Arenethiolate-Catalyzed 1,5-HAT of Aryl Halides: Synthesis of γ-Spirolactams

γ-Spirolactam is a privileged building block that is found in a wide range of natural products and bioactive compounds. Herein, we report an arenethiolate-catalyzed 1,5-HAT of aryl halides to obtain γ-spirolactams through a SET reduction/intramolecular 1,5-HAT/cyclization/HAT process. This protocol features metal-free conditions and a broad substrate scope, furnishing the γ-spirolactams in moderate to excellent yields. Notably, aryl bromides, aryl chlorides and even aryl fluorides are well tolerated in this transformation. A mechanism involving arenethiolate as a SET catalyst is proposed based on DFT calculation.

Copper-catalyzed intramolecular dearomative aza-Wacker reaction of indole

Herein, we describe a copper-catalyzed intramolecular dearomative amination of indoles via a formal aza-Wacker reaction. This protocol provides an efficient method to access aza-polycyclic indoline molecules bearing exocyclic CC bonds in moderate to excellent yields in the presence of molecular oxygen as an oxidant. It is worth noting that indolin-3-ones are achieved when employing C3-non-substituted indoles as substrates.

Regioselective Dearomative Amidoximation of Nonactivated Arenes Enabled by Photohomolytic Cleavage of N-nitrosamides

Dearomative spirocyclization reactions represent a promising means to convert arenes into three-dimensional architectures; however, controlling the regioselectivity of radical dearomatization with nonactivated arenes to afford the spirocyclizative 1,2-difunctionalization other than its kinetically preferred 1,4-difunctionalization is exceptionally challenging. Here we disclose a novel strategy for dearomative 1,2- or 1,4-amidoximation of (hetero)arenes enabled by direct visible-light-induced homolysis of N-NO bonds of nitrosamides, giving rise to various highly regioselective amidoximated spirocycles that previously have been inaccessible or required elaborate synthetic efforts. The mechanism and origins of the observed regioselectivities were investigated by control experiments and density functional theory calculations.

Tandem Synthesis of 2-Azaspiro[4.5]deca-1,6,9-trien-8-ones Based on Tf2O-Promoted Activation of N-(2-Propyn-1-yl) Amides

Structurally novel 2-azaspiro[4.5]deca-1,6,9-trien-8-ones were synthesized from N-(2-propyn-1-yl) amides and 1,3,5-trimethoxybenzenes by a tandem method consisting of a Tf₂O-promoted amide activation and a TfOH-promoted Friedel-Crafts ipso-cyclization. The method offered the first example of using N-(2-propyn-1-yl) amides as substrates in both Tf₂O-promoted secondary amide activation and the synthesis of azaspiro[4.5]deca-6,9-diene-8-ones.

Rh(III)-Catalyzed Oxidative Domino C-H/N-H Annulation: Diarylureas as Arylamine Donors for the Assembly of Indolo[2,1-a]isoquinolines

An efficient and convenient Rh(III)-catalyzed double aryl C(sp2)-H bond and N-H activation and annulation reaction is reported for the synthesis of indolo[2,1-a]isoquinolines in the presence of the Cu(OAc)₂ oxidant under heating conditions. Distinct from previous works with other arylamine donors, one molecule of 1,3-diarylurea can serve as a precursor of two molecules of arylamine in the reaction with diaryl-substituted alkynes.

Highly congested spiro-compounds via photoredox-mediated dearomative annulation cascade

Photo-mediated radical dearomatization involving 5-exo-trig cyclizations has proven to be an important route to accessing spirocyclic compounds, whereas 6-exo-trig spirocyclization has been much less explored. In this work, a dearomative annulation cascade is realized through photoredox-mediated C-O bond activation of aromatic carboxylic acids to produce two kinds of spirocyclic frameworks. Mechanistically, the acyl radical is formed through oxidation of triphenylphosphine and subsequent C-O bond cleavage, followed by a 6-exo-trig cyclization/SET/protonation sequence to generate the spiro-chromanone products in an intramolecular manner. Furthermore, the protocol was extended to more challenging intermolecular tandem sequences consisting of C-O bond cleavage, radical addition to an alkene substrate, and 5-exo-trig cyclization to yield complex spirocyclic lactams.

Visible-light induced dearomatization reactions

Dearomatization reactions provide rapid access to structurally complex three-dimensional molecules from simple aromatic compounds. Plenty of reports have demonstrated their utilities in the synthesis of natural products, medicinal chemistry, and materials science in the last decades. Recently, visible-light mediated photocatalysis has emerged as a powerful tool to promote many kinds of transformations. The dearomatization reactions induced by visible-light have also made significant progress during the past several years. This review provides an overview of visible-light induced dearomatization reactions classified based on the manner in which aromaticity is disrupted.

Electrochemically-promoted synthesis of benzo[b]thiophene-1,1-dioxides via strained quaternary spirocyclization

We report an approach for the synthesis of benzothiophene motifs under electrochemical conditions by the reaction of sulfonhydrazides with internal alkynes. Upon the formation of a quaternary spirocyclization intermediate by the selective ipso-addition instead of an ortho-attack, the S-migration process was rationalized to lead to the products. Computational studies revealed the selectivity and the compatibility of drug molecules showcased the potential application of the protocols.

We report an approach for the synthesis of benzothiophene motifs under electrochemical conditions by the reaction of sulfonhydrazides with internal alkynes.

Visible-light enabled synthesis of cyclopropane-fused indolines via dearomatization of indoles

An efficient synthesis of methylene-unsubstituted cyclopropane-fused indolines via photoredox catalyzed dearomative cyclopropanation of indole derivatives was developed. A broad range of indoles bearing a variety of functional groups were compatible...

Visible-Light Induced C(sp2 )-H Amidation with an Aryl-Alkyl σ-Bond Relocation via Redox-Neutral Radical-Polar Crossover

We report an approach for the intramolecular C(sp² )-H amidation of N-acyloxyamides under photoredox conditions to produce δ-benzolactams with an aryl-alkyl σ-bond relocation. Computational studies on the designed reductive single electron transfer strategy led us to identify N-[3,5-bis(trifluoromethyl)benzoyl] group as the most effective amidyl radical precursor. Upon the formation of an azaspirocyclic radical intermediate by the selective ipso-addition with outcompeting an ortho-attack, radical-polar crossover was then rationalized to lead to the rearomative ring-expansion with preferential C-C bond migration.

Dearomatizing Amination Reactions

Dearomatization reactions allow the direct synthesis of structurally complex sp³ -rich molecules from readily available "flat" precursors. Established dearomatization processes commonly involve the formation of new C-C bonds, whereas methods that enable the introduction of C-N bonds have received less attention. Because of the privileged position of nitrogen in drug discovery, significant recent methodological efforts have been directed towards addressing this deficiency. Consequently, a variety of new processes are now available that allow the direct preparation of sp³ -rich amino-containing building blocks and scaffolds. This review gives an overview of C-N bond forming dearomatization reactions, particularly with respect to scaffold assembly processes. The discussion gives historical context, but the main focus is on selected methods that have been reported recently.

One-Pot Transformation of Furans into 1-Azaspirocyclic Alkaloid Frameworks Induced by Visible Light

High-value 1-azaspirocyclic scaffolds have been made from simple and readily accessible furan precursors in a single operation. The protocol is a one-pot sequence using highly sustainable conditions (oxygen, visible light, and a favored green solvent) that leads to a dramatic increase in molecular complexity. The initial substrates can include functionalities that are suitable for further elaboration; in this way, the pruned polycyclic skeletons of the stemonamine, cylindricine, and lepadiformine natural products were rapidly accessed.

Visible-Light Photoredox Catalyzed Dehydrogenative Synthesis of Allylic Carboxylates from Styrenes

The visible-light photoredox/[Co(III)] cocatalyzed dehydrogenative functionalization of cyclic and acyclic styryl derivatives with carboxylic acids is documented. The methodology enables the chemo- and regioselective allylic functionalization of styryl compounds, leading to allylic carboxylates (32 examples) under stoichiometric acceptorless conditions. Intermolecular as well as intramolecular variants are documented in high yields (up to 82%). A mechanistic rationale is also proposed on the basis of a combined experimental and spectroscopic investigation.

Visible-light induced divergent dearomatization of indole derivatives: controlled access to cyclobutane-fused polycycles and 2-substituted indolines

A visible-light-induced catalytic, divergent dearomative functionalization of indole derivatives is achieved, thereby leading to the formation of cyclobutane-fused polycycles and 2-substituted indolines in a controllable fashion.