Rhodium- and Platinum-Catalyzed [4+3] Cycloaddition with Concomitant Indole Annulation: Synthesis of Cyclohepta[b]indoles

Cascade radical cyclization on 3-propargyl-2-alkenyl indole gives stereoselective access to cyclohepta[b]indole over carbazole

A protecting group-dependent diastereoselective synthesis of cyclohepta[b]indole over carbazole derivatives is developed. This strategy involves a regioselective 6-exo-trig radical cyclization-cyclopropanation-ring expansion cascade of 3-propargyl-2-alkenyl indole. The cascade radical cyclization was also performed on indole derivatives possessing alkyne, acrylate and vinylogous carbamate moieties, which delivered pyridocarbazole giving credence to the mechanistic hypothesis. Furthermore, cyclohepta[b]indole could be selectively converted to sulfone and sulfoxide as well as benzoazulenoindole via intramolecular Friedel-Crafts acylation.

Transition-Metal-Catalyzed Cycloaddition Reactions to Access Seven-Membered Rings

The efficient and selective synthesis of functionalized seven-membered rings remains an important pursuit within synthetic organic chemistry, as this motif appears in numerous drug-like molecules and natural products. Use of cycloaddition reactions remains an attractive approach for their construction within the perspective of atom and step economy. Additionally, the ability to combine multiple components in a single reaction has the potential to allow for efficient combinatorial strategies of diversity-oriented synthesis. The inherent entropic penalty associated with achieving these transformations has impressively been overcome with development of catalysis, whereby the reaction components can be pre-organized through activation by transition-metal-catalysis. The fine-tuning of metal/ligand combinations as well as reaction conditions allows for achieving chemo-, regio-, diastereo- and enantioselectivity in these transformations. Herein, we discuss recent advances in transition-metal-catalyzed construction of seven-membered rings via combination of 2-4 components mediated by a variety of metals. An emphasis is placed on the mechanistic aspects of these transformations to both illustrate the state of the science and to highlight the unique application of novel processes of transition-metals in these transformations.

Divergent Reactivity of Rhodium(I) Carbenes Derived from Indole Annulations

Rhodium(I) carbenes were generated from propargylic alcohol derivatives as the result of a dehydrative indole annulation. Depending on the choice of the electron-withdrawing group on the aniline nitrogen nucleophile, either a cyclopropanation product or dimerization product was obtained chemoselectively. Intramolecular hydroamidation occurred for the same type of propargylic alcohol derivatives when other transition-metal catalysts were employed.

Rhodium enalcarbenoids: direct synthesis of indoles by rhodium(II)-catalyzed [4+2] benzannulation of pyrroles

Disclosed herein is the design of an unprecedented electrophilic rhodium enalcarbenoid which results from rhodium(II)-catalyzed decomposition of a new class of enaldiazo compounds. The synthetic utility of these enalcarbenoids has been successfully demonstrated in the first transition-metal-catalyzed [4+2] benzannulation of pyrroles, thus leading to substituted indoles. The new benzannulation has been applied to the efficient synthesis of the natural product leiocarpone as well as a potent adipocyte fatty-acid binding protein inhibitor.