Syntheses of 9‐Vinylated Benzotropolones by a Sequence of Acylation, Ring‐Closing Enyne Metathesis, and Oxidation

Abstract

Previous studies from our group revealed that 6,7‐benzannulated nona‐1,8‐diene‐5‐ones of a variety of substitution patterns undergo ring‐closing diene metatheses in the presence of the Grubbs‐II catalyst; the benzocycloheptadienones formed thereby gave benzotropolones in between one and four follow‐up steps. The present study discloses that exemplary 6,7‐benzannulated non‐1‐en‐8‐yn‐5‐ones undergo ring‐closing enyne metatheses in the presence of the Grubbs‐II catalyst; the vinylated benzocycloheptadienones formed thereby gave 9‐vinylbenzotropolones in four steps: silyl enol ether formation, Rubottom‐type oxidation, desilylation, and Dess‐Martin oxidation. The metathesis substrates were synthesized either from (ortho‐bromoaryl)ethynes with an unsubstituted sp‐carbon by a Br→Li exchange and an acylation with an H2C=CH‐containing Weinreb amide; or, they were synthesized from (ortho‐bromoaryl)ethynes with a trimethylsilylated sp‐carbon by a Br→Li→ZnHal exchange, an acylation with an H2C=CH‐containing thioester, and a desilylation.

One-Pot Synthesis of Functionalized Benzotropones via a Phthalide Ring-Opening/Intramolecular Aldol Condensation Cascade

“…Ebine and co-workers have described a ring-expansion strategy (Scheme b), while our group has developed a dual-ring-expansion strategy using a cyclopropyl benzocyclobutene derivative (Scheme c) . More recently, Brückner and co-workers have described a concise and practical approach to access benzotropolones via ring-closing olefin metathesis (Scheme d) . Although the synthetic accessibility of benzotropone derivatives has thus been demonstrated, all hitherto reported methods require multistep transformations and are limited in terms of generality and applicability.…”