Catalytic Asymmetric Epoxidation of Aldehydes with Two VANOL‐Derived Chiral Borate Catalysts

Abstract

A highly diastereo‐ and enantioselective method for the epoxidation of aldehydes with α‐diazoacetamides has been developed with two different borate ester catalysts of VANOL. Both catalytic systems are general for aromatic, aliphatic, and acetylenic aldehydes, giving high yields and inductions for nearly all cases. One borate ester catalyst has two molecules of VANOL and the other only one VANOL. Catalysts generated from BINOL and VAPOL are ineffective catalysts. An application is shown for access to the side‐chain of taxol.

A Structurally Robust Chiral Borate Ion: Molecular Design, Synthesis, and Asymmetric Catalysis

“…This is probably attributed to the inherent structural instability of B(OR) 4 − resulting from the reversibility of boron-oxygen (B-O) bond formation, allowing the exchange of the oxy-ligand (OR), especially under acidic conditions. [25][26][27] In fact, previously reported chiral borates, most of which adopt a bora-spiro core consisting of two bidentate chiral backbones, are not stable enough to be isolated by a standard purification procedure such as silica gel column chromatography; these borates are known to be structurally fluxional in solution in the absence of basic substrates such as imines or amines. 28 These profiles, while crucial for the in situ assembly of thermodynamically favorable borate catalysts, reflect the lack of a strategy for the development of chiral borates with structural robustness and sufficient diversity for eliciting the untapped potential as chiral WCAs in the arena of asymmetric catalysis.…”