Seiki Saito

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.

One-pot transformation of azido-group to N-(t-butoxycarbonyl)amino group

Abstract A convenient one-pot protocol for the conversion of azido-group to N-Boc amino group has been described.

A facile cleavage of oxirane with hydrazoic acid in dmf A new route to chiral β-hydroxy-α-amino acids

Abstract Chiral β-hydroxy-α-amino acids such as erythro -β-hydroxy-L-aspartic acid and erythro-β-hydroxymethyl-L-serine derivatives have been synthesized in optically pure form from L- and D-tartaric acids through a novel oxirane ring-opening reaction and a selective reduction of α-hydroxy ester as key steps.

Highly active Pd(O) catalyst from Pd(OAc)2Bu3P combination in untapped 1:1 ratio: Preparation, reactivity, and 31P-NMR

Abstract Reaction of Pd(OAc) 2 with Bu 3 P (1:1) in benzene or THF afforded extraordinarily active zerovalent palladium species as a pale yellow solution, the nature of which has been discussed on the basis of the significant difference in catalytic reactivity and 31 P{ 1 H}-NMR features between such a catalyst and one prepared from Pd 2 (dba) 3 and excess Bu 3 P.

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.

Highly nucleophilic tributyltin azide in oxirane ring cleavage leading to 1,2-azido alcohol

Abstract Enhanced reactivity of tributyltin azide has been demonstrated in nucleophilic ring cleavage of oxiranes without solvent and promoter to give 1,2-azido alcohols in good to excellent yields.

Preparation and rearrangement of 2-allyloxyethyl aryl sulfoxides; A mercury-free Claisen sequence

Abstract Alkali metal hydride promoted addition of allylic alcohols to aryl vinyl sulfoxides at room temperature gives 2-allyloxyethyl aryl sulfoxides, which, on heating, are converted into γ,δ-unsaturated aldehydes via allylic vinyl ethers.

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.

Tandem Pd-catalyzed carbonylation and intramolecular ene reaction of 1-(2-methoxycarbonylethynyl)-4-alkenyl methyl carbonates

Abstract The palladium-catalyzed carbonylation of 1-(2-methoxycarbonylethynyl)-4-alkenyl methyl carbonates 6 and 9 provides allenyl geminal diesters as reactive intermediates which induce an intramolecular ene reaction to give 6- and 5-membered products.