| The first chapter of this thesis describes a highly enantioselective DA reaction catalyzed by a salen-Cr(III) complex. A very board range of dienes, including 1-amino-3-siloxybutadienes and 1-aminobutadienes has been affected by this catalyst. The reaction proceeds in high yield, with complete endo selectivity and up to >97% ee. This methodology provides a very powerful asymmetric process for synthesis of six-membered carbon cycles containing a quaternary stereogenic center. The reaction can be easily performed in large scale, allowing gram-scale total syntheses of (+)-tabersonine and (+)-16-methoxytabersonine.; The second chapter discusses the 2nd generation silylsalen-Co(III) catalysts for enantioselective DA reactions of aminobutadienes, as inspired by a systematic survey of catalytic activities of metal-salen complexes and an X-ray structure of a salen-cobalt-benzaldehyde complex. These newly synthesized complexes are the most effective catalysts for the asymmetric DA reaction, achieving the world-record high substrate/catalyst ratio. Detailed discussion also includes some unique features of these silysalen-cobalt complexes that greatly expand the scope of this DA reaction: these catalysts can response to the changes of functional groups and “intelligently” adopt the desired coordination conformation. Application of this methodology has been demonstrated by the total synthesis of (γ)-(−)-lycorane.; Chapter 3 summarizes a highly efficient, Lewis-acid-free hetero Diels-Alder reaction. The reaction can be conveniently carried out at room temperature in the absence of any catalyst or additive, allowing convenient synthesis of dihydropyrones and dihydropyridones. The mild, Lewis-acid-free reaction conditions offer obvious advantages over conventional Lewis acid catalyzed processes. Hydrogen bonding has been proposed to explain the unusual acceleration of this reaction observed for certain solvents.; In chapter 4, A highly enantioselective HDA reaction taking advantage of the hydrogen bonding activation is discussed in details. A TADDOL derivative has been found to catalyze this reaction with excellent enantioselectivity and good yield.; The last chapter of this thesis contains X-ray data of complex 179 and NMR spectra of all the important and new compounds appeared in previous chapters. |
