Showing papers by "Hisashi Yamamoto published in 2002"

Enantio- and diastereoselective stepwise cyclization of polyprenoids induced by chiral and achiral LBAs. A new entry to (-)-ambrox, (+)-podocarpa-8,11,13-triene diterpenoids, and (-)-tetracyclic polyprenoid of sedimentary origin.

Abstract: An enantio- and diastereoselective stepwise cyclization of polyprenoids induced by Lewis acid-assisted chiral Bronsted acids (chiral LBAs) and achiral LBAs is described. In particular, the absolute stereocontrol in the initial cyclization of polyprenoids to form an A-ring induced by chiral LBAs and the importance of the nucleophilicity of the internal terminator in polyprenoids for the relative stereocontrol in subsequent cyclization are demonstrated. (−)-Ambrox was synthesized via the enantioselective cyclization of (E,E)-homofarnesyl triethylsilyl ether with tin(IV) chloride-coordinated (R)-2-(o-fluorobenzyloxy)-2‘-hydroxy-1,1‘-binaphthyl ((R)-BINOL-o-FBn) and subsequent diastereoselective cyclization with CF3CO2H·SnCl4 as key steps. Protection of (E,E)-homofarnesol by a triethylsilyl group increased the enantioselectivity of chiral LBA-induced cyclization and both the chemical yield and diastereoselectivity in the subsequent cyclization. The enantioselective cyclization of homo(polyprenyl)arenes posses...

Asymmetric Carbon−Carbon Coupling of Phenols or Anilines with Aryllead Triacetates

Abstract: The asymmetric coupling of various phenol or aniline derivatives with bulky aryllead triacetates was thoroughly investigated using optically active amines, including strychnine and brucine. We found that conformationally restricted tertiary amines, as well as lithium aryloxides and molecular sieves, are essential for accelerating the rate of phenol coupling. Consequently, the reaction can be carried out at a low temperature (−40 to −20 °C) and gives a high degree of diastereo- and enantioselectivity. In contrast to the effectiveness of lithiation in phenol coupling, magnesation of anilines was a critical technique for aniline coupling with aryllead triacetates. Using these coupling methods, a diverse set of di-, tri, and polyaryl compounds with axial chirality can be easily obtained, and these should be useful for the construction of a variety of aryl−aryl frameworks involved in metal ligands, natural products, and artificial helical polymers.

Lewis acid-activated chiral leaving group: enantioselective electrophilic addition to prochiral olefins.

Abstract: A new strategy using a BINOL derivative as a chiral leaving group and Lewis acid has been developed for enantioselective alkylation of prochiral olefins. (R)-2,2‘-Bis[2-(trimethylsilyl)ethoxymethyl]-1,1‘-binaphthol is demonstrated to be an effective reagent for enantioselective hydroxymethylation of silyl enol ethers and trisubstituted alkenes. Electrophilic addition to prochiral olefins is accompanied by cleavage of an acetal that is dual activated by SnCl4 and the δ-effect of silicon through the SN2 substitution process. Enantioselective synthesis of cyclic terpenes is also described using this strategy.

A fluorous super Brønsted acid catalyst: Application to fluorous catalysis without fluorous solvents

Abstract: The preparation of a fluorous super Bronsted acid catalyst, 4-(1H,1H-perfluorotetradecanoxy)-2,3,5,6-tetrafluorophenylbis(trifluoromethanesulfonyl)methane (3d), is described. The fluorous catalyst 3d can be recycled by using liquid/solid phase separation without fluorous solvents. A perfluorocarbon solvent is not essential for fluorous biphasic catalysis.

Single-pass reaction column system with super Brønsted acid-loaded resin

Abstract: Various acid-promoted reactions gave the desired products in high yields by passing a solution of reactants through a reaction column packed with polystyrene-bound super Bronsted acid (1) just once. Polar and nonpolar organic solvent-swellable 1 is much superior to Nafion® SAC-13 (2) as a Bronsted acid-loaded resin packed in the column.

Simple asymmetric construction of a carbocyclic framework. direct coupling of dimenthyl succinate and 1,ω-dihalides

Abstract: Die Umsetzung der aus den Dimenthylsuccinaten (I) gebildeten Dianionen mit den l,w Dihalogeniden bzw. -tosylaten (II) bzw. (V) liefert stereoselektiv die Hans-konfigurierten (S,S)-[(III),(VI)] bzw. (R,R)-Diester (IV) [(IVa) bzw. (IVb) werden auch aus (Ia)′beim Zusatz von HMPA zum Reaktionsansatz gebildet] (Tabelle).

Asymmetric Carbon—Carbon Coupling of Phenols or Anilines with Aryllead Triacetates

Abstract: The asymmetric coupling of various phenol or aniline derivatives with bulky aryllead triacetates was thoroughly investigated using optically active amines, including strychnine and brucine. We found that conformationally restricted tertiary amines, as well as lithium aryloxides and molecular sieves, are essential for accelerating the rate of phenol coupling. Consequently, the reaction can be carried out at a low temperature (−40 to −20 °C) and gives a high degree of diastereo- and enantioselectivity. In contrast to the effectiveness of lithiation in phenol coupling, magnesation of anilines was a critical technique for aniline coupling with aryllead triacetates. Using these coupling methods, a diverse set of di-, tri, and polyaryl compounds with axial chirality can be easily obtained, and these should be useful for the construction of a variety of aryl−aryl frameworks involved in metal ligands, natural products, and artificial helical polymers.