Weng SS, Shen MW, Kao JQ, Munot YS, Chen CT. Chiral N-salicylidene vanadyl carboxylate-catalyzed enantioselective aerobic oxidation of alpha-hydroxy esters and amides.
Proc Natl Acad Sci U S A 2006;
103:3522-7. [PMID:
16501046 PMCID:
PMC1382168 DOI:
10.1073/pnas.0511021103]
[Citation(s) in RCA: 77] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/06/2005] [Indexed: 11/18/2022] Open
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.
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