Lewis Acid-Catalyzed Intramolecular [3+2] Cross-Cycloaddition of Aziridine 2,2-Diesters with Conjugated Dienes for Construction of Aza-[n .2.1] Skeletons

Diastereoselective Cascade Double Michael Addition to Access Bridged Coumarins, Oxindoles and Spirooxindoles: A Sustainable Strategy for Synthesis of Anticancer Molecules

An efficient and concise synthesis of highly functionalized bridged coumarins has been developed through a diastereoselective double Michael addition reaction of p-quinols with various 4-hydroxy coumarins under catalyst-free conditions in H2O-DMSO (8 : 2). The method has been applied to oxindoles for the synthesis of a variety of bridged-oxindoles and bridged-spiroxindoles in presence of a DABCO base using H2O-EtOH (8 : 2) as solvent medium. The strategy is simple, highly atom economical as there is no by-product and environmentally benign (E-factor=0.1-0.9). The synthesized compounds were screened against triple-negative breast cancers and found that bridged coumarin (3 a) and oxindole (5 d) compounds exhibit potent anti-cancer activity at 6.6 and 8.8 μM (IC50) concentrations respectively. Further analysis revealed that 3 a and 5 d caused elevated early and total apoptosis by arresting the MDA-MB-468 cells in G2/M phase of the cell cycle. Overall, our results demonstrate that bridged coumarin (3 a) and oxindole (5 d) compounds-based approach attenuates the cancer progression and may pave a path for the translational outcome.

Au(I)/Sc(III)-co-catalyzed tandem spiroannulation/cycloisomerization of 3-(2-ethynylaryl)-N-tosylaziridines with indoles to access 5H-benzo[b]carbazoles

An unusual spiroannulation/cycloisomerization cascade of 3-(2-ethynylaryl)-N-tosylaziridines with indoles enabled by cooperative gold/scandium catalysis is presented, which facilitates the synthesis of 5H-benzo[b]carbazoles in moderate to excellent yields. This protocol features a broad substrate scope and good functional-group compatibility. Additionally, the resulting 5H-benzo[b]carbazoles exhibit good fluorescence properties, demonstrating the synthetic practicality of this method. Moreover, control experiments were performed to illustrate the reaction mechanism.

Enantioselective Iridium-Catalyzed Reductive Coupling of Dienes with Oxetanones and N-Acyl-Azetidinones Mediated by 2-Propanol

Cyclometallated iridium-PhanePhos complexes generated in situ from [Ir(cod)Cl]2 and (R)-PhanePhos catalyze 2-propanol-mediated reductive couplings of 2-substituted dienes with oxetanone and N-acyl-azetidinones to form branched homoallylic oxetanols and azetidinols with excellent control of regio- and enantioselectivity without C-C cleavage of the strained ring via enantiotopic π-facial selection of transient allyliridium nucleophiles. This work represents the first systematic study of enantioselective additions to symmetric ketones.

Synthesis of polycyclic spiroindolines via the cascade reaction of 3-(2-isocyanoethyl)indoles

Tandem reactions of the yttrium(iii) catalyzed ring-opening reaction of 2,2'-diester aziridines with 3-(2-isocyanoethyl)indoles and the subsequent Friedel-Crafts/Mannich/desulfonylation were reported. A series of polycyclic spiroindolines containing tetrahydro-β-carbolines were obtained in moderate to excellent yields (56-92%) in one step under mild reaction conditions. A possible catalytic mechanism was also proposed.

Synthetic Approaches to Non-Tropane, Bridged, Azapolycyclic Ring Systems Containing Seven-Membered Carbocycles

This Short Review highlights various synthetic approaches to bridged azabicyclic ring systems containing seven-membered carbocyclic rings. Such ring systems are common to a number of biologically active natural products. The seven-membered ring in such systems is generally formed in one of four ways: 1) cyclization of an acyclic precursor; 2) ring expansion or rearrangement of a different ring size; 3) cycloaddition; and 4) use of a synthetic building block with the seven-membered ring already present. Representative examples of each approach from both total synthesis and methodological studies are discussed, with an emphasis on work published

in the last twenty years.

1 Introduction

2 Cyclization Reactions

3 Ring Expansions and Rearrangements

4 Cycloadditions

5 Strategies Involving Seven-Membered Ring Building Blocks

6 Conclusion

Mechanistic Insights into the Ru(II)-Catalyzed Intramolecular Formal [3 + 2] Cycloaddition of (E)-1,6-Enynes

Design of a unique reaction pathway in transition-metal-catalyzed 1,6-enynes cyclization to construct valuable synthetic motifs is a significant challenge in organic chemistry. Herein, we report a Ru(II)-catalyzed formal [3 + 2] cycloaddition as an efficient method to prepare unprecedented bicyclo[3.3.0]octenes from readily available (E)-1,6-enynes. Mechanistic studies based on the deuterium labeling experiments and the DFT calculation disclose a reasonable mechanistic pathway, where a ruthenacyclopentene generated by an ene-yne oxidative cyclization undergoes a sequential ß-hydride elimination and intramolecular hydroruthenation to form a ruthenacyclohexene, producing the desirable bicyclo[3.3.0]octenes.

(3+3)-Annulation of Donor-Acceptor Cyclopropanes with Diaziridines

The first example of (3+3)-annulation of two different three-membered rings is reported herein. Donor-acceptor cyclopropanes in reaction with diaziridines were found to afford perhydropyridazine derivatives in high yields and diastereoselectivity under mild Lewis acid catalysis. The disclosed reaction is applicable for the broad substrate scope and exhibits an excellent functional group tolerance.

Accessing polysubstituted oxazolidines, pyrrolidines and imidazolidines by regioselective [3 + 2] annulations of ketenimines with donor–acceptor oxiranes and aziridines

Diverse five-membered rings result from formal [3 + 2] cycloadditions of ketenimines with carbonyl and azomethine ylides.