Indoline-3-carboxylic acid derived organocatalysts for the anti-Mannich reaction.

Mannich type reactions of a preformed aldimine with various carbonyl compounds were investigated with a series of functionalised indoline derivatives as catalysts: indoline-3-carboxylic acid, the diphenylcarbinol analogue and O-protected silyl ether analogues. All compounds were readily prepared in enantiopure form by using an enzymatic kinetic resolution as a key step (E≫100). The alcohol and ether catalysts failed to induce complete chirality transfer but did afford the Mannich bases in good yields and high diastereomeric ratios, whereas the acid catalyst gave the products in a highly diastereo- and enantioselective manner. The absolute configuration of the products was determined by a syn-anti isomerisation protocol, initiated by the sterically demanding base 1,8-diazabicyclo[5.4.0]undec-7-ene.