Chapter 1 reviews the copper- and palladium-mediated aryl-N bond formations used in the synthesis of biological active compounds. Examples using the palladium-catalysed Buchwald-Hartwig method, the copper-mediated Ullmann-Goldberg method and the Cham-Lan method were compared concerning their reaction conditions, their scope for a wide spectra of substituent and functional groups and their scale-up prospects. The discussed examples show that palladium-catalysed reactions are favoured for large scale applications and tolerate sterical demanding substituents on the coupling partners better than Cham-Lan conditions. Cham-Lan N-arylations are particular mild and do not require the addition of ligands, which facilitates the work-up. However, reaction times can be very long. Ullmann- and Buchwald-Hartwig-type reactions have been used in intramolecular reactions giving access to complex ring structures. All three N-arylation methods have specific advantages and disadvantages that should be considered when selecting the reaction conditions for a desired C-N bond formation in the course of a total synthesis or drug synthesis. Chapters 2, 3 and 4 deal with the modulation of the membrane located ABC-transporter ABCG2, which is an effective tool for the treatment of multi-drug resistant cancer cells. Starting from the recently discovered tariquidar-derived selective ABCG2 modulator (lead structure), in chapter 2 different fluorescent labelled ABCG2 modulators were synthesised. The modified modulators show emission in the red part of the spectrum and reveal quantum yields up to 31.2 %. Moderate potency was obtained by the replacement of the tetrahydroisoquinoline part of the tariquidar-derived lead-structure by a pyrylium label. All synthesised derivatives showed selectivity for ABCG2 over ABCB1. Studies at the physiological expressed ABCG2 concentrations on rat brain capillaries revealed that the parent lead structure is also influencing the rat ABCG2 transporter, confirming the results of previous in vitro experiments with MCF-7/Topo cells and Hoechst 33342 as substrate. Due to lower affinity of the fluorescent labelled compounds, this could not be proven for the fluorescent modulators. Chapter 3 shows structural variations of the lead structure to improve the drug-like properties and the potency. Incubation in mouse plasma proved the stability of the ester functionality, but one of the peptide bonds was cleaved, also if N-methylated. Structural changes of different parts of the molecule decreased the potency only moderately as long as there was no negative charge introduced under physiological conditions. The phenylbenzamide group turned out to be essential for the interaction with the transporter. Chapter 4 discribes photochromic tariquidar-derived compounds as potential ABCG2 modulators which may be useful as tool in mechanistic investigation. The synthetic approach was focused on the replacement of characteristic structural moieties of a potent and selective ABCG2 inhibitor by photochromic dithienylethenes. Photochemical properties, thermostability, cycle performance, photostationary state and photostability at assay conditions are suitable for all four compounds for pharmacological in vitro investigations. Chapter 5 reports the synthesis of secondary amines containing two carbon stereocentres in both alpha-positions. These are typical substructure of ACE inhibitors and other biological active compounds. The stereoselective synthesis was aspired in different approaches, but industrial applications still mostly apply the separation of diastereomers. Here, two synthetic methods, a SN2 reaction and an oxazolidine mediated reaction sequence, were compared concerning their selectivity. The oxazolidine intermediate-mediated amine formation gave only one entantiomer for the substituted secondary amine. The synthesised compounds showed weak or no cytotoxicity. Selected compounds were tested as inhibitors of ACE-2, but revealed no biological activity.