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N Iwasawa

Publications and source records attributed to N Iwasawa.

9 recordsLinked to original sources
PubMed2001

A novel reaction for annulation onto alpha,beta-unsaturated ketones: W(CO)(5).L promoted exo- and endo-selective cyclizations of omega-acetylenic silyl enol ethers prepared by 1,4-addition of propargyl malonate to enones.

A highly useful method for five- and six-membered ring annulation onto alpha,beta-unsaturated ketones is described. 1,4-Addition of propargylmalonate to alpha,beta-unsaturated ketones in the presence of silyl triflate gives 7-siloxy-6-en-1-yne derivatives in good yield. W(CO)(5).L-catalyzed cyclization of these substrates can be induced to give preferentially either exo- or endo-cyclized products in good yield simply by changing the reaction solvent. [reaction: see text]

Journal Article↗
PubMed2000

Reactions of novel reactive species generated by Samarium(II)-mediated one-electron reduction of fischer-type carbene complexes

Two types of novel reactive species were generated by one-electron reductions of Fischer-type carbene complexes. In the case of tungsten complexes, one-electron reductions generated anion radical species, which added to electron-deficient olefins to give addition products. In the case of chromium complexes, carbonyl insertion occurred to give acyl chromate complexes, which underwent 1, 4-addition to various electron-deficient olefins.

Journal Article↗
PubMed2000

Complexation-Initiated Intramolecular Diels-Alder Reaction.

A complexation-initiated intramolecular Diels-Alder reaction was demonstrated for the first time. On adsorbing the alkyne-Co(2)(CO)(6) complexes having a diene and a dienophile on opposite ends of the alkyne on silica gel, the Diels-Alder reaction was accelerated and the equilibrium was shifted toward the adduct formation to a greater extent compared to that in solution. Seven-membered ring formation was apparently favorable in this system.

Journal Article↗
PubMed1991

The carbohydrate domain of calicheamicin gamma I1 determines its sequence specificity for DNA cleavage.

We have investigated the DNA cleaving properties of calicheamicinone, the synthetic core aglycone of calicheamicin gamma I1, a natural product with extremely potent antitumor activity. Our experiments have shown that the synthetic analog binds and cleaves DNA, albeit without any sequence selectivity and with less efficiency than the natural compound. We propose that a key element in the sequence recognition process is the thiobenzoate ring present in the natural compound. We have demonstrated by one-dimensional NMR that there is direct hydrogen abstraction from DNA by calicheamicinone, with enhanced binding affinity contributed by the carbohydrate domain. The reduced efficiency of hydrogen abstraction from DNA by bound calicheamicinone, compared with the natural compound, implicates the carbohydrate moiety in positioning the drug for hydrogen abstraction.

Aminoglycosides↗
PubMed1988

Structures and actions of Mytilus inhibitory peptides.

Two congeneric peptides that inhibit contraction of the anterior byssus retractor muscle of Mytilus edulis were isolated from the pedal ganglia of the mussel. Their structures were determined to be H-Gly-Ser-Pro-Met-Phe-Val-NH2 and H-Gly-Ala-Pro-Met-Phe-Val-NH2. These hexapeptides also showed inhibitory action on contractions in several other molluscan muscles, such as the cardiac muscle of Meretrix lusoria and the penis retractor muscle of Achatina fulica.

Amino Acid Sequence↗