GaCl3 promotes addition reactions of carbon nucleophiles to a C-C triple bond. Interaction of alkyne with GaC13 generates a highly reactive electrophile, which aromatic hydrocarbon attacks to give an alkenylated arene. Silylethyne reacts predominantly at the p-position of toluene, while disilylated 1, 3-butadiyne exhibits o-selectivity. The behavior of a silylated 1, 2-propadiene is intermediate between that of the silylethyne and the disilylated 1, 3-butadiyne. In the presence of GaCl3, electrophilic trimerization of silylethyne takes place to give a conjugated hexatriene. In this reaction, silylethyne attacks the GaCl3-activated C-C triple bond. Carbometalation is another interesting addition reaction of an organogallium compound to alkyne. Alkynyldichlorogallium dimerizes in hydrocarbon solvents to give 1, 1-dimetallo-l-buten-3-yne. In the presence of GaCl3, silyl enol ether is ethenylated at the α-carbon atom with trimethylsilylethyne. Treatment of lithium phenoxide with silylethyne in the presence of GaCl3 gives o- (β-silylethenyl) phenol. These reactions involve carbogallation of alkynylgallium, gallium enolate, or gallium phenoxide.
Highly enantioselective catalyses for preparing α, α-difluoro-β-hydroxycarboxylates and secondary α-trifluoromethyl alcohols have been developed. Difluoroketene ethyl trimethylsilyl acetal reacts with aldehydes in the presence of chiral boron complexes to afford the aldol adducts with up to 98% ee. The hydrogenation of 1, 1, 1-trifluoroalkan-2-one enol acetates using chiral ruthenium catalysts provides the secondary α-trifluoromethyl alcohols of excellent enantiopurity. The trifluoromethylation of carbonyl compounds with TMSCF3catalyzed by chiral quaternary ammonium fluorides or triaminosulfonium salts was assessed for synthesizing optically active α-trifluoromethyl alcohols including the tertiary alcohols which, unfortunately, gave moderate enantioselection. Throughout the course of our search for asymmetric catalysts for the trifluoromethylation, two types of Lewis bases, chiral phosphoramides and formamides, have been newly synthesized, and their successful application to the allylation of aldehydes with allylic trichlorosilanes is also described, which provide the corresponding homoallylic alcohols with good-to-excellent enantioselectivity.
This review summarizes the generation and characterization of monocarbonyl iodonium ylides and their alkylidene-transfer reactions to aldehydes and activated imines. Exposure of Z- (2-acetoxy-l-alkenyl) -λ3-iodanes, prepared stereoselectively by sodium acetate-catalyzed Michael addition of acetic acid to 1-alkynyl-λ3-iodanes, to EtOLi in THE at 78 °C results in ester exchange to generate the labile monocarbonyl iodonium ylides with the liberation of ethyl acetate. The monocarbonyl iodonium ylide acts as an alkylidene-transfer agent to carbonyl compounds to give α, β-epoxy ketones with E-isomers as a major product. Reaction with activated imines affords 2-acylaziridines in good yields. The stereochemical outcome of this aziridination depends on both the activating groups of the imines and the reaction solvents. Measurement of the relative rates of these alkylidene-transfer reactions indicates that the monocarbonyl iodonium ylide is moderately nucleophilic in nature. Wittig olefination of aldehydes with Z- (2-acetoxy-l-alkenyl) -λ3-iodanes, which involves transylidation between monocarbonyl iodonium and phosphonium ylides, is also discussed.
Synthetic methods based on phosphite chemistry to efficiently establish the total synthesis of inositol phosphates and inositol phospholipids, that are physiologically important in the intracellular signal transduction are discussed. Using these and our other new synthetic methodologies, various inositol derivatives have been synthesized.
Novel chemical derivatizations of C60, higher fullerenes and endohedral metallofullerenes with activated organosilicon compounds have been made to afford the mono- and bis-silylated fullerenes (silyl-fullerenes) in order to understand the basic chemical properties of fullerenes and to obtain new derivatives with interesting physical and chemical properties.
Novel macrocyclic saccharide receptors that possess terpyridine skeletons as a hydrogen-bonding site are presented. On modification of the hydrogen-bonding sites and the macrocyclic bridges, the receptors showed selective bindings for ribofuranosides, deoxyribofuranosides, and glucopyranosides, respectively. The binding affinities of the receptors were very high, so that native monosaccharides were solubilized into non-polar solvents in the presence of the receptors. The hydrogen-bonding interactions and the binding modes between them were characterized by 1H NMR spectroscopy.
New approaches to organo-transition-metal catalyzed-polymerizations of methyl methacrylate (MMA), ethylene, and 1-olefins using modified methylalumoxane (MMAO) as cocatalyst are described. Ni (acac) 2, (C5H5) Ni (η3-C3H5), [NiMe4Li2 (THF) 2] 2 coupled with MMAO showed a good catalytic activity toward the polymerization of MMA. A variety of (C5R5dP) (η3-C3H5) also showed good catalytic activities for the polymerization of MMA, while these initiators exhibited no catalysis for the polymerizations of ethylene and 1-olefins. V (1-R, 3-R'-acac) 3/MMAO systems showed a moderate catalytic activity for the polymerization of ethylene, while the polymerization using (2-dimethylaminoethyl-C5Me4) VCl2 produced high-molecular-weight polyethylenes. [6-Thiobis (2, 4-di-tert-butyl-phenolate)] VOCl2, and (N, N'-di-tert-butyl-1, l'binaphthyl-2, 2'-diamino) VOCl2 exhibited a higher catalytic activity for the polymerization of ethylene. The (Ind) CH2CH2NMe2CrCl2/MMAO system also showed a high catalytic activity. Me2Si bridged lanthanocene complexes showed high catalytic activity for both block copolymerizations of ethylene with polar monomers such as MMA or ε-caprolactone and the block copolymerizations of 1-hexene with polar monomers.
N-Boc-N-MOM-α-amino acid derivatives (Phe, Val, Leu, His, Trp, Tyr, Dopa) underwent asymmetric α-methylation in 76-93% ee by treatment with KHMDS followed by methyl iodide in the absence of any external chiral source. A chiral nonracemic enolate with dynamic axial chirality is proposed as the crucial intermediate whose racemization barrier is 16.0 kcal/mol and the corresponding half-life is 22h at -78°C.
Highly regio- and stereospecific isomerization reactions of siloxy- and alkoxycyclopropanes were effected under mild reaction conditions by a variety of metal-catalyzed systems using such as Zn (II), Pt (II), Rh (I), and Ni (0). It is noteworthy that the conversion of siloxycyclopropanes to allyl silyl ethers was established by the use of either zinc iodide or Zeise's dimer. The scope, limitations, and mechanisms of isomerization of siloxy- and alkoxy-substituted cyclopropanes are discussed.
To elucidate the structure required as the ligands of neural Siglecs at the level of the sugar residue, α-series polysialogangliosides, GD1α, GT1aα and GQ1bα, were successfully synthesized. In addition, a series of GD 1α analogues which contain the modified sialic acids, were synthesized to elucidate the structure-activity relationship of the MAG ligands at the level of the functional group.