Synthesis of Structurally and Stereochemically Diverse Tetrahydropyran Structures via DDQ-Mediated Oxidative Carbon‒Hydrogen Bond Activation

摘要

The 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)-mediated oxidative carbon¨Chydrogen bond cleavage of allylic ethers has been studied. The generated ¦Á,¦Â-unsaturated oxocarbenium ions can be captured by appended enol acetate nucleophiles to stereoselectively provide cis-2,6-disubstituted tetrahydropyrones. Alkenes with a wide assortment of substitution patterns undergo oxidative cyclizations efficiently, and commonly encountered functional groups on either side of the ether linkage are well tolerated.The scope of this method has successfully been expanded to (silyl)allylic and propargylicethers. The generated vinylsilane-substituted tetrahydropyrans are versatile precursors for a wide range of functional group interconversions and stereocontrolled additions. The cyclization of(silyl)allylic ethers proceeds efficiently to generate cis-2,6-disubstituted tetrahydropyrones with excellent stereocontrol, and therefore is preferable for target-oriented syntheses. The cyclization of propargylic ethers results in a mixture of cis- and trans-2,6-disubstituted tetrahydropyrones, and it is applicable in diversity-oriented syntheses.Two models of the geometries of 1,1-disubstituted oxocarbenium ions (A) and the conformations of oxocarbenium ions that contain a tertiary stereocenter (B) have been designed. Both models have been applied to highly stereoselective syntheses of tetrahydropyrans containing tertiary ethers.DDQ-catalyzed oxidative cyclizations for tetrahydropyran synthesis have been achievedby using MnO2 as an inexpensive and environmentally benign terminal oxidant. This catalytic system is also applicable to other commonly encountered DDQ-mediated reactions, such as PMB ether cleavages and dehydrogenations. The products are quite easy to purify, and the yields are comparable with the corresponding reactions using stoichiometric DDQ.