Targeted and Logical Discovery of Piperazic Acid-Bearing Natural Products Based on Genomic and Spectroscopic Signatures.

A targeted and logical discovery method was devised for natural products containing piperazic acid (Piz), which is biosynthesized from ornithine by l-ornithine N-hydroxylase (KtzI) and N-N bond formation enzyme (KtzT). Genomic signature-based screening of a bacterial DNA library (2020 strains) using polymerase chain reaction (PCR) primers targeting ktzT identified 62 strains (3.1%). The PCR amplicons of KtzT-encoding genes were phylogenetically analyzed to classify the 23 clades into two monophyletic groups, I and II. Cultivating hit strains in media supplemented with 15NH4Cl and applying 1H-15N heteronuclear multiple bond correlation (HMBC) along with 1H-15N heteronuclear single quantum coherence (HSQC) and 1H-15N HSQC-total correlation spectroscopy (HSQC-TOCSY) NMR experiments detected the spectroscopic signatures of Piz and modified Piz. Chemical investigation of the hit strains prioritized by genomic and spectroscopic signatures led to the identification of a new azinothricin congener, polyoxyperuin B seco acid (1), previously reported chloptosin (2) in group I, depsidomycin D (3) incorporating two dehydropiperazic acids (Dpz), and lenziamides A and B (4 and 5), structurally novel 31-membered cyclic decapeptides in group II. By consolidating the phylogenetic and chemical analyses, clade-structure relationships were elucidated for 19 of the 23 clades. Lenziamide A (4) inhibited STAT3 activation and induced G2/M cell cycle arrest, apoptotic cell death, and tumor growth suppression in human colorectal cancer cells. Moreover, lenziamide A (4) resensitized 5-fluorouracil (5-FU) activity in both in vitro cell cultures and the in vivo 5-FU-resistant tumor xenograft mouse model. This work demonstrates that the genomic and spectroscopic signature-based searches provide an efficient and general strategy for new bioactive natural products containing specific structural motifs.

Genomic and Spectroscopic Signature-Based Discovery of Natural Macrolactams.

The logical and effective discovery of macrolactams, structurally unique natural molecules with diverse biological activities, has been limited by a lack of targeted search methods. Herein, a targeted discovery method for natural macrolactams was devised by coupling genomic signature-based PCR screening of a bacterial DNA library with spectroscopic signature-based early identification of macrolactams. DNA library screening facilitated the efficient selection of 43 potential macrolactam-producing strains (3.6% of 1,188 strains screened). The PCR amplicons of the amine-deprotecting enzyme-coding genes were analyzed to predict the macrolactam type (α-methyl, α-alkyl, or ß-methyl) produced by the hit strains. 1H-15N HSQC-TOCSY NMR analysis of 15N-labeled culture extracts enabled macrolactam detection and structural type assignment without any purification steps. This method identified a high-titer Micromonospora strain producing salinilactam (1), a previously reported α-methyl macrolactam, and two Streptomyces strains producing new α-alkyl and ß-methyl macrolactams. Subsequent purification and spectroscopic analysis led to the structural revision of 1 and the discovery of muanlactam (2), an α-alkyl macrolactam with diene amide and tetraene chromophores, and concolactam (3), a ß-methyl macrolactam with a [16,6,6]-tricyclic skeleton. Detailed genomic analysis of the strains producing 1-3 identified putative biosynthetic gene clusters and pathways. Compound 2 displayed significant cytotoxicity against various cancer cell lines (IC50 = 1.58 µM against HCT116), whereas 3 showed inhibitory activity against Staphylococcus aureus sortase A. This genomic and spectroscopic signature-based method provides an efficient search strategy for new natural macrolactams and will be generally applicable for the discovery of nitrogen-bearing natural products.

Iron-Catalyzed Oxidative Cyclization of 2-Amino Styrenes with Alcohols and Methyl Arenes for the Synthesis of Polysubstituted Quinolines.

Herein, we present the iron-catalyzed oxidative cyclization of alcohol/methyl arene with 2-amino styrene to synthesize polysubstituted quinoline. Low-oxidation level substrates such as alcohols and methyl arenes are converted to aldehydes in the presence of an iron catalyst and di-t-butyl peroxide. Then, the quinoline scaffold is synthesized through imine condensation/radical cyclization/oxidative aromatization. Our protocol showed a broad substrate scope, and various functionalization and fluorescence applications of quinoline products demonstrated its synthetic ability.

Chemoselective α-Alkylation and α-Olefination of Arylacetonitriles with Alcohols via Iron-Catalyzed Borrowing Hydrogen and Dehydrogenative Coupling.

α-Alkyl and α-olefin nitriles are very important for organic synthesis and medicinal chemistry. However, different types of catalysts are employed to achieve either α-alkylation of nitriles by borrowing hydrogen or α-olefination by dehydrogenative coupling methods. Designing and developing high-performance earth-abundant catalysts that can procure different products from the same starting materials remain a great challenge. Herein, we report an iron(0) catalyst system that achieves chemoselectivity between borrowing hydrogen and dehydrogenative coupling protocols by simply changing the base. A broad range of nitriles and alcohols, including benzylic, linear aliphatic, cycloaliphatic, heterocyclic, and allylic alcohols, were selectively and efficiently converted to the corresponding products. Mechanistic studies reveal that the reaction mechanism proceeds through a dehydrogenative pathway. This iron catalytic protocol is environmentally benign and atom-efficient with the liberation of H2 and H2O as green byproducts.

Synthesis of 4-Aryl Pyrrolo[1,2-α]quinoxalines via Iron-Catalyzed Oxidative Coupling from an Unactivated Methyl Arene.

Herein, we describe the direct synthesis of pyrrolo[1,2-α]quinoxaline via oxidative coupling between methyl arene and 1-(2-aminophenyl) pyrroles. Oxidation of the benzylic carbon of the methyl arene was achieved by di-t-butyl peroxide in the presence of an iron catalyst, followed by conversion to an activated aldehyde in situ. Oxygen played a crucial role in the oxidation process to accelerate benzaldehyde formation. Subsequent Pictet-Spengler-type annulation completed the quinoxaline structure. The protocol tolerated various kinds of functional groups and provided 22 4-aryl pyrrolo[1,2-α]quinoxalines when various methyl arene derivatives were used. The developed method proceeded in air, and all catalysts, reagents, and solvents were easily accessible.

Enantioselective Total Synthesis of Nitraria Alkaloids: (+)-Nitramine, (+)-Isonitramine, (-)-Isonitramine, and (-)-Sibirine via Asymmetric Phase-Transfer Catalytic α-Allylations of α-Carboxylactams.

Many optically active 2-azaspirocyclic structures have frequently been found in biologically active natural products. In particular, Nitraria alkaloids, (+)-nitramine, (+)-isonitramine, (-)-isonitramine, and (-)-sibirine, have stereogenicity on their quaternary carbon of the 2-azaspiro[5,5]undecane-7-ol structure. To synthesize Nitraria alkaloids, we developed a new enantioselective synthetic method for chiral α-quaternary lactams via the α-alkylation of α-tert-butoxycarbonyl lactams. α-Alkylation of α-tert-butoxycarboxylactams in the circumstances of phase-transfer catalytic (PTC) system (solid KOH, toluene, and -40 °C) by virtue of the catalytic action of (S,S)-NAS bromide (5 mol %) furnished the corresponding α-alkyl-α-tert-butoxycarbonyl lactams in very high chemical (<99%) and enantioselectivity (<98% ee). Our catalytic methodology was successfully applied for the enantioselective total synthesis of Nitraria alkaloids. (+)-Isonitramine was obtained in 12 steps (98% ee, 43% yield) from δ-valerolactam through enantioselective phase-transfer catalytic allylation, Dieckmann condensation, and diastereoselective reduction as the key reactions. (-)-Sibirine and (+)-nitramine were prepared from (-)-isonitramine or its intermediate. Switching the phase-transfer catalyst from (S,S)-NAS bromide to (R,R)-NAS bromide afforded (-)-isonitramine (98% ee, 41% yield). (-)-Sibirine was synthesized by N-ethoxycarbonylation of (-)-isonitramine followed by reduction (98% ee, 14 steps, 32% yield). Furthermore, the diastereoselective reduction of (R)-2-benzhydryl-2-azaspiro[5.5]undecane-1,7-dione [(R)-15] followed by reductive removal of the diphenylmethyl group successfully gave (+)-nitramine (98% ee, 11 steps, 40% yield).

Synthesis and biological activity of selenopsammaplin A and its analogues as antitumor agents with DOT1L inhibitory activity.

Disruptor of telomeric silencing-1 like (DOT1L) is a histone H3 methyltransferase which specifically catalyzes the methylation of histone H3 lysine-79 residue. Recent findings demonstrate that DOT1L is abnormally overexpressed and the upregulated DOT1L evokes the proliferation and metastasis in human breast cancer cells. Therefore, the DOT1L inhibitor is considered a promising strategy to treat breast cancers. Non-nucleoside DOT1L inhibitors, selenopsammaplin A and its analogues, were firstly reported in the present study. Selenopsammaplin A was newly designed and synthesized with 25% overall yield in 8 steps from 3-bromo-4-hydroxybenzaldahyde, and thirteen analogues of selenopsammaplin A were prepared for structure-activity relationship studies of their cytotoxicity against cancer cells and inhibitory activity toward DOT1L for antitumor potential. All synthetic selenopsammaplin A analogues exhibited the higher cytotoxicity compared to psammaplin A with up to 6 - 60 times depending on cancer cells, and most analogues showed significant inhibitory activities against DOT1L. Among the prepared analogues, the phenyl analogue (10) possessed the most potent activity with both cytotoxicity and inhibition of DOT1L. Compound 10 also exhibited the antitumor and antimetastatic activity in an orthotopic mouse metastasis model implanted with MDA-MB-231 human breast cancer cells. These biological findings suggest that analogue 10 is a promising candidate for development as a cancer chemotherapeutic agent in breast cancers.

First Total Synthesis of Gaylussacin and Its Stilbene Derivatives.

Gaylussacin (1), a stilbene glucoside, has been isolated from Pentarhizidium orientale and is used in Korean folk medicine. Although it was first isolated in 1972, the synthesis of gaylussacin has never been reported. Herein, we report the first total synthesis of gaylussacin in six steps with an overall yield of 23.8%, as well as the synthesis of its derivatives. Structurally, gaylussacin contains a carboxylic acid and a glycoside along with a free phenol on the same benzene ring, making selective functionalization for the synthesis of 1 difficult. Heck cross-coupling was employed as a key step to introduce the stilbene moiety. Glycosylation followed by global deprotection provided natural product 1.

Unprecedented Noncanonical Features of the Nonlinear Nonribosomal Peptide Synthetase Assembly Line for WS9326A Biosynthesis.

Systematic inactivation of nonribosomal peptide synthetase (NRPS) domains and translocation of the thioesterase (TE) domain revealed several unprecedented nonlinear NRPS assembly processes during the biosynthesis of the cyclodepsipeptide WS9326A in Streptomyces sp. SNM55. First, two sets of type ΙΙ TE (TEΙΙ)-like enzymes mediate the shuttling of activated amino acids between two sets of stand-alone adenylation (A)-thiolation (T) didomain modules and an "A-less" condensation (C)-T module with distinctive specificities and flexibilities. This was confirmed by the elucidation of the affinities of the A-T didomains for the TEΙΙs and its structure. Second, the C-T didomain module operates iteratively and independently from other modules in the same protein to catalyze two chain elongation cycles. Third, this biosynthetic pathway includes the first example of module skipping, where the interpolated C and T domains are required for chain transfer.

Iron(0)-Catalyzed Transfer Hydrogenative Condensation of Nitroarenes with Alcohols: A Straightforward Approach to Benzoxazoles, Benzothiazoles, and Benzimidazoles.

The iron-catalyzed hydrogen transfer strategy has been applied to the redox condensation of o-hydroxynitrobenzene with alcohol, leading to the formation of benzoxazole derivatives. A wide range of 2-substituted benzoxazoles were synthesized in good to excellent yields without the addition of an external redox agent. A series of control experiments provided a plausible mechanism. Furthermore, the reaction system was successfully extended to the synthesis of benzothiazoles and benzimidazoles.

Direct Synthesis of Pyrrolo[1,2-α]quinoxalines via Iron-Catalyzed Transfer Hydrogenation between 1-(2-Nitrophenyl)pyrroles and Alcohols.

Herein, we describe novel iron-catalyzed transfer hydrogenation between alcohols and 1-(2-nitrophenyl)pyrroles for the synthesis of pyrrolo[1,2-α]quinoxalines. The tricarbonyl (η4-cyclopentadienone) iron complex catalyzed the oxidation of alcohols and the reduction of nitroarenes, and the corresponding aldehydes and aniline were generated in situ. The resulting Pictet-Spengler-type annulation/oxidation completed the quinoxaline structure formation. The protocol tolerated various kinds of functional groups and provided 29 samples of 4-substituted pyrrolo[1,2-α]quinoxalines. The developed method was also applied for the synthesis of additional polyheterocycles.

Synthesis of 2-aryl quinazolinones via iron-catalyzed cross-dehydrogenative coupling (CDC) between N-H and C-H bonds.

Herein, we describe the direct synthesis of quinazolinones via cross-dehydrogenative coupling between methyl arenes and anthranilamides. The C-H functionalization of the benzylic sp3 carbon is achieved by di-t-butyl peroxide under air, and the subsequent amination-aerobic oxidation process completes the annulation process. Iron catalyzed the whole reaction process and various kinds of functional groups were tolerated under the reaction conditions, providing 31 examples of 2-aryl quinazolinones using methyl arene derivatives in yields of 57-95%. The synthetic potential has been demonstrated by the additional synthesis of aryl-containing heterocycles.

Absolute Configuration and Antibiotic Activity of Piceamycin.

The cultivation of a Streptomyces sp. SD53 strain isolated from the gut of the silkworm Bombyx mori produced two macrolactam natural products, piceamycin (1) and bombyxamycin C (2). The planar structures of 1 and 2 were identified by a combination of NMR, MS, and UV spectroscopic analyses. The absolute configurations were assigned based on chemical and chromatographic methods as well as ECD calculations. A new chromatography-based experimental method for determining the configurations of stereogenic centers ß to nitrogen atoms in macrolactams was established and successfully applied in this report. These compounds exhibited significant bioactivities against the silkworm entomopathogen Bacillus thuringiensis and various human pathogens as well as human cancer cell lines. In particular, piceamycin potently inhibited Salmonella enterica and Proteus hauseri with MIC values of 0.083 µg/mL and 0.025 µg/mL, respectively. The biosynthetic pathway involved in the formation of the cyclopentenone moiety in piceamycin is discussed.

Donghaecyclinones A-C: New Cytotoxic Rearranged Angucyclinones from a Volcanic Island-Derived Marine Streptomyces sp.

Chemical profiling of the Streptomyces sp. strain SUD119, which was isolated from a marine sediment sample collected from a volcanic island in Korea, led to the discovery of three new metabolites: donghaecyclinones A-C (1-3). The structures of 1-3 were found to be rearranged, multicyclic, angucyclinone-class compounds according to nuclear magnetic resonance (NMR) and mass spectrometry (MS) analyses. The configurations of their stereogenic centers were successfully assigned using a combination of quantum mechanics-based computational methods for calculating the NMR shielding tensor (DP4 and CP3) as well as electronic circular dichroism (ECD) along with a modified version of Mosher's method. Donghaecyclinones A-C (1-3) displayed cytotoxicity against diverse human cancer cell lines (IC50: 6.7-9.6 µM for 3).

Enantioselective Synthesis of Chiral α-Thio-Quaternary Stereogenic Centers via Phase-Transfer-Catalyzed α-Alkylation of α-Acylthiomalonates.

An efficient synthetic method for establishing chiral α-thio-α-quaternary stereogenic center was successfully developed. The enantioselective α-alkylation of α-acylthiomalonates under phase-transfer catalytic conditions [50% aq. KOH, toluene, -20 °C, and (S,S)-3,4,5-trifluorophenyl-NAS bromide] provided the corresponding α-acylthio-α-alkylmalonates in high chemical yields (up to 99%) and high optical yields (up to 98% ee).

Enantioselective Synthesis of Chiral α-Azido and α-Aryloxy Quaternary Stereogenic Centers via the Phase-Transfer-Catalyzed α-Alkylation of α-Bromomalonates, Followed by SN2 Substitution.

A new efficient synthetic method for chiral α-azido-α-alkylmalonates and α-aryloxy-α-alkylmalonates was developed. The enantioselective α-alkylation of diphenylmethyl tert-butyl α-bromomalonate under phase-transfer catalytic conditions [(S,S)-3,4,5-trifluorophenyl-NAS bromide, 50% KOH, toluene, and -40 °C) provided the corresponding α-bromo-α-alkylmalonates in high chemical yields (≤98%) and high optical yields (≤99% ee). The resulting α-alkylated products were converted to α-azido-α-alkylmalonates (≤96%, ≤97% ee) and α-aryloxy-α-alkylmalonates (≤79%, ≤93% ee) by SN2 substitution with sodium azide and aryloxides, respectively.

Total Synthesis of Swinholide A: An Exposition in Hydrogen-Mediated C-C Bond Formation.

Diverse hydrogen-mediated C-C couplings enable construction of the actin-binding marine polyketide swinholide A in only 15 steps (longest linear sequence), roughly half the steps required in two prior total syntheses. The redox-economy, chemo- and stereoselectivity embodied by this new class of C-C couplings are shown to evoke a step-change in efficiency.

Diastereo- and Enantioselective Iridium Catalyzed Carbonyl (α-Cyclopropyl)allylation via Transfer Hydrogenation.

The first examples of diastereo- and enantioselective carbonyl α-(cyclopropyl)allylation are reported. Under the conditions of iridium catalyzed transfer hydrogenation using the chiral precatalyst (R)-Ir-I modified by SEGPHOS, carbonyl α-(cyclopropyl)allylation may be achieved with equal facility from alcohol or aldehyde oxidation levels. This methodology provides a conduit to hitherto inaccessible inaccessible enantiomerically enriched cyclopropane-containing architectures.

Construction of chiral α-amino quaternary stereogenic centers via phase-transfer catalyzed enantioselective α-alkylation of α-amidomalonates.

An efficient enantioselective synthetic method for α-amido-α-alkylmalonates via phase-transfer catalytic α-alkylation was successfully developed. The α-alkylation of α-amidomalonates under phase-transfer catalytic conditions (50% KOH, toluene, -40 °C) in the presence of (S,S)-3,4,5-trifluorophenyl-NAS bromide afforded the corresponding α-amido-α-alkylmalonates in high chemical yields (up to 99%) and optical yields (up to 97% ee), which could be readily converted to versatile chiral intermediates bearing α-amino quaternary stereogenic centers. The synthetic potential of this methodology was demonstrated via the synthesis of chiral azlactone, oxazoline, and unnatural α-amino acid.

Enantioselective synthesis of α-halo-α-alkylmalonates via phase-transfer catalytic α-alkylation.

A new enantioselective synthetic method for α-halo-α-alkylmalonates is reported. α-Alkylation of diphenylmethyl tert-butyl α-halomalonates under phase-transfer catalytic conditions (solid KOH, toluene, -40 °C) in the presence of (S,S)-3,4,5-trifluorophenyl-NAS bromide (5 mol%) afforded diphenylmethyl tert-butyl α-halo-α-alkylmalonates in very high chemical yields (up to 99%) and optical yields (up to 93% ee).