Teruhiko Ishikawa

A revised mechanism for chemoselective reduction of esters with borane dimethyl sulfide complex and catalytic sodium tetrahydroborate directed by adjacent hydroxyl group

Abstract The plausible mechanism for the reduction of the ester groups with a strong preference for one located α to the hydroxyl groups of S -malates and R,R -tartrate-based derivatives has been proposed together with some results with regard to its applications to the syntheses of chiral synthons.

Domino Double Michael−Claisen Cyclizations: A Powerful General Tool for Introducing Quaternary Stereocenters at C(4) of Cyclohexane-1,3-diones and Total Synthesis of Diverse Families of Sterically Congested Alkaloids

Reactions of substituted acetone derivatives with acrylic acid esters (>200 mol %) in the presence of t-BuOK (200 mol %) in t-BuOH-THF (1:1 by volume) turned out to proceed as a cascade process consisting of the first Michael addition, the second Michael addition, and the last Claisen reaction to afford 4,4-disubstituted cyclohexane-1,3-diones. Only more substituted enolates play the role of a Michael donor in this cascade process, and therefore the ketone took up two alkoxycarbonylethyl groups on the same carbon bearing more substituents. Such intermediates were followed by intramolecular Claisen reactions leading to cyclohexane-1,3-diones bearing quaternary stereogenic centers at C(4), which bears an alkoxycarbonylethyl group and the substituent of the starting acetone derivatives. Thus-obtained 4,4-disubstituted cyclohexane-1,3-diones were successfully employed for total syntheses of intricate alkaloids of biological interest such as (+)-aspidospermidine, (+/-)-galanthamine, (+/-)-lycoramine, and (+/-)-mesembrine, all featuring quaternary stereogenic centers. DFT calculations provided us with clear-cut explanations for the observed chemoselectivity of the cascade process involving ketone-based enolates under thermodynamically controlled conditions.

Selective C-2 opening of 2,3-epoxyesters with HN3-amine system: A viable route to β-hydroxy-α-amino acids

Abstract The combination of hydrogen azide with amines has proven to effect the C-2 opening of 2,3-epoxyester with high regioselectivity uniformly for trans -epoxyesters and depending on their structures for cis -2,3-epoxyesters.

HEMIAMINAL GENERATED BY HYDRATION OF KETONE-BASED NITRONE AS AN N,O-CENTERED NUCLEOPHILE IN ORGANIC SYNTHESIS

Isoxazolidines that are useful precursors for β-amino acids have been prepared relying on intermolecular amino Michael addition-intramolecular SN2 displacement employing hydroxylamine or hydrate of ketone-based nitrone as an N- and O-centered binucleophile.

Hydrogen azide-amine systems as an azide nucleophile for substitutions of sulfonates, halides, and vicinal disulfonates

Abstract Hydrogen azide-alkylamine combination turned out to ensure the displacement of simple primary or secondary sulfonates, halides, and vicinal disulfonates with an azido-group in which an enhanced reactivity is generally achieved as compared with a case of traditional alkali metal azides.

Synthesis of A-ring fragments of 1α,25-dihydroxyvitamin D3 and taxane diterpenoids: Effective construction of conjugated formylcyclohexene frameworks from isoxazolines

Abstract This paper have delineated the effectiveness of [3+2] cycloaddition process in the synthesis of multifunctional 6-membered rings. The novel synthesis of chiral A-ring fragment of 1α,25-dihydroxyvitamin D 3 and also a novel route to A-ring fragment of taxane diterpenoids have been established.