Rh(iii)-catalyzed synthesis of dibenzo[b,d]pyran-6-ones from aryl ketone O-acetyl oximes and quinones via C–H activation and C–C bond cleavage

A redox-neutral synthesis of dibenzob,d]pyran-6-ones from aryl ketone O-acetyl oximes and quinones has been realized via Rh(iii)-catalyzed cascade C–H activation annulation. A possible Rh(iii)–Rh(v)–Rh(iii) mechanism involving an unprecedented β-C elimination step was proposed.

A novel procedure for the synthesis of dibenzob,d]pyran-6-ones via Rh(iii)-catalyzed C–H activation and C–C bond cleavage is described.

The dibenzob,d]pyran-6-one is one of the most important structural motifs widely present in natural products with pharmacological relevance,¹ such as gut microbiota metabolites urolithins (1–4) that show anti-inflammatory, antiglycative and neuroprotective effects,^2–4 and the extracts of an endophytic fungus Cephalosporium acremonium IFB-E007 (5–7) that have pronounced anticancer activities.⁵ In addition, the related heterocyclic structure benzod]naphtho1,2-b]pyran-6-one is found in some bactericidal and antitumor natural products including gilvocarcins^6,7 (8–10) chrysomycins^8,9 (11–13), etc. (Fig. 1). Therefore, a number of approaches to access dibenzob,d]pyran-6-ones have been developed via the intra- or inter-molecular biaryl formation as the key step.¹⁰ However, many of these methodologies require multi-step reactions, and the development of new efficient synthetic methods, especially those easy one-step reactions that are still of great interest.Open in a separate windowFig. 1Selected representative natural products.In the past decade, transition-metal-catalyzed C–H bond activation has proven to be a powerful tool in organic syntheses¹¹ and several methods for the synthesis of dibenzob,d]pyran-6-ones via C–H activation have been reported.¹² Actually, in 2015, our group reported Rh(iii)-catalyzed synthesis of dibenzob,d]pyran-6-ones from N-methoxybenzamides and quinones through C–H activation annulation.¹³ Interestingly, we obtained the same products using aryl ketone O-acetyl oximes as substrates to react with quinones under Rh(iii)-catalyzed conditions in this work. Rh(iii)-catalyzed C–H activation using ketoximes as substrates has been developed for synthesis of various substituted heterocycles.¹⁴ Compared to the previous reports, this reaction undergoes a novel mechanism involving an unexpected C–C bond cleavage, which is attractive. Moreover, our study demonstrated that solvent is vital to these reactions. In 2018, we reported Rh(iii)-catalyzed annulation of aryl ketone O-acetyl oximes with quinones to synthesize 6H-benzoc]chromenes with acetone as a co-solvent.¹⁵ Herein, we described Rh(iii)-catalyzed synthesis of dibenzob,d]pyran-6-ones using the same substrates without acetone (Scheme 1).Open in a separate windowScheme 1Rh(iii)-catalyzed divergent C–H activation annulation with quinones.Initially, the reaction of acetophenone O-acetyl oxime 1a with benzoquinone 2a was employed to optimize the reaction conditions (