Ring Contraction Reactions of a Non-Benzenoid Aromatic Cation and a Neutral Homoaromatic System into Benzene Derivatives

Aromaticity is one of the most intriguing concepts in organic chemistry. Simple and extended benzenoid aromatic systems have been very well established in undergraduate textbooks, and there are also mentions of non-benzenoid aromatic structures such as cyclopropenium, cyclopentadienide and cycloheptatrienylium (tropylium) ions. However, the structural relationship and the comparison of stabilization energy of such aromatic ions to benzene ring have been rarely studied and remained an underexplored area of advanced organic chemistry research. To contribute some insights into this topic, we focused on the chemical transformation, namely a ring contraction reaction, of the tropylium ion to benzene ring in this work. With an approach combining computational studies with experimental reactions, we also aim to turn this transformation into a synthetically useful tool. Indeed, this work led to the development of a new synthetic protocol, which involved an oxidative ring-contraction of tropylium ion, to formally introduce the phenyl ring onto a range of organic structures. Furthermore, the homoaromatic cycloheptatrienyl precursors of tropylium salts used in these reactions can also be rearranged to valuable benzhydryl or benzyl halides, enriching the synthetic utility of this ring-contraction protocol.

A Comparative Study of Thallium(III) and Iodine(III)-Mediated Ring Contraction Reactions for the Synthesis of Indane

Reported herein, is a comparative study for the synthesis of indane via ring contraction reaction, mediated by iodine(III) and thallium(III). A series of protected 1,2-dihydronaphthalenes were synthesized, and subjected to...

Iodine-promoted synthesis of pyrazoles from 1,3-dicarbonyl compounds and oxamic acid thiohydrazides

A novel approach to 3,4-dicarbonyl-substituted pyrazoles from 1,3-dicarbonyl compounds and oxamic acid thiohydrazides was developed via iodine-promoted cascade imination/halogenation/cyclization/ring contraction reaction.

Attempted Synthesis of Vinca Alkaloids Condensed with Three-Membered Rings

Our successful work for the synthesis of cyclopropanated vinblastine and its derivatives by the Simmons–Smith reaction was followed to build up further three-membered rings into the 14,15-position of the vindoline part of the dimer alkaloid. Halogenated 14,15-cyclopropanovindoline was prepared by reactions with iodoform and bromoform, respectively, in the presence of diethylzinc. Reactions of dichlorocarbene with vindoline resulted in the 10-formyl derivative. Unexpectedly, in the case of the dimer alkaloids vinblastine and vincristine, the rearranged products containing an oxirane ring in the catharanthine part were isolated from the reactions. The attempted epoxidation of vindoline and catharanthine also led to anomalous rearranged products. In the epoxidation reaction of vindoline, an o-quinonoid derivative was obtained, in the course of the epoxidation of catharanthine, a hydroxyindolenine type product and a spiro derivative formed by ring contraction reaction, were isolated. The coupling reaction of vindoline and the spiro derivative obtained in the epoxidation of catharanthine did not result in a bisindole alkaloid. Instead, two surprising vindoline trimers were discovered and characterized by NMR spectroscopy and mass spectrometry.