Synthesis Of Substituted Fluorenes By Cascade Allenylation/Electrocyclization/Intramolecular Friedel-crafts Reaction Of 1,3-diene-substituted Propargylic Alcohols

Fluorenes and its derivatives are being extensively used as ligands in organometallic chemistry for their high donor ability and varied coordination modes.In addition,fluorene derivatives are important structural moieties found in various functional materials and biologically active molecules.Although many viable methods for construction of fluorenes have been developed,they usually suffer from the use of expensive transition-metal catalysts or multistep preparation of the starting materials.Thus,the development of a new strategy for the direct synthesis of substituted fluorenes from readily available starting materials represents a valuable but challenging target.A practical way to form allenes is using suitable nucleophiles to trap the allenic carbocation intermediates,which could be generated in situ by the Meyer-Schuster rearrangement of propargylic alcohols.Either Br(?)ster acids or Lewis acids could efficiently catalyze this rearrangement.Using this strategy,a number of substituted allenes were synthesized.Several heterocyclic and carboncyclic compounds were also constructed via a cascade process from propargylic alcohols.In this context,a cascade allenylation/electrocyclization/IFC reaction of 1,3-diene-substituted propargylic alcohols is designed to synthesis the substituted fluorenes.The main contents are as follow:1.The Synthesis of 1,3-diene-substituted propargylic alcohols1,3-diene-substituted propargylic alcohols were easily synthesized in good yield by the aldol reaction of cinnamaldehyde with ketones followed by treated with lithium alkynes.All new compounds were characterised by 1H NMR and 13C NMR.2.The synthesis of fluorene derivativesThe optimum reaction conditions for the cascade Meyer-Schuster rearrangement/electro.cyclization/intramolecular F-C reaction is using chlorobenzene as solvent and TiCl4 as a catalyst under room temperature for 8 h.10 examples were given.All of them were confirmed by 1H NMR,13C NMR and HRMS,and two compounds were further confirmed by X-ray crystallography analysis.Interestingly,it was found that the electronic property of the substituents on the aryl ring of the R1 play significant effect on the reaction regioselectivity and a proposed mechanism for the present cascade reaction is proposed.In.summary,a concise and versatile cascade allenylation/electrocyclization/intramolecular Friedel-Crafts reaction of 1,3-diene-substituted propargylic alcohols has been established.This transformation provide facile and direct strategy for construction of substituted fluorenes from easily accessible starting materials.We also found that the regioselectivity of the cascade reation was depended on the substitued group of the propargylic alcohols.