Asymmetric hydrogenation of amino acid precursors promoted by a new type of cholesterol amphiphiles: Investigation of aggregation behaviour and stereoselective effects

Reaction of 2-hydroxyethanesulfonic acid sodium salt, 3-hydroxypropanesulfonic acid sodium salt or 2-aminoethanesulfonic acid sodium salt with cholest-5-en-3 beta-ol-3-chloroformate yields the chiral amphiphilic compounds 2-(cholest-5-en-3 beta)oxy-carbonyloxy-ethanesulfonic acid sodium salt 1, 2-(cholest-5-en-3 beta)oxy-carbonyloxy-propanesulfonic acid sodium salt 2 and 2-(cholest-5-en-3 beta)oxy-carbonylaminoethanesulfonic acid sodium salt 3, respectively. Another chiral amphiphile 2-(cholest-5-en-3 beta)oxy-carbonylethanesulfonic acid sodium salt 4 was synthesized by treating cholest-5-en-3 beta-ol with sulfopropionic acid anhydride. The surfactants form vesicles in aqueous solutions, as shown by electron microscopy. Stepwise destruction of these vesicles by addition of ethanol or methanol was proved by Circular Dichroism (CD) measurements. Especially compound 3 shows pronounced induced CD effects with the achiral dye 3,6-diamino-acridine-sulfate (proflavine). The synthesized chiral amphiphiles were used in the asymmetric hydrogenation reaction of methyl (Z)-alpha-acetamidocinnamate. In the case of an achiral rhodium complex as catalyst the prepared surfactants 1, 2, and 4 are able to provide an enantiomer excess by themselves. The best ee-value reached was 8.5% methyl (R)-N-acetylphenyl-alaninate in connection with compound 4.