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

Abstract

Ahighly diastereo-and enantioselective method for the epoxidation of aldehydes with a-diazoacetamides has been developed with two different borate ester catalysts of VA NOL. Both catalytic systems are general for aromatic,aliphatic, and acetylenic aldehydes,g iving high yields and inductions for nearly all cases.One borate ester catalyst has two molecules of VA NOL and the other only one VA NOL. Catalysts generated from BINOL and VA POL are ineffective catalysts.A n application is shown for access to the side-chain of taxol.The tactical repertoire for the conversion of either an aldehyde or ketone into an epoxide largely consists of two transformations (Scheme 1): 1) the Darzens condensation of an a-halo stabilized carbanion with the ac arbonyl compound, [1,2] 2) the Corey-Chaykovskyr eaction which involves as ulfur ylide as ac arbene surrogate in the synthesis of epoxides. [3][4][5][6][7] At hird method involving the formation of epoxides from the reactions of diazo compounds are not that common and have not been particularly useful. [8][9][10] However, Scheme 1. Epoxidesf rom ketones and aldehydes.EWG = electron-withdrawing group.Scheme 2. The cis-and trans-aziridination with BOROX catalysts.

Absolute stereochemical determination of 1,2‐diols via complexation with dinaphthyl borinic acid

“…In fact, without the water molecule, the dioxaborolane ring would fragment in calculations (see Figures S1 and S2 for computational details). The choice to have a dative bond between the hydroxyl and the boron atom is confirmed from NMR studies, [18][19][20] where the boron is clearly tricoordinated, with a chemical shift (22.6 ppm) consistent with the latter assignment (see Figure 4a,b for 11 B-NMR of DBA and (S)-6/DBA complex). As expected, the five member dioxaborolane ring adopts a chair conformation, placing the larger phenyl group in the pseudo equatorial position.…”

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

“…Ar' [b] rr [c] dr [c] ee [%] [8][9][10][11]. Vinylic ethers with asubstituent at the terminal carbon of the nucleophilic olefinic component were amenable to this protocol, and 2,3-disubstituted dihydropyrans 3m-p were obtained with virtually complete diastereoselectivity and excellent enantioselectivity .…”

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

“…Ar' [b] rr [c] dr [c] ee [%] [8][9][10][11]. Vinylic ethers with asubstituent at the terminal carbon of the nucleophilic olefinic component were amenable to this protocol, and 2,3-disubstituted dihydropyrans 3m-p were obtained with virtually complete diastereoselectivity and excellent enantioselectivity (Entries 12-15).…”

“…Nevertheless, the rational development of boron‐based chiral anions that can precisely control prochiral cations remains elusive. 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 . In fact, previously reported chiral borates, most of which adopt a bora‐spiro core consisting of two bidentate chiral backbones, are known to be structurally fluxional in solution in the absence of basic substrates such as imines or amines .…”

“…[9] Nevertheless,t he rational development of boron-based chiral anions that can precisely control prochiral cations remains elusive.T his is probably attributed to the inherent structural instability of B(OR) 4 À resulting from the reversibility of boron-oxygen (BÀO) bond formation, allowing the exchange of the oxyligand (OR), especially under acidic conditions. [10,11] In fact, previously reported chiral borates,m ost of which adopt ab ora-spiro core consisting of two bidentate chiral backbones,are known to be structurally fluxional in solution in the absence of basic substrates such as imines or amines. [12] These profiles,w hile crucial for the in situ assembly of thermodynamically favorable borate catalysts,r eflect the lack of as trategy for the development of chiral borates with structural robustness and sufficient diversity for eliciting the untapped potential as chiral WCAsi nt he arena of asymmetric catalysis.Here,wedescribe asolution to this problem; that is,t he molecular design, synthesis,a nd characterization of structurally robust chiral borates,w hich act as highly effective chiral WCA catalysts for achieving ah ighly regioand enantioselective protonative Prins-type cyclization.…”