N-to-S Acyl Transfer as an Enabling Strategy in Asymmetric and Chemoenzymatic Synthesis

The observation of thioester-mediated acyl transfer processes in nature has inspired the development of novel protein synthesis and functionalization methodologies. The chemoselective transfer of an acyl group from S-to-N is the basis of several powerful ligation strategies. In this work, we sought to apply the reverse process, the transfer of an acyl group from N-to-S, as a method to convert stable chiral amides into more reactive thioesters. To this end, we developed a novel cysteine-derived oxazolidinone that serves as both a chiral imide auxiliary and an acyl transfer agent. This auxiliary combines the desirable features of rigid chiral imides as templates for asymmetric transformations with the synthetic applicability of thioesters. We demonstrate that the auxiliary can be applied in a range of highly selective asymmetric transformations. Subsequent intramolecular N-to-S acyl transfer of the chiral product and in situ trapping of the resulting thioester provides access to diverse carboxylic acid derivatives under mild conditions. The oxazolidinone thioester products can also be isolated and used in Pd-mediated transformations to furnish highly valuable chiral scaffolds, such as noncanonical amino acids, cyclic ketones, tetrahydropyrones, and dihydroquinolinones. Finally, we demonstrate that the oxazolidinone thioesters can also serve as a surrogate for SNAC-thioesters, enabling their seamless use as non-native substrates in biocatalytic transformations.

Enantioselective synthesis of spirooxindole-pyran derivatives via a remote inverse-electron-demand Diels-Alder reaction of β,γ-unsaturated amides

Novel construction methods for obtaining 3,4'-pyran spirooxindole heterocyclic skeletons have always been the focus of attention. Herein, we report a highly enantioselective inverse-electron-demand oxa-Diels-Alder cycloaddition reaction of a β,γ-unsaturated pyrazole amide and a N-diphenyl isatin-derived oxodiene using a bifunctional catalyst. In addition, large-scale experiments confirmed the reliability of the reaction. The resultant products of this study can be further transformed.

Copper-Catalyzed Direct Asymmetric Vinylogous Mannich Reaction between β,γ-Alkynyl-α-ketimino Esters and β,γ-Unsaturated N-Acylpyrazoles

We report a Cu(I)-Ph-BPE-catalyzed asymmetric vinylogous Mannich reaction of β,γ-alkynyl-α-ketimino esters with β,γ-unsaturated N-acylpyrazoles. In this process, the Cu(I)-Ph-BPE catalyst activates the β,γ-alkynyl-α-ketimino ester through N,O-coordination, enabling the subsequent nucleophilic addition of a dienolate generated from the β,γ-unsaturated N-acylpyrazole via α-position deprotonation with a catalytic amount of tertiary amine. The reactions gave useful products with very high enantioselectivities. A broad range of substrates with various substituents are tolerated in this reaction. The versatility of this method was demonstrated by a gram-scale reaction, and subsequent elaboration of the Mannich adducts was also provided.

Scale-up of Sodium Persulfate Mediated, Nitroxide Catalyzed Oxidative Functionalization Reactions

BACKGROUND

Oxidation is a valuable tool in preparative organic chemistry. Oxoammonium salts and nitroxides have proven valuable as reagents and catalysts in this endeavor.

OBJECTIVE

The objective of this study is to scale up the oxidative amidation, ester formation, and nitrile formation using nitroxide as an organocatalyst.

METHODS

Oxidative functionalization reactions were scaled from the 1 mmol to the 1 mole level. Sodium persulfate was used as the primary oxidant, and a nitroxide was employed as a catalyst. The products of the reactions were isolated in analytically pure form by extraction with no need for column chromatography.

RESULTS

The oxidative amidation and esterification of aldehydes can be scaled up from 1 mmol to 1 mole effectively, with comparable product yields being obtained at each increment. This work shows that conditions developed on a small scale can be transferred to a larger scale without reoptimization. The oxidative functionalization of aldehydes to prepare nitriles is not amenable to direct scale-up due to the concomitant formation of significant quantities of the corresponding carboxylic acid, thereby compromising the product yield.

CONCLUSION

Two of the three oxidative transformations studied here can be scaled up successfully from the 1 mmol to the 1 mole level.

Catalytic Enantioselective Entry to Triflones Featuring a Quaternary Stereocenter

A highly enantioselective one-pot synthesis of functionalized triflones, bearing a quaternary stereocenter, has been developed, exploiting the Michael reaction of α-(trifluoromethylsulfonyl) aryl acetic acid esters with N-acryloyl-1H-pyrazole catalyzed by commercially available Takemoto’s catalyst, followed by nucleophilic acyl substitution with alcohols. Preliminary investigations highlighted the attractive potential of the triflinate anion as the leaving group for stereocontrolled postfunctionalizations.

Solvent- and additive-free oxidative amidation of aldehydes using a recyclable oxoammonium salt

A range of acyl azoles have been prepared from aromatic, heteroaromatic, and aliphatic aldehydes by means of an oxidative amidation reaction. The methodology employs a substoichiometric quantity of an oxoammonium salt as the oxidant. It avoids the need for additives such as a base, is run solvent-free, and the oxoammonium salt is recyclable.

Continuous Flow Synthesis of α-Trifluoromethylthiolated Esters and Amides from Carboxylic Acids: a Telescoped Approach

A continuous flow approach to access α-trifluoromethylthiolated esters and amides using commercially available arylacetic acids and N-(trifluoromethylthio)phthalimide as the electrophilic reagent is described. The experimental protocol involves the in-flow conversion of the carboxylic acid into N-acylpyrazole followed by the α-trifluoromethylthiolation in a PTFE coil reactor and final reaction with primary or secondary amines, or alcohols, to afford in a telescoped process α-substituted SCF₃ amides and esters, respectively, in good overall yield and short reaction times.