In this account we give an overview of the synthesis and properties of heterocyclic [8]circulenes. Much of the interest in studying heterocyclic [8]circulenes stems from the planar cyclooctatetraene core often contained in these compounds, which in principle is antiaromatic. We start with a short introduction to the hydrocarbon [n]circulenes and proceed to describe the synthetic chemistry involved in creating tetraoxa[8]circulenes, with particular focus on the acid-mediated oligomerization of benzo-or naphthoquinones, resulting in some simple rules for predicting the outcome of the oligomerization reactions. These rules have guided the synthetic strategies for the preparation of azatrioxa[8]circulenes and diazadioxa[8]circulenes, which will be described in separate sections of this account. More traditional synthetic strategies have been applied in the preparation of octathia[8]circulene, tetrathiatetraselena[8]circulene, and a number of other heterocyclic [8]circulenes, and these synthetic efforts will be highlighted. Finally, a section describing structures that are closely related to the heterocyclic [8]circulenes will be presented, and at the end we will comment on the extensive theoretical work regarding the question of aromaticity/antiaromaticity of the central cyclooctatetraene of heterocyclic[8]circulenes. 1 Introduction 2 Synthesis of [n]circulenes 2.1 [4]Circulene 2.2 [5]Circulene 2.3 [6]Circulene 2.4 [7]Circulene 2.5 [8]Circulene 3 Synthesis of Tetraoxa[8]circulenes: A Historical Perspective 4 Synthesis of Azatrioxa[8]circulenes 5 Synthesis of Diazadioxa[8]circulenes 6 Synthesis of Other Heterocyclic [8]Circulenes 7 Synthesis of Structurally Related Compounds 8 Hetero[8]circulenes and Related Compounds as Tools to Study Aromaticity and Antiaromaticity 9 Conclusion and Outlook

Insights to the subtle reactivity patterns of hydroxy-substituted carbazoles allows the precise synthesis of unsymmetrical azatrioxa[8]circulenes by the reaction of N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with two different 1,4-benzoquinones in the presence of an oxidant (chloranil) and a Lewis acid (BF3OEt2). The unique synthetic control obtained originates from the selectivity obtained upon reacting N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with an electron-rich benzoquinone to give first the C-C bond formation and then subsequently the dibenzofuran formation with high regioselectivity. Herein the first synthesis of unsymmetrical antiaromatic azatrioxa[8]circulenes and the full characterization using NMR spectroscopy, optical spectroscopy, electrochemistry, computational techniques and single crystal X-ray crystallography is reported. The controlled stepwise condensation of N-benzyl-2,7-di-tert-butyl-3,6-dihydroxycarbazole with two different 1,4-benzoquinones gives selectively the unsymmetrical azatrioxa[8]circulenes.

We have efficiently synthesized 36 arylopeptoid dimers with ortho‐, meta‐, and para‐substituted aromatic backbones and tert‐butyl or phenyl side chains. The dimers were synthesized by using a “submonomer method” on solid phase, by applying a simplified common set of reaction conditions. X‐ray crystallographic analysis of two of these dimers disclosed that the tert‐butyl side chain invokes a cis amide conformation with a comparatively more closely packed structure of the surrounding aromatic backbone while the phenyl side chain results in a trans amide conformation with a more open, extended structure of the surrounding aromatic backbone.

Synthesis of Heterocyclic [8]Circulenes and Related Structures -[68 refs. + subrefs.]. -(HENSEL, T.; ANDERSEN, N. N.; PLESNER, M.; PITTELKOW*, M.; Synlett 27 (2016) 4, 498-525, http://dx.doi.org/10.1055/s-0035-1560524 ; Dep. Chem., Univ. Copenhagen, DK-2100 Copenhagen, Den.; Eng.) -Y. Steudel