Apparent chelation control in aldol reactions of chiral (Me2CHO)3Ti-enolates
作者:Maryellen Nerz-Stonies、Edward R. Thornton
DOI:10.1016/s0040-4039(00)84131-3
日期:1986.1
Directed aldolreactions of chiral (Me2CHO)3Ti-enolates give the opposite diastereofacial selectivity compared with the corresponding boronenolates. This stereochemical reversal is best explained if the Ti reactions involve chelation (not possible with boron, and previously thought to be unfavorable with (Me2CHO)3Ti-enolates). Changing the solvent from THF to diethyl ether significantly enhances the
Asymmetric aldol reactions. Use of the titanium enolate of a chiral N-acyloxazolidinone to reverse diastereofacial selectivities
作者:Maryellen Nerz-Stormes、Edward R. Thornton
DOI:10.1021/jo00007a042
日期:1991.3
Aldol reactions of the titanium enolate of (S)-N-propionyl-4-isopropyl-2-oxazolidinone (readily derived from L-valine) with representative aldehydes give high diastereofacial selectivities for the syn aldol adducts expected from chelation control. This represents a remarkable reversal in selectivity compared with the corresponding boron enolate, thus permitting either enantiomeric form of beta-hydroxy-alpha-methyl carboxylic acids to be made from a single, readily available oxazolidinone simply by changing the metal. A lithium interference effect is shown to be easily prevented by use of excess titanium. Use of diethyl ether as solvent rather than THF significantly enhances the stereoselectivity. Mechanistically, the observed stereochemical reversal constitutes very strong evidence that chelation is operative with titanium, presumably through a chelated chairlike transition structure. In this transition structure, the conformation would be rigidly locked by chelation and the titanium would be at least hexacoordinate, resulting in a ''superaxial'' ligand, thus nicely explaining the high stereocontrol.
A general synthesis of homochiral β-hydroxy N-acetylcysteamine thioesters
作者:Christine Le Sann、Thomas J. Simpson、David I. Smith、Paul Watts、Christine L. Willis
DOI:10.1016/s0040-4039(99)00657-7
日期:1999.5
A convenient and efficient route for the enantioselective synthesis of functionalised B-hydroxy N-acetyicysteamine thioesters is described. The route allows the facile incorporation of vicinal C-13 labelling to produce intermediates required for biosynthetic studies on a wide range of polyketide metabolites, e.g. 6-MSA, monocerin, colletodiol and strobilurins. (C) 1999 Elsevier Science Ltd. All rights reserved.