Catalyst-free And Atom-economic Synthesis Of 1-substituted Carbazoles

Carbazole is an important structural unit of natural products alkaloids and drug fragments,and is a very important nitrogen-containing aromatic heterocyclic compound.Because Carbazole and its derivatives have good biological activity and special fused ring structure,allowing such compounds are widely used in pharmaceuticals,dye and photoelectric materials,and other fields.Carbazole compounds have a bigger conjugate system,special fused ring structure,high fluorescence quantum yield and electronic and charge-transport properties,prompting carbazole compounds are widely used in the photoelectric material field,are one of the hotspot in material field in recent years.Carbazole and its derivatives can be used as a pesticide stabilizers,plant growth regulator and important drug intermediates for their good biological activities including antioxidant,anti-inflammatory,antihistamines,antimicrobial and antitumor.More importantly,Carbazole is also easy to introduce substituents,which is convenient for structural modification.Therefore,the synthesis and application of carbazole and its derivatives have attracted significant attention of chemists.As a result,chemists have developed numerous synthetic strategies to prepare the functionalized carbazole compounds.A number of these approaches for the preparation of carbazoles focus on C–C and/or C–N bond forming reactions including coupling reactions,cyclization reactions,and Diels–Alder reactions,etc.A synthesis strategy is based on amines and its derivatives,which form C–N bond to synthesize carbazole through the arylation of amines.But the strategy requires expensive transition metals,needs to be added to the ligand stable intermediates and higher temperature activated substrate.Recently,indole-to-carbazole cyclization reactions have become an attractive synthetic tool,primarily because indoles are cheap and readily available.However,the use of harsh reaction conditions,expensive transition metals or bronsted-acid and moderate yields are three further drawbacks.With the destruction of ecological environment,more and more people realize the importance of green chemistry in recent years.Chemists advocate chemical methods to reduce the use and production of reagents such as raw materials,catalysts,additives and solvents that are harmful to human health,ecological environment and community safety.Consequently,it is of great significance to develop a concise,efficient and green synthetic route for the preparation of functionalized carbazoles.Therefore,a Diels–Alder reaction of 3-alkenyl indole with conjugated alkynyl ketone is studied in this paper.These reactions proceed in an atom-economic cycloadditional manner without a catalyst and give various substituted 1-acetyl carbazoles in moderate to excellent yields.The methodology has a broad substrate scope under mild reaction conditions,and the cyclization products can be converted into 1-hydroxyl carbazoles with good yields in a simple and efficient one-pot process.