Enantioselective Au-Catalyzed Synthesis of Thia[5]- and Thia[6]helicenes and Their Transformation into Bowl-shaped Pleiadenes

A series of helically shaped benzo[b]chryseno[4,3-d]thiophenes, naphtho[1,2-b]phenanthro[4,3-d]thiophenes, and chryseno[3,4-b]naphtho[1,2-d]thiophenes is synthesized via a highly enantioselective Au-catalyzed intramolecular alkyne hydroarylation reaction. The inversion barriers of the structures obtained are determined both theoretically and experimentally, and their chiroptical properties are reported. Preliminary studies on the post-synthetic functionalization of these thiahelicenes and their transformation into azahelicenes are also presented. In addition, a straightforward one-step protocol is developed, which wraps the initially obtained chryseno[3,4-b]naphtho[1,2-d]thiophenes into bowl-shaped pleiadene derivatives without erosion of the enantiopurity. The number of structurally related products that are obtained with high enantioselectivity enables the establishment of comprehensive correlations between the structure and conformational stability or (chir)optical properties.

Room Temperature C-H Arylation of Benzofurans by Aryl Iodides

A robust method of room temperature direct arylation for benzofuran is reported. This discovery allows for mild arylation by commercially available aryl iodides with complete C-2 regioselectivity and tolerates a range of functional groups, including heat sensitive groups. Mechanistically, a Heck-type oxyarylation product from a direct arylation process is reported as a key piece of evidence for a carbopalladation intermediate.

Synthesis of [6,6,m]-Tricyclic Compounds via [4+2] Cycloaddition with Au or Cu Catalyst

We synthesized [6,6,6]- and [6,6,7]-tricyclic compounds via intramolecular [4+2] cycloaddition by gold or copper catalysts. Substrates for cyclization were prepared by coupling reactions between eight types of diyne and four types of aromatic moieties. We have successfully synthesized eleven tricyclic compounds.

Aromatic Metamorphosis of Thiophenes by Means of Desulfurative Dilithiation

A new mode of aromatic metamorphosis has been developed, which allows thiophenes and their benzo-fused derivatives to be converted to a variety of exotic heteroles. This transformation involves 1) the efficient generation of key 1,4-dianions by means of desulfurative dilithiation with lithium powder and 2) the subsequent trapping of the dianions with heteroatom electrophiles in a one-pot manner. Via the desulfurative dilithiation, the sulfur atoms of thiophenes are replaced also with a carbon-carbon double bond or a 1,2-phenylene for the construction of benzene rings.

Asymmetric systematic synthesis, structures, and (chir)optical properties of a series of dihetero[8]helicenes

A series of dihetero[8]helicenes have been systematically synthesized in enantiomerically enriched forms by utilizing the characteristic transformations of the organosulfur functionality. The synthetic route begins with assembling a ternaphthyl common synthetic intermediate from 2-naphthol and bissulfinylnaphthalene through an extended Pummerer reaction followed by facile multi-gram-scale resolution. The subsequent cyclization reactions into dioxa- and dithia[8]helicenes take place with excellent axial-to-helical chirality conversion. Dithia[8]helicene is further transformed into the nitrogen and the carbon analogs by replacing the two endocyclic sulfur atoms via SNAr-based skeletal reconstruction. The efficient systematic synthesis has enabled comprehensive evaluation of physical properties, which has clarified the effect of the endocyclic atoms on their structures and (chir)optical properties as well as the unexpected conformational stability of the common helical framework.

Cut and sew: benzofuran-ring-opening enabled cyclopentenone ring formation

A facile approach to the fully substituted cyclopentenones involving an unprecedented benzofuran-ring-opening is described. The cleavage of a benzofuran endocyclic C2-O bond proceeded smoothly in the absence of any transition metal catalyst or highly reactive organometallic reagent. Such benzofuran-ring-opening is delicately incorporated into an acid-catalyzed cascade process, orchestrating a novel synthetic strategy for complex cyclopentenones with excellent yields and diastereoselectivities.

New reductive rearrangement of N-arylindoles triggered by the Grubbs-Stoltz reagent Et3SiH/KO t Bu

N-Arylindoles are transformed into dihydroacridines in a new type of rearrangement, through heating with triethylsilane and potassium tert-butoxide. Studies indicate that the pathway involves (i) the formation of indole radical anions followed by fragmentation of the indole C2–N bond, and (ii) a ring-closing reaction that follows a potassium-ion dependent hydrogen atom transfer step. Unexpected behaviors of ‘radical-trap’ substrates prove very helpful in framing the proposed mechanism.

N-Arylindoles are transformed into dihydroacridines in a new type of rearrangement, through heating with triethylsilane and potassium tert-butoxide.

Synthesis of N-Alkyl and N-H-Carbazoles through SN Ar-Based Aminations of Dibenzothiophene Dioxides

Alkyl amines have become available for the synthesis of diverse N-alkyl carbazoles through twofold S_N Ar aminations of dibenzothiophene dioxides by using alkali metal bases. Of particular importance is the choice of counter cations on alkali metal bases, that is, i) the use of Li base for the efficient intermolecular reaction and ii) the sequential addition of heavier alkali metal bases (Na, K, or Cs) to promote intramolecular cyclization in a one-pot manner. This protocol also enables the cascade synthesis of N-H-carbazoles by using 2-phenylethylamine by removal of the 2-phenethyl group from N-(2-phenethyl) carbazoles in a single operation.