Construction Of Pyranoquinoline Derivatives Through Annulation Reactions Catalyzed By Rhodium

3,4-Dihydro-pyrano[4,3-b]quinolin-1-one is an important alkaloid extracted from the Rutaceae family and possesses a wide range of pharmacological and biological activities.However,lack of the efficient method to build such core limits its widespread applications.Traditional procedures suffer from narrow scope,low efficiency,difficultly available starting materials and harsh reaction conditions.Therefore,it is highly desirable to develop a highly efficient and simple synthetic method to build such core.Herein,we disclose the first example of Rh(III)-catalyzed redox-neutral [4+2] annulation of 3-carboxyquinolines with internal alkynes,synthesis of a series of 3,4-dihydro-pyrano[4,3-b]quinolin-1-ones in one-pot manner in a yield of up to 95%(Scheme 1).Additionally,most diastereomers can be separated,which provide the possibility of asymmetric synthesis.Exploration of reaction mechanism reveal that protonation metathesis process is involved rather than reduction elimination,the valent state of the rhodium center always is kept as Rh(III).Remarkably,this work is the first redox-neutral [4+2] annulation of carboxylic acid and alkynes as-C-C-unit for construct non-conjugated ethylarenes based on the C-H activation.These current protocol features external oxidant free,high atom-economy,good functional group compatibility and excellent regioselectivity.According to the experimental results and related literature,a plausible pathway was proposed as showed in(Scheme 2).