Tin- or magnesium-mediated diastereoselective aldol-type reactions for the asymmetric synthesis of α-substituted serines

Diastereoselective aldol-type reactions of ethyl (5S)-3,6-diethoxy-2,5-dihydro-5-isopropyl-2-pyrazine-carboxylate (5S)-3 with achiral aldehydes 8a-d was investigated by using Sn(OSO2CF3)(2)-N-ethylpiperizine and MgBr2-triethylamine. The reaction with Sn(II) between (5S)-3 and aliphatic aldehydes 8a,b proved to be quite different from that with Mg(II). On the other hand, Sn(II)- or Mg(II)-mediated aldol-type reactions of (5S)-3 with benzaldehyde Sc and 3-methyl-2-butenal 8d each afforded the same diastereomer as the major product. These aldol products were each converted to the corresponding alpha-substituted serines 6 and 7 as enantiomerically pure compounds. (C) 1998 Elsevier Science Ltd. AII rights reserved.