Palladium-catalyzed cyclizations of polyenynes. A palladium zipper
作者:Barry M. Trost、Yian Shi
DOI:10.1021/ja00074a008
日期:1993.10
The cycloisomerization of polyenynes with a catalyst derived from Pd(0), acetic acid, and a ligand to polycycles depends upon the juxtaposition of unsaturation. The process involves the stages of initiation (by addition of Pd-H to an acetylene), propagation (by intramolecular carbopalladation), and termination (by beta-hydrogen elimination). With 2-homopropargylated 1,5- and 1,6-dienes, monocyclizations via 5-exo and 6-endo modes dominate with the ratio dependent upon ligand. The 5-exo mode is favored by tri-o-tolylphosphine, whereas triphenylstibine favors the 6-endo mode. On the other hand, a 3-homoallyl-3-homopropargylcyclopentene undergoes smooth bicyclization even when triphenylphosphine is used as ligand. Combining Pd(0)-catalyzed allylic alkylation to make the substrates with Pd(0)-catalyzed cycloisomerization simplifies triquinane and azatriquinane synthesis. A2-allyl-2-homopropargyl-1-methylenecycloalkane array cycloisomerizes to [4.3.3]propellanes and [3.3.3]propellanes. Methallyl alcohol serves as a basic building block to construct acyclic substrates for construction of spirocycles. Both terminal and internal acetylenes serve as suitable initiators provided the proper ligand is employed. With a terminal acetylene as initiator, triphenylphosphine proves satisfactory, but an internal acetylene requires triphenylstibine. High diastereoselectivity may accompany formation of the spirocycle. Increasing the number of double bonds increases the number of rings formed. Substrates bearing 3, 4, 5, 6, and 7 double bonds generate polyspiranes consisting of 3, 4, 5, 6, and 7 rings. The regio- and diastereoselectivity may be understood on the basis of a conformational analysis of the reactive intermediate. This atom economical approach for construction of polycycles can be considered as the equivalent of a palladium zipper in which the pi orbitals are the teeth and the palladium complex is the tab. Closing the zipper stitches the pi bonds into sigma bonds with creation of the polycycles.