title: Chiral N-salicylidene vanadyl carboxylate-catalyzed enantioselective aerobic oxidation of alpha-hydroxy esters and amides.
abstract: A series of chiral vanadyl carboxylates derived from N-salicylidene-L-alpha-amino acids and vanadyl sulfate has been developed. These configurationally well defined complexes were examined for the kinetic resolution of double- and mono-activated 2 degrees alcohols. The best chiral templates involve the combination of L-tert-leucine and 3,5-di-t-butyl-, 3,5-diphenyl-, or 3,4-dibromo-salicylaldehyde. The resulting vanadyl(V)-methoxide complexes after recrystallization from air-saturated methanol serve as highly enantioselective catalysts for asymmetric aerobic oxidation of alpha-hydroxyl-esters and amides with a diverse array of alpha-, O-, and N-substituents at ambient temperature in toluene. The asymmetric inductions of the oxidation process are in the range of 10 to >100 in terms of selectivity factors (k(rel)) in most instances. The previously undescribed aerobic oxidation protocol is also applicable to the kinetic resolution of C-13 taxol side chain with high selectivity factor (k(rel) = 35). X-ray crystallographic analysis of an adduct between a given vanadyl complex and N-benzyl-mandelamide allows for probing the stereochemical origin of the nearly exclusive asymmetric control in the oxidation process.
authors.affiliation: Department of Chemistry, National Taiwan Normal University, No. 88, Section 4, Ding-jou Road, Taipei 11650, Taiwan.
authors.forename: Shiue-Shien
authors.initials: SS
authors.lastname: Weng
authors.forename: Mei-Wen
authors.initials: MW
authors.lastname: Shen
authors.forename: Jun-Qi
authors.initials: JQ
authors.lastname: Kao
authors.forename: Yogesh S
authors.initials: YS
authors.lastname: Munot
authors.forename: Chien-Tien
authors.initials: CT
authors.lastname: Chen