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Kazuhiko Saigo

Publications and source records attributed to Kazuhiko Saigo.

At least 37 records · Page 2Linked to original sources
PubMed2004

Stereocontrolled synthesis of phosphorothioate RNA by the oxazaphospholidine approach.

Stereoregulated diribonucleoside phosphorothioates were synthesized by the use of 2'-O-TBDMS-protected ribonucleoside 3'-O-oxazaphospholidine derivatives as monomers and N-(cyanomethyl)ammonium salts as activators. Diastereoselectivity of the condensation reaction was found to be highly dependent on the substituent groups of the oxazaphospnolidine ring as well as the structure of the activators. By the use of the optimized oxazaphospholidine monomers and activators, diastereopure diribonucleoside phosphorothioate bearing Rp and Sp configurations were obtained in good yields.

Oxazoles↗
PubMed2003

Template polymerization of columnar architectures based on the salts of a carboxylic acid and 2-amino alcohols: application to the molecular recognition of 2-amino alcohols.

The salts of trialkoxybenzoic acids and 2-amino alcohols showed a columnar liquid crystalline phase; in the case of the salt of a polymerizable acid and norephedrine, the photopolymerization proceeded efficiently in the liquid crystalline state, and the resultant solid adsorbed 2-amino alcohols size, regio-, and enantio-selectively.

Journal Article↗
PubMed2003

Synthesis and structure of macrocyclic bis(hydroxynaphthoic amide)s connected by an achiral or chiral diamine.

Macrocyclic bis(hydroxynaphthoic amide)s 6, connected by an achiral or chiral diamine, were synthesized by the tandem Claisen rearrangement. CD spectra, X-ray crystallographic analyses, and variable-temperature NMR measurements of the chiral bis(hydroxynaphthoic amide)s revealed that the two hydroxynaphthalene rings in these macrocycles adopt a twisted conformation both in solution and in the crystalline state because of the steric hindrance between the two hydroxynaphthalene rings and that the chirality of the twisted conformation is generated by that of the chiral linker. Theoretical calculations revealed that the chiral linker works effectively to favor energetically one conformer of the diastereomers, although a flipping process was possible and can be observed to occur on the NMR time scale in variable-temperature experiments.

Journal Article↗
PubMed2003

[60]fullerenoacetyl chloride as a versatile precursor for fullerene derivatives: efficient ester formation with various alcohols.

[reaction: see text] [60]Fullerenoacetyl chloride, one of the reactive derivatives of [60]fullerenoacetic acid, was isolated and identified for the first time. This acid chloride was easily synthesized in good yield from tert-butyl [60]fullerenoacetate through two steps. In the presence of 4-(dimethylamino)pyridine as a base, the acid chloride smoothly reacted with various alcohols under mild conditions to give the corresponding esters including [60]fullerene-biomolecule hybrids in moderate to high yields.

Journal Article↗
PubMed2003

An oxazaphospholidine approach for the stereocontrolled synthesis of oligonucleoside phosphorothioates.

The stereocontrolled synthesis of oligodeoxyribonucleoside phosphorothioates (PS-ODNs) using nucleoside 3'-O-oxazaphospholidine derivatives as monomer units is described. 2-Chloro-1,3,2-oxazaphospholidine derivatives were prepared from six kinds of enantiopure 1,2-amino alcohols and used for the phosphitylation reactions of 5'-O-protected nucleosides. A detailed study of these reactions revealed that the diastereoselectivity of the reaction depended on the structure of the enantiopure 1,2-amino alcohol, the reaction temperature, and the amine used as a scavenger of HCl. In addition, ab initio molecular orbital calculations for the 2-chlorooxazaphospholidine derivatives were carried out to elucidate the mechanism of these diastereoselective phosphitylation reactions. The LUMO of the 2-chloro-5-phenyloxazaphospholidine derivatives on the phosphorus atom was found to be almost orthogonal to the P-Cl bond. This LUMO may be involved in the phosphitylation reactions with predominant retention of the P-configuration. A series of dialkyl(cyanomethyl)ammonium salts were developed and used as activators for the condensation reactions of the diastereopure nucleoside 3'-O-oxazaphospholidines with 3'-O-protected nucleosides. In the presence of the new activators, the reactions proceeded rapidly to give the corresponding dinucleoside phosphite triesters. The diastereoselectivity of the condensation reaction did not depend on the counteranion but on the structure of the dialkyl(cyanomethyl)amine. In the presence of the activator, which consists of a relatively small dialkyl(cyanomethyl)amine, the condensation proceeded with excellent diastereoselectivity. After sulfurization and deprotection, diastereopure (R(p))- and (S(p))-dinucleoside phosphorothioates were obtained in excellent yields. The present methodology was also applied to the solid-phase synthesis of stereoregulated PS-ODNs. all-(R(p))-[T(PS)](3)T, all-(S(p))-[T(PS)](3)T, all-(R(p))-d[G(PS)A(PS)C(PS)]T, and all-(R(p))-[T(PS)](9)T were synthesized on a highly cross-linked polystyrene resin.

Models, Molecular↗
PubMed2003

Unexpected switch in regioselectivity of tether-directed Bingel-type biscyclopropanations depending on the leaving groups at tethered active ethylene moieties.

The regioselectivity of tether-directed Bingel-type biscyclopropanations of [60]fullerene was switched depending on the leaving groups at tethered active methylene moieties; the reactions of [60]fullerene with unhalogenated tethered bis(active methylene) derivatives/I2 and with brominated derivatives in the presence of 1,7-diazabicyclo[5.4.0]undec-7-ene gave trans-4-adducts predominantly, while the reactions with chlorinated derivatives afforded equatorial-adducts almost exclusively.

Journal Article↗
PubMed2003

Membrane cholesterol but not putative receptors mediates anandamide-induced hepatocyte apoptosis.

The endogenous cannabinoid anandamide, a lipid mediator, induces various physiologic events such as vascular relaxation, inhibition of gap-junctions formation, tumor proliferation, neurologic analgesia, and apoptosis. Although increased concentration of anandamide in plasma has been implicated in pathophysiologic states including endotoxin-induced hypotension, the effects of anandamide on hepatocytes still remain unclear. In this study, we present evidence that plasma anandamide concentration is highly increased in severe hepatitis and cirrhosis patients. In addition, concentrations of anandamide within the pathophysiologic range potently induced apoptosis of hepatoma cell line (Hep G2) and primary hepatocytes, suggesting a possible link between increased anandamide level and hepatocyte damage. Anandamide-induced cell death was preceded by G0/G1 cell-cycle arrest, activation of proapoptotic signaling (i.e., p38 MAPK and JNK), and inhibition of antiapoptotic signaling (i.e., PKB/Akt) pathways. Moreover, anandamide increased susceptibility to oxidative stress-induced hepatocyte damage. In this context, methyl-beta-cyclodextrin (MCD), a membrane cholesterol depletor, or mevastatin, an HMG-CoA reductase inhibitor, or N-acetyl cysteine, an antioxidant, potently inhibited the anandamide-induced proapoptotic events and cell death, whereas putative cannabinoid receptor antagonists did not exhibit an inhibitory effect on anandamide-induced cell death. Furthermore, binding assay using polymyxin beads revealed that anandamide could interact with cholesterol. In conclusion, our data suggest that cholesterol present in the cell membrane determines the fate of hepatocytes exposed to anandamide, possibly functioning as an anandamide receptor.

Animals↗
PubMed2003

Stereocontrolled synthesis of phosphorothioate DNA by an oxazaphospholidine approach.

Stereocontrolled synthesis of oligodeoxyribonucleoside phosphorothioates using nucleoside 3'-O-oxazaphospholidine derivatives as monomer units is described. N-(Cyanomethyl)pyrrolidinium triflate (CMPT) was found to be effective as an activator for the highly diastereoselective internucleotidec bond formation. The present method was applied to the solid-phase synthesis of stereoregulated oligodeoxyribonucleoside phosphorothioates.

Chromatography, High Pressure Liquid↗
PubMed2003

Decoding property of C5 uridine modification at the wobble position of tRNA anticodon.

Post-transcriptional modification at the first (wobble) position of the tRNA anticodon participates in precise decoding of the genetic code. We recently identified a novel taurine-containing modified uridine (tau m5U; 5-taurinomethyluridine) at the wobble position of mammalian mitochondrial tRNAs and found lack of this modification in mutant mitochondrial tRNAs from human pathogenic cells of the mitochondrial encephalomyopathies, investigate molecular pathogenesis of the diseases, decoding activity of wobble uridines with or without C5 modification was measured using E. coli cell-free translation system. It has been revealed that C5 modification has a functional role for stabilizing U:G wobble base pair.

Amino Acid Sequence↗
PubMed2003

Highly chemo- and regioselective phosphitylation of 2'-deoxyribonucleosides.

We developed a novel method for the highly chemo- and regioselective phosphitylation of the 5'-hydroxy group of 2'-deoxyribonuleosides with bis(O-tert-butyl) N,N-diethylphosphoramidite. The tert-butyl groups introduced in the thymidine 5'-O-phosphite and 2'-deoxycytidine 5'-O-phosphite were simultaneously removed within 5 min by treatment with trimethylsilyl triflate in acetonitrile to give the corresponding H-phosphonate monoesters without any side reactions.

Deoxyribonucleosides↗
PubMed2002

Taurine as a constituent of mitochondrial tRNAs: new insights into the functions of taurine and human mitochondrial diseases.

Taurine (2-aminoethanesulphonic acid), a naturally occurring, sulfur-containing amino acid, is found at high concentrations in mammalian plasma and tissues. Although taurine is involved in a variety of processes in humans, it has never been found as a component of a protein or a nucleic acid, and its precise biochemical functions are not fully understood. Here, we report the identification of two novel taurine-containing modified uridines (5-taurinomethyluridine and 5-taurinomethyl-2-thiouridine) in human and bovine mitochondrial tRNAs. Our work further revealed that these nucleosides are synthesized by the direct incorporation of taurine supplied to the medium. This is the first reported evidence that taurine is a constituent of biological macromolecules, unveiling the prospect of obtaining new insights into the functions and subcellular localization of this abundant amino acid. Since modification of these taurine-containing uridines has been found to be lacking in mutant mitochondrial tRNAs for Leu(UUR) and Lys from pathogenic cells of the mitochondrial encephalomyopathies MELAS and MERRF, respectively, our findings will considerably deepen our understanding of the molecular pathogenesis of mitochondrial encephalomyopathic diseases.

Anticodon↗
PubMed2002

A supramolecular oscillator composed of carbon nanocluster C(120) and a rhodium(III) porphyrin cyclic dimer.

A rhodium(III) porphyrin cyclic dimer (1) included carbon nanocluster C120 (2) to form a pi-electronic supramolecular complex (1 superset2), in which 2 oscillated back and forth within its cavity. Spectroscopic titration of 1 with 2 and line shape analysis on variable-temperature 1H NMR spectral profiles of 1 superset2 and reference inclusion complexes in toluene-d8, chlorobenzene-d5, and dichlorobenzene-d4 indicated that the association constant (Kassoc) of 1 superset2 and oscillation activity of included 2 are both larger as the affinity of the solvent toward fullerenes is smaller. The DeltaS values for the oscillation were all positive, indicating that the oscillation of 2 involves desolvation of a protruding C60 moiety of included 2 from the host cavity.

Journal Article↗
PubMed2002

Diastereocontrolled synthesis of dinucleoside phosphorothioates using a novel class of activators, dialkyl(cyanomethyl)ammonium tetrafluoroborates.

A novel class of activators, dialkyl(cyanomethyl)ammonium tetrafluoroborates 1a-c, has been developed and applied to the condensations of diastereopure 5'-O-tert-butyldiphenylsilylthymidine 3'-cyclic phosphoramidites 3a-d with 3'-O-tert-butyldimethylsilylthymidine (4a). Among them, the condensation of 3a with 4a in the presence of 1a completed within 5 min and gave only one diastereoisomer of the corresponding phosphite 5a. After sulfurization and deprtection, almost diastereopure (Rp)-TpsT 7 was obtained (d.r. = 99:1). Next the 5'-O-(DMTr)nucleoside 3'-phosphoramidites 8a-d containing thymine, N6-benzoyladenine, N4-benzoylcytosine, and N2-phenylacetylguanine have been synthesized and allowed to condense with 3'-O-protected thymidine and 2'-deoxyadenosine. The 5'-O-DMTr group and the N-acyl groups of the nucleobases were compatible with the reaction conditions and the condensations completed quickly with excellent diastereoselectivity.

Amino Alcohols↗
PubMed2002

Chemical synthesis of novel taurine-containing uridine derivatives.

Recently, novel taurine-containing uridine derivatives were discovered in mammalian mitochondrial tRNAs, and these modified ribonucleosides existed at the first position of the anti-codon. This paper describes the chemical synthesis of these novel uridine derivatives, 5-taurinomethyluridine (tau m5U) and 5-taurinomethyl-2-thiouridine (tau m5s2U). These taurine-containing uridine derivatives were synthesized in the good yields by the reaction of the corresponding 5-hydroxymethyluridine derivatives with taurine under basic conditions.

Chromatography, High Pressure Liquid↗
PubMed2002

Reaction mechanism for the stereoselective internucleotidic bond formation by the oxazaphospholidine approach.

This paper describes the reaction mechanisms for the stereoselective synthesis of deoxyribnucleoside 3'-cyclic phosphoramidites as well as dinucleoside phosphite derivatives by the oxazaphospholidine approach. All the key reactions in the present approach were analyzed on the basis of ab initio molecular orbital calculations for the model compounds at the HF/6-31G* level.

Nucleotides↗
PubMed1998

An Activated Germanium Metal-Promoted, Highly Diastereoselective Reformatsky Reaction.

Activated germanium metal, prepared by the reduction of germanium(II) iodide with potassium metal, was found to promote the Reformatsky reaction effectively under mild conditions. In the presence of activated germanium metal, the reactions of alpha-bromo ketones 2a and 2b and alpha-bromo imides 2e and 2f with benzaldehyde (1a) proceeded smoothly to give the corresponding beta-hydroxy carbonyl compounds 3a, 3b, 3e and 3f, respectively, in good yields and with good syn diastereoselectivity. The activated germanium metal-promoted, asymmetric Reformatsky reaction of enantiomerically pure-oxazolidinone derivatives 2g-j with various aldehydes 1a-d was also examined; the highest diastereoselectivity was achieved when (1S,2R)-2-amino-1,2-diphenylethanol-derived 2j was used as the Reformatsky donor. The excellent diastereoselectivity could be explained in terms of the formation of a chairlike, six-membered transition state between the aldehyde and enolate as in the Zimmerman-Traxler model. A single recrystallization of the Reformatsky adducts, followed by hydrolysis and subsequent esterification, led to enantiomerically pure methyl 3-hydroxy-2-methylalkanoates 10j-m, with almost quantitative recovery of the enantiomerically pure 2-oxazolidinone 14.

Journal Article↗