Asymmetric Synthesis of Five-membered Compounds by Transition Metal-Catalyzed Asymmetric Cyclizations of Enyne Systems

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Enyne compounds undergo cyclization reactions, upon treatment with transition metal catalysts such as palladium, nickel, cobalt, rhodium and so on. The use of chiral auxiliaries involved in the systems or chiral ligands provides a new strategy for asymmetric synthesis of cyclic compounds. Our recent results of asymmetric cyclizations with transition metals along these lines are as follows.1. The use of symmetric substrates involving two olefinic parts with the same substituents and an yne group in palladium-catalyzed reactions provided enantiotopic differentiating reaction products by using chiral sulfinyl or (S)-proline-derived groups involed in the substrates or chiral ligands.2. The palladium-catalyzed asymmetric cycloisomerizations were applicable to 1,6-enyne systems involving chiral sulfinyl, (S)-proline or (S)-tetrahydroisoquinoline-derived groups in the systems as chiral auxiliaries or using chiral ligands, providing a new entry to chiral five-membered compounds. Introduction o … More f a carboxyl group into the 1,6-enyne substrates provided higher diastereo- or enantioselectivity, presumably due to stereocontrol of the conformation of the resultant intermediary palladium complexes by coordination of the carboxyl groups to the palladium catalysts involved in the systems.3. The first catalytic asymmetric Pauson-Khand reaction has been developed by using chiral phosphine ligands, in which dramatic effects of the substituents were observed in achieving high enantioselectivity. Introduction of chiral organosulfur functionality such as sulfinyl or sulfinamide groups has been attempted. Slightly low efficiency was observed with the chiral organosulfur groups. Participation of organosulfur functionality was also revealed by the use of (S)-methionine-derived substrates for the Pauson-Khand reactions. The role of the sulfenyl groups involved in the chiral substrates derived from (S)-methionine and (S)-S-methylcystein in the cobatlt catalysis was reveiled in comparison with the results obtained from the substrates without organosulfur functionality, derived from (S)-norvaline and (S)-norleucine. The structure of the product was determined by the X-ray christallographic analysis. The reaction mechanism is proposed on the basis of the stereochemical results obtained.4. The stereochemistry of the palladium-catalyzed conversion of (1,3-butadienyl) cyclopentene derivatives into cyclopentenes was determined. The participation of the chiral sulfinyl groups in the palladium catalysis was revealed by the comparison with the stereochemical results between two geometrical isomers of the chiral vinylic sulfoxides employed as starting substrates. Less