Cationic rhodium(I)-catalyzed cyclodimerization of oxabenzonorbornadienes produced naphtho[1,2-b]furan ring systems in a single step with excellent yields and excellent enantioselectivities. The effect of various Rh(I) catalysts, Ag(I) salts, solvents, and phosphine ligands on the yield and enantioselectivity of the reaction was investigated, and the scope and limitations of this reaction with various oxabicyclic alkenes were studied. Similar results were obtained with the azabenzonorbornadiene analogues, providing the corresponding cyclodimerization products in excellent yields and excellent enantioselectivities.

Julia olefination between alpha-halomethyl sulfones and a variety of aldehydes afforded alkenyl halides in good to excellent yields with high E/Z stereoselectivities. Sulfones were readily prepared in two or three steps from commercially available reagents in good yields. Optimization revealed that the nature of the solvent, the base, and the additive were crucial to obtain the desired alkenyl halides.

Fe-catalyzed cross-coupling reactions between a bicyclic alkenyl triflate and Grignard reagents were investigated. Under the optimized reaction conditions, various 2-substituted bicyclic alkenes were synthesized in moderate to excellent yields (52-93%). This method provided an efficient route for the synthesis of 2-substituted bicyclic alkenes with secondary alkyl groups which cannot be synthesized using previous methods such as Pd-catalyzed coupling reactions and lithium-halide exchange reactions.

asymmetric synthesis · chiral ligands · cycloadditions · palladium · trimethylenemethane Cycloaddition reactions are among the most powerful and efficient methods for the construction of rings. Unfortunately, unactivated substrates usually require extreme reaction conditions, such as high temperatures and pressures, in order to achieve good yield of the cycloadduct. Transition-metal catalysts provide new opportunities for highly selective cycloaddition reactions. These novel reactions are possible as the complexation of the metal to the substrates temporarily polarizes and activates the otherwise unreactive species.[1] In addition to the rate enhancements observed in the presence of the metal catalyst, the opportunity to achieve asymmetric transformations by the use of chiral ligands is another attractive feature of this strategy.Since natural products containing five-membered rings are widespread, much attention has been directed to developing methods for their construction.[2] A number of metalcatalyzed cycloadditions have been developed for the formation of five-membered rings (Scheme 1). [3][4][5][6][7][8][9][10][11][12][13] Trost and co-workers reported the first examples of Pdcatalyzed trimethylenemethane (TMM) [3+2] cycloadditions in 1979.[7a] Since then, various aspects of the cycloadditions have been studied, including chemo-, regio-and stereoselectivites, [7b-e] intramolecular [7f,g] and heterocyclic [7h,i] variants, and applications in the total synthesis of natural products.[7j-l] The investigation of the asymmetric version for control over absolute stereochemistry was limited to the use of chiral auxiliaries.[7m] In 1989, Hayashis group reported the first examples of the asymmetric Pd-catalyzed cycloadditions of this type; however, the level of asymmetric induction was only low to moderate (4-78 % ee).[14] The major difficulty in designing a catalyst for this reaction is that the initial nucleophilic attack of the zwitterionic intermediate to the alkene (which is believed to be the selectivity-determining step) occurs distal to the coordinated chiral ligand on the palladium. When Trost and co-workers recently used bulky chiral phosphoramidite ligands (Figure 1) to overcome this problem, high levels of asymmetric induction were observed for the Pd-catalyzed TMM [3+2] cycloadditions. [8][9][10] For example, in the presence of 5 mol % of [Pd(dba) 2 ] (dba = dibenzylideneacetone) and 10 mol % of chiral phosphorami-

The ruthenium(II)-catalyzed [2+2] cycloadditions of anti 7-substituted norbornenes with an alkyne were investigated. The cycloadditions were found to proceed with a high degree of stereoselectivity, giving only the exo stereoisomers in moderate to good yields using an improved protocol. Comparative rate studies between a variety of anti 7-substituted nor-