Nitrile Ylides: Diastereoselective Cycloadditions using Chiral Oxzolidinones Without Lewis Acid

Synthesis of Spiropyrrolines via One-Pot Tf2O-Mediated Amide Activation/Formal [3 + 2]-Cycloaddition of α-Formylamino Ketones

An efficient method for the synthesis of spiropyrrolines from readily accessible α-formylamino ketones is reported. The method involves amide activation using Tf2O, followed by a formal [3 + 2]-cycloaddition of the resulting enolic nitrilium intermediate with Michael acceptors, ultimately affording spiropyrrolines. Mechanistic insights were gained through NMR studies, elucidating the precise role of the base additive and suggesting the formation of an enolic nitrilium intermediate.

Triplet Energy Transfer (EnT)-Promoted 1,3-Dipolar Cycloaddition Reactions of N-(Trimethylsilyl)methylphthalimide with Electron-Deficient Alkynes and Alkenes

Triplet energy transfer (EnT)-promoted photochemical pathways, arisen by visible light and its photosensitizers, have gained significant attention as a complementary strategy for initiating organic transformations in photochemical reactions that are unlikely to occur through a single electron transfer (SET) process. In the present study, we investigated the triplet EnT-promoted 1,3-dipolar cycloaddition reactions of N-(trimethylsilyl)methylphthalimide with electron-deficient alkynyl and alkenyl dipolarophiles. The triplet excited state of N-(trimethylsilyl)methylphthalimide, promoted by the triplet EnT from thioxanthone (TXA) photosensitizer, underwent sequential intramolecular SET and carbon-to-oxygen migration of the silyl group to form azomethine ylide. This generated ylide cycloadded to alkynes or alkenes to regioselectively and stereospecifically produce a nitrogen-containing benzopyrrolizidine scaffold with multiple stereogenic centers. Crucially, the stereoselectivity of these cycloaddition reactions (i.e., endo versus exo addition) was influenced by the nature of the dipolarophiles.

Tin(iv) chloride mediated (3 + 2) annulation of trans-2-aroyl-3-styrylcyclopropane-1,1-dicarboxylates with nitriles: diastereoselective access to 5-vinyl-1-pyrroline derivatives

A tin(iv) chloride promoted (3 + 2) annulation of trans-2-aroyl-3-styrylcyclopropane-1,1-dicarboxylates with nitriles is reported. The transformation involves the Lewis acid assisted formation of 1,5-dipolar intermediates from the cyclopropane dicarboxylates and nitriles followed by cyclization. The reactions proceed in a highly diastereoselective manner and afford 5-vinyl-1-pyrroline derivatives in 60-88% yields.

Novel asymmetric photodimerization reaction of coumarin derivatives bearing a chiral 2-oxazolidinone auxiliary

A novel asymmetric photodimerization reaction of coumarin derivatives bearing the (S)-4-benzyl-2-oxazolidinone auxiliary provides only the syn-head-to-tail (syn-HT) dimer with moderate diastereoselectivity (up to 75 : 25). The mechanism of complete syn-HT selectivity and moderate diastereoselectivity is proposed based on the result of density functional theory (DFT) calculation. The benzyl group of the (S)-4-benzyl-2-oxazolidinone auxiliary in combination with a Lewis acid exerts effective diastereofacial shielding of the reaction site.

Visible light photocatalytic asymmetric synthesis of pyrrolo[1,2-a]indoles via intermolecular [3+2] cycloaddition

The asymmetric synthesis of pyrrolo[1,2-a]indoles is developed through a [3+2] cycloaddition between silyl-indole derivatives and α,β-unsaturated N-acyl oxazolidinones by merging photocatalysis and Lewis acid catalysis.

Cu(i)-catalyzed Michael addition of ketiminoesters to β-trifluoromethyl β,β-disubstituted enones: rapid access to 1-pyrrolines bearing a quaternary all-carbon stereocenter

Chiral pyrrolines with two contiguous stereocenters, including a trifluoromethylated quaternary all-carbon stereocenter, were provided from β-trifluoromethyl β,β-disubstituted enones and glycine ketoimino ester.

An MEDT study of the carbenoid-type [3 + 2] cycloaddition reactions of nitrile ylides with electron-deficient chiral oxazolidinones

A combination of the bonding evolution theory and non-covalent interactions analyses of TSs makes it possible to characterise both the carbenoid-type 32CA reaction between a NY and a chiral oxazolidinone, and the diastereoselectivity experimentally observed.

Synthesis of spiro[indane-1,3-dione-1-pyrrolines] via copper-catalyzed heteroannulation of ketoxime acetates with 2-arylideneindane-1,3-diones

A cuprous cyanide-catalyzed heteroannulation reaction of 2-arylideneindane-1,3-dione with ketoxime acetates has been developed for the synthesis of novel spiro[indane-1,3-dione-1-pyrrolines] through the cleavage of N–O and C–H bonds and formation of C–C and C–N bonds.

A highly stereoselective Diels-Alder cycloaddition of enones with chiral cyclic 2-amidodienes derived from allenamides

Lewis acid promoted Diels-Alder cycloadditions of a series of de novo chiral cyclic 2-amidodienes are described. These cyclic 2-amidodienes are derived from chiral α-allyl allenamides via a sequence of E-selective 1,3-H shift and 6π-electron pericyclic ring closure. With enones serving as effective dienophiles, these cycloadditions can be highly diastereoselective depending upon the chiral amide substituent, thereby representing a facile entry to optically enriched [2.2.2]bicyclic manifolds.

Catalytic enantioselective 1,3-dipolar cycloaddition of C,N-cyclic azomethine imines with alpha,beta-unsaturated aldehydes

A yet-unexploited class of azomethine imines, C,N-cyclic azomethine imines, could be successfully employed in highly enantioselective 1,3-dipolar cycloaddition with enals catalyzed by titanium-BINOLate to give pharmaceutically attractive tetrahydroisoquinoline and piperidine motifs.

Stereoselectivity in Oxyallyl-Furan 4+3 Cycloadditions: Control of Intermediate Conformations and Dispersive Stabilisation with Evans' Oxazolidinones

Chiral oxazolidinones were previously thought to control cycloaddition stereoselectivity by steric crowding of one face of the substrate. We have discovered that in 4+3 cycloaddition reactions of oxallyls, the stereoinduction is caused instead by stabilising CH-π interactions that lead to reaction at the more crowded face of the oxazolidinone. Density functional theory calculations on the 4+3 cycloadditions of oxazolidinone-substituted oxyallyls with furans establish unexpected transition state conformations and a new explanation of selectivity.