Catalytic Asymmetric Roskamp Reaction of Silyl Diazoalkane: Synthesis of Enantioenriched α-Silyl Ketone

Reaction dynamics as the missing puzzle piece: the origin of selectivity in oxazaborolidinium ion-catalysed reactions

The selectivity in a group of oxazaborolidinium ion-catalysed reactions between aldehyde and diazo compounds cannot be explained using transition state theory. VRAI-selectivity, developed to predict the outcome of dynamically controlled reactions, can account for both the chemo- and the stereo-selectivity in these reactions, which are controlled by reaction dynamics. Subtle modifications to the substrate or catalyst substituents alter the potential energy surface, leading to changes in predominant reaction pathways and altering the barriers to the major product when reaction dynamics are considered. In addition, this study suggests an explanation for the mysterious inversion of enantioselectivity resulting from the inclusion of an orthoiPrO group in the catalyst.

Catalytic Asymmetric Homologation of 4-Substituted Cyclohexanones with CF3 CHN2 : Enantioselective Synthesis of α-Trifluoromethyl Cycloheptanones

Introduction of the trifluoromethyl group (CF₃ ) into organic molecules in an enantioselective manner has attracted significant attention, but still remains a challenging problem. We herein report a catalytic asymmetric trifluoromethylation of cyclic ketones via a Sc^III /chiral bisoxazoline-catalyzed homologation reaction by employing 2,2,2-trifluorodiazoethane (CF₃ CHN₂ ) as the CF₃ source. This desymmetrization process is highly efficient and generates two chiral centers with excellent diastereoselectivity and enantioselectivity, affording chiral α-trifluoromethyl cyclic ketones in a straightforward manner.

Catalytic Asymmetric Darzens-Type Epoxidation of Diazoesters: Highly Enantioselective Synthesis of Trisubstituted Epoxides

Highly enantioselective Darzens-type epoxidation of diazoesters with glyoxal derivatives was accomplished using a chiral boron-Lewis acid catalyst, which facilitated asymmetric synthesis of trisubstituted α,β-epoxy esters. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction proceeded in high yield (up to 99 %) with excellent enantio- and diastereoselectivity (up to >99 % ee and >20:1 dr, respectively). The synthetic potential of this method was illustrated by conversion of the products to various compounds such as epoxy γ-butyrolactone, tertiary β-hydroxy ketone and epoxy diester.

Enantioselective Acyloin Rearrangement of Acyclic Aldehydes Catalyzed by Chiral Oxazaborolidinium Ion

A catalytic enantioselective acyloin rearrangement of acyclic aldehydes to synthesize highly optically active acyloin derivatives is described. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction provided chiral α-hydroxy aryl ketones in high yield (up to 95%) and enantioselectivity (up to 98% ee). In addition, the enantioselective acyloin rearrangement of α,α-dialkyl-α-siloxy aldehydes produced chiral α-siloxy alkyl ketones in high yield (up to 92%) with good enantioselectivity (up to 89% ee).

Catalytic Asymmetric Homologation of Ketones with α-Alkyl α-Diazo Esters

The homologation of ketones with diazo compounds is a useful strategy to synthesize one-carbon chain-extended acyclic ketones or ring-expanded cyclic ketones. However, the asymmetric homologation of acyclic ketones with α-diazo esters remains a challenge due to the lower reactivity and complicated selectivity. Herein, we report the enantioselective catalytic homologation of acetophenone and related derivatives with α-alkyl α-diazo esters utilizing a chiral scandium(III) N,N'-dioxide as the Lewis acid catalyst. This reaction supplies a highly chemo-, regio-, and enantioselective pathway for the synthesis of optically active β-keto esters with an all-carbon quaternary center through highly selective alkyl-group migration of the ketones. Moreover, the ring expansion of cyclic ketones was accomplished under slightly modified conditions, affording a series of enantioenriched cyclic β-keto esters. Density functional theory calculations have been carried out to elucidate the reaction pathway and possible working models that can explain the observed regio- and enantioselectivity.

Effective synthesis of a hexacyclic-steroid derivative from 4-hydroxyestrone

Several studies have been reported for the preparation of hexacyclic-steroid derivatives; however, some reagents are expensive and require special conditions for handling. In this way, the objective of this study was to synthesize a hexacyclic-steroid derivative from 4-hydroxyestrone. The chemical structure was evaluated through both ¹H NMR and ¹³C NMR spectroscopic analysis. The results showed good performance of the hexacyclic-steroid derivative. In conclusion in this study, an easy method for the preparation of the hexacyclic-steroid derivative is reported.

Enantioselective Strecker and Allylation Reactions with Aldimines Catalyzed by Chiral Oxazaborolidinium Ions

Chiral oxazaborolidinium ion (COBI)-catalyzed enantioselective nucleophilic addition reactions of aldimines using tributyltin cyanide and allyltributylstannane have been developed. Various α-aminonitriles and homoallylic amines were synthesized in high yield (up to 98%) with high to excellent enantioselectivity (up to 99% ee). A rational mechanistic model for the complex of COBI and aldimine is provided to account for these enantioselective reactions.

Enantioselective Carbonyl 1,2- or 1,4-Addition Reactions of Nucleophilic Silyl and Diazo Compounds Catalyzed by the Chiral Oxazaborolidinium Ion

Boron Lewis acid catalysis has a long history and has become one of the most powerful methods for organic synthesis. In addition to achiral boron catalysts such as BX₃ (X = F, Cl, Br) and B(C₆F₅)₃, chiral boron catalysts are also significant synthetic tools used by organic chemists in academic laboratories and industry. Since first reported by Corey et al. in 2002 ( Corey et al. J. Am. Chem. Soc. 2002 , 124 , 3808 ), the chiral oxazaborolidinium ion (COBI), an activated form of proline-derived oxazaborolidine, has been used as a strong Lewis acid catalyst. Although the early examples of asymmetric synthesis through COBI-catalyzed nucleophilic 1,2- or 1,4-carbonyl additions were reported in 2004-2006, Diels-Alder and cycloaddition reactions of various carbonyl compounds were mostly developed over the next several years to afford enantioenriched cyclized products. The power of COBI in catalyzing carbonyl 1,2- or 1,4-addition reactions triggered our interest in developing asymmetric synthetic methodologies to generate versatile enantiomerically enriched compounds. In this Account, we summarize our recent studies on COBI-catalyzed asymmetric nucleophilic carbonyl addition and tandem reactions. Logical mechanistic explanations of asymmetric COBI catalysis are also discussed. The proton-activated COBI catalyst, which can activate various carbonyl compounds such as aldehydes, ketones, acroleins, and enones through Lewis acid-base interactions and synergistic hydrogen bonds, facilitates asymmetric 1,2- or 1,4-carbonyl additions of nucleophiles. Nucleophiles bearing trialkylsilyl groups successfully reacted with aromatic, aliphatic, and α,β-unsaturated aldehydes through 1,2-addition reactions resulting in chiral β-hydroxy esters. In addition, efficient asymmetric hydrosilylation of ketones was achieved with a TfOH-activated COBI catalyst. Optically active β-keto esters and all-carbon quaternary aldehydes were synthesized successfully through asymmetric 1,2-addition of diazo compounds and tandem H- or C-migration, respectively. In some cases, epoxide products were obtained as side products via the Darzens reaction pathway. Solvent and π-π interactions played important roles in favoring C-migration over H-migration. Nucleophilic 1,4-addition of diazo compounds and chemoselective ring-closure afforded an efficient approach to cyclopropanes, and their tandem rearrangements provided four- and seven-membered cyclic compounds with excellent stereoselectivity. After a Michael addition of diazo compounds, the selective β-hydride shift pathway afforded the β-substituted cyclic enones with high diastereo- and enantioselectivity. The presence of π-bond(s) in the substituents at the α-position of the diazo compound hindered the β-hydride shift pathway and, as a result, favored the cyclopropanation pathway. While there still remain challenges to be overcome, these results further understanding of COBI catalysis and open a window for future development of new asymmetric synthetic methods using carbonyl addition and tandem reactions.

Asymmetric Synthesis of Cyclobutanone via Lewis Acid Catalyzed Tandem Cyclopropanation/Semipinacol Rearrangement

Chiral Lewis acid catalyzed asymmetric formation of cyclobutanones from α-silyloxyacroleins and α-alkyl or α-aryl diazoesters has been developed. In the presence of a chiral oxazaborolidinium ion catalyst, various α-silyloxycyclobutanones possessing a chiral β-quaternary center were synthesized in high yield (up to 91%) with excellent enantio- and diastereoselectivity (up to 98% ee and up to >20:1 dr) through tandem cyclopropanation/semipinacol rearrangement. The synthetic potential of this method was illustrated by conversion of the product to various cyclic compounds such as γ-lactone, cyclobutanol, and cyclopentanone.