Enantioselektive Synthese von (2R)-2-Methylserinen

Aldehydes and ketones react with the lithiated bislactim ether 3 of cyc[o-(L-Ala-L-Ala) with 81 to > 95% asymmetric induction (d. e. = diastereomeric excess) at C-3; (R) configuration is formed predominantly. - A model concept for the asymmetric induction is proposed. - With aldehydes or unsymmetrical ketones C-7 of the adducts 6 becomes a chiral center, too. (R) configuration is induced here [for the (3R)-isomers] with d. e. 47-73%. Hydrolysis of the addition products 6 (0.25 N HCI, room temperature) gives L-Ala-OCH, and (2R)-2-methylserine methyl esters 7. Both compounds can be separated either at the ester stage by distillation or - if 7 is thermolabile - after further hydrolysis at the amino acid stage