Flexible, phase-transfer catalyzed approaches to 4-substituted prolines.

A range of 4-substituted prolines can be rapidly synthesized from a protected glycine Schiff base in only four steps and in 27-55% overall yield. Phase transfer catalysis allows direct access to both enantiomeric series, and the relative stereochemistry at the 4-position is readily controlled (>10:1 dr) through the choice of hydrogenation conditions.