Carbohydrate-derived thiols as protic polarity-reversal catalysts for enantioselective radical-chain reactions.
J CHEM SOC PERK T 1
1376 - 1386.
A variety of novel homochiral carbohydrate-derived thiols, in which the SH group is attached to the anomeric carbon atom, have been prepared and characterised. These thiols have been evaluated as protic polarity-reversal catalysts to mediate the enantioselective radical-chain addition of triphenylsilane to the H2C=(CRR2)-R-1 group in prochiral methylenelactones to give chiral adducts of the general type (Ph3SiCH2CHRR2)-R-1; chemical yields were uniformly high. Systematic changes in the structures of the thiols were made with the aim of increasing the enantioselectivity of hydrogen-atom abstraction from the SH group by the prochiral alkyl radical (Ph3SiCH2CRR2)-R-1. Although adducts could be obtained in high enantiomeric excess in reactions carried out at 60degreesC, no significant improvement in enantioselectivity could be achieved over that obtainable using simple tetra-O-acetyl-beta-glucopyranose and -beta-mannopyranose thiols as catalysts. It was found that the alpha-anomers of the pyranose thiols were ineffective at mediating enantioselective hydrogen-atom transfer to the radical (Ph3SiCH2CRR2)-R-1. All the beta-pyranose thiols gave asymmetric induction in the same sense, but two beta-mannofuranose thiols with less polar substituents gave asymmetric induction in the opposite sense. It is concluded that both steric and dipole-dipole interactions between the prochiral carbon-centred radical and the thiol are important in determining enantioselectivity and that these interactions can act in opposition as well as co-operatively; solvent effects are also shown to be important.
|Title:||Carbohydrate-derived thiols as protic polarity-reversal catalysts for enantioselective radical-chain reactions|
|Keywords:||SYNTHETIC ANTIVIRAL AGENTS, O-SPECIFIC POLYSACCHARIDE, ALKENES, HYDROSILYLATION, HYPONITRITE, DERIVATIVES, SULFIDES, SILANES, HALIDES, ACIDS|
|UCL classification:||UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry|
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