Total synthesis of chloropeptin II (complestatin) and chloropeptin I

Solving the supply of resveratrol tetramers from Papua New Guinean rainforest anisoptera species that inhibit bacterial type III secretion systems

The supply of (-)-hopeaphenol (1) was achieved via enzymatic biotransformation in order to provide material for preclinical investigation. High-throughput screening of a prefractionated natural product library aimed to identify compounds that inhibit the bacterial virulence type III secretion system (T3SS) identified several fractions derived from two Papua New Guinean Anisoptera species, showing activity against Yersinia pseudotuberculosis outer proteins E and H (YopE and YopH). Bioassay-directed isolation from the leaves of A. thurifera, and similarly A. polyandra, resulted in three known resveratrol tetramers, (-)-hopeaphenol (1), vatalbinoside A (2), and vaticanol B (3). Compounds 1-3 displayed IC50 values of 8.8, 12.5, and 9.9 μM in a luminescent reporter-gene assay (YopE) and IC50 values of 2.9, 4.5, and 3.3 μM in an enzyme-based YopH assay, respectively, which suggested that they could potentially act against the T3SS in Yersinia. The structures of 1-3 were confirmed through a combination of spectrometric, chemical methods, and single-crystal X-ray structure determinations of the natural product 1 and the permethyl ether analogue of 3. The enzymatic hydrolysis of the β-glycoside 2 to the aglycone 1 was achieved through biotransformation using the endogenous leaf enzymes. This significantly enhanced the yield of the target bioactive natural product from 0.08% to 1.3% and facilitates ADMET studies of (-)-hopeaphenol (1).

C7-derivatization of C3-alkylindoles including tryptophans and tryptamines

A versatile strategy for C7-selective boronation of tryptophans, tryptamines, and 3-alkylindoles by way of a single-pot C2/C7-diboronation-C2-protodeboronation sequence is described. The combination of a mild iridium-catalyzed C2/C7-diboronation followed by an in situ palladium-catalyzed C2-protodeboronation allows efficient entry to valuable C7-boroindoles that enable further C7-derivatization. The versatility of the chemistry is highlighted by the gram-scale synthesis of C7-boronated N-Boc-L-tryptophan methyl ester and the rapid synthesis of C7-halo, C7-hydroxy, and C7-aryl tryptophan derivatives.

Enzymatic glycosylation of vancomycin aglycon: completion of a total synthesis of vancomycin and N- and C-terminus substituent effects of the aglycon substrate

Studies on the further development of the sequential glycosylations of the vancomycin aglycon catalyzed by the glycosyltransferases GtfE and GtfD and the observation of unusual, perhaps unexpected, aglycon substrate substituent effects on the rate and efficiency of the initial glycosylation reaction are reported.

Antibacterial serrulatane diterpenes from the Australian native plant Eremophila microtheca

Chemical investigations of the aerial parts of the Australian plant Eremophila microtheca resulted in the isolation of three serrulatane diterpenoids, 3-acetoxy-7,8-dihydroxyserrulat-14-en-19-oic acid (1), 3,7,8-trihydroxyserrulat-14-en-19-oic acid (2) and 3,19-diacetoxy-8-hydroxyserrulat-14-ene (3) as well as the previously reported compounds verbascoside (4) and jaceosidin (5). Acetylation and methylation of the major serrulatane diterpenoid 2 afforded 3,8-diacetoxy-7-hydroxyserrulat-14-en-19-oic acid (6) and 3,7,8-trihydroxyserrulat-14-en-19-oic acid methyl ester (7), respectively. The antibacterial activity of 1-7 was assessed against a panel of Gram-positive and Gram-negative bacterial isolates. All of the serrulatane compounds exhibited moderate activity against Streptococcus pyogenes (ATCC 12344) with minimum inhibitory concentrations (MICs) ranging from 64-128 μg/mL. Serrulatane 1 demonstrated activity against all Gram-positive bacterial strains (MICs 64-128 μg/mL) except for Enterococcus faecalis and Enterococcus faecium. This is the first report of natural products from E. microtheca.

Investigation into the functional impact of the vancomycin C-ring aryl chloride

A vancomycin aglycon analogue that possesses a reduced C-ring and an intact E-ring chloride was prepared and its antimicrobial activity towards Staphylococcus aureus and binding affinity to model cell wall ligands were established. Comparison of the derivative with a series of vancomycin aglycon analogues that possess and lack the chloro substituents on the aryl C- and E-rings defines the impact and further refines the role the C-ring chloride plays in promoting both target binding affinity and binding selectivity for d-Ala-d-Ala and its impact on antimicrobial activity.

Probing the role of the vancomycin e-ring aryl chloride: selective divergent synthesis and evaluation of alternatively substituted E-ring analogues

The selective functionalization of vancomycin aglycon derivatives through conversion of the E-ring aryl chloride to a reactive boronic acid and its use in the synthesis of a systematic series of vancomycin E-ring analogues are described. The series was used to examine the E-ring chloride impact in binding d-Ala-d-Ala and on antimicrobial activity. In contrast to the reduced activity of the unsubstituted E-ring derivatives, hydrophobic and relatively nonpolar substituents approach or match the chloro-substituted vancomycin and were insensitive to the electronic character of the substituent (e.g., Cl vs CN/OMe), whereas highly polar substituents fail to provide the enhancements. Moreover, the active permethylated vancomycin aglycon derivatives exhibit VanB VRE antimicrobial activity at levels that approach (typically within 2-fold) their activity against sensitive bacteria. The robust borylation reaction also enabled the functionalization of a minimally protected vancomycin aglycon (N-Boc-vancomycin aglycon) and provides a direct method for the preparation of previously inaccessible analogues.

Copper-catalyzed diastereoselective arylation of tryptophan derivatives: total synthesis of (+)-naseseazines A and B

A copper-catalyzed arylation of tryptophan derivatives is reported. The reaction proceeds with high site- and diastereoselectivity to provide aryl pyrroloindoline products in one step from simple starting materials. The utility of this transformation is highlighted in the five-step syntheses of the natural products (+)-naseseazine A and B.

Pyridocoumarin, aristolactam and aporphine alkaloids from the Australian rainforest plant Goniothalamus australis

Chemical investigation of the CH(2)Cl(2)/CH(3)OH extracts from aerial parts of the Australian plant Goniothalamus australis has resulted in the isolation of two pyridocoumarin alkaloids, goniothalines A (1) and B (2) as well as eight known natural products, aristolactam AII (3), enterocarpam II (4), caldensine (5), sauristolactam (6), (-)-anonaine (7), asimilobine (8), altholactone (9) and (+)-goniofufurone (10). The chemical structures of all compounds were determined by extensive spectroscopic and spectrometric analysis. Methylation of 2 using TMS-diazomethane afforded 1, which unequivocally established that both 1 and 2 possessed a 10-methyl-2H-pyrano[2,3-f]quinolin-2-one skeleton. These pyridocoumarin alkaloids are putatively proposed to arise biosynthetically from an aporphinoid precursor. Compounds 1-10 were evaluated for in vitro antimalarial activity against a chloroquine-sensitive Plasmodium falciparum line (3D7). Sauristolactam (6) and (-)-anonaine (7) exhibited the most potent antiparasitic activity with IC(50) values of 9 and 7 μM, respectively.

A Pd(0)-mediated indole (macro)cyclization reaction

Herein we report a systematic study of the Larock indole annulation designed to explore the scope and define the generality of its use in macrocyclization reactions, its use in directly accessing the chloropeptin I versus II DEF ring system as well as key unnatural isomers, its utility for both peptide-derived and more conventional carbon-chain based macrocycles, and its extension to intramolecular cyclizations with formation of common ring sizes. The studies define a powerful method complementary to the Stille or Suzuki cross-coupling reactions for the synthesis of cyclic or macrocyclic ring systems containing an embedded indole, tolerating numerous functional groups and incorporating various (up to 28-membered) ring sizes. As a result of the efforts to expand the usefulness and scope of the reaction, we also disclose a catalytic variant of the reaction, along with a powerful Pd(2)(dba)(3)-derived catalyst system, and an examination of the factors impacting reactivity and catalysis.

Redesign of glycopeptide antibiotics: back to the future

The glycopeptide antibiotics are the most important class of drugs used in the treatment of resistant bacterial infections including those caused by methicillin-resistant Staphylococcus aureus (MRSA). After more than 50 years of clinical use, the emergence of glycopeptide-resistant Gram-positive pathogens such as vancomycin-resistant enterococci (VRE) and vancomycin-resistant Staphylococcus aureus (VRSA) presents a serious global challenge to public health at a time few new antibiotics are being developed. This has led to renewed interest in the search for additional effective treatments including the development of new derivatives of the glycopeptide antibiotics. General approaches have been explored for modifying glycopeptide antibiotics, typically through the derivatization of the natural products themselves or more recently through chemical total synthesis. In this Perspective, we consider recent efforts to redesign glycopeptide antibiotics for the treatment of resistant microbial infections, including VRE and VRSA, and examine their future potential for providing an even more powerful class of antibiotics that are even less prone to bacterial resistance.

Silver(I)-promoted conversion of thioamides to amidines: divergent synthesis of a key series of vancomycin aglycon residue 4 amidines that clarify binding behavior to model ligands

Development of a general Ag(I)-promoted reaction for the conversion of thioamides to amidines is disclosed. This reaction was employed to prepare a key series of vancomycin aglycon residue 4 substituted amidines that were used to clarify their interaction with model ligands of peptidoglycan precursors and explore their resulting impact on antimicrobial properties.

Total synthesis of [Ψ[C(═S)NH]Tpg4]vancomycin aglycon, [Ψ[C(═NH)NH]Tpg4]vancomycin aglycon, and related key compounds: reengineering vancomycin for dual D-Ala-D-Ala and D-Ala-D-Lac binding

The total synthesis of [Ψ[C(═S)NH]Tpg(4)]vancomycin aglycon (8) and its unique AgOAc-promoted single-step conversion to [Ψ[C(═NH)NH]Tpg(4)]vancomycin aglycon (7), conducted on a fully deprotected substrate, are disclosed. The synthetic approach not only permits access to 7, but it also allows late-stage access to related residue 4 derivatives, alternative access to [Ψ[CH(2)NH]Tpg(4)]vancomycin aglycon (6) from a common late-stage intermediate, and provides authentic residue 4 thioamide and amidine derivatives of the vancomycin aglycon that will facilitate ongoing efforts on their semisynthetic preparation. In addition to early stage residue 4 thioamide introduction, allowing differentiation of one of seven amide bonds central to the vancomycin core structure, the approach relied on two aromatic nucleophilic substitution reactions for formation of the 16-membered diaryl ethers in the CD/DE ring systems, an effective macrolactamization for closure of the 12-membered biaryl AB ring system, and the defined order of CD, AB, and DE ring closures. This order of ring closures follows their increasing ease of thermal atropisomer equilibration, permitting the recycling of any newly generated unnatural atropisomer under progressively milder thermal conditions where the atropoisomer stereochemistry already set is not impacted. Full details of the evaluation of 7 and 8 along with several related key synthetic compounds containing the core residue 4 amidine and thioamide modifications are reported. The binding affinity of compounds containing the residue 4 amidine with the model D-Ala-D-Ala ligand 2 was found to be only 2-3 times less than the vancomycin aglycon (5), and this binding affinity is maintained with the model d-Ala-d-Lac ligand 4, representing a nearly 600-fold increase in affinity relative to the vancomycin aglycon. Importantly, the amidines display effective dual, balanced binding affinity for both ligands (K(a)2/4 = 0.9-1.05), and they exhibit potent antimicrobial activity against VanA resistant bacteria ( E. faecalis , VanA VRE) at a level accurately reflecting these binding characteristics (MIC = 0.3-0.6 μg/mL), charting a rational approach forward in the development of antibiotics for the treatment of vancomycin-resistant bacterial infections. In sharp contrast, 8 and related residue 4 thioamides failed to bind either 2 or 4 to any appreciable extent, do not exhibit antimicrobial activity, and serve to further underscore the remarkable behavior of the residue 4 amidines.

Synthesis and stereochemical determination of complestatin A and B (neuroprotectin A and B)

Recently, we reported the first total synthesis of chloropeptin II (1, complestatin), the more strained and challenging of the two naturally occurring chloropeptins. Central to the design of the approach and by virtue of a single-step, acid-catalyzed ring expansion rearrangement of chloropeptin II to chloropeptin I, the route also provided a total synthesis of chloropeptin I. Herein, we report a complementary and divergent oxidation of chloropeptin II (1, complestatin) to either complestatin A (2, neuroprotectin A) or complestatin B (3, neuroprotectin B), providing the first synthesis of the natural products and establishing their remaining stereochemical assignments. Key to the approach to complestatin A (2, neuroprotectin A) was the development of two different single-step indole oxidations (HCl-DMSO and NBS, THF-H(2)O) that avoid the rearrangement of chloropeptin II (1) to chloropeptin I (4), providing the 2-oxindole 2 in superb yields (93% and 82%). With a mechanistic understanding of features that impact the latter oxidation and an appreciation of the intrinsic reactivity of the chloropeptin II indole, its modification (NCS, THF-H(2)O; Cs(2)CO(3), DMF-H(2)O) provided a two-step, single-pot oxidation of chloropeptin II (1) to afford directly the 3-hydroxy-2-oxindole complestatin B (3, neuroprotectin B). Extensive studies conducted on the fully functionalized synthetic DEF ring system of chloropeptin II were key to the unambiguous assignment of the stereochemistry as well as the exploration and subsequent development of the mild oxidation conditions used in the synthesis of complestatin A and B.

A redesigned vancomycin engineered for dual D-Ala-D-ala And D-Ala-D-Lac binding exhibits potent antimicrobial activity against vancomycin-resistant bacteria

The emergence of bacteria resistant to vancomycin, often the antibiotic of last resort, poses a major health problem. Vancomycin-resistant bacteria sense a glycopeptide antibiotic challenge and remodel their cell wall precursor peptidoglycan terminus from d-Ala-d-Ala to d-Ala-d-Lac, reducing the binding of vancomycin to its target 1000-fold and accounting for the loss in antimicrobial activity. Here, we report [Ψ[C(═NH)NH]Tpg(4)]vancomycin aglycon designed to exhibit the dual binding to d-Ala-d-Ala and d-Ala-d-Lac needed to reinstate activity against vancomycin-resistant bacteria. Its binding to a model d-Ala-d-Ala ligand was found to be only 2-fold less than vancomycin aglycon and this affinity was maintained with a model d-Ala-d-Lac ligand, representing a 600-fold increase relative to vancomycin aglycon. Accurately reflecting these binding characteristics, it exhibits potent antimicrobial activity against vancomycin-resistant bacteria (MIC = 0.31 μg/mL, VanA VRE). Thus, a complementary single atom exchange in the vancomycin core structure (O → NH) to counter the single atom exchange in the cell wall precursors of resistant bacteria (NH → O) reinstates potent antimicrobial activity and charts a rational path forward for the development of antibiotics for the treatment of vancomycin-resistant bacterial infections.

Rhodium(III)-catalyzed heterocycle synthesis using an internal oxidant: improved reactivity and mechanistic studies

Directing groups that can act as internal oxidants have recently been shown to be beneficial in metal-catalyzed heterocycle syntheses that undergo C-H functionalization. Pursuant to the rhodium(III)-catalyzed redox-neutral isoquinolone synthesis that we recently reported, we present in this article the development of a more reactive internal oxidant/directing group that can promote the formation of a wide variety of isoquinolones at room temperature while employing low catalyst loadings (0.5 mol %). In contrast to previously reported oxidative rhodium(III)-catalyzed heterocycle syntheses, the new conditions allow for the first time the use of terminal alkynes. Also, it is shown that the use of alkenes, including ethylene, instead of alkynes leads to the room temperature formation of 3,4-dihydroisoquinolones. Mechanistic investigations of this new system point to a change in the turnover limiting step of the catalytic cycle relative to the previously reported conditions. Concerted metalation-deprotonation (CMD) is now proposed to be the turnover limiting step. In addition, DFT calculations conducted on this system agree with a stepwise C-N bond reductive elimination/N-O bond oxidative addition mechanism to afford the desired heterocycle. Concepts highlighted by the calculations were found to be consistent with experimental results.

Synthesis and evaluation of selected key methyl ether derivatives of vancomycin aglycon

A select series of methyl ether derivatives of vancomcyin aglycon were prepared and examined for antimicrobial activity against vancomycin-sensitive Staphylococcus aureus and vancomycin-resistant Enterococci faecalis as well as their binding affinity for D-Ala-D-Ala and D-Ala-D-Lac. The intent of the study was to elucidate the role selected key methyl groups may play in the improvement of the in vitro antimicrobial profile of the tetra methyl ether derivative of vancomycin aglycon against vancomycin-resistant Enterococci faecalis previously reported. In these studies, methodology for selective derivatization of the A-, B-, and D-ring was developed that defines the relative reactivity of the four phenols of vancomycin aglycon, providing a foundation for future efforts for site-directed modification of the vancomycin aglycon core.

Total synthesis of complestatin: development of a Pd(0)-mediated indole annulation for macrocyclization

Full details of the initial development and continued examination of a powerful intramolecular palladium(0)-mediated indole annulation for macrocyclization closure of the strained 16-membered biaryl ring system found in complestatin (1, chloropeptin II) and the definition of factors impacting its intrinsic atropodiastereoselectivity are described. Its examination and use in an alternative, second-generation total synthesis of complestatin are detailed in which the order of the macrocyclization reactions was reversed from our first-generation total synthesis. In this approach and with the ABCD biaryl ether ring system in place, the key Larock cyclization was conducted with substrate 36 (containing four phenols, five secondary amides, one carbamate, and four labile aryl chlorides) and provided the product 37 (56%) exclusively as a single atropisomer (>20:1, detection limits) possessing the natural (R)-configuration. In this instance, the complexity of the substrate and the reverse macrocyclization order did not diminish the atropodiastereoselectivity; rather, it provided an improvement over the 4:1 selectivity that was observed with the analogous substrate used to provide the isolated DEF ring system in our first-generation approach. Just as significant, the atroposelectivity represents a complete reversal of the diasteroselectivity observed with analogous macrocyclizations conducted using a Suzuki biaryl coupling.

Scalable total syntheses of N-linked tryptamine dimers by direct indole-aniline coupling: psychotrimine and kapakahines B and F

This report details the invention of a method to enable syntheses of psychotrimine (1) and the kapakahines F and B (2, 3) on a gram scale and in a minimum number of steps. Mechanistic inquiries are presented for the key enabling quaternization of indole at the C3 position by electrophilic attack of an activated aniline species. Excellent chemo-, regio-, and diastereoselectivities are observed for reactions with o-iodoaniline, an indole cation equivalent. Additionally, the scope of this reaction is broad with respect to the tryptamine and aniline components. The anti-cancer profiles of 1-3 have also been evaluated.