Asymmetrische Synthesen über heterocyclische Zwischenstufen, V Asymmetrische Synthese von α-Methylaminosäuren durch Alkylieren des lithiierten Lactimethers von cyclo-(L-Ala-L-Ala)

The (3S,6S)-2,5-dimethoxy-3,6-dimethyl-3,6-dihydropyrazi[nlaec tim ether of cyclo-(L-Ala-LAla)] (7) is obtained from cyclo-(L-Ala-L-Ala) 5 (93 - 95% optically pure) and trimethyloxonium tetrafluoroborate. With butyllithium the lithio derivative 8 is formed which reacts with alkyl halides in good chemical yields and with more than 90% diastereoselectivity, whereby R-configuration is induced at C-3. A model concept is discussed, which explains the remarkably high asymmetric induction. - Hydrolysis (0.25 N HCI, room temp.) gives L-alanine methyl ester (4) and the (R)-a-methyl amino acid methyl esters 13. The two amino acid esters are separable by distillation or chromatography