How To Achieve High Regioselectivity in Barrier-less Nucleophilic Addition to p-Benzynes Generated via Bergman Cyclization of Unsymmetrical Cyclic Azaenediyne?

Functionalized 10-Membered Aza- and Oxaenediynes through the Nicholas Reaction

The scope and limitations of the Nicholas-type cyclization for the synthesis of 10-membered benzothiophene-fused heterocyclic enediynes with different functionalities were investigated. Although the Nicholas cyclization through oxygen could be carried out in the presence of an ester group, the final oxaenediyne was unstable under storage. Among the N-type Nicholas reactions, cyclization via an arenesulfonamide functional group followed by mild Co-deprotection was found to be the most promising, yielding 10-membered azaendiynes in high overall yields. By contrast, the Nicholas cyclization through the acylated nitrogen atom did not give the desired 10-membered cycle. It resulted in the formation of a pyrroline ring, whereas cyclization via an alkylated amino group resulted in a poor yield of the target 10-membered enediyne. The acylated 4-aminobenzenesulfonamide nucleophilic group was found to be the most convenient for the synthesis of functionalized 10-membered enediynes bearing a clickable function, such as a terminal triple bond. All the synthesized cyclic enediynes exhibited moderate activity against lung carcinoma NCI-H460 cells and had a minimal effect on lung epithelial-like WI-26 VA4 cells and are therefore promising compounds in the search for novel antitumor agents that can be converted into conjugates with tumor-targeting ligands.

Nucleophilic Addition of Enolates to 1,4-Dehydrobenzene Diradicals Derived from Enediynes: Synthesis of Functionalized Aromatics

Alkylation of aromatics and formation of a new C-C bond is usually achieved by the electrophilic attack of an activated carbon species on an electron-rich aromatic ring. Herein, we report an alternative method for alkylation of aromatics via nucleophilic addition of enolates of active methylene compounds to 1,4-dehydrobenzene diradicals derived from enediynes cyclodec-1,5-diyne-3-ene, benzo[3,4]-cyclodec-1,5-diyne-3-ene, and cyclohexeno[3,4]-cyclodec-1,5-diyne-3-ene. The benzo-substituted enediyne produces slightly higher yields of alkylation products than do the other two enediynes, but the differences are not substantial. The reaction produces a new C-C bonded aromatic alkylation product, which allows the construction of complex polyfunctional structures in a few steps. Moreover, this reaction provides solely C-arylated products, and no O-arylation products were observed.

Cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene: A Convenient Enediyne

Enediynes are widely studied to understand their cycloaromatization and the trapping of the resulting p-dehydrobenzene diradical. However, few model substrates are known, and they are hard to synthesize and difficult to handle. Herein we report cyclohexeno[3,4]cyclodec-1,5-diyne-3-ene as a convenient model for studying the reactivity of enediynes. It can be easily synthesized from 1,2-diethynylcyclohexene and 1,4-diiodobutane. It is a solid that is stable at room temperature. In solution the p-dehydrobenzene diradical derived from its cycloaromatization can be trapped by nucleophiles. The rate-limiting step is the cyclization, which is slightly slower than that of the parent cyclodec-1,5-diyne-3-ene but faster than that of its benzo analogue, consistent with the distances between the reacting carbon atoms.

Sterically hindering the trajectory of nucleophilic attack towards p-benzynes by a properly oriented hydrogen atom: an approach to achieve regioselectivity

Nucleophilic addition to p-benzynes derived via Bergman cyclization has become a topic of keen interest. Studying the regioselectivity in such addition can reveal important information regarding the parameters controlling such addition. Recently, high regioselectivity has been achieved in nucleophilic addition to a p-benzyne derived from an ortho substituted benzo fused cyclic azaenediyne. Rather than having a freely rotating substitution, a rigid hydrogen atom coming from a suitable naptho fused enediyne and residing in the plane of the p-benzyne ring can offer hindrance to the trajectory of the nucleophile. This can lead to regioselectivity provided the other side remains relatively free of such hindrance. Based on that approach, halide addition to p-benzynes derived from naphtho fused cyclic azaenediynes was studied and a high level of regioselectivity was observed. Steric hindrance to the trajectory of nucleophile by the bay hydrogen was found to be the main cause of such regioselectivity; however, differential electrostatic potential as well as distortions at reactive centres have a minor role in controlling the regioselectivity. The products of such high yielding addition are the halo naphtho tetrahydroisoquinolines.

Regioselective Synthesis of Benzo-Fused Tetrahydroisoquinoline-Based Biaryls through a Tandem One-Pot Halogenation of p-Benzynes from Enediynes and Suzuki-Miyaura Coupling

A regioselective halogenation of p-benzyne derived from a nonaromatic enediyne core via Bergman cyclization and Suzuki-Miyaura coupling of the resulting haloarene in one-pot is disclosed. For the one-pot protocol to work, the reaction conditions were modified compared to an earlier reported procedure ( J. Org. Chem. 2019 , 84 , 2911 - 2921 ) by reducing the amount of lithium iodide and exclusion of pivalic acid. Under these modified conditions, the products, benzo-fused tetrahydroisoquinoline-based biaryl derivatives were obtained in overall high to excellent yields (71-90%).