Efficient Synthesis of a New Structural Phenanthro[9,10,3‘,4‘]indolizidine Starting from Pyrrole

The Scholl Reaction as a Powerful Tool for Synthesis of Curved Polycyclic Aromatics

The past decade has witnessed remarkable success in the synthesis of curved polycyclic aromatics through Scholl reactions which enable oxidative aryl-aryl coupling even in company with the introduction of significant steric strain. These curved polycyclic aromatics are not only unique objects of structural organic chemistry in relation to the nature of aromaticity but also play an important role in bottom-up approaches to precise synthesis of nanocarbons of unique topology. Moreover, they have received considerable attention in the fields of supramolecular chemistry and organic functional materials because of their interesting properties and promising applications. Despite the great success of Scholl reactions in synthesis of curved polycyclic aromatics, the outcome of a newly designed substrate in the Scholl reaction still cannot be predicted in a generic and precise manner largely due to limited understanding on the reaction mechanism and possible rearrangement processes. This review provides an overview of Scholl reactions with a focus on their applications in synthesis of curved polycyclic aromatics with interesting structures and properties and aims to shed light on the key factors that affect Scholl reactions in synthesizing sterically strained polycyclic aromatics.

Rearrangements in Scholl Reaction

Rearrangements in Scholl reaction are mostly serendipitous. The design of molecular precursors is what seems to guide the course of rearrangement. This review consolidates different classes of precursors used in Scholl reaction and their accompanying rearrangements that include aryl migration, migration followed by cyclization and skeletal rearrangements involving ring expansion, ring contraction and both, under the reaction conditions. The attempt in collating heretofore-reported examples in this review is to guide designing appropriate precursors to predictably achieve complex molecular structures or nanographenes or defect-nanographenes via rearrangement.

Intramolecular Dehydrogenative Coupling of 2,3-Diaryl Acrylic Compounds: Access to Substituted Phenanthrenes

A simple, facile, and environmentally benign intramolecular dehydrogenative coupling of various 1,2-diarylethylenes for the synthesis of phenanthrenes in excellent yield has been described. This new methodology uses ceric ammonium nitrate (CAN) as a promoter at room temperature and has been extended to intermolecular synthesis of biaryl compounds. The electron transfer from methoxyarene to cerium leads to cationic radical formation, which further proceeds to intramolecular coupling. Preliminary mechanistic investigation by EPR spectroscopy and density functional theory calculation suggested a similar view.

Preparation of dibenzo[e,g]isoindol-1-ones via Scholl-type oxidative cyclization reactions

A flexible synthesis of dibenzo[e,g]isoindol-1-ones has been developed. Dibenzo[e,g]isoindol-1-ones represent simplified benzenoid analogues of biological indolo[2,3-a]pyrrolo[3,4-c]carbazol-5-ones (indolocarbazoles), compounds that have demonstrated a wide range of biological activity. The synthesis of the title compounds involved tetramic acid sulfonates. Different aryl groups were introduced at C4 of the heterocyclic ring via Suzuki-Miyaura cross-coupling reactions. Finally, mild Scholl-type oxidative cyclizations mediated by phenyliodine(III) bis(trifluoroacetate) (PIFA) converted some of the latter compounds into the corresponding dibenzo[e,g]isoindol-1-ones. A systematic study of the oxidative cyclization revealed the following reactivity trend: 3,4-dimethoxyphenyl ≫ 3-methoxyphenyl > 3,4,5-trimethoxyphenyl > 4-methoxyphenyl ≈ phenyl. Overall, the oxidative cyclization required at least two methoxy groups distributed in the aromatic rings, at least one of which had to be located para to the site of the cyclization.

Access to pyrrole-based heterocyclic compounds via addition of pyrrole to C=C and C=N bonds

A series of metal triflate-catalyzed addition reactions of pyrrole to C=C and C=N bonds have been investigated to access pyrrole-based heterocyclic compounds. The addition of pyrrole to different α,β-unsaturated compounds or N-tosyl imines afforded suitable structures for the construction of [5-5] bicyclic systems or porphyrins, respectively. Intramolecular cyclization reactions were applied for the synthesis of pyrrolizine derivatives. In the other reaction mode, intermolecular cyclization reactions gave A4- and trans-A2B2-meso-substituted porphyrins under mild reaction conditions with low scrambling.

An enantioselective strategy for the total synthesis of (S)-tylophorine via catalytic asymmetric allylation and a one-pot DMAP-promoted isocyanate formation/Lewis acid catalyzed cyclization sequence

A novel total synthesis of (S)-tylophorine is reported, featuring asymmetric allylation and cascade isocyanate formation and cyclization.

Palladium-catalyzed annulation of 2,2'-diiodobiphenyls with alkynes: synthesis and applications of phenanthrenes

A range of phenanthrene derivatives were efficiently synthesized by the palladium-catalyzed annulation of 2,2'-diiodobiphenyls with alkynes. The scope, limitations and regioselectivity of the reaction were investigated. The described method was adopted to synthesize 9,10-dialkylphenanthrenes, sterically overcrowded 4,5-disubstituted phenanthrenes and phenanthrene-based alkaloids. Reactions of highly methoxy-substituted biphenyls with 2-(2-propynyl)pyrrolidine and 2-(2-propynyl)piperidine gave 2-(9-phenanthylmethyl)pyrrolidines and 2-(9-phenanthylmethyl)piperidines, respectively. The products were transformed to phenanthroindolizidine and phenanthroquinolizidine alkaloids by the Pictet-Spengler reaction.