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

Abstract

(5aS,10aS)‐Octahydro‐1H,5H‐dipyrrolo[1,2‐a:1′,2′‐d]pyrazine «DPP» as Auxiliary Reagent for Enantioselective Alkylations of Prochiral Carbonyl CompoundsIn 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 13C‐NMR spectroscopy.