The natural product engelhardione is an underexplored chemotype for developing novel

The natural product engelhardione is an underexplored chemotype for developing novel treatments for bacterial infections; we therefore explored this natural product scaffold for chemical diversification and structure-activity relationship studies. ether analogs. An extended macrocyclic chemical library was then produced by oxime formation reductive amination and XR9576 and Gram-positive pathogens as well as anti-Gram-negative activity against an efflux impaired strain. These results provide validated leads for further optimization and development. and other pathogenic bacterial infections there is an XR9576 urgent need to discover new chemotype antitubercular and antibacterial brokers with novel mechanisms of action.1 2 Only five novel chemical classes of antibiotics exemplified by linezolid (Zyvox?) daptomycin (Cubicin?) retapamulin (Altabax?) fidaxomicin (Difcid?) and bedaquiline (Sirturo?) have been introduced into the clinic since the early 1960s.3 Among antibacterial discovery XR9576 strategies whole cell-based TMUB2 phenotypic screens of small molecule and/or natural product-like libraries followed by target deconvolution and identification remain a stylish and efficient approach.4 Natural products represent one of the most valuable sources XR9576 for novel bioactive molecules and chemical diversity in drug discovery.5 Indeed most antibiotics in clinical use are natural products semisynthetic and/or natural product-inspired derivatives.6 Notably most clinically used natural product antibiotics are derived from microorganisms such as bacteria and fungi; and no plant-derived antibacterial brokers have been used clinically.3 Macrocyclic diarylheptanoids belong to a chemical class of bioactive naturally occurring phytochemicals which display a characteristic diphenyl ether motif linked by a seven carbon bridge.7 Since acerogenin A the first member in the cyclic diarylheptanoid class was isolated and reported by the Nagai group in 1976 8 diverse diarylheptanoids9-16 have been isolated and found to mediate a variety of biological activities (Determine 1). One such example engelhardione was recently isolated from the roots of and reported to show potent antituberculosis activity with a minimum inhibitory concentration (MIC) of 0.2 μg ml?1.17 Determine 1 Chemical structures of bioactive diarylheptanoids with a cyclic diphenyl ether moiety. As our continued effort to develop natural products-derived novel antibacterial brokers we have been interested in chemical modification of emerging natural product scaffolds. Inspired by the reported potent antitubercular activity of engelhardione and the limited attention given to exploring this macrocyclic molecule we directed medicinal chemistry efforts toward this promising natural product scaffold. Consequently we recently reported the first total synthesis of the published structure (1a) of engelhardione and this effort led to its structural revision to that of pterocarine (1b Fig. 1).18 The structural revision was also subsequently confirmed by the Natarajan group9 and the Chen group19. To further improve the efficiency of macrocyclization we developed an efficient and modular microwave-assisted macrocyclization via intramolecular Ullmann coupling and investigated the scope and generality of a panel of substrates with different linkers ring sizes and substitution patterns.20 To extend the medicinal chemistry effort of this work and to investigate if this cyclic diarylheptanoid architecture possesses any tractable antibacterial activity herein we report the synthesis antibacterial evaluation and preliminary structure-activity relationships (SAR) of this class of macrocyclic diarylheptanoids against and a broad panel of Gram-positive XR9576 and Gram-negative pathogens. This work constitutes the first systematic report describing the antitubercular and antibacterial evaluation of synthetic engelhardione pterocarine and related structural analogs. Our preliminary mechanistic study identified that lead compounds inhibited several key macromolecular processes (DNA RNA and protein). RESULTS AND DISCUSSION Chemistry As XR9576 illustrated in scheme 1 starting from 1 7 2 18 the proposed structure (1a) of engelhardione pterocarine (1b) and their regioisomer 1c were synthesized by a series of cross aldol condensations and selective hydrogenations affording linear 1 7 as key intermediate 3a-c followed by intramolecular Ullmann reactions to give the macrocyclic architectures 4a-c and final and oxime isomers in an approximate ratio of 1 1:1 and 1:2 was obtained respectively. Initial attempts to prepare Schiff base imines from the reaction of 4a with amines were unsuccessful due to the facile decomposition of.