Akio Ohsawa

Reductive amination of aldehydes and ketones by a Hantzsch dihydropyridine using scandium triflate as a catalyst

Direct reductive amination of aldehydes and ketones was carried out using a Hantzsch dihydropyridine as a reducing agent in the presence of a catalytic amount of a Lewis acid.

The aromatization of Hantzsch dihydropyridines with nitric oxide (NO)

Abstract Hantzsch dihydropyridines were readily oxidized by nitric oxide to give corresponding pyridines in quantitative yields. This reaction system required no work-up procedure. In the presence of oxygen, it was possible to reduce nitric oxide to less than an euimolar amount. This is the first reaction of nitric oxide itself that is practical for synthetic use.

Synthesis of 2-Substituted Benzimidazoles by Reaction of o-Phenylenediamine with Aldehydes in the Presence of Sc(OTf)3 (Heterocycles 30th in Anniversary Issue)

2-Substituted benzimidazoles were synthesised under mild conditions by reaction of o-phenylenediamine with aldehydes in the presence of Sc(OTf) 3 as a Lewis acid catalyst.

Synthesis of 2-Arylbenzothiazoles from 2-Aminobenzenethiol and Aryl Aldehydes Catalyzed by Scandium Triflate

2-Aminobenzenethiol and an aryl aldehyde reacted to give a benzothiazoline via an imine intermediate, and the benzothiazoline was aromatized by oxygen or hydrogen peroxide to give a high yield of 2-arylbenzothiazole in the presence of a catalytic amount of scandium triflate Sc(OTf) 3 . The intermediary benzothiazoline was isolated and allowed to react with O 2 or H 2 O 2 in the presence of Sc(OTf) 3 , and the Lewis acid was found to also catalyze the oxidative process other than the ring closing step.

Oxidation of 1-aminopyrazoles and synthesis of 1,2,3-triazines

Oxydation par Pb(OAc) 4 , PbO 2 /acide trifluoroacetique et/ou peroxyde de nickel/acide acetique

Asymmetric addition of nucleophiles to C-1 position of isoquinolines using (S)-alanine derivatives as chiral auxiliaries

Abstract 5,8-Dibromoisoquinoline derivatives were allowed to react with a nucleophile (silyl enol ether or allyltributyltin) in the presence of an acyl chloride derived from (S)-alanine to afford the 1,2-addition products in good chemical yields and high stereoselectivity. The bromo groups were readily removed by a reduction process in which the double bond at C3–C4 was also reduced. Thus the reaction system provided a general method to synthesize asymmetric 1-substituted tetrahydroisoquinolines. In order to determine the absolute configuration of the reaction product, (−)-homolaudanosine was synthesized in an enantiopure form. The stereoselectivity was rationally understood from the conformation of intermediary N-acylated isoquinolinium salts.

Reaction of amides with nitric oxide (NO)

Amides were allowed to react with nitric oxide in aprotic and non-ethereal solvents to give the corresponding N-nitroso derivatives. The reaction was accelerated by addition of oxygen. The solvent effect revealed that the reaction did not proceed in the presence of protic media.

Radical scavenging by N-aminoazaaromatics

N-Aminoazaaromatics were found to react with nitric oxide in the presence of oxygen to afford deaminated products in high yields. The reaction proceeded almost instantaneously in various solvents including water, and one to two equivalent of NO was consumed depending upon the amount of oxygen coexisted, and 1 equivalent of N2O was released in the reaction. In addition, N-aminoazoles were deaminated by potassium superoxide to give parent azoles in good yields. Two equivalents of superoxide was consumed, and about half equivalents of both nitrite and nitrate ion were released. The results demonstrated that N-aminoazoles have ability to protect the biological system against the oxidation promoted by radicals such as nitrogen oxides and superoxide.

Reductive deamination of aromatic amines with nitric oxide (NO)

Abstract Aromatic amines were treated with nitric oxide in tetrahydrofuran or chloroform under argon atmosphere to afford deaminated aromatic compounds in good yields. The reaction is suggested to proceed via aryl radicals, which are supposed to be formed by reduction of aryldiazonium salts with NO.

A general method for the asymmetric synthesis of both enantiomers of 1-substituted 1,2,3,4-tetrahydro-β-carbolines employing pyroglutamic acid derivatives as chiral auxiliaries

9-(S)-Pyroglutaminyl-β-carbolines were allowed to react with a nucleophile (allyltributyltin or a silyl enol ether) in the presence of 2,2,2-trichloroethyl chloroformate to give 1,2-addition products in good yields and high diastereoselectivity. The chiral auxiliary at N-9 was readily removed by a mild hydrolysis. The same chiral source afforded both enantiomers by simply altering a protecting group of the amide nitrogen. That is, (S)-pyroglutaminyl groups which had an N-alkyl group afforded the (S) isomer, whereas the ones having an N-acyl group produced the (R) isomer of the addition products.