Construction of CF3-Containing Tetrahydropyrano[3,2-b]indoles through DMAP-Catalyzed [4+1]/[3+3] Domino Sequential Annulation

Phosphine-Catalyzed Domino Annulation of γ-Vinyl Allenoates: Synthesis of Tetrahydrofuro[3,2-c]quinoline Derivatives

A novel phosphine-catalyzed domino annulation reaction of γ-vinyl allenoates and o-aminotrifluoacetophenones for the construction of terahydrofuro[3,2-c]quinoline derivatives has been developed. In this domino reaction, two kinds of terahydrofuro[3,2-c]quinoline compounds containing CF3 groups were obtained with good yields under mild conditions, three new C-N, C-C, and C-O bonds can be built in one step, and the reaction selectivity is achieved by adjusting the reaction conditions. Furthermore, preliminary studies on an asymmetric variant of this reaction proceeded with moderate enantioselectivity.

Switchable Selectivity in the Annulation of o-Trifluoroacetylanilines and Activated Terminal Alkynes Based on Transition Metal and Phosphine Catalysis

In this work, we have developed selective methods for the synthesis of quinoline-2-carboxylates and quinoline-3-carboxylates as well as (indolin-2-ylidene)acetates through copper-, silver-, or phosphine-catalyzed reaction of propiolates with 2'-amino-2,2,2-trifluoroacetophenones. The approaches proposed ensure synthesis of substituted quinoline carboxylates and (indolin-2-ylidene)acetates in good yields. Introduction of alkynones into the reaction with 2'-amino-2,2,2-trifluoroacetophenones gives acyl substituted derivatives in good yields.

Pyridine vs DABCO vs TBAB in Annulations of δ-Acetoxy Allenoates with Thioamides Leading to Dihydrothiophene, Thiopyran, and Thiazole Scaffolds

The same δ-acetoxy allenoates and thioamides, under DABCO, pyridine, or tetra-n-butyl ammonium bromide (TBAB) catalysis, undergo distinctly different annulations giving chemoselective routes to dihydrothiophene, thiopyran, or thiazole motifs. Thus, using pyridine in [3 + 2] annulation, dihydrothiophenes are obtained as essentially single diastereomers. By contrast, under DABCO catalysis, allenoates deliver thiopyran motifs in good to high yields through 6-exo-dig cyclization. In the thiazole forming [3 + 2] annulation, tetra-n-butyl ammonium bromide (TBAB) facilitates addition-elimination and 5-exo-trig cyclization in which β- and γ-carbons of allenoates participate to deliver thiazole cores exclusively with a Z-isomeric exocyclic double bond. A possible rationale for these observations is delved into.

Organocatalytic diastereo- and enantioselective oxa-hetero-Diels-Alder reactions of enones with aryl trifluoromethyl ketones for the synthesis of trifluoromethyl-substituted tetrahydropyrans

Tetrahydropyran derivatives are found in bioactives, and introduction of the trifluoromethyl group into molecules often improves biofunctions. Here we report diastereo- and enantioselective oxa-hetero-Diels-Alder reactions catalyzed by amine-based catalyst systems that afford trifluoromethyl-substituted tetrahydropyranones. Catalyst systems and conditions suitable for the reactions to provide the desired diastereomer products with high enantioselectivities were identified, and various trifluoromethyl-substituted tetrahydropyranones were synthesized with high diastereo- and enantioselectivities. Mechanistic investigation suggested that the reactions involve a [4 + 2] cycloaddition pathway, in which the enamine of the enone acts as the diene and the ketone carbonyl group of the aryl trifluoromethyl ketone acts as the dienophile. In this study, tetrahydropyran derivatives with the desired stereochemistry that are difficult to synthesize by previously reported methods were concisely obtained, and the range of tetrahydropyran derivatives that can be synthesized was expanded.

Divergent Reactivity of δ- and β'-Acetoxy Allenoates with 2-Sulfonamidoindoles via Phosphine Catalysis: Entry to Dihydro-α-carboline, α-Carboline, and Spiro-cyclopentene Motifs

The reactivity of 2-sulfonamidoindoles with acetoxy allenoates under phosphine catalysis depends on the disposition of the acetoxy (OAc) group on the allenoate. In the temperature-controlled [3 + 3] annulations, δ-acetoxy allenoates afforded dihydrocarboline and carboline scaffolds with carbon-nitrogen nucleophilic 2-sulfonamidoindoles, in which allenoate serves as a β-, γ-, and δ-carbon donor. At room temperature (25 °C), dihydro-α-carboline motifs were obtained exclusively through Michael addition, 1,4-proton shift, isomerization, 1,2-proton transfer, phosphine elimination, and aza-Michael addition. The higher temperature (80 °C) cascade protocol using Ph₃P-Cs₂CO₃ combination involves addition-elimination, aza-Claisen rearrangement, tosyl migration, and aromatization as key steps to give α-carbolines containing tosyl functionality at the γ-carbon. In contrast, with β'-acetoxy allenoate, 2-sulfonamidoindole acts only as a carbo-nucleophile in (p-tolyl)₃P-directed [4 + 1] spiro-annulation, leading to five-membered spiro-carbocyclic motifs essentially as single diastereomers (dr >20:1) via chemoselective carbo-annulation.

A formal [3 + 3] cycloaddition of allenyl imide and activated ketones for the synthesis of tetrasubstituted 2-pyrones

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives have been demonstrated. The allenyl imide was utilized as a C3-synthon, and a ketenyl intermediate was proposed via the process of 1,4-addition of carbon anion to allene followed by elimination of the 2-oxazolidinyl group.

CsOH·H₂O-catalyzed formal [3 + 3] cycloadditions of allenyl imide with β-ketoesters, 1,3-diketones or β-ketonitriles for the synthesis of tetrasubstituted 2-pyrone derivatives were reported.

Contrasting Carboannulation Involving δ-Acetoxy Allenoate as a Four-Carbon Synthon Using DABCO and DMAP: Access to Spiro-carbocyclic and m-Teraryl Scaffolds

Spiro-annulation involving δ-acetoxy allenoate and alkyl benzoisothiazole dioxide (N-sulfonyl ketimine) triggered by DABCO/MeCO₂H combination leads to an essentially single diastereomer via chemo- and regiospecific [4 + 2]-carboannulation and a new hydroxyl group is introduced. In contrast, DMAP-catalyzed benzannulation using the same reactants affords unsymmetrical m-teraryls via Mannich coupling, sequential proton transfers, and C-N bond cleavage. Here, δ-acetoxy allenoate serves as a 4C-synthon and the carboannulation is completely base dependent and mutually exclusive.