Tandem Processes inC-Aryl Ketenes and Ketenimines Triggered by [1,5]-Hydride-Like Migration of an Acetalic Hydrogen Atom

Stereoselective synthesis of 6/7/6-fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder reaction sequence

A highly stereoselective synthesis of fused heterocycles with multiple stereocenters via an internal redox reaction/inverse electron-demand hetero-Diels-Alder (IEDHDA) reaction sequence is described. The present reaction sequence has three interesting features: (1) complete control of two potentially competitive processes, i.e., hetero-Diels-Alder reaction and [1,5]-hydride shift; (2) one-shot construction of the complicated 6/7/6-fused heterocyclic structure having multiple stereocenters; and (3) high control of its stereoselectivity. When alkenylidene barbiturates with an allyl benzyl ether moiety were treated with a catalytic amount of Sc(OTf)3 and 2,2'-bipyridine, the internal redox reaction/IEDHDA reaction proceeded successively to afford 6/7/6-fused heterocycles in good chemical yields with good to excellent diastereoselectivities.

Divergent synthesis of multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence

We report an effective synthetic route to multi-substituted phenanthrenes via an internal redox reaction/ring expansion sequence. The interesting feature of the present system is that it allows for the divergent synthesis of the target skeleton depending on the selected Lewis acid catalyst. When benzylidene malonates with a cyclic structure at the ortho-position were treated with BF3·OEt2, three sequential processes (internal redox reaction/elimination of the alkoxy group/ring expansion) proceeded to give phenanthrene derivatives in which the alkoxycarbonyl (CO2R) group and the alkyl (R) group were in close proximity to each other, in good chemical yields. In sharp contrast, treatment with Bi(OTf)3 exclusively led to the formation of another type of phenanthrene, whose R group was positioned distal to the CO2R group.

Synthesis of Polysubstituted Naphthalenes by a Hydride Shift Mediated C-H Bond Functionalization/Aromatization Sequence

A synthetic strategy for forming multisubstituted naphthalenes based on hydride shift mediated C(sp3)-H bond functionalization was developed. This strategy consists of three successive transformations: (1) an intramolecular hydride shift mediated C(sp3)-H bond functionalization; (2) a decarboxylative fragmentation; and (3) an oxidation reaction. When benzylidene malonates having a 2-alkoxyethyl group at the ortho position were treated with a catalytic amount of Al(OTf)3, the hydride shift/cyclization reaction proceeded smoothly to afford tetralin derivatives in good chemical yields. The resulting tetralins were easily converted into naphthalenes by exposing them to modified Krapcho decarboxylation reaction conditions (LiCl, DMSO, and heating under an O2 atmosphere). The one-pot operation of these two reactions was also realized.

Unmasking the elusive 1,4-diazabutatrienes: the stabilizing role of the N-substituents

Challenging 1,4-diazabutatrienes are computed to be more stabilized by substituents with either good π-donor or σ-acceptor capability.

Expeditious Synthesis of Multisubstituted Quinolinone Derivatives Based on Ring Recombination Strategy

We achieved a concise construction of 3-substituted quinolinone derivatives based on a ring recombination strategy. In this process, seven transformations involving two types of cyclization proceeded in one pot to afford various quinolinone derivatives in good to excellent chemical yields (up to 98%).

Diastereoselective Synthesis of CF3-Substituted Spiroisochromans by [1,5]-Hydride Shift/Cyclization/Intramolecular Friedel-Crafts Reaction Sequence

Developed herein is a diastereoselective synthesis of CF₃-substituted spiroisochromans via C(sp³)-H bond functionalization involving sequential transformations ([1,5]-hydride shift/cyclization/elimination of MeOH/intramolecular Friedel-Crafts reaction).

Highly diastereoselective synthesis of tricyclic fused-pyrans by sequential hydride shift mediated double C(sp3)-H bond functionalization

Described herein is the Brønsted acid-catalyzed double C(sp3)-H bond functionalization of alkyl phenethyl ether derivatives. In this process, a [1,5]-[1,5]-hydride shift occurred successively to afford tricyclic fused pyran derivatives in excellent chemical yields with excellent diastereoselectivities (up to >20 : 1). The key to achieve this reaction was the introduction of two methyl groups at the benzylic position, which was effective in both hydride shift processes: (1) the Thorpe-Ingold effect for the first hydride shift and (2) conformational control in the second hydride shift.

Construction of 1,3-Dithio-Substituted Tetralins by [1,5]-Alkylthio Group Transfer Mediated Skeletal Rearrangement

A novel skeletal rearrangement involving a [1,5]-alkylthio group transfer/cyclization sequence is described. Treatment of benzylidene malonates having a thioketal moiety at the homobenzyl position with a catalytic amount of Sc(OTf)₃ afforded alkylthio group rearranged adducts in good chemical yields. Detailed investigation of the reaction mechanism revealed that an intramolecular conjugate addition/ring opening sequence (not through-space transfer) is the key to achieving this reaction.

Accessing polysubstituted oxazolidines, pyrrolidines and imidazolidines by regioselective [3 + 2] annulations of ketenimines with donor–acceptor oxiranes and aziridines

Diverse five-membered rings result from formal [3 + 2] cycloadditions of ketenimines with carbonyl and azomethine ylides.

Expeditious synthesis of multisubstituted indoles via multiple hydrogen transfers

Expeditious synthesis of multisubstituted indoles was achieved by multiple Lewis acid promoted hydrogen transfers.

Divergent synthesis of CF3-substituted polycyclic skeletons based on control of activation site of acid catalysts

We report a divergent synthesis of CF₃-substituted fused skeletons based on precise control of the activation site through the selection of acid catalysts.

Hf(OTf)4-Catalyzed highly diastereoselective synthesis of 1,3-disubstituted tetralin derivatives via benzylic C(sp3)–H bond functionalization

A highly diastereoselective synthesis of 1,3-disubstituted tetralin derivatives was achieved via Hf(OTf)₄-catalyzed benzylic C(sp³)–H bond functionalization.

Tandem processes promoted by a hydrogen shift in 6-arylfulvenes bearing acetalic units at ortho position: a combined experimental and computational study

6-Phenylfulvenes bearing (1,3-dioxolan or dioxan)-2-yl substituents at ortho position convert into mixtures of 4- and 9-(hydroxy)alkoxy-substituted benz[f]indenes as result of cascade processes initiated by a thermally activated hydrogen shift. Structurally related fulvenes with non-cyclic acetalic units afforded mixtures of 4- and 9-alkoxybenz[f]indenes under similar thermal conditions. Mechanistic paths promoted by an initial [1,4]-, [1,5]-, [1,7]- or [1,9]-H shift are conceivable for explaining these conversions. Deuterium labelling experiments exclude the [1,4]-hydride shift as the first step. A computational study scrutinized the reaction channels of these tandem conversions starting by [1,5]-, [1,7]- and [1,9]-H shifts, revealing that this first step is the rate-determining one and that the [1,9]-H shift is the one with the lowest energy barrier.

Lewis acid catalyzed [3 + 2] annulation of ketenimines with donor–acceptor cyclopropanes: an approach to 2-alkylidenepyrrolidine derivatives

C,C,N-Trisubstituted ketenimines undergo [3 + 2] annulation reactions via its CN bond with donor–acceptor cyclopropanes triggered by Sc(OTf)₃ to yield a diversity of 2-alkylidene pyrrolidines.