Intramolecular Alder-ene Reaction Of Cyclopropene

Alder-ene reaction takes place through a six membered ring transition state of ene having allyl hydrogen donor and enophile with various unsaturated bonds.Because it is a pericyclic reaction,the conversion of each bond takes place synchronously,and the product has good selectivity.Generally,the transition state energy of product is the lowest.However,this reaction needs high reaction temperature to fracture hydrocarbon bond,which greatly limits the development of this reaction.With the development of hyperactive enophile and the application of lewis acid and transition metal catalysts,the reaction temperature has been greatly reduced and the selectivity can be controlled.Cyclopropene is the smallest unsaturated cyclic compound in nature.Due to its high ring tension and high reactivity,it can undergo many types of transformation.In 2018,our research group published an article on the synthesis of azaspirocyclic compounds by using alkenyl azide to produce azacyclopropene intermediates,and explored the factors affecting the regioselectivity and diastereoselectivity of products.On this basis,we have successfully synthesized spirocyclic compounds with complex structure by Alder-ene reaction using cyclopropene instead of azacyclopropene.In this paper,the applicability of substrates for this reaction was explored.All kinds of non-cyclic olefins,exo-cyclic olefins and endo-cyclic olefins can participate in this reaction as ene.We not only get azaspiro products,but also get all-carbon spiro products and multi ring products.The reaction can take place on cis allyl hydrogen with high regioselectivity,and the product has a single trans diastereoselective.We also applied computational chemistry to explore the reaction mechanism.By calculating the energy of various transition states of the substrate,the transition state with the lowest energy can be obtained,and the most reasonable reaction path in theory can be determined.The reasons of regioselectivity and diastereoselectivity were explained more clearly by comparing the theoretical results with the experimental results.