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The discovery and implementation of antibiotics for clinical use was unquestionably the greatest medical breakthrough of the 20th century. However, the widespread misuse and overuse of these antibiotics, has led to the rapid emergence and spread of antibiotic resistance. The 'ESKAPE' pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) represent a critical threat in multidrug-resistant infections. The Gram-negative species (such as E. coli, K. pneumoniae, P. aeruginosa, and A. baumannii) are especially difficult to combat due to their dual-membrane and efficient efflux pumps, which limit the efficacy of many antibiotics. Despite significant efforts, no new antibiotic class with a new mechanism of action has been approved for Gram-negative pathogens in over five decades. New chemical classes of antibacterial compounds targeting distinct mechanisms within Gram-negative bacteria are therefore urgently called for. The studies outlined in this thesis addresses these challenges by designing and synthesising new antibacterial compounds of three distinct chemical classes, which interact with two unrealized targets, LepB and LpxH, in Gram-negative bacteria, including E. coli and K. pneumoniae.  This thesis investigates the effect of macrocyclization of type I signal peptidase (LepB) inhibitors by optimizing previously studied linear lipopeptide boronic acids and esters to address their cytotoxic and hemolytic liabilities while retaining activity. This resulted in the synthesis of first-in-class P2-P1' boronic ester-linked macrocycles with modest improvement of cytotoxicity but at the cost of reduced antibacterial activity (paper I). In another optimization attempt, isosteric modification of LepB inhibitors was explored by introducing the sulfonimidamide motif into oligopeptide boronic esters, displaying potent LepB inhibitors. Prior to the synthesis of these pseudopeptides novel methods were developed to introduce sulfonimidamides into peptides on solid-phase (paper II and paper III). These studies demonstrated the potential of sulfonimidamides to alter the drug properties and which was herein compared to a corresponding sulfonamide. Additionally, the thesis describes how a new series of LpxH inhibitors, meta-sulfonamidobenzamide-based sulfonyl piperazine derivatives, were identified and prepared. This resulted in inhibitors with wild-type activity without causing hemolysis, cell toxicity or inhibition of hERG ion channel (paper IV). In summary, strategies suitable for the synthesis and optimization of new antibacterial compounds targeting two distinct Gram-negative bacterial targets, LepB and LpxH, are described in this thesis. While there was no success in separating toxicity from antibacterial activity of the LepB inhibitors, these results highlight the challenge in this task and contribute to a better understanding of the structure-activity and toxicity relationships of such inhibitors and provide strategies that could be of use in antibacterial drug discovery. The identification of the meta-sulfonamidobenzamide derivatives offer promising LpxH-targeting hits with the potential for further development in future hit-to-lead antibacterial programs.