Direct one-pot synthesis of phenanthrenes via Suzuki-Miyaura coupling/aldol condensation cascade reaction

Transition metal-catalyzed transformations of 2-formylarylboronic acids

2-Formylarylboronic acids are easily available precursors in organic chemistry. Different types of transition metal catalysts, such as Pd(0), Pd(II), Rh(I), Ir(I), Ni(II), Cu(I), Cu(II), and Co(II), can efficiently catalyze coupling reactions of 2-formylarylboronic acids with other organic reactants. In this review, we describe the synthesis of a diverse range of carbocyclic and heterocyclic compounds, as well as acyclic compounds, via transition metal-catalyzed reactions of 2-formylarylboronic acids over the past two decades.

Controllable Chemoselectivity Cascade Reactions for the Synthesis of Phenanthrenols via Palladium-Catalyzed-Suzuki/Heck Reaction and Michael Addition

Palladium serves as a multi-functional catalyst which is controllable by tuning reaction conditions. This work demonstrated the utilization of a palladium catalyst for the synthesis of phenanthrenols by cascade palladium-catalyzed Suzuki/Heck reaction between chalcone and 2-bromophenylboronic acid, followed by Michael addition. The sequential reaction could be controlled by reactivity of the palladium catalyst in different solvents and concentrations of reagents. This protocol could be applied to a broad range of substrates to give products in low to good yields.

Lead Optimization: Synthesis and Biological Evaluation of PBT-1 Derivatives as Novel Antitumor Agents

Phenanthrene-based tylophorine-1 (PBT-1) was identified previously as a lead compound in an anticancer drug discovery effort based on natural Tylophora alkaloids. An expanded structural optimization using a new more efficient synthetic route provided 14 PBT-derivatives. Eleven compounds displayed obvious antiproliferative activities in cellular assays (GI₅₀ 0.55-9.32 μM). The most potent compounds 9c, 9g, and 9h (GI₅₀ < 1 μM) contained a 7-hydroxy group on the phenanthrene B-ring in addition to a pendant piperidine E-ring with different 4-substituents. Compound 9h with NH₂ as the piperidine substituent was at least 4-fold more potent against triple-negative breast cancer MDA-MB-231 than estrogen-responsible breast cancer MCF-7 cell growth. In further biological evaluations, the new active compounds induced cell cycle accumulation in the late S and G2/M phase without interfering with microtubule formation or cell morphology. These results on the optimization of the B- and E-rings of PBT-1 should benefit further development of novel antitumor agents.

Reactivity of 5-(Alkynyl)dibenzothiophenium Salts: Synthesis of Diynes, Vinyl Sulfones, and Phenanthrenes

The reactivity of 5-(alkynyl)dibenzothiophenium salts 1 is explored in the presence of different nucleophiles, dienes, and under photochemical conditions. Reaction with lithium acetylides affords diynes in moderate yields; while depending on the substitution pattern, the reaction with sulfinates delivers either the alkyne transfer products, alkynyl sulfones, or β-(sulfonium) vinyl sulfones through addition to the C-C triple bond. Similar behavior is observed when tosylamines are used as nucleophiles. Salts of general formula 1 also react with dienes to render the corresponding Diels-Alder cycloadducts. The vinyl sulfonium salts obtained by these routes further react with nucleophiles through a Michael addition, dibenzothiophene elimination sequence. Alternatively, they also engage in photoinduced radical cyclizations to produce substituted phenanthrenes. Attempts to use this specific addition/radical cyclization sequence for the construction of the 6a,7-dehydroaporphine skeleton present in several families of alkaloids are also described.

Palladium-Catalyzed, Copper(I)-Promoted Methoxycarbonylation of Arylboronic Acids with O-Methyl S-Aryl Thiocarbonates

Here, we report O-methyl S-aryl thiocarbonates as a versatile esterification reagent for palladium-catalyzed methoxycarbonylation of arylboronic acid in the presence of copper(I) thiophene-2-carboxylate (CuTC). The reaction condition is mild, and a variety of substituents including sensitive -Cl, -Br, and free -NH₂ could be tolerated. Further applications in the late-stage esterification of some pharmaceutical drugs demonstrate the broad utility of this method.

Cu(I)-Catalyzed Ligand-Free Tandem One-Pot or Sequential Annulation via Knoevenagel Intermediates: An Entry into Multifunctional Naphthalenes, Phenanthrenes, Quinolines, and Benzo[b]carbazoles

A simple but efficient one-pot or sequential copper-catalyzed protocol using 2-bromoaldehydes and active methylene group containing substrates that affords multifunctional naphthalenes, phenanthrenes, quinolines, and benzo[b]carbazoles via Knoevenagel condensation, C-arylation, and decarboxylation, followed by aromatization, is developed. The reaction utilizes the potential of Knoevenagel intermediates and does not require any ancillary ligand. The phenanthrene products thus obtained show moderate fluorescence activity. Structural elaboration of the products to obtain dihydrobenzoquinazolines is also highlighted.

One-Pot, Three-Component Approach to Diarylmethylphosphonates: A Direct Entry to Polycyclic Aromatic Systems

A new type of three-component reaction was developed consisting of aldehydes, electron-rich (hetero)arenes, and trialkyl phosphite, which provided facile access to a wide range of diarylmethylphosphonates under mild reaction conditions. Simple one- or two-step synthetic manipulation of the resulting compounds enabled us to reach several polycyclic (hetero)aromatic systems efficiently.

A palladium-catalyzed multi-component annulation approach towards the synthesis of phenanthrenes

An efficient palladium-catalyzed three-component cascade reaction has been developed for the facile construction of phenanthrene frameworks. The transformation is driven by a controlled reaction sequence of Suzuki-Miyaura coupling followed by the insertion of alkynes, and finally, annulation to yield phenanthrene derivatives via C-H activation. This methodology is able to accommodate a variety of substrates and affords the anticipated products in good to excellent yields.

N-Bromosuccinimide-Induced C-H Bond Functionalization: An Intramolecular Cycloaromatization of Electron Withdrawing Group Substituted 1-Biphenyl-2-ylethanone for the Synthesis of 10-Phenanthrenol

An NBS-induced intramolecular cycloaromatization for the synthesis of 10-phenanthrenols from electron-withdrawing group substituted 1-biphenyl-2-ylethanones is described. The in situ generated bromide was designed to act as an initiator for the radical C-H bond activation. An oxidative cross-dehydrogenative coupling reaction of a highly active C-H bond with an inert C-H bond readily occurs under mild conditions without the need for transition metals or strong oxidants.

Ag(i)-Catalyzed cycloisomerization reactions: synthesis of substituted phenanthrenes and naphthothiophenes

Ag-Catalyzed synthesis of substituted phenanthrenes and naphthothiophenes from ortho-alkylated biaryl derivatives via 6-endo-dig intramolecular alkyne–arene coupling has been described.

Pd catalyzed insertion of alkynes into cyclic diaryliodoniums: a direct access to multi-substituted phenanthrenes

Cyclic diaryliodoniums remain unexplored compared to linear iodoniums. In our current work, internal alkynes were for the first time applied to react with cyclic iodoniums, catalyzed by Pd, resulting in a [4 + 2] benzannulation. Our work offers a new strategy to synthesize multi-substituted phenanthrene derivatives which are not easily accessed by conventional methods.

Preparation of dibenzo[e,g]isoindol-1-ones via Scholl-type oxidative cyclization reactions

A flexible synthesis of dibenzo[e,g]isoindol-1-ones has been developed. Dibenzo[e,g]isoindol-1-ones represent simplified benzenoid analogues of biological indolo[2,3-a]pyrrolo[3,4-c]carbazol-5-ones (indolocarbazoles), compounds that have demonstrated a wide range of biological activity. The synthesis of the title compounds involved tetramic acid sulfonates. Different aryl groups were introduced at C4 of the heterocyclic ring via Suzuki-Miyaura cross-coupling reactions. Finally, mild Scholl-type oxidative cyclizations mediated by phenyliodine(III) bis(trifluoroacetate) (PIFA) converted some of the latter compounds into the corresponding dibenzo[e,g]isoindol-1-ones. A systematic study of the oxidative cyclization revealed the following reactivity trend: 3,4-dimethoxyphenyl ≫ 3-methoxyphenyl > 3,4,5-trimethoxyphenyl > 4-methoxyphenyl ≈ phenyl. Overall, the oxidative cyclization required at least two methoxy groups distributed in the aromatic rings, at least one of which had to be located para to the site of the cyclization.

Total synthesis of the aristolochic acids, their major metabolites, and related compounds

Plants from the Aristolochia genus have been recommended for the treatment of a variety of human ailments since the time of Hippocrates. However, many species produce the highly toxic aristolochic acids (AAs), which are both nephrotoxic and carcinogenic. For the purposes of extensive biological studies, a versatile approach to the synthesis of the AAs and their major metabolites was devised based primarily on a Suzuki-Miyaura coupling reaction. The key to success lies in the preparation of a common ring-A precursor, namely, the tetrahydropyranyl ether of 2-nitromethyl-3-iodo-4,5-methylendioxybenzyl alcohol (27), which was generated in excellent yield by oxidation of the aldoxime precursor 26. Suzuki-Miyaura coupling of 27 with a variety of benzaldehyde 2-boronates was accompanied by an aldol condensation/elimination reaction to give the desired phenanthrene intermediate directly. Deprotection of the benzyl alcohol followed by two sequential oxidation steps gave the desired phenanthrene nitrocarboxylic acids. This approach was used to synthesize AAs I-IV and several other related compounds, including AA I and AA II bearing an aminopropyloxy group at position-6, which were required for further conversion to fluorescent biological probes. Further successful application of the Suzuki-Miyaura coupling reaction to the synthesis of the N-hydroxyaristolactams of AA I and AA II then allowed the synthesis of the putative, but until now elusive, N-acetoxy- and N-sulfonyloxy-aristolactam metabolites.

Synthesis, docking, and biological studies of phenanthrene β-diketo acids as novel HIV-1 integrase inhibitors

In the present study we report the synthesis of halogen-substituted phenanthrene β-diketo acids as new HIV-1 integrase inhibitors. The target phenanthrenes were obtained using both standard thermal- and microwave-assisted synthesis. 4-(6-Chlorophenanthren-2-yl)-2,4-dioxobutanoic acid (18) was the most active compound of the series, inhibiting both 3'-end processing (3'-P) and strand transfer (ST) with IC50 values of 5 and 1.3 μM, respectively. Docking studies revealed two predominant binding modes that were distinct from the binding modes of raltegravir and elvitegravir, and suggest a novel binding region in the IN active site. Moreover, these compounds are predicted not to interact significantly with some of the key amino acids (Q148 and N155) implicated in viral resistance. Therefore, this series of compounds can further be investigated for a possible chemotype to circumvent resistance to clinical HIV-1 IN inhibitors.

Synthesis of dibenzoxepine lactams via a Cu-catalyzed one-pot etherification/aldol condensation cascade reaction: application toward the total synthesis of aristoyagonine

A general synthesis of dibenzoxepine lactams has been developed using a one-pot Cu-catalyzed etherification/aldol condensation cascade reaction. The reaction of 4-hydroxyisoindolin-1-one with a wide range of 2-bromobenzaldehydes in the presence of a copper catalyst provided various aristoyagonine derivatives in good yields.

Copper-catalyzed recycling of halogen activating groups via 1,3-halogen migration

A Cu(I)-catalyzed 1,3-halogen migration reaction effectively recycles an activating group by transferring bromine or iodine from a sp(2) to a benzylic carbon with concomitant borylation of the Ar-X bond. The resulting benzyl halide can be reacted in the same vessel under a variety of conditions to form an additional carbon-heteroatom bond. Cross-over experiments using an isotopically enriched bromide source support intramolecular transfer of Br. The reaction is postulated to proceed via a Markovnikov hydrocupration of the o-halostyrene, oxidative addition of the resulting Cu(I) complex into the Ar-X bond, reductive elimination of the new sp(3) C-X bond, and final borylation of an Ar-Cu(I) species to turn over the catalytic cycle.