. "true intercalators". Compounds 5.21 and 5.31 displayed moderate activity against the P388 cancer cell line. The 5-(hydroxymethyl)pyrrole substructure analogue (5.41) elicited no activity in the P388 assay. Compound 5.31 did not react with aniline, which suggested that compounds of this type would not effect DNA alkylation in vivo. A DNA binding assay revealed that 5.31 had a low affinity for the genetic material, which rationalised its moderate P388 activity. Compounds 5.21 and 5.31 displayed minimal antiviral and antimicrobial activity.