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Hisashi Yamamoto

Publications and source records attributed to Hisashi Yamamoto.

At least 19 recordsLinked to original sources
PubMed2006

Catalytic enantioselective hetero-Diels-Alder reactions of an azo compound.

This communication describes studies in which an azo hetero-Diels-Alder adduct was furnished in high regio- and enantioselectivity using azopyridine as a reagent and silver as a catalyst. The obtained hetero-Diels-Alder adduct was easily converted to the corresponding chiral 1,4-diamino alcohol.

Azo Compounds↗
PubMed2006

Catalytic diastereoselective polycyclization of homo(polyprenyl)arene analogues bearing terminal siloxyvinyl groups.

Highly diastereoselective polycyclization of homo(polyprenyl)arene analogues bearing terminal siloxyvinyl groups was catalyzed by tin(IV) chloride (10 mol %). The cyclizations of tert-butyldiphenylsilyl and triisopropylsilyl polyenol ethers gave 4alpha(equatorial)- and 4beta(axial)-siloxypolycycles as major isomers, respectively. The strong nucleophilicity of pro-C(9), a (6E) geometry, and a bulky silyl group effectively favored the alpha-preference, whereas the weak nucleophilicity of pro-C(9), a (6Z)-geometry, and less steric hindrance of a silyl group favored the beta-preference. [reaction: see text].

Catalysis↗
PubMed2006

Design of Brønsted acid-assisted chiral Brønsted acid catalyst bearing a bis(triflyl)methyl group for a Mannich-type reaction.

[Structure: see text] A new Brønsted acid-assisted chiral Brønsted (chiral BBA) acid catalyst (1) was developed by substituting a hydroxy group of optically active 1,1'-bi(2-naphthol) with a stronger Brønsted acidic group such as a bis(trifluoromethanesulfonyl)methyl group. The enantioselective Mannich-type reaction of ketene silyl acetals with aldimines catalyzed by (R)-1 in the presence of stoichiometric achiral proton sources gave (S)-beta-amino esters in high yield with moderate to good enantiomeric excesses.

Journal Article↗
PubMed2006

PEGylated albumin-heme as an oxygen-carrying plasma expander: Exchange transfusion into acute anemia rat model.

Poly(ethylene glycol) (PEG) conjugated recombinant human serum albumin (HSA) incorporating the synthetic iron-porphyrin (FeP) [PEGylated albumin-heme, PEG(HSA-FeP)] is a unique albumin-based oxygen carrier as a red blood cell (RBC) substitute. The physiological responses to an exchange transfusion with PEG(HSA-FeP) into an acute anemia rat model were investigated. After a 65% isovolemic hemodilution with HSA, a 30% volume of the circulatory blood was withdrawn, affording a hemorrhaged state. The circulation parameters, blood parameters, renal cortical oxygen partial pressure [PtO(2)(R)], and muscle tissue oxygen partial pressure [PtO(2)(M)] were continuously monitored. The intravenous infusion of PEG(HSA-FeP) restored the reduced levels of the mean arterial pressure, heart rate, respiration rate, mixed venous PO(2), and arterial PCO(2). The increased arterial PO(2) and pH also returned to their basal values. These effects were almost to the same extent as those observed after the administration of the RBC suspension. The relatively low recovery in PtO(2)(R) and PtO(2)(M) might be due to the Langmuir-type oxygen binding profile of PEG(HSA-FeP) (Hill coefficient: 1.0). All the animals survived during the experiments. In contrast, those injected with HSA died within 41 min. The PEG(HSA-FeP) solution is an oxygen-carrying plasma expander which can be used as a resuscitative fluid for hemorrhagic shock.

Acute Disease↗
PubMed2006

4,5,6,7-Tetrachlorobenzo[d][1,3,2]dioxaborol- 2-ol as an effective catalyst for the amide condensation of sterically demanding carboxylic acids.

[reaction: see text] 4,5,6,7-Tetrachlorobenzo[d][1,3,2]dioxaborole (4a) and 4,5,6,7-tetrachlorobenzo[d][1,3,2]dioxaborol-2-ol (4b) are effective catalysts for the dehydrative amide condensation between an equimolar mixture of carboxylic acids and amines. In particular, these catalysts are greatly superior to 3,5-bis(trifluoromethyl)phenylboronic acid (1) for the amide condensation of sterically demanding carboxylic acids. In contrast, 4c, which is prepared from a 1:2 molar mixture of B(OH)(3) and tetrachlorocatechol, is effective as a Lewis acid-assisted Brønsted acid (LBA) catalyst for Ritter reaction.

Journal Article↗
PubMed2006

Catalytic enantioselective Nozaki-Hiyama allylation reaction with tethered bis(8-quinolinolato) (TBOx) chromium complex.

The utility of the new class of chiral ligand, tethered bis(8-quinolinol) (TBOxH), is further explored. Its chromium complex, TBOxCr(III)Cl, effectively catalyzes the Nozaki-Hiyama allylation reactions of various aldehydes at room temperature with high yield (up to 95%) and high enantioselectivity (up to 99% ee). The scope of the present method is shown to be wide, and this method represents an efficient access to chiral homoallylic alcohols.

Journal Article↗
PubMed2006

Tris(trimethylsilyl)silyl-governed aldehyde cross-aldol cascade reaction.

The use of the tris(trimethylsilyl)silyl (TTMSS) group in aldehyde-derived silyl enol ethers affords aldehyde cross-aldol products with high yields and allows for unprecedented reactivity. The reaction is catalyzed by 0.05 mol % of HNTf2, and can easily be managed to give beta,delta-bis-, beta,delta,gamma-tris-, and beta,delta,zeta-tris-hydroxy-aldehydes with extremely high selectivity by simple stoichiometric control. High diastereoselectivity is obtained in all cases, and the use of chiral aldehydes affords Felkin products when there are nonchelating substituents, chelation products when there is a chelating sbustituent, and syn products when there is beta-substitution. HNTf2 is proposed to be an initiator, and highly Lewis acidic TTMSSNTf2 is the true catalyst.

Aldehydes↗
PubMed2005

N-alkyl-4-boronopyridinium salts as thermally stable and reusable amide condensation catalysts.

[reaction: see text] N-Alkyl-4-boronopyridinium salts are highly effective and reusable catalysts for the dehydrative amide condensation reaction between equimolar mixtures of carboxylic acids and amines. N-Alkylboronopyridinium salts are thermally stabilized in the order N-alkyl-2-boronopyridinium salt << N-alkyl-3-boronopyridinium salt < N-alkyl-4-boronopyridinium salt. Homogeneous catalysts, such as 4-borono-N-methylpyridinium iodide, are more effective in the presence of ionic liquid and can be recovered by extraction with ionic liquid. In contrast, heterogeneous catalysts, such as polystyrene-bound 4-boronopyridinium salts, are effective even in the absence of ionic liquid and can be recovered by filtration.

Journal Article↗
PubMed2005

N-alkyl-4-boronopyridinium halides versus boric acid as catalysts for the esterification of alpha-hydroxycarboxylic acids.

[reaction: see text] Boric acid is a highly effective catalyst for the dehydrative esterification reaction between equimolar mixtures of alpha-hydroxycarboxylic acids and alcohols. In contrast, N-methyl-4-boronopyridinium iodide (2a) is a more effective catalyst than boric acid for the similar esterification in excess alcohol. A heterogeneous catalyst, such as N-polystyrene-bound 4-boronopyridinium chloride, is also an effective catalyst and can be recovered by filtration.

Journal Article↗
PubMed2005

Silver-catalyzed asymmetric Sakurai-Hosomi allylation of ketones.

The complex of AgF and (R)-DIFLUORPHOS has been shown to be an effective catalyst for the asymmetric Sakurai-Hosomi allylation of simple ketones. A significaant improvement of the reactivity was observed by using THF as the solvent. The catalyst turnover was increased by addition of 1 equiv of MeOH. AgF and (R)-DIFLUORPHOS predominantly formed a 1:1 complex that provided high enantioselectivity. This catalyst system can be applied to various simple ketones, and corresponding tertiary homoallylic alcohols were obtained with excellent enantioselectivities (up to 96% ee). Only 1,2-adducts were obtained from both acyclic and cyclic conjugate ketones. The regio-, diastereo-, and enantioselective crotylation has also been achieved. E- or Z-crotyltrimethoxysilane gave a similar diastereomer ratio with high enantioselectivities. This finding introduces the utility of racemic allylsilanes for the enantioselective Sakurai-Hosomi allylation reaction.

Catalysis↗
PubMed2005

Biomimetic synthesis of acid-sensitive (-)- and (+)-caparrapi oxides, (-)- and (+)-8-epicaparrapi oxides, and (+)-dysifragin induced by artificial cyclases.

Asymmetric total syntheses of acid-sensitive (-)- and (+)-caparrapi oxides (1) and (+)-8-epicaparrapi oxide (2) from farnesol (10) are achieved using Sharpless-Katsuki epoxidation and Lewis acid-assisted chiral Brønsted acid (chiral LBA)-induced polyene cyclization as key steps. The relative configuration of (+)-dysifragin (4) is determined by a single-crystal X-ray diffraction and its total synthesis is accomplished by the diastereoselective epoxidation of (+)-1. Furthermore, (-)-1 can be directly synthesized from (S)-nerolidol (3) and (R)-LBA with 88% ds by reagent control, which overcame substrate control, while (-)-2 is obtained from (R)-3 and (R)-LBA with >99% ds by the double asymmetric induction.

Acids↗
PubMed2005

Cyanuric chloride as a mild and active Beckmann rearrangement catalyst.

The first general organocatalytic Beckmann rearrangement of ketoximes into amides has been realized by the catalytic use of cyanuric chloride. Furthermore, acids such as HCl and ZnCl2 are effective as cocatalysts with cyanuric chloride. For example, azacyclotridecan-2-one, which is synthetically useful as a starting material for nylon-12, was prepared in quantitative yield by the Beckmann rearrangement of cyclododecanone oxime (100 mmol scale) catalyzed by cyanuric chloride (0.5 mol %) and ZnCl2 (1 mol %).

Journal Article↗