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Chiral NHC-Catalyzed Annulations of Ynals and Enals
Z.-Q. Zhu, X.-L. Zheng, N.-F. Jiang, X. Wan, J.-C. Xiao*
Shanghai Institute of Organic Chemistry and Changsha University of Science and Technology, P. R. of China
19 August 2011 (online)
Xiao and co-workers describe chiral N-heterocyclic carbene (NHC) catalyzed annulations of ynals and enals with 1,3-dicarbonyl compounds. The nucleophilic Breslow intermediates starting from enals or ynals afforded α,β-unsaturated acylazoliums, and subsequent annulation led to the formation of 3,4-dihydropyranones. Molecular sieves were crucial to improve yields and enantioselectivities. In the case of enal annulation, 3,3′,5,5′-tetra-tert-butyldiphenoquinone was added to generate the critical unsaturated acylazolium salt and the desired products were also obtained in high yields and enantioselectivities.