This thesis documents a new approach for the identification of a small biologically active molecule’s site of interaction, through the rapid synthesis of molecular probes. A marked library approach has been developed whereby a biocompatible marker is attached onto the small molecule’s molecular scaffold. This marker plays no role in the screening process itself, but facilitates the formation of a range of molecular probes from active marked library members. As an example of molecular probe formation, site selective biotinylation will be discussed in the introduction. This marked library concept has been applied to the natural product anisomycin A. Investigations focused on development of a detailed structure activity relationship for anisomycin’s activation of the stress activated protein kinase (SAPK) pathway, along with the synthesis of a number of marked library analogues. The active marked library members were then converted to a range of functional molecular probes utilising the copper(I) catalysed Huisgen cycloaddition as the key coupling step. These molecular probes are being used in the elucidation of anisomycin’s biological target for activation of the SAPK pathway. In a further demonstration of this strategy, a focused library of marked steroids has been synthesised based on the functionalisation of dehydroepiandrosterone B. Directed by the results of preliminary biological screening, a number of marked library members have been converted into fluorescent molecular probes. These probes will be used in future applications to probe the biological action of the dehydroepiandrosterone.