DOI: 10.25442/hku.28181402
title: Supporting data for "Total Synthesis and Medicinal Chemistry Study of Klebsazolicin"

description: Although the discovery and improvement of antibiotics have saved numerous lives from deadly infections and extended our lifespan, the emergence of antibiotic resistance casts new challenges to human health, dramatically weakening the efficacy of present antibiotics in clinical practice. Worse still, the rise of multidrug-resistant pathogens and superbugs exacerbates the situation. With fewer and fewer effective antibiotics, we have been approaching the post-antibiotic era, which will lead to countless deaths and incalculable economic losses.Science, politics, economy, health and environment, etc. should be considered to handle the worldwide threat caused by antibiotic resistance but exploring new antibiotics and improving existing antibiotics through modification are mainstream strategies. This thesis mainly focuses on the total synthesis and medicinal chemistry study of a novel antibiotic, named klebsazolicin, which was discovered from <i>Klebsiella pneumoniae</i> subspecies <i>ozaenae</i> in 2017.Total synthesis of klebsazolicin was achieved by combining SPPS and new strategies developed for synthesizing thiazole, oxazole and amidine moieties. Combining thioamide formation, Mitsunobu reaction, and oxidative dehydrogenation of MnO<sub>2</sub>, a novel synthetic approach for thiazole-containing building blocks was developed, avoiding possible epimerization of the Hantzsch thiazole synthesis and providing an epimerization-free option for synthesizing polypeptide antibiotics with thiazole moieties. Constructing diastereomers provides a new method to solve the easily ignored epimerization problem of oxazole-containing building blocks. The combination of thiazole and oxazole synthetic strategies makes it possible to access TOMMs with various structures, increasing the diversity of TOMMs and other azole-containing polypeptides for promising antibiotic screening. To increase the efficacy of amidine formation by pre-activating HgCl<sub>2</sub>, the biomimetic on-resin ring-closing amidine formation strategy was developed. This novel amidine formation strategy not only overcomes the obvious drawback of biosynthesis, Ser-dependent YcaO catalyzed amidine formation, but also avoids excess usage of free amine in the previous chemical protocols. Lots of successful synthetic examples including natural and artificial products with complex structures have confirmed the general applicability of this strategy, which provides a useful tool in designing and synthesizing amidine-containing products with a variety of structures for developing new antibiotics.The structure-activity relationship of klebsazolicin was explored and it was found that π-π stacking interaction could be replaced with H-bonding at the corresponding position, which provides a unique insight into modifying antibiotics with aromatic moieties. The necessity of amidine in presenting effective antimicrobial activity was also confirmed. In addition, the introduction of a thiol group at certain positions could surprisingly compensate for the worse antimicrobial performance induced by the absence of amidine. Unfortunately, attempts including tuning conformation constrain via cyclization and installing penetration-enhancing fragments didn’t give satisfactory results in improving antimicrobial performance of klebsazolicin. Nonetheless, these attempts provided meaningful experiences for studying the medicinal chemistry of other TOMMs and RiPPs.Although no ideal analogues with promising antimicrobial performance were obtained, most analogues, especially <b>92</b> and <b>93</b> with simple structures, showed excellent antimicrobial activity against <i>Bacillus cereus</i> and <i>Vibrio</i> in M9 + glucose culture, which may provide an alternative option for treating related infectious diseases such as keratitis, chronic skin infections, vibriosis and cholera.