(5a S ,10a S )‐Octahydro‐1 H ,5 H ‐dipyrrolo[1,2‐ a :1′,2Prime;‐ d ]pyrazin «DPP» als Hilfsreagenz bei der enantioselektiven 1,2‐Addition von Grignard‐Reagenzien an prochirale Carbonylverbindungen

(5a S ,10a S )‐Octahydro‐1 H ,5 H ‐dipyrrolo[1,2‐ a :1′,2′‐ d ]pyrazine «DPP» as Auxiliary Reagent for Enantioselective Alkylations of Prochiral Carbonyl Compounds In the presence of one equivalent of the title compound «DPP» ( 1 ) prochiral carbonyl compounds 2 react with Grignard reagents 3 in THF to form enantioselectively alcohols 4 with up to 98% ee, whereas the addition of one equivalent of triethylamine yields the opposite enantiomers with up to 97% ee. The optimal molar ratio of 1 and magnesium reagent 3 was determined by cryoscopic measurements. Asymmetric induction is supposed to arise from transition states involving monomeric octahedral complexes of Grignard reagent, chiral solvent, and carbonyl compound. DPP ( 1 ) also catalyses Grignard alkylations, because a tenth of its stochiometric amount gives an enantiomeric eccess four times more than as expected. Prepared from ( S )‐proline, only the boat conformation of 1 induces the observed selectivity as detected by 13 C‐NMR spectroscopy.