Synthesis of Estrone Methyl Ether

Significance

Canales and Corey report a ­remarkably short and efficient synthesis of estrone methyl ether which exploits a novel N-methyl­oxazaborolidinium cation as a Lewis acid catalyst for an asymmetric Diels-Alder reaction. The paper cites ten further examples all proceeding with excellent yields and ee values. The reactions typically proceed in dichloromethane at -78 ˚C but in the estrone synthesis depicted, the cycloaddition was performed at room temperature. The ee of the adduct D (82%) was raised to 99% by one recrystallization.