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.
Weinreb amides are a privileged, multi-functional group with well-established utility in classical synthesis. Recently, several studies have demonstrated the use of Weinreb amides as interesting substrates in transition metal-catalyzed C-H functionalization reactions. Herein, we review this part of the literature, including the metal catalysts, transformations explored so far and specific insights from mechanistic studies.
A novel rhodium-catalyzed dienamine activation of diazoenals resulted in a new class of γ-functionalized donor-acceptor dienamines. The synthetic utility of these dienamines has been demonstrated in a cooperative rhodium(II)/Brønsted acid and gold(I)-catalyzed direct [3+3] annulation of enaldiazo ketones with N-propargyl anilines, thus leading to highly substituted enal-functionalized 1,4-oxazines. The reaction is proposed to involve dienamine activation through the diacceptor rhodium enalcarbenoid NH-insertion and a gold-catalyzed intramolecular site-selective 6-exo-dig heterocyclization. The methodology was applied to the efficient synthesis of structurally complex [1,4]oxazino[4,3-a]quinolone, which is present in the antibacterial agent PNU-286607.
An efficient AgOTf catalyzed tandem intramolecular transannulation of ((2-alkynyl)aryl)cyclopropyl ketones leading to the 2,3-dihydronaphtho[1,2-b]furans has been developed. The reaction features a regioselective alkyne hydration, cyclopropylketone-2,3-dyhydrofuran rearrangement, and benzannulation. The methodology gives direct access to the tricyclic core structure of biologically important 2,3-dihydronaphtho[1,2-b]furan natural products.
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