LEWIS-ACID-MEDIATED AND CATIONIC LITHIUM-MEDIATED DIASTEREOSELECTIVE ALDOL-TYPE REACTION-BASED ON A DOUBLE CHIRAL RECOGNITION MANNER FOR THE ASYMMETRIC-SYNTHESIS OF ALPHA-SUBSTITUTED SERINES
Diastereoselective aldol-type reaction of ethyl (5R or 5S)-3,6-diethoxy-2,5-dihydro-5-isopropyl-2-pyrazinecarboxylate (5) with chiral aldehyde 7 was investigated by using Sn(OSO2CF3)(2)-N-ethylpiperizine, MgBr2-Et(3)N, n-butyllithium, and lithium diisopropylamide. The mediation mode with Sn(II) between (5R or 5S)-5 and 7 proved to be quite different from that with Mg(II). The two aldol products were converted to the corresponding gamma-lactonic alpha-substituted serines (2S)- and (2R)-10.