Carbohydrate-derived thiols as protic polarity-reversal catalysts for enantioselective radical-chain reactions

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 H2CCR1R2 group in prochiral methylenelactones to give chiral adducts of the general type Ph3SiCH2CHR1R2; 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 Ph3SiCH2ĊR1R2. Although adducts could be obtained in high enantiomeric excess in reactions carried out at 60 °C, no significant improvement in enantioselectivity could be achieved over that obtainable using simple tetra-O-acetyl-β-glucopyranose and -β-mannopyranose thiols as catalysts. It was found that the α-anomers of the pyranose thiols were ineffective at mediating enantioselective hydrogen-atom transfer to the radical Ph3SiCH2ĊR1R2. All the β-pyranose thiols gave asymmetric induction in the same sense, but two β-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.