Oxidative Ring-Opening Transformation of 5-Acyl-4-pyrones as an Approach for the Tunable Synthesis of Hydroxylated Pyrones and Furans

Synthesis of Rapicone Based on Acyl Ketene-Alkyne Cycloaddition

We report herein a concise route for the total synthesis of rapicone. The key strategy to form the ynone intermediate involves an Fe(III)/TEMPO-catalyzed aerobic oxidation of a 1,3-dihydroisobenzofuran moiety. This ether oxidation for the simultaneous installation of the keto aldehyde allowed the effective formation of the required ynone intermediate. The concise total synthesis of rapicone is substantiated by the unique formal [4 + 2] cycloaddition of acyl ketene with alkynone and is completed in 7 steps with 9.4% overall yield.

Acid-Promoted Multicomponent Reaction To Synthesize 4-Phosphorylated 4H-Chromenes

An effective multicomponent reaction for the synthesis of 4-phosphorylated 4H-chromenes via a tandem phosphorylation/alkylation/cyclization/dehydration sequence with water as the only byproduct was developed. Extensive mechanistic investigations involving in situ NMR experiments, time control experiments, and in situ HRMS experiment allowed us to elucidate the order of each subreaction to arrive at a complete understanding of the underlying mechanism of this multicomponent reaction. Mechanistic data confirm that the reaction begins with a phospha-aldol-elimination, followed by addition of a ketone enolate, intermolecular alkylation, intramolecular cyclization, and dehydration under acidic conditions.

Cyclization Reactions of In Situ-Generated Acyl Ketene with Ynones to Form Oxacycles

Acyl ketenes react with polar unsaturated functional groups to give unique heterocyclic rings, yet reactions with unpolarized unsaturated functional groups have not been reported. Herein, we describe two effective ring-forming reactions between acetyl ketene and electron-deficient alkynes. The first reaction involves in situ tethering between acetyl ketene and nucleophile-containing 1,3-diynones, which promotes sequential intramolecular 1,6/1,4-additions to generate 2-methylene-2H-pyrans in various yields (24-91%). The other involves a zwitterionic intermediate generated from acetyl ketene and DABCO, which undergoes a Michael addition with terminal alkynyl ketones to generate 3-acyl-4-pyrones (11-79%).