Indium Tribromide Catalyzed Cross-Claisen Condensation between Carboxylic Acids and Ketene Silyl Acetals Using Alkoxyhydrosilanes

Organocatalytic Activation of Inert Hydrosilane for Peptide Bond Formation

We describe the development of a reliable catalytic protocol for peptide bond formation that is generally applicable to natural and unnatural α-amino acids, β-amino acids, and peptides bearing various functional groups. A 10 mol % loading of HSi[OCH(CF₃)₂]₃ as a catalyst was sufficient to guarantee a consistently high yield of the resulting peptide. This method facilitates the sustainable utilization of natural resources by using a catalyst and an auxiliary based on earth-abundant silicon.

Non-metal Lewis acid-catalyzed cross-Claisen condensation for β-keto esters

In this work, we disclose a new catalytic and highly chemoselective cross-Claisen condensation of esters. In the presence of TBSNTf₂ as a non-metal Lewis acid, various esters can undergo cross-Claisen condensation to form β-keto esters which are important building blocks. Compared with the traditional Claisen condensation, this process, employing silyl ketene acetals (SKAs) as carbonic nucleophiles to achieve cross-Claisen condensation, requires mild conditions and has good tolerance of functional groups.

Convergent Total Synthesis of Yaku'amide A

Total synthesis of the anticancer peptide natural product yaku'amide A is reported. Its β-tert-hydroxy amino acids were prepared by regioselective aminohydroxylation involving a chiral mesyloxycarbamate reagent. Stereospecific construction of the E- and Z-ΔIle residues was accomplished through a one-pot reaction featuring anti dehydration, azide reduction, and O→N acyl transfer. Alkene isomerization was negligible during this process. These methods enabled a highly convergent and efficient synthetic route to the natural product.

Air-Tolerant Direct Thiol Esterification with Carboxylic Acids Using Hydrosilane via Simple Inorganic Base Catalysis

Direct thioesterification of carboxylic acids with thiols using nontoxic activation agents is highly desirable. Herein, an efficient and practical protocol using safe and inexpensive industrial waste polymethylhydrosiloxane as the activation agent and K₃PO₄ with 18-crown-6 as a catalyst is described. Various functional groups on carboxylic acid and thiol substituents can be tolerated by the present system to afford thioesters in yields of 19-100%.

Indium-Catalyzed Annulation of o-Acylanilines with Alkoxyheteroarenes: Synthesis of Heteroaryl[b]quinolines and Subsequent Transformation to Cryptolepine Derivatives

We disclose herein the first synthetic method that is capable of offering heteroaryl[b]quinolines (HA[b]Qs) with structural diversity, which include tricyclic and tetracyclic structures with (benzo)thienyl, (benzo)furanyl, and indolyl rings. The target HA[b]Q is addressed by the annulation of o-acylanilines and MeO–heteroarenes with the aid of an indium Lewis acid that effectively works to make two different types of the N–C and C–C bonds in one batch. A series of indolo[3,2-b]quinolines prepared here can be subsequently transformed to structurally unprecedented cryptolepine derivatives. Mechanistic studies showed that the N–C bond formation is followed by the C–C bond formation. The indium-catalyzed annulation reaction thus starts with the nucleophilic attack of the NH₂ group of o-acylanilines to the MeO-connected carbon atom of the heteroaryl ring in an S_NAr fashion, and thereby the N–C bond is formed. The resulting intermediate then cyclizes to make the C–C bond through the nucleophilic attack of the heteroaryl-ring-based carbon atom to the carbonyl carbon atom, providing the HA[b]Q after aromatizing dehydration.

Tetramethyl Orthosilicate (TMOS) as a Reagent for Direct Amidation of Carboxylic Acids

Tetramethyl orthosilicate (TMOS) is shown to be an effective reagent for direct amidation of aliphatic and aromatic carboxylic acids with amines and anilines. The amide products are obtained in good to quantitative yields in pure form directly after workup without the need for any further purification. A silyl ester as the putative activated intermediate is observed by NMR methods. Amidations on a 1 mol scale are demonstrated with a favorable process mass intensity.

Gallium-catalyzed reductive lactonization of γ-keto acids with a hydrosilane

Gallium chloride efficiently catalyzes cyclization of γ-keto carboxylic acids in the presence of PhSiH₃ to produce the corresponding γ-lactone derivatives.

Chemistry of Fluorinated Carbon Acids: Synthesis, Physicochemical Properties, and Catalysis

The bis[(trifluoromethyl)sulfonyl]methyl (Tf2CH; Tf=SO2CF3) group is known to be one of the strongest carbon acid functionalities. The acidity of such carbon acids in the gas phase is stronger than that of sulfuric acid. Our recent investigations have demonstrated that this type of carbon acids work as novel acid catalysts. In this paper, recent achievements in carbon acid chemistry by our research group, including synthesis, physicochemical properties, and catalysis, are summarized.

Metal-catalyzed formation of 1,3-cyclohexadienes: a catalyst-dependent reaction

A metal-dependent and complementary catalytic method to synthesize the cyclohexadienes has been developed. When gold or indium salts were used as catalysts, 1,3-cyclohexadiene (1,3-CHD) could be obtained; when Cu(OTf)₂ was used as the catalyst, however, another isomer 2,4-cyclohexadiene (2,4-CHD) was furnished instead.