Synthesis of Seven-Membered Cross-Conjugated Cyclic Trienes by 8π Electrocyclic Reaction

Synthesis of metalla-dual-azulenes with fluoride ion recognition properties

Azulene-based conjugated systems are of great interests due to their unusual structures and photophysical properties. Incorporation of a transition metal into azulene skeleton presents an intriguing opportunity to combine the dπ-pπ and pπ-pπ conjugated properties. No such metallaazulene skeleton however has been reported to date. Here, we describe our development of an efficient [5 + 2] annulation reaction to rapid construction of a unique metal-containing [5-5-7] scaffold, termed metalla-dual-azulene (MDA), which includes a metallaazulene and a metal-free organic azulene intertwined by sharing the tropylium motif. The two azulene motifs in MDA exhibit distinct reactivities. The azulene motif readily undergoes nucleophilic addition, leading to N-, O- and S-substituted cycloheptanetrienyl species. Demetalation of the metallaazulene moiety occurs when it reacts with nBu4NF, which enables highly selective recognition of fluoride anion and a noticeable color change. The practical [5 + 2] annulation methodology, facile functional-group modification, high and selective fluoride detection make this new π-conjugated polycyclic system very suitable for potential applications in photoelectric and sensing materials.

Recent advances in 8π electrocyclization reactions

Medium-ring systems, which constitute a class of structurally intriguing and biologically important molecules, are present in many natural products and pharmaceuticals. However, the construction of these skeletons tends to be difficult because of the torsional strain of the medium-sized ring, and control of the selectivity is also challenging in these flexible skeletons. Electrocyclization is one of the most straightforward methods to construct medium-sized rings and this process typically proceeds in a stereospecific manner, resulting in the stereo-controlled formation of two neighboring stereocenters. At present, there are few studies on 8π electrocyclization, mainly focusing on the synthesis of small molecules, while the applications in the synthesis of functional materials and biological contexts are rare. This feature article highlights recent advances, from 2000 to 2022, in the 8π electrocyclization reaction. This study is organized into four sections based on the size/composition of the target ring, including the synthesis of aza-seven-membered, cycloheptene, cyclooctene and bicyclo[4,2,0]octane frameworks. We expect that this feature article will provide beneficial guidance for the selective construction of medium-ring skeletons.

Total Synthesis and Structural Revision of the 6,11-Epoxyisodaucane Natural Sesquiterpene Using an Anionic 8π Electrocyclic Reaction

A new synthetic strategy that forms a seven-membered carbocycle using an anionic 8π electrocyclic reaction facilitated the first total synthesis of the 6,11-epoxyisodaucane natural sesquiterpene in 9.0% yield over 10 steps in the longest linear sequence. The misassigned proposed stereochemistry was corrected by the synthesis of both the proposed structure and its C6 epimer. In addition, the 5-7-fused ring system was concisely constructed by tandem decyanation/five-membered-ring formation from an epoxynitrile.

Assembly of 5H-dibenzo[a,d]cycloheptenes by a formal [5 + 2] annulation of ortho-aryl alkynyl benzyl alcohols with arenes

A new synthetic methodology for the synthesis of 5H-dibenzo[a,d]cycloheptenes from ortho-aryl alkynyl benzyl alcohols and arenes via a Tf₂O-mediated formal [5 + 2] annulation reaction has been achieved. From this transformation, structurally diverse 5H-dibenzo[a,d]cycloheptenes were achieved in moderate to good yields. This transformation probably involves an intermolecular Friedel-Crafts-type alkylation and a subsequent intramolecular 7-endo-dig cyclization in one pot, highlighting the high efficiency, regioselectivity, and step-economy of this protocol.

8π Electrocyclic Reaction of Phosphonate Derivatives: Access to Seven-Membered Cross-Conjugated Cyclic Trienes

An anionic 8π electrocyclic reaction of 4-(diethoxyphosphoryl)-1,3,6-heptatriene derivatives was developed. Under the influence of a base, the substrate underwent deprotonation at the C5 position followed by the 8π electrocyclization of the resulting heptatrienyl anion. The subsequent Horner-Wadsworth-Emmons reaction in one pot, following the addition of an aldehyde, resulted in the production of the 3-alkylidene-1,4-cycloheptadiene derivative. The electrocyclic reaction proceeded in a stereospecific manner, resulting in the stereocontrolled formation of two neighboring stereogenic centers.