| Abstract
| - The use of protonated l-prolinethioamide instead of the free base derivative 1 as the organocatalyst forthe direct aldol addition has a profound and appreciable effect on both the yield and the stereochemicalcourse of the reaction. 4-Nitrobenzaldehyde (2) reacts with acetone in the presence of the protonatedcatalyst 1·TFA, affording aldol product 3 with a yield up to 99% and an ee up to 98%. The catalystloading can be lowered to 2.5 mol %. More than 20 different acids were investigated as an additive, andits role as cocatalyst has been discussed. Furthermore, reactions of l-prolinethioamide salts with acetonehave been monitored using ESI-MS and 1H NMR techniques, giving insight into the mechanism of thedirect aldol reaction. The presumed formation of the iminium salt 10 has been unambiguously confirmed.
|
