New Methods and Reagents in Organic Synthesis 89. Studies on Reaction Conditions and New Entry to Chiral Ligands in the Chiral Lithium Amide-Mediated Enantioselective Aldol Reaction.
作者:Akira ANDO、Toshiaki TATEMATSU、Takayuki SHIOIRI
DOI:10.1248/cpb.39.1967
日期:——
Reaction conditions for the enantioselective aldol reaction of 2, 2-dimethyl-3-pentanone (3) and benzaldehyde using the chiral lithium amide 1b as a chiral auxiliary were thoroughly investigated. All three procedures, that is, (1) the combined use of lithium diisopropylamide and the chiral lithium amide 1b, (2) the use of an excess of the chiral lithium amide 1b, and (3) the regeneration of the chiral lithium amide 1b, afforded the aldol 4 in about 90% yield and 70% enantiomeric excess (ee). Investigation of the effects of solvent by utilizing 1-naphthaldehyde revealed that in tetrahydrofuran, (S, S)-aldol 5 of 77% ee was obtained as the major product, while in ether (R, R)-5 became the major isomer (38% ee). Furthermore, addition of hexamethylphosphoric triamide caused a dramatic change of stereoselectivity, and (S, S)-5 of 70% ee was obtained in ether with 20 eq of hexamethylphosphoric triamid. The aldol 4 of 74% ee was obtained when the new chiral lithium amide 6b was used.
对使用手性锂胺1b作为手性辅助剂的2,2-二甲基-3-戊酮(3)和苯甲醛的立体选择性醛醇反应的反应条件进行了彻底的研究。所有三种程序,即(1)使用二异丙基锂胺和手性锂胺1b的组合,(2)过量使用手性锂胺1b,和(3)再生手性锂胺1b,都能以约90%的产率和70%的旋光纯度(ee)得到醛醇产物4。通过利用1-萘甲醛研究溶剂影响,发现在四氢呋喃中,主要产物是旋光纯度为77%的(S, S)-醛醇5,而在乙醚中,主要异构体变为旋光纯度为38%的(R, R)-5。此外,加入六甲基磷酸三胺会导致立体选择性的显著变化,在乙醚中加入20 eq的六甲基磷酸三胺,得到了旋光纯度为70%的(S, S)-5。当使用新手性锂胺6b时,得到了旋光纯度为74%的醛醇4。