Pd-catalyzed three-component [2 + 2 + 1] cycloamination toward carbazoles.

Conventional approaches using hydroxylamine derivatives as single nitrogen sources for the preparation of N-heterocyclic molecules rely on two chemical processes involving sequential nucleophilic and electrophilic C-N bond formations. Herein, we report a novel Suzuki reaction/C-H activation/amination sequence for building a myriad of carbazoles in a single transformation using bifunctional secondary hydroxylamines. It is noteworthy that the synthetic utility of this methodology is highlighted by the total synthesis of clausine V and glycoborine by incorporating the title [2 + 2 + 1] cycloamination as the key step. Control experiments were performed to gain a better understanding of the reaction mechanism.

Dearomatization/Deiodination of o-Iodophenolic Compounds with α,ß-Unsaturated Imines for Accessing Benzofuran Derivatives.

A dearomatization/deiodination/rearomatization strategy for the [3 + 2] cyclization of o-iodophenolic substrates with α,ß-unsaturated imines to construct various dihydrobenzofuran-related skeletons has been established. Tolerance to different functional groups has been tested. Mechanistic studies revealed that this domino reaction was possibly realized by the deiodination and tautomerization of the key dearomatized intermediate to generate a free phenolic O radical. Moreover, an anticancer agent 4 and an α-glucosidase inhibitor 5 with high bioactivities were successfully synthesized using this novel protocol.

Hydroxylamines As Bifunctional Single-Nitrogen Sources for the Rapid Assembly of Diverse Tricyclic Indole Scaffolds.

Conventional approaches on using hydroxylamine derivatives as single nitrogen sources, for the construction of n-membered (n > 3) N-heterocycles, rely upon two chemical operations by involving sequential nucleophilic and electrophilic C-N bond formations. Here, we report a highly efficient cascade of alkyne insertion/C-H activation/amination for the rapid preparation of a myriad of tricyclic indoles, in a single-step transformation, by using bifunctional secondary hydroxylamines. It is noteworthy that judicious selection of applicable amino agents, for enabling the prior oxidative addition of aryl iodide to initial Pd(0) species and subsequent two C-N bonds formation, was the key to the success of this reaction. Control experiments indicated that a five-membered palladacyclic intermediate played a crucial role in promoting the final aminative ring closure.

Unified Asymmetric Total Syntheses of (-)-Alotaketals†A-D and (-)-Phorbaketal†A.

The novel tricyclic spiroketal alotane-type sesterterpenoids showed strikingly different biological activities and potency with subtle structural alterations. Asymmetric total syntheses of the tricyclic sesterterpenoids (-)-alotaketals A-D and (-)-phorbaketal A were accomplished [29-31 steps from (-)-malic acid] in a collective way for the first time. The key features of the strategy included 1) a new cascade cyclization of vinyl epoxy δ-keto-alcohols to forge the common tricyclic spiroketal intermediate, 2) a late-stage allylic C-H oxidation, and 3) olefin cross-metathesis to install the different side chains.

Total Syntheses of Sesterterpenoid Ansellones†A and B, and Phorbadione.

Ansellane-type sesterterpenoids including, ansellones A-G and (+)-phorbadione are structurally novel marine secondary metabolites which exhibit anticancer and anti-HIV activity. The first, asymmetric total syntheses of three structurally representative members, (-)-ansellones A and B and (+)-phorbadione, were accomplished in 16-23 steps from (+)-sclareolide. The route features the first regioselective cyclization of vinyl epoxides with internal alcohol nucleophiles in a 1,4-addition manner (SN 2'). Additionally, the allylic C-H oxidation was exploited at a late stage of the synthesis of (-)-ansellone A and (+)-phorbadione. This strategy is expected to be applicable to the synthesis of other ansellane sesterterpenoids.

Regioselective and Stereospecific Copper-Catalyzed Deoxygenation of Epoxides to Alkenes.

Two copper salts (Cu(CF3CO2)2 and IMesCuCl) were identified as earth-abundant, inexpensive, but effective metal catalysts together with diazo malonate for chemo-/regioselective and stereospecific deoxygenation of various epoxides with tolerance of common functional groups (alkene, ketone, ester, p-methoxybenzyl, benzyl, tert-butyldimethylsilyl, and triisopropylsilyl). In particular, the unprecedented regioselectivity allowed for the first time monodeoxygenation of diepoxides to alkenyl epoxides. Density functional theory mechanistic studies showed that the deoxygenation occurred by collapsing the free ylide, unfavoring the possible intuitive pathway via cycloreversion of possible oxetane.

Ligand-promoted oxidative cross-coupling of aryl boronic acids and aryl silanes by palladium catalysis.

The first cross-coupling reaction between aryl silanes and aryl boronic acids is described. This transformation represents one of the very few examples of coupling reactions between two nucleophilic organometallic reagents and provides a new method for the formation of biaryl compounds. The successful development of this reaction was enabled by the use of commercially available 2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (BINAP) as the ligand. A small amount of BINAP (3 mol %) was sufficient to suppress the formation of the homocoupling products, and the reaction yielded the cross-coupling products with high selectivity under mild conditions, even when the ratio of the two coupling partners was 1:1.

Silver-catalyzed C-H trifluoromethylation of arenes using trifluoroacetic acid as the trifluoromethylating reagent.

Direct trifluoromethylation of arenes using TFA as the trifluoromethylating reagent was achieved with Ag as the catalyst. This reaction not only provides a new protocol for aryl C-H trifluoromethylation, but the generation of CF3· from TFA may prove useful in other contexts and could potentially be extended to other trifluoromethylation reactions.