Migratory cycloisomerization of 1,3-dien-5-ynes conjugated with pseudopeptides in assembly of benzo[7]annulenes

Synthesis of dihydrotropone derivatives using an anionic 8π-electrocyclic reaction

A new method to synthesise functionalised cycloheptatrienes was established using the anionic 8π-electrocyclic reaction of β-ketoester-derived dienyne substrates. The cyclised products were converted to a variety of cycloheptatriene derivatives including tropones. Using this method, a concise, first total synthesis of (-)-orobanone, a natural sesquiterpenoid, was achieved.

Construction of Unique Spiro[dibenzo[a,f]azulene-6,2'-indenes] via Unprecedented Annulation of ortho-C-H Bond of Benzylidene Group

The domino reaction of alkyl and aryl isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C resulted in unique functionalized spiro[dibenzo[a,f]azulene-6,2'-indenes] in good yields, in which the two 2-arylidene-1,3-indanediones acted as different building blocks to construct the polycyclic system. More importantly, the unprecedented anticipation of the ortho-position of benzylidene group to form a novel dibenzo[a,f]azulene ring through a formal [5 + 2] cycloaddition process was first observed. On the other hand, DABCO-promoted reaction of the isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C afforded functionalized spiro[cyclopenta[a]-indene-2,2'-indene] derivatives.

Synthesis of Polysubstituted Pyrimidines through Palladium-Catalyzed Isocyanide Insertion to 2H-Azirines

The domino process of the palladium-catalyzed coupling reaction of isocyanides with 2H-azirine provides various tetrasubstituted pyrimidines via one C-C bond and two C-N bond formations with satisfactory yields. The title compounds are obtained with good functional group tolerance, high atom economy, and broad substrate scopes.

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.

Decarboxylative Cyclization of Proline with o-Alkynylbenzaldehyde through an Unexploited 8π-Electrocyclization: A DFT Study

We employed density functional theory (DFT) methods to investigate the most plausible mechanism of cyclization/ring expansion of proline with o-alkynylbenzaldehyde. This one-pot reaction starts with the in situ formation of azomethine ylide, which can undergo three different reaction pathways to form the final product. Two mechanisms are based on nucleophilic addition and 4π-electrocyclization of the azomethine ylide, and our results indicate that the rate-determining step (RDS) of these two cyclizations are 40.1 and 40.2 kcal/mol, respectively. The third novel pathway relies upon 8π-electrocyclization as the key step of this reaction; interestingly, the RDS of this cyclization is ∼20.6 kcal/mol, which shows this route to be most feasible. Thus, we introduce a novel mechanism for the electrocyclization reaction of conjugated azomethine ylides that can help chemists to design and access a new series of compounds.