Researches On Phosphine-Catalyzed (4+n) Cycloaddition Reactions

This thesis focused on the phosphine-catalyzed cycloaddition reactions, and two novel reaction protocols, including phosphine-catalyzed (4+n) annulations and PPh3-catalyzed [4+2] annulations, have been successfully developed.1. Phosphine-catalyzed (4+n) cycloadditions of 2-(acetoxymethyl)buta-2,3-dienoate with 1, n-bis-nucleophiles In general, the related 1,3-zwitterionic intermediates, generated by the interaction of phosphine catalyst and buta-2,3-dienoate, work as nucleophiles to reaction with various of electrophiles, accomplish cycloadditions. On the base of the Umpolung strategy, 2-(acetoxymethyl)buta-2,3-dienoate, which is modified through the introduction of the acetate group as a leaving group into the classical buta-2,3-dienoate subtrate, is proved to be a versatile 1,4-biselectrophile in the presence of phosphine catalyst to realize (4+n) annulations with 1, n-bis-nucleophiles. Furthermore, a reasonable mechanism for this transformation has been proposed on the base of several deuterium-labeling experiments.2. PPh3-catalyzed [4+2] annulations of activated terminal alkynes and Oxodienes A new PPh3-catalyzed [4+2] annalution of activated terminal alkynes and oxodienes has been developed, which allows the efficient construction of highly functionalized 2H-dihydropyrans. The reaction mechanism has been established with two-step sequences, initial PPh3-catalyzed [4+2] annalution which is evidented by the isolation of the intermediacy 4H-pyran and subsequent [1,3]-proton transfer which shows a large isotopic effect. The flexibility for direct synthesis of pyridine derivatives in one-pot way through initial PPh3-catalyzed [4+2] annulations, followed by the addition of ammonium acetate, has also been demonstrated.