Shinichi Itsuno

Asymmetric reduction of aromatic ketones with the reagent prepared from (S)-(–)-2-amino-3-methyl-1,1-diphenylbutan-1-ol and borane

Asymmetric reduction of prochiral aromatic ketones with the reagent prepared from (S)-(–)-2-amino-3-methyl-1,1-diphenylbutan-1-ol [(S)-(1)] and borane afforded the corresponding aromatic secondary alcohols in high optical (94–100% enantiomeric excess) and chemical (100%) yields.

Main-Chain Ionic Chiral Polymers: Synthesis of Optically Active Quaternary Ammonium Sulfonate Polymers and Their Application in Asymmetric Catalysis

A condensation reaction between quaternary ammonium halide and sulfonate was first successfully applied to the synthesis of the chiral polymers. Reaction of bis(quaternary ammonium halide) with disulfonate afforded a polymer containing ionic bonds in its main chain. The chiral ionic polymers were applied to the asymmetric benzylation of N-diphenylmethylidene glycine tert-butyl ester to prepare (S)-phenylalanine derivatives in high yield with a high level of enantioselectivity.

Asymmetric reduction of ketoxime O-alkyl ethers with chirally modified NaBH4–ZrCl4

Reducing agents prepared from sodium borohydride (NaBH4), zirconium tetrachloride (ZrCl4), and chiral amino alcohols have been successfully applied to the enantioselective reduction of oxime ethers. Optically active primary amines were obtained in high enantiomeric excess (⩽95% e.e.) with good chemical yield. The extent of asymmetric synthesis was dependent on the solvent, the temperature, the structure of chiral amino alcohol, and the proportions [NaBH4] : [ZrCl4] : [chiral amino alcohol] : [oxime ether].

Asymmetric synthesis using chirally modified borohydrides. Part 2. Enantioselective reduction of ketones with polymeric (S)-prolinol–borane reagent

Chiral polymeric reagents have been prepared from polymeric (S)-prplinols and borane and used in the enantioselective reduction of a series of prochiral ketones under various conditions. Reductions with these polymeric reagents are shown to give alcohols of reasonably good optical purity (up to 80%). The results are compared to those obtained with (S)-N-benzylprolinol–borane complex which is a soluble model reagent.

Synthesis of polymer-supported chiral N-sulfonylamino acids and their use in asymmetric diels-alder reaction of cyclopentadiene with methacrolein

Abstract Crosslinked polymer-supported chiral N -sulfonylamino acids have been synthesized by two methods; namely the chemical modification method and the copolymerization method. In the chemical modification method, the chlorosulfonyl group was introduced into styrenedivinylbenzene copolymers 1 by sulfonation with chlorosulfonic acid followed by chlorination with thionyl chloride. The degree of functionalization could be well controlled by the feed ratio of chlorosulfonic acid to aromatic rings of the polymer. Treatment of l -valine with the polymer having the chlorosulfonyl group 4 gave polymer-supported N -sulfonylated l -valine 5 . An alternative approach to the synthesis of chiral polymers involves the copolymerization of monomers containing the desired chiral groups with styrene and divinylbenzene (DVB). Suspension copolymerization of chiral monomers 6 , styrene, and DVB afforded the chiral polymers 7 in good yield. The N -sulfonylamino acid group in the polymer reacted with borane or monobromoborane to form chiral oxazaborolidinone 8 , which catalyzed the Diels-Alder reaction of cyclopentadiene with methacrolein, leading to optically active cycloadduct having enantiomeric purity of 65% ee.

Polymer-immobilized chiral catalysts

Polymer immobilization of chiral catalysts has progressed extensively over the past years. Recent intensive development of chiral organocatalysts resulted in the identification of numerous highly active catalysts, which are, however, still far less effective than transition metal catalysts. Separation of relatively large amounts of organocatalysts from the reaction mixture causes problems during product isolation. In the case of chirally modified metal catalysts, recovery of valuable metal species and suppression of metal leaching are perpetually important requirements in the design of environmentally friendly chemical processes. Various types of chiral organocatalysts and metal catalysts have been immobilized as pendant groups onto the side chains of polymer supports. Another important polymer immobilization technique is the incorporation of a chiral catalyst into its main chain, with several types of chiral catalyst monomers having been copolymerized with achiral monomers for their production. Recently, the synthesis of chiral main-chain polymeric catalysts has progressed extensively. Moreover, many examples of polymer-immobilized catalysts exhibit higher enantioselectivities in comparison to those of the corresponding low-molecular-weight catalysts. The development of these polymer-immobilized chiral catalysts, which have largely been reported in the last five years, is reviewed in this article.

Asymmetric synthesis using chirally modified borohydrides. Part 4. Enantio-selective reduction of ketones and oxime ethers with the reagent prepared from borane and polymer-supported (S)-(–)-2-amino-3-(p-hydroxyphenyl)-1,1-diphenylpropan-1-ol

The polymer-supported chiral amino alcohol (3) has been prepared by the reaction of chloromethylated polystyrene resin and (S)-(–)-2-amino-3-(p-hydroxy)phenyl-1,1-diphenylpropan-1-ol (2); the chiral polymeric reagent has been prepared from (3) and borane. The asymmetric reductions of ketones and oxime ethers with the polymeric reagent have been shown to give optically active alcohols (up to 97% optical purity) and amines (up to 67% optical purity), respectively. The results are compared with those obtained with (S)-(–)-2-amino-1,1,3-triphenylpropan-1-ol (4) or (S)-(–)-2-amino-3-(p-benzyl-oxyphenyl)-1,1 -diphenylpropan-1-ol (5) which are soluble model reagents.

Asymmetric synthesis using chirally modified borohydrides. Part 3. Enantioselective reduction of ketones and oxime ethers with reagents prepared from borane and chiral amino alcohols

The asymmetric reduction of aromatic and aliphatic ketones, halogeno ketones, hydroxy ketones, keto esters, and ketone oxime ethers with reagents prepared from borane and chiral amino alcohols has been investigated. When α,α-diphenyl β-amino alcohols, such as (2S,3R)-(–)-2-amino-3-methyl-1,1 -diphenylpentanol (2d), were used as a chiral auxiliary, very high enantioselectivities (ca. 90 % e.e.) were obtained in the reduction of various ketones and oxime ethers.

Reactive polymers: design considerations, novel preparations and selected applications in organic chemistry

Crosslinked reactive polymers with novel structures have been prepared from the corresponding monomers or by newly developed chemical modification reactions. These include the preparation of polymers with two or three carbon spacer groups between backbone and functionality, the first preparation of a Grignard reagent on an insoluble polymer, and some new copper m ediated c hemical modification reactions resulting in C-C bond formation. Novel applications of these polymers in asymmetric syntheses, regenerable polymeric protecting groups, supernucleophili c catalysts, polymeric separation media for HPLC, as well as aids in the determination of reaction mechanisms, have been investigated. Microenvironment effects within the polymer beads are shown to be important when considering the use of an insoluble reactive polymer,

Polymer-supported chiral Lewis acids as asymmetric catalysts for diels-alder reactions of methacrolein with cyclopentadiene

Abstract Insoluble polymer-supported chiral Lewis acids were prepared from monobromoborane or borane with crosslinked polymers having a chiral moiety such as an amino alcohol, dial, or N-sulfonylamino acid. These polymers were used as chiral catalysts for Diels-Alder reactions of methacrolein with cyclopentadiene to give optically active cycloadduct.