Poly(Ethylene Glycol)-Supported Proline: A Versatile Catalyst for the Enantioselective Aldol and Iminoaldol Reactions
作者:Maurizio Benaglia、Mauro Cinquini、Franco Cozzi、Alessandra Puglisi、Giuseppe Celentano
DOI:10.1002/1615-4169(200207)344:5<533::aid-adsc533>3.0.co;2-y
日期:2002.7
(2S,4R)-4-Hydroxyproline has been anchored to the monomethyl ether of poly(ethylene glycol), M-w 5000, by means of a succinate spacer to afford a soluble, polymer-supported catalyst (PEG-Pro) for enantioselective aldol and iminoaldol condensation reactions. This organic catalyst can be considered as a minimalistic version of a type I aldolase enzyme, with the polymer chain replacing the enzyme's peptide backbone, and the proline residue acting as the enzyme's active site. In the presence of PEG-Pro (0.25 - 0.35 mol equiv.), acetone reacted with enolizable and non-enolizable aldehydes and imines to afford beta-ketols and beta-aminoketones in good yield and high enantiomeric excess (ee), comparable to those obtained using non-supported proline derivatives as the catalysts. Extension of the PEG-Pro-promoted condensation to hydroxyacetone as the aldol donor opened an access to synthetically relevant anti-alpha,beta-dihydroxyketones and syn-alpha-hydroxy- beta-aminoketones, that were obtained in moderate to good yields, and good to high diastereo- and enantioselectivity. Exploiting its solubility properties, the PEG-Pro catalyst was easily recovered and recycled to promote all of the above-mentioned reactions, that occurred in slowly diminishing yields but virtually unchanged ee's.
(2S,4R)-4-羟基脯氨酸通过succinateSpacer键合到分子量为5000的单甲醚聚乙二醇(PEG)上,制备成可溶性、载体化催化剂(PEG-Pro),用于具有立体选择性的aldol和iminoaldol缩合反应。该有机催化剂可被认为是I型aldol酶的极简版本,其中聚乙二醇链替代了酶的肽主链,脯氨酸残基则充当了酶的活性中心。在PEG-Pro(0.25-0.35mol当量)存在下,丙酮与可烯醇化和不可烯醇化的醛类以及亚胺反应,以良好收率和高对映体过量值得到了β-酮醇和β-氨基酮;其结果与采用非载体化脯氨酸衍生物作为催化剂时所获得的结果相当。将PEG-Pro催化的缩合反应推广到羟基丙酮作为aldol供体,即可获得具有合成价值的anti-α,β-二羟基酮和syn-α-羟基-β-氨基酮;这类产物具有中等到良好的收率,并表现出良好至极佳的双和对映选择性。利用其溶解性能,PEG-Pro催化剂易于回收并循环再利用,可用来推动上述所有反应;这些反应的收率虽逐步减小,但对映体过量值几乎不变。