Total syntheses of (±)-cryptopleurine, (±)-antofine and (±)-deoxypergularinine

Rapid entry to phenanthroindolizidine alkaloids via an acid-catalysed acyliminium ion-electrocyclization cascade

A rapid total synthesis of seco-phenanthroindolizidine alkaloids was achieved involving a one-pot acid catalyzed deprotection- condensation-electrocyclization strategy. This synthetic route provided a concise synthesis of (±)-seco-antofine and (±)-septicine in only 4 steps with an overall yield of 22% and 17%, respectively.

Synthesis of Intricate Fused N-Heterocycles via Ring-Rearrangement Metathesis

Herein, a facile synthesis of intricate fused N-heterocycles is disclosed by employing C-H activation and ring-rearrangement metathesis/enyne ring-rearrangement metathesis as key steps. Interestingly, some of these N-heterocyclic products possess the tricyclic core of epimeloscine, deoxycalyciphylline B, daphlongamine H, isodaphlongamine H, and a bioactive alkaloid, annotinolide A, which shows antiaggregation activity against amyloid-β (Aβ)_1-42 peptide aggregation. Moreover, various starting materials required in this protocol are easily assembled via C-X bond annulation of 2-bromo-N-protected aniline with norbornadiene or directing group-assisted ruthenium-catalyzed C-H activation of N-methoxybenzamide.

In vitro activity of (-)-deoxypergularinine, on its own and in combination with anti-tubercular drugs, against resistant strains of Mycobacterium tuberculosis

BACKGROUND

The increasing incidence of multidrug-resistant tuberculosis (MDR-TB) infections has created a need for new effective drugs that also target extensively drug-resistant tuberculosis (XDR-TB) and/or augment the activities of existing drugs against tuberculosis.

AIM

This study searched natural products for a new lead compound that targets MDR/XDR-TB.

METHODS

An active compound was purified from the roots of Cynanchum atratum Bunge (Asclepiadaceae) after screening 1640 plant extracts, and its inhibitory effects against MDR/XDR strains and synergistic effects with existing anti-TB drugs were assessed using the resazurin, MGIT, and checkboard assays.

RESULTS

(-)-Deoxypergularinine, purified from the roots of C. atratum, inhibited not only M. tuberculosis but also MDR/XDR strains. The minimum inhibitory concentrations (MICs) of (-)-deoxypergularinine for H37Ra, H37Rv, MDR, and XDR strains were all about 12.5 µg/ml. Moreover, combinations of (-)-deoxypergularinine with the first-line standard drugs rifampicin or isoniazid afforded six- and eight-fold reductions in drug MIC values, respectively, against strain H37Ra.

CONCLUSIONS

(-)-Deoxypergularinine exerts anti-tubercular activities not only against normal tuberculosis strains but also MDR/XDR strains, and synergic effects with rifampicin and isoniazid for the H37Ra strain. The alkaloid may be valuable for targeting M/XDR M. tuberculosis.

Direct synthesis of arenecarboxamides through Friedel-Crafts acylation using ureas

The reaction of urea derivatives that contain the phenothiazine unit with trifluoromethanesulfonic anhydride in the presence of electron-rich aromatic compounds leads to the formation of arenecarboxamides. The reaction has been successfully demonstrated for several inter- and intramolecular systems.

Total synthesis of (+)-antofine and (-)-cryptopleurine

The tylophorine alkaloid anticancer compounds antofine and cryptopleurine have been synthesized in optically active form. Both syntheses employ optically pure α-amino acids as the starting materials, require only seven steps from known 2-ethynylpyrrolidine or 2-ethynylpiperidine derivatives, and are free of protecting groups. Key steps include an alkyne hydration and a chromium carbene complex based net [5+5]-cycloaddition step. Alkyne hydration was accompanied by racemization of the resulting β-aminoketone under most of the conditions examined, and successful minimization of this side reaction was achieved through careful pH control and choice of metal additive. Final ring closure involves a Bischler-Napieralski reaction using a carbamate (antofine) or urea (cryptopleurine) precursor.

Synthesis of Seco-Chlorinated Derivatives of Phenanthroindolizidine Precursors via Friedel-Crafts Reaction

In the course of synthesizing 3-demethyltylophorine (1) by Lewis acid catalyzed intramolecular Friedel-Crafts reaction starting from N-(3-hydroxy-2,6,7-trimethoxy-phenanthr-9-ylmethyl)-2-chloromethylpyrrolidine, two chlorinated phenanthrene derivatives N-(4,10-dichloro-3-hydroxy-2,6,7-trimethoxyphenanthr-9-ylmethyl)-2-chloromethylpyrrolidine (4) and N-(4-chloro-3-hydroxy-2,6,7-trimethoxyphenanthr-9-ylmethyl)-2-chloromethylpyrrolidine (5) were obtained. The structures of these compounds were determined by spectroscopic analysis.

Total synthesis of phenanthroindolizidine alkaloids (+/-)-antofine, (+/-)-deoxypergularinine, and their dehydro congeners and evaluation of their cytotoxic activity

Due to their limited natural abundance and significant biochemical effects, we synthesized the alkaloids (+/-)-antofine (1a), (+/-)-deoxypergularinine (1b), and their dehydro congeners (2 and 3) starting from the corresponding phenanthrene-9-carboxaldehydes. We also evaluated their in vitro cytotoxic activity. Compounds 1a and 1b showed significant potency against various human tumor cell lines, including a drug-resistant variant, with EC(50) values ranging from 0.16 to 16ng/mL. Structure-activity correlations of these alkaloids and some of their synthetic intermediates were also ascertained. The non-planar structure between the two major moieties, phenanthrene and indolizidine, plays a crucial role in the cytotoxic activity of phenanthroindolizidines. Increasing the planarity and rigidity of the indolizidine moiety significantly reduced potency. A methoxy group at the 2-position (1a) was more favorable for cytotoxic activity than a hydrogen atom (1b).

Antitumor agents 253. Design, synthesis, and antitumor evaluation of novel 9-substituted phenanthrene-based tylophorine derivatives as potential anticancer agents

C9-Substituted phenanthrene-based tylophorine derivatives (PBTs) (13-36) were synthesized and evaluated as in vitro anticancer agents against the human A549 lung cancer cell line. Twelve active compounds were further examined against DU-145 (prostate), ZR-751 (breast), KB (nasopharyngeal), and KB-Vin (multidrug resistant KB subline) human cancer cell lines. They showed potent cytotoxic activity against both wild type and matched multidrug resistant KB cell lines, and displayed notable selectivity toward DU-145 (prostate) and ZR-751 (breast) cancer cell lines. The mode of action of this class may be distinctly different from that of other cancer chemotherapeutic compounds. Three PBT analogs were also evaluated in a murine model. Compound 24b showed modest in vivo antitumor activity against human A549 xenograft in nude mice as well as potent in vitro cytotoxic activity, and thus, is a promising anticancer lead compound.

Total syntheses of the tylophora alkaloids cryptopleurine, (-)-antofine, (-)-tylophorine, and (-)-ficuseptine C

A concise, efficient and modular approach to the tylophora alkaloids is described, a family of potent cytotoxic agents that are equally effective against drug sensitive and multidrug resistant cancer cell lines. The advantages of the chosen route are illustrated by the total syntheses of the phenanthroquinolizidine cryptopleurine (1) and the phenanthroindolizidines (-)-antofine (2), (-)-tylophorine (3), and their only recently isolated congener (-)-ficuseptine C (4). The key steps consist in a Suzuki cross-coupling between a (commercial) boronic acid and a simple aryl-1,2-dihalide followed by elaboration of the resulting products into the corresponding 2-alkynyl-biphenyl derivatives 27, 33, 41 and 46. The latter undergo PtCl2-catalyzed cycloisomerizations with formation of the functionalized phenanthrenes 28, 34, 42 and 47, which were transformed into the targeted alkaloids by a deprotection/Pictet-Spengler annulation tandem. Due to the flexibility and robust character of this approach, it might enable a systematic exploration of the pharmacological profile of this promising class of bioactive natural products.

Expedient Syntheses of Antofine and Cryptopleurine via Intramolecular 1,3-Dipolar Cycloaddition

The practical and expedient total syntheses of the representative phenanthroindolizidine and phenanthroquinolizidine alkaloids, antofine and cryptopleurine, are described. Construction of the pyrrolidine and piperidine ring of each alkaloid was achieved by using an intramolecular 1,3-dipolar cycloaddition of an azide onto an alkene and subsequent reduction of the resulting imine and aziridine.

Synthesis and structure–activity studies of antofine analogues as potential anticancer agents

Due to the profound cytotoxicities and interesting biochemical aspects, phenanthroindolizidine alkaloids have received an attention as potential therapeutic leads. To define the features of the molecule that are essential for cytotoxicity, we have synthesized and evaluated a series of phenanthroindolizidine alkaloid, antofine, analogues with different substituents on the phenanthrene ring. The systematic structure activity relationship studies elucidate the essential functional group requirement of phenanthrene ring, providing the basis for further development of phenanthroindolizidine alkaloids.

Intramolecular amidation — An efficient synthesis of 3-aryl-2-quinolinones

To reveal the scope of the syntheses of 3-aryl-2-quinolinones from 2-nitro-α-phenylcinnamic acids, the isomerization of (E)-2-amino-α-phenylcinnamic acids was studied. The results showed that (E)-2-amino-α-phenylcinnamic acids were isomerized to its (Z)-forms under sunlight in organic solvents. The reaction temperature and the functional groups at both phenyl rings have no effect on the isomerization of (E)-2-amino-α-phenylcinnamic acids and the following intramolecular spontaneous amidation of (Z)-2-amino-α-phenylcinnamic acids. Various 3-aryl-2-quinolinones prepared in high total yields indicated that the syntheses of 3-aryl-2-quinolinones from Perkin condensation products 2-nitro-α-phenylcinnamic acids via reduction, sunlight-induced isomerization of (E)-2-amino-α-phenylcinnamic acids, and the following intramolecular amidation is an efficient procedure. Key words: 3-aryl-2-quinolinones, isomerization, amidation.

Total Synthesis of Antofine Using the Net [5+5]-Cycloaddition of γ,δ-Unsaturated Carbene Complexes and 2-Alkynylphenyl Ketones as a Key Step

A compound containing all of the carbons of the anticancer agent antofine was produced in a single step from the coupling of a gamma,delta-unsaturated carbene complex with a 2-alkynylphenyl ketone derivative. Subsequent conversion to antofine was effected in three steps.

Antitumor agents 251: synthesis, cytotoxic evaluation, and structure-activity relationship studies of phenanthrene-based tylophorine derivatives (PBTs) as a new class of antitumor agents

Polar phenanthrene-based tylophorine derivatives (PBTs) were designed, synthesized and evaluated as potential antitumor agents. These compounds contain a core phenanthrene structure and can be synthesized efficiently in excellent yield. The newly synthesized PBTs were evaluated for cytotoxic activity against the A549 human cancer cell line. Among them, N-(2,3-methylenedioxy-6-methoxy-phenanthr-9-ylmethyl)-L-2-piperidinemethanol (34) and N-(2,3-methylenedioxy-6-methoxy-phenanthr-9-ylmethyl)-5-aminopentanol (28) showed the highest potency with IC50 values of 0.16 and 0.27 microM, respectively, which are comparable to those of currently used antitumor drugs. A structure-activity relationship (SAR) study was also explored to facilitate the further development of this new compound class.

Asymmetric Total Syntheses of (−)-Antofine and (−)-Cryptopleurine Using (R)-(E)-4-(Tributylstannyl)but-3-en-2-ol

The asymmetric total syntheses of the representative phenanthroindolizidine and phenanthroquinolizidine alkaloids, (-)-antofine and (-)-cryptopleurine, are described. An efficient synthetic pathway to the key intermediate 12, in enantiomerically pure form, was achieved by using a chiral building block (R)-9 and the Overman rearrangement with a total transfer of chirality. The problem of constructing the pyrrolidine and piperidine rings was successfully addressed, primarily by using a ring-closing metathesis reaction and a cross-metathesis reaction, respectively.

A concise total synthesis of the azaphenanthrene alkaloid eupolauramine

A six-step total synthesis of the azaphenanthrene alkaloid eupolauramine 1 has been achieved using combinational metalation-cyclization tactics. The synthetic route involved first the construction of the azaisoindolinone 9 by aryne-mediated cyclization of he phosphorylated pyridocarboxamide 7 and subsequent dephosphorylation. Metalation of 9 followed by connection of the hydroxybenzyl appendage and E(1)CB anti-elimination allowed the formation of the halogenoarylmethylene azaisoindolinone 4 in the exclusive E-form. Oxidative radical cyclization gave rise to the azaphenanthrene skeleton and regioselective bromination of 3 induced the incorporation of the bromine atom at the 6-position of the azaphenanthrene lactam. Ultimate replacement of the bromine atom of 2 by the methoxy functionality by sequential transmetalation, in situ oxidation, and O-methylation of the phenolic derivative 14 completed the synthesis of the target natural product eupolauramine.

First synthesis and pharmacological evaluation of benzoindolizidine and benzoquinolizidine analogues of alpha- and beta-peltatin

The benzoindolizidine and quinolizidine analogues of alpha- and beta-peltatin were designed and synthesized by two different synthetic routes involving as the key step the Bischler-Napieralski cyclization of suitably substituted N-acyl-2-arylmethylpyrrolidine and -piperidine derivatives. The in vitro biological activity of these analogues as well as some of their derivatives was subsequently evaluated.