Gold-Catalyzed Synthesis of Heterocycles from Alkynyl Ethers. Synthesis of Leucascandrolide A. Mechanistic Analysis of Oxidative C–H Cleavages using Inter- and Intramolecular Kinetic Isotope Effects.

摘要

Gold complexes catalyze the cyclyzation of homopropargylic ethers to prepare saturated hetercyclic ketones through a sequence of alkyne hydration, alkoxy elimination, and intramolecular conjugate addition of pendant oxygen or nitrogen nucleophiles. This reaction was used in an efficient total synthesis of the natural product andrachcinidine. Regioselective hydration of internal alkynes on propargylic ethers rather than homopropargylic ethers expanded the scope of products. Leucascandrolide A was synthesized through an Electron Transfer-Initiated Cyclization (ETIC) reaction as a key step. The reaction sequence also had highlights as stereoselective BiBr3-mediated allylation, acetal formation as a fragment-coupling reaction, and a rhenium-mediated allylic alcohol transposition leading to stable macrolactol formation. Intra- and intermolecular kinetic isotope effects of oxidative carbon-hydrogen bond cleavage in DDQ-mediated cyclization reations were also explored. The carbon-hydrogen cleavage is rate determining and that a radical cation is most likely a key intermediate in the reaction mechanism.