Chiral schiff bases synthesized from terpenes of pinane series in asymmetric metall complex oxidation of sulfides

Abstract

The absolute confi guration of α-hydroxyaldehyde obtained from verbenol epoxide in the presence of clay was determined. Two new Schiff bases were synthesized from the aldehyde obtained. The compounds can be used as ligands in the asymmetric vanadium-catalyzed oxidation of sulfi des to sulfoxides. The structure of initial sulfi des signifi cantly affects the value of the enantiomeric excess in the obtained sulfoxides: the highest enantiomeric excess is observed in the oxidation of thioanisole. OH R R S VO(acac) 2 , Iа _ Ic [O] S O II (S)-III + SO 2 Me IVOptically active sulfoxides play an important role in the modern organic chemistry. They are widely used in the asymmetric synthesis [1-5], a number of chiral sulfoxides exhibits a large pharmacologic activity [6-10]. The preparation methods of nonracemic sulfoxides were treated in detail in several recent reviews [11][12][13][14]. One among the most effi cient and general synthetic procedures for the synthesis of optically active sulfoxides is the asymmetric metal complex oxidation of prochiral sulfi des. During recent years a constant attention was paid to the use as ligands in the asymmetric vanadium-catalyzed oxidation of sulfi des of compounds prepared from monoterpenoids [15][16][17][18][19][20][21][22] preferably with pinane skeleton [15][16][17][19][20][21].We showed recently [16] that the use of ligands Ia-Ic in the vanadium-catalyzed oxidation of methyl phenyl sulfi de (II) led to the formation of the corresponding sulfoxide III with the enantiomeric excess (ee) up to 32% (Scheme 1).The aim of this study was the synthesis of new ligands on the basis of the terpenoid of the pinane series, verbenol epoxide (V) and the investigation of the applicability of these ligands to the vanadium-catalyzed asymmetric Scheme 1.oxidation of several sulfi des as compared with the previously obtained ligands Ia-Ic.In the isomerization of verbenol epoxide (V) alongside diol VI and hydroxyketone VII α-hydroxyaldehyde VIII with a cyclopentane skeleton formed as a minor product [23-25] (Scheme 2). Although the confi guration at the atom C 6 was not established, it was shown [23] that the isomerization of epoxide V into compound VIII proceeded stereospecifi cally giving a single diastereomer.We formerly demonstrated [15] that the reaction

Modern Stereoselective Synthesis of Chiral Sulfinyl Compounds

“…Modified Schiff bases were introduced by several research groups as chiral tridendate ligands for vanadium­(IV), typically applied as VO­(acac) 2 in a two-phase system with hydrogen peroxide as oxidant. − Manganese was used in combination of salen , or another tetradendate, N4 ligand 8 ; the latter, with carboxylic acid 9 as additive and H 2 O 2 as oxidant, allowed highly enantioselective, gram-scale preparation of various sulfoxides also in the flow system (Scheme ). − Chiral salen-type Fe­(III) complexes were also used in successful sulfoxidations with peroxides. , Nishiguchi et al applied iron­(III) complex with a Schiff base 11 in preparation of proton pump inhibitors and received esomeprazole 2 in 87% yield (12% of sulfone was formed) and >99% ee (Scheme ).…”

Catalytic Applications of Vanadium: A Mechanistic Perspective

“…Bulky substrates such as benzylphenyl sulfide also produced lower product yields than less sterically demanding substrates such as methylphenyl sulfide. Many other examples of sulfide oxidation with tridentate Schiff base vanadium complexes were also found, − and these motifs seem to be the most popular for this transformation. A report by Pessoa and co-workers compared tridentate amino alcohol-derived Schiff base ligands with reduced variants in which the Schiff base imine functionality has been converted to an amine .…”