β-Homoamino acids as catalysts in enantioselective intra- and intermolecular aldol reactions

β³-Homoamino acids catalyze the intra- (cf. the Hajos-Parrish-Eder-Sauer-Wiechert reaction) as well as the intermolecular aldol reaction. The stereochemical outcome with selectivities of up to 83% ee is reversed in the intramolecular reaction, when we go from the proteinogenic amino acids to the homologues, and reaction time increases dramatically for both reactions. In contrast, in the intermolecular reaction Me-β³hPhe-OH gives the same enantiomer as (S)-proline does, but with lower enantiomeric excess (54% vs 48% ee).