Study On Novel Stereoselective Ring Opening Reactions Of Bicyclobutanones

Cycloaddition reactions are among the most important reactions in organic synthesis. Because of their high stereoselectivity and regioselectivity, easy elaboration of the products, this kind of reactions has been widely used in the synthesis of a diverse range of natural products and other targets. The theory of orbital symmetry conversation developed by Woodward and others are proved so useful in explaining many processes of the reaction. Since it was discovered hundred years ago, cycloaddition reactions have been one of the research focuses because of the inherent high efficiency.Cycloaddition reaction studied in this thesis mainly includes [π2s+π2a] cycloaddition of ketenes and cycloreversion reactions.Ketene is an active synthon, which can react with various unsaturated compounds, and always afford stereoselective and regiodelective products. By means of [π2s+π2a] ketene cycloaddition, we could conveniently prepare the required cyclobutanones, which were used as synthons in organic synthesis. Cyclobutanones undergo a diverse range of rearrangement and ring opening reactions because of the ring strain in the four-membered ring. Three topics are addressed, which are:1. We studied the anion-accelerated [2+2] cycloreversion reaction. Triggered by an in situ alkylation with alkyl or aryl lithium reagents in the presence of catalytic amounts of DMPU or HMPT as Li-chelating agents, the ketene adducts4,5and6underwent cycloreversion to yield the functionalized homo-polyene ketones10,12and13with excellent stereoselectivity. The (Z)-configured double bond of the starting cyclopentenyl unit retained its configuration with high selectivity. And the configuration of C1-C7also retained its configuration. Thus, we have uncovered an efficient anion-accelerated metathetic cycloreversion giving rise to linear polyene ketones in moderate yields with high stereoselectivity and offered a fascinating two-step route to olfactory useful compounds.2. We studied base-promoted ring opening reaction of dialkyl substituented bicyclobutanones. Treating cyclobutanones with alkoxides, we found a base-promoted ring cleavage and established a novel method for the preparation of cyclopentenecarboxylates. Under these conditions, the products are epimerized resulting in the formation of a mixture of epimers. In a few cases, the sequence affords stereoselective products by employing the sterically hindered base. By investigation on its scope and limitations, we found the protocol works well for the α-vinyl/pheny substituted cyclobutanones to provide cyclopentenecarboxylate23and24in moderated to good yields.3. In the last part of this dissertation, we tested the effects of four-membered ring and the ring strain on the Ireland-Claisen rearrangement and Au(I)-catalyzed reactions. We found the quaternary propargyl substituted cyclobutanol29provides enyne cycloisomerization product31with Au(I). For the Ireland-Claisen rearrangement, no prospective results were obtained. This may be attributed to the difficulty in formation of cyclobutane-bearing six-membered transition-state.