Asymmetric Synthesis of δ-Chloro-β-amino-N-sulfinyl Imidates as Versatile Chiral Building Blocks for the Synthesis of 2,3-Disubstituted Piperidines

Asymmetric hydrogenation of O-/N-functional group substituted arenes

Asymmetric hydrogenation of aromatic compounds represents one of the most straightforward synthetic methods to construct important chiral cyclic skeletons that are often found in biologically active agents and natural products. So far, the most successful examples in this field are largely limited to aromatics containing alkyl and aryl substituted groups due to the poor functional-group tolerance of hydrogenation. Direct asymmetric hydrogenation of functionalized aromatics provides enormous potential for expanding the structural diversity of reductive products of planar aromatic compounds, which is highly desirable and has not been well studied. This feature article focuses on the progress in catalytic asymmetric hydrogenation and transfer hydrogenation of O/N substituted arenes.

Mannich-Type Reaction of α-Sulfanyl N-tert-Butanesulfinylimidates: Diastereoselective Access to α-Mercapto-β-amino Acid Derivatives

A series of α-mercapto-β-amino acid derivatives were synthesized diastereoselectively in good yields through the aza-enolization of α-sulfanyl N-tert-butanesulfinylimidates, followed by their nucleophilic addition to N-tosyl imines via a Mannich-type reaction. The resulting derivatives bearing a β-sulfonylamino sulfide moiety participated in further inter- and intramolecular transformations involving episulfonium ion intermediates generated through neighboring-group participation.

Discovery of Annulating Reagents Enabling the One-Step and Highly Stereoselective Synthesis of Cyclopentyl and Cyclohexyl Cores

The use of the unprecedented annulating reagents methyl N-(tert-butylsulfinyl)-4-chlorobutanimidate and methyl N-(tert-butylsulfinyl)-5-bromopentanimidate enables the diastereoselective preparation of 5- and 6-membered carbocycles bearing three contiguous stereocenters. These synthons undergo cycloaddition with a variety of Michael acceptors to form cyclopentane/cyclohexane rings with excellent stereochemical control, generating only one of the eight possible diastereomers. This novel methodology has enabled the highly enantioselective and high yielding synthesis of novel chemotypes of pharmacological relevance.

Diastereoselective α-Amination of N- tert-Butanesulfinyl Imidates Using N-Aryl- N-diphenylphosphinyldiazenes

Diastereoselective α-amination of N- tert-butanesulfinyl imidates has been developed using N-aryl (or N- tert-butyl) N-diphenylphosphinyldiazenes as nitrogen sources. The chiral 1-azaenolates derived from imidates undergo nucleophilic addition with diazenes to give α-hydrazino imidates in good yields.

Divergent synthesis of polysubstituted cyclopropanes and β-silyoxy imidates via switchable additions of N-tert-butanesulfinylimidates to acylsilanes

A switchable addition of N-tert-butanesulfinylimidates to acylsilanes leads to divergent synthesis of polysubstituted cyclopropanes and β-silyloxy imidates.

Diastereoselective α-Fluorination of N- tert-Butanesulfinyl Imidates

A diastereoselective α-fluorination of N- tert-butanesulfinyl imidates was developed. Deprotonation of N- tert-butanesulfinyl imidates with lithium hexamethyldisilazide generates aza-enolates that can be intercepted, with excellent diastereocontrol, by the inexpensive electrophilic fluorinating agent NFSI. This protocol was applied to the preparation of synthetically useful trans-2-fluoro-cyclohexamine with high enantiomeric purity (99.5% ee).

Diastereoselective α-Sulfenylation of N- tert-Butanesulfinyl Imidates

A diastereoselective α-sulfenylation of chiral α-aryl/alkyl N- tert-butanesulfinyl imidates has been developed. Suitable sulfur electrophiles can be used as sulfenylating reagents to intercept aza-enolates generated from imidate deprotonation, giving α-thiofunctionalized imidates in good yields with high diastereocontrol. This protocol for C-S bond formation can efficiently synthesize enantioenriched 1,2-sulfanyl amine derivatives such as sulconazole.

Construction of α-methoxyimidoyl ketonitrones via phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes

A dimethyl phosphite-mediated addition of α-keto N-tert-butanesulfinyl imidates to nitrosoarenes was developed. Nitrosoarenes were successfully used as electrophiles to trap aza-enolate intermediates that were generated from nucleophilic addition of deprotonated phosphite to α-keto N-tert-butanesulfinyl imidates and following phospha-Brook rearrangement, allowing efficient construction of ketonitrones with excellent (Z)-geometries.

Diastereoselective α-Hydroxylation of N-tert-Butanesulfinyl Imidates and N'-tert-Butanesulfinyl Amidines with Molecular Oxygen

Diastereoselective α-hydroxylation using molecular oxygen has been achieved with chiral α-alkyl N-tert-butanesulfinyl imidates and α-aryl N'-tert-butanesulfinyl amidines. The aza-enolates generated from deprotonation of imidates/amidines can be intercepted by O₂ with excellent diastereocontrol and subsequently transformed into α-hydroxylation products in the presence of the reductant trimethyl phosphite.

Aldol Reaction of N-tert-Butanesulfinyl Imidates under Basic Conditions for Diastereoselective Synthesis of anti-Aldols

Diastereoselective aldol reaction of N-tert-butanesulfinyl imidates under typical hard enolization conditions is reported. Potassium bis(trimethylsilyl)amide (KHMDS) effectively promotes the aldol reaction of α-aryl- and α-alkyl-substituted imidates, providing anti-aldol adducts in high yields with good to excellent diastereoselectivities. In the case of α-aryl imidates, high conversion depends on adding trimethylsilyl chloride (TMSCl) to the reaction mixture. In the presence of a suitable Lewis acid, cyclohexanone is a good electrophile in the aldol reaction of imidates.

Diastereoselective Electrophilic α-Hydroxyamination of N-tert-Butanesulfinyl Imidates

Diastereoselective α-hydroxyamination of N-tert-butanesulfinyl imidates using nitrosoarenes is reported. A catalytic amount of base effectively promotes the hydroxyamination reaction of α-aryl-substituted imidates, and the resulting α-hydroxyamino imidates can be transformed into a range of synthetically useful intermediates.

Diastereoselective synthesis of 2-methoxyimidoyloxiranes via dimethyl phosphite-mediated coupling of α-keto N-sulfinyl imidates with aldehydes

A dimethyl phosphite-triggered coupling of α-keto N-tert-butylsulfinyl imidates with aldehydes provides access to enantioenriched 2-methoxyimidoyloxiranes with excellent diastereoselectivities.