Dearomative spirocyclization via visible-light-induced reductive hydroarylation of non-activated arenes

Visible-light-induced dearomative 1,4-carbamoylpyridinylation of nonactivated naphthalenes

A visible-light-induced dearomative 1,4-carbamoylpyridinylation of nonactivated naphthalenes is described. The protocol provides rapid access to a series of pyridinylated spiro 1,2-dihydronaphthalenes in moderate yields by using naphthyl-substituted oxamic acids and 4-cyanopyridines as substrates through radical-radical cross-coupling followed by base-mediated alkene tautomerization. In addition, this method enabled late-stage functionalization of several drug derivatives, demonstrating the practical utility of this reaction.

Investigation of Dearomatizing Spirocyclizations and Spirocycle Functionalization En Route to Spirocalcaridines A and B-Some Trials and Tribulations

Spirocalcaridines A and B are among the most challenging members of the marine invertebrate-derived Leucetta alkaloids. Approaches to the construction and elaboration of the highly compact spirocyclic core are described. The synthesis of tricyclic guanidine via tandem oxidative amination dearomatizing spirocyclization (TOADS) using hypervalent iodine set the stage for total synthesis via the migration of the C4/C8 double bond to the C4/C5 position, followed by oxidation. The undesired but not surprising propensity of the spirocyclic cyclohexadienone to undergo rearrangement to the phenol hindered the desired olefin migration. Furthermore, initial efforts to install the oxidation sequentially, first at C5 and then at C4 in the complete carbon skeleton, were fraught with unforeseen challenges and unusual outcomes. In addition, the scope and limitations of hypervalent iodine-mediated tandem oxidative dearomatizing spirocyclization on various substrates were explored. Urethanes and thiourethanes underwent spirocyclization with an excellent yield, whereas the reaction with allylic substrates and species lacking the p-methoxy substituent did not proceed. Attempts to prepare other guanidine precursors are briefly discussed.

CO2-promoted photocatalytic aryl migration from nitrogen to carbon for switchable transformation of N-arylpropiolamides

Photocatalytic N-to-C aryl migration allows for quick construction of highly useful amide derivatives from readily available compounds. By developing the reactions of sodium sulfinates with the N-aryl-propiolamides, we herein demonstrate that the CO2-promoted visible-light-induced method enables a large variety of aryl groups on nitrogen atoms of the N-arylamides to undergo efficient aryl migration from N atom to C atom to synthesize tetra- and tri-substituted alkenyl amides selectively. 1,4-N-to-C aryl migration is a key step in this transformation which is achieved through photocatalytic radical-polar crossover pathway. The protocol exhibits the remarkably tolerant of the electronic properties of the migrating aryl substituent, as both electron-rich and -poor arenes are compatible with the migration process. As a result, this protocol features with a broad substrate scope, as demonstrated by more than 90 examples including complex bioactive compounds. Notably, abundant, nontoxic and low-cost CO2 acted as an essential and irreplaceable additive to enable the tetra- and tri-substituted alkenyl amides to be synthesized with excellent selectivity.

Photochemical Divergent Ring-Closing Metathesis of 1,7-Enynes: Efficient Synthesis of Spirocyclic Quinolin-2-ones

A photocatalytic method for the ring-closing 1,7-enyne metathesis using the α-amino radical as an alkene deconstruction auxiliary is present. Preliminary mechanistic studies suggest that intramolecular 1,5-hydrogen atom transfer is the key to the generation and β-scission of the α-amino radical, while the dearomatization of arenes and ring opening of cyclopropanes are the key to construct spirocyclic quinolin-2-ones. This approach highlights the potential of ring-closing 1,7-enyne metathesis, providing a green, efficient, and step-economical way for the synthesis of spirocyclic quinolin-2-ones.

Recent Advances in Dearomative Partial Reduction of Benzenoid Arenes

Dearomative partial reduction is an extraordinary approach for transforming benzenoid arenes and has been well-known for many decades, as exemplified by the dehydrogenation of Birch reduction and the hydroarylation of Crich addition. Despite its remarkable importance in synthesis, this field has experienced slow progress over the last half-century. However, a revival has been observed with the recent introduction of electrochemical and photochemical methods. In this Minireview, we summarize the recent advancements in dearomative partial reduction of benzenoid arenes, including dihydrogenation, hydroalkylation, arylation, alkenylation, amination, borylation and others. Further, the intriguing utilization of dearomative partial reduction in the synthesis of natural products is also emphasized. It is anticipated that this Minireview will stimulate further progress in arene dearomative transformations.

Visible-Light-Promoted Radical Cascade Cyclization of 2-Vinyl Benzimidazoles: Access to Benzo[4,5]imidazo[1,2-b]isoquinolin- 11(6H)-ones

A visible-light-enabled photoredox radical cascade cyclization of 2-vinyl benzimidazole derivatives is developed. This chemistry is applicable to a wide range of N-aroyl 2-vinyl benzimidazoles as acceptors, and halo compounds, including alkyl halides, acyl chlorides and sulfonyl chlorides, as radical precursors. The Langlois reagent also serves as an effective partner in this photocatalytic oxidative cascade process. This protocol provides a robust alternative for rendering highly functionalized benzo[4,5]imidazo[1,2-b]isoquinolin-11(6H)-ones.

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.