Synthesis and Antimycobacterial Activity of 2,1′-Dihydropyridomycins

Abstract

Dihydropyridomycins 2 and 3, which lack the characteristic enol ester moiety of the potent antimycobacterial natural product pyridomycin (1), have been prepared from L-Thr, R-and S-hydroxy isovaleric acid, and 3-pyridinecarboxaldehyde. The 2R isomer 2 shows only 4-fold lower anti-Mtb activity than 1, indicating that the enol ester moiety in the natural product is not critical for its biological activity. This finding establishes 2 as a potent and more practical lead for anti-TB drug discovery.KEYWORDS: natural products, InhA, pyridomycin, total synthesis, tuberculosis T uberculosis (TB) is a frequently fatal infectious disease that causes more than 1.4 million deaths annually. TB was considered to be well contained in the 1960s, but recent decades have witnessed a resurgence of the disease, even in industrialized countries, due to comorbidity with AIDS and the emergence of multi-and extensively drug-resistant (MDR, XDR) strains of the causative pathogen Mycobacterium tuberculosis (Mtb). 1 These developments were paralleled by a decline in TB-directed drug discovery efforts; thus, as of today, the last TB drug with a novel mode of action was launched more than 40 years ago, and current combination therapy is insufficient to eliminate XDR Mtb. 2 As a consequence, there is an urgent need for the development of new anti-TB agents that can shorten the duration of treatment (current standard first line therapy of TB extends over 6 months) and/or are active against MDR and XDR bacteria.Pyridomycin (1) (Chart 1) is a bacterial natural product that was first isolated from the Streptomyces strain 6706 in 1953. 3,4 The compound was subsequently shown to exhibit significant in vitro antimycobacterial activity and low systemic toxicity in mice. 5 While these findings were not further explored for decades to come, we have recently confirmed the in vitro antimycobacterial activity of 1, which we found to inhibit Mtb growth with a minimal inhibitory concentration (MIC) of 0.3 μg/mL. 6 Moreover, we have identified the molecular target of 1 as the mycobacterial NADH-dependent enoyl-[acyl-carrierprotein] reductase (InhA), 6 which is also the target of the clinical TB drug isoniazid (INH) (after metabolic activation and formation of a NADH adduct as the effective inhibitory species). 7,8 Pyridomycin (1) is a competitive inhibitor at the NADH-binding site of InhA but has not shown cross-resistance with INH. 6 This suggests that the exact molecular interactions of 1 with the NADH-binding site differ from those of the NADH adduct of INH. Together with the fact that the structure of 1 does not resemble any known TB drug, these findings render pyridomycin an auspicious starting point for TB drug discovery.Only a single total synthesis of 1 has been reported in the literature; significant difficulties were encountered in that work 9 with regard to the establishment of the enol ester double bond between C2 and C1′, 10 a problem that could not be solved in a fully satisfactory manner. In light of these difficulties, we felt th...

Functional Characterization of PyrG, an Unusual Nonribosomal Peptide Synthetase Module from the Pyridomycin Biosynthetic Pathway

“…The fact that many isoniazide‐resistant clinical isolates of Mycobacterium tuberculosis are sensitive to pyridomycin strongly highlights the efficacy of this drug, thus it has attracted considerable interest from chemists and biologists. For instance, much effort has been made to synthesize the pyridomycin scaffold to resemble the crystal structure of pyridomycin, thereby retaining the ability to bind to InhA and for combined therapy with other compounds …”

“…Pyridomycin is a structurally unique antimycobacterial antibiotic with a novel biosynthetic mechanism. Significant efforts have been made to synthesize chemical analogues . This deep understanding of the hybrid NRPS/PKS PyrG from the pyridomycin gene cluster promises the potential to develop more efficient pyridomycin‐derived drug candidates for TB treatment through rational metabolic engineering.…”

“…For instance, much effort has been made to synthesize the pyridomycin scaffold to resemble the crystal structure of pyridomycin, thereby retaining the ability to bind to InhA and for combined therapy with other compounds. [4,5] The 12-membered ring of pyridomycin consists of four moieties in the order N-3-hydroxypicolinyl-l-threonine, 3-(3-pyridyl)-l-alanine, propionic acid, and 2-hydroxy-3-methylpent-2enoic acid, which is at automer of a-keto-b-methylvaleric acid [6] (Figure 1). Feeding experiments with isotopically labeled precursors [7] and characterization of the biosynthetic gene cluster from S. pyridomyceticus NRRL B-2517 [8] established that the pyridomycin ring structure is assembled by ah ybrid NRPS/PKS system (Figure 1).…”

Recent developments in natural product-based drug discovery for tuberculosis

“…One of the major difficulties in this synthesis was the establishment of the structurally unique enol-ester moiety that is part of the pyridomycin macrocycle. To bypass these difficulties, dihydropyridomycins 11 and 12 have been studied; these have an iso-propyl substituent attached to C2 by a single, rather than a double bond [35]. Intriguingly, the 2R analog 11 retained most of the activity of natural pyridomycin (a fourfold increase in MIC against H37Rv), thus demonstrating that the enol-ester double bond is not a prerequisite for the anti-Mtb activity of 10.…”

Synthetic scope and DFT analysis of the chiral binap–gold(I) complex-catalyzed 1,3-dipolar cycloaddition of azlactones with alkenes

“…The preparation of azlactones is very simple and their reactivity is very diverse due to their functional groups [ 2 – 5 ]. Many enantioselective and/or diastereoselective processes have been focussed on the elaboration of enantiomerically enriched new non-proteinogenic α-amino acids, such as Michael-type additions [ 6 – 7 ], transition metal-catalyzed allylations [ 8 ], Mannich-type additions [ 9 ], aldol-type reactions [ 10 ], and for other different purposes [ 11 – 17 ]. These substrates can be easily transformed in münchnones, which are potential 1,3-dipoles, after deprotonation and imine-activation with a chiral Lewis acid.…”