Asymmetric Synthesis of Chiral N-Sulfinyl 3-Alkyl- and 3-Arylpiperidines by α-Alkylation of N-Sulfinyl Imidates with 1-Chloro-3-iodopropane

Palladium-Catalyzed Allylation of Endocyclic 1-Azaallyl Anions

Endocyclic 1-azaallyl anions engage allyl acetates in a palladium-catalyzed allylation followed by reduction to give unprotected 2-(hetero)aryl-3-allylpiperidines and 2-allyl-3-arylmorpholines, products not easily accessible by other means. The allyl group is then readily transformed into a variety of functional groups. Preliminary studies on the asymmetric variant of the reaction using an enantiomerically pure BI-DIME-type ligand provide the product with moderate enantioselectivity. Computational studies suggest that energy barriers of inner-sphere reductive elimination and outer-sphere nucleophilic substitution are almost the same, which makes both of them possible reaction pathways. In addition, the inner-sphere mechanism displays an enantiodiscriminating C-C bond forming step, while the outer-sphere mechanism is much less selective, which combined to give the asymmetric variant of the reaction moderate enantioselectivity.

Transition-metal catalyzed C-H activation as a means of synthesizing complex natural products

Over the past few decades, the advent of C-H activation has led to a rethink among chemists about the synthetic strategies employed for multi-step transformations. Indeed, deploying innovative and masterful tricks against the numerous classical organic transformations has been the need of the hour. Despite this, the immense importance of C-H activation remains unfulfilled unless the methodology can be deployed for large-scale industrial processes and towards the concise, step-economic synthesis of prodigious natural products and pharmaceutical drugs. Lately, the growing potential of C-H activation methodology has indeed driven the pioneers of synthetic organic chemists into finding more efficient methods to accelerate the synthesis of such complex molecular scaffolds. This review aims to draw a general overview of the various C-H activation procedures that have been adopted for synthesizing these vast majority of structurally complicated natural products. Our objective lies in drawing a complete picture and taking the readers through the synthesis of a series of such complex organic compounds by simplified techniques, making it step-economic on a larger scale and thus instigating the readers to trigger the use of such methodology and uncover new, unique patterns for future synthesis of such natural products.

Palladium-Catalyzed Arylation of Endocyclic 1-Azaallyl Anions: Concise Synthesis of Unprotected Enantioenriched cis-2,3-Diarylpiperidines

Unprotected cis-2,3-diarylpiperidines are synthesized through an unprecedented palladium-catalyzed cross-coupling reaction between aryl halides and elusive endocyclic 1-azaallyl anions. These intermediates are generated in situ by the deprotonation of 2-aryl-1-piperideines, precursors that are readily prepared in two operations from simple piperidines. An asymmetric version of this reaction with (2R, 3R)-iPr-BI-DIME as the ligand provides products in moderate to good yields and enantioselectivities. This study significantly expands the synthetic utility of endocyclic 1-azaallyl anions.

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.

Iridium-catalyzed asymmetric hydrogenation of racemic α-substituted lactones to chiral diols

We report a protocol for the highly efficient iridium-catalyzed asymmetric hydrogenation of racemic α-substituted lactones via dynamic kinetic resolution. Using Ir-SpiroPAP (R)-1d as a catalyst, a wide range of chiral diols were prepared in a high yield (80-95%) with a high enantioselectivity (up to 95% ee) under mild reaction conditions. This protocol was used for enantioselective syntheses of (-)-preclamol and a chiral 2,5-disubstituted tetrahydropyran.

Asymmetric Michael Addition Induced by (R)-tert-Butanesulfinamide and Syntheses of Chiral Pyrazolidinone Derivatives

A highly diastereoselective Michael addition of (R)-N-tert-butanesulfinyl imidates 8 to α,β-unsaturated pyrazolidinone 3a has been developed to afford pyrazolidinones 10 possessing three contiguous stereocenters with good to excellent yield and excellent diastereoselectivity. A two-step conversion of reduction and cyclization provides the bicyclic pyrazolopiperidine 12 in a good yield. A series of pyrazolopiperidine derivatives 18 with a quaternary carbon center at C-3a are stereoselectively synthesized via alkylation or Michael addition.

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.

Total syntheses of (-)-isoschizogamine and (-)-2-hydroxyisoschizogamine

Total syntheses of (-)-isoschizogamine and (-)-2-hydroxyisoschizogamine are described. The synthesis employs two asymmetric Michael additions to establish chiral centers at C7 and the quaternary carbon C20. Regioselective reduction of the methylthioiminium cation rather than the enamine generates an isoschizogamine-type pentacyclic skeleton. Acidic hydrolysis of the isoschizogamine-type intermediate in the absence of oxygen provides natural (-)-isoschizogamine. Conducting the reaction in the presence of oxygen leads to a multistep oxidative hydrolysis cascade that affords unnatural (-)-2-hydroxyisoschizogamine.

[Asymmetric syntheses of azacycloalkanes by one-pot [1+2+n] cyclization]

Enantio- and diastereoselective one-pot synthesis of three- to seven-membered cis-azaheterocycles was achieved using a triggered asymmetric conjugate addition reaction of lithium amide with an enoate, followed by C- and N-alkylation of the resulting lithium 3-aminoenolate with α, ω-dihaloalkane, which can be regarded as a [1+2+n] cyclization. The usefulness of the developed methodology was clearly demonstrated by the short-step asymmetric syntheses of azirinomycin ester, nemonapride, and kopsinine.

Enantioselective synthesis of 2-substituted and 3-substituted piperidines through a bromoaminocyclization process

A catalytic enantioselective bromocyclization of olefinic amides using amino-thiocarbamates as the catalysts has been developed. The resulting enantioenriched 2-substituted 3-bromopiperidines can readily be transformed to 3-substituted piperidines through a silver salt-mediated rearrangement. This process has been applied to the synthesis of a dopaminergic drug, Preclamol.

General entry to asymmetric one-pot [N + 2 + n] cyclization for the synthesis of three- to seven-membered azacycloalkanes

Enantio- and diastereoselective one-pot synthesis of three- to seven-membered cis-azaheterocycles was achieved using a triggered asymmetric conjugate addition reaction of lithium amide with an enoate, followed by alkylation of the resulting lithium enolate with α,ω-dihaloalkane and N-alkylation. Isomerization of cis-azaheterocycles with a base yielded the trans-product, constituting a one-pot synthesis of cis-azacycles and a two-step synthesis of trans-azacycles. The four-step asymmetric synthesis of nemonapride highlights the general utility of the method.