Origins of opposite syn-anti diastereoselectivities in primary and secondary amino acid-catalyzed intermolecular aldol reactions involving unmodified alpha-hydroxyketones.
The effects of different amino acid catalysts on the stereoselectivity of the direct intermolecular aldol reactions between alpha-hydroxyketones and isobutyraldehyde or 4-nitrobenzaldehyde have been studied with the aid of density functional theory methods. The transition states of the crucial C-C bond-forming step with the enamine intermediate addition to the aldehyde for the proline and threonine-catalyzed asymmetric aldol reactions are reported. B3LYP/6-31+G** calculations provide a good explanation for the opposite syn vs anti diastereoselectivity of these two kinds of amino acid catalysts (anti-selectivity for the secondary cyclic amino acids proline, syn-selectivity for the acyclic primary amino acids like threonine). Calculated and observed diastereomeric ratio and enantiomeric excess values are in good agreement.
Fu
Aiping
A
Institute for Computational Science and Engineering, Laboratory of New Fiber Materials and Modern Textile, Qingdao University, Qingdao, Shandong 266071, China. faplhl@eyou.com
Li
Hongliang
H
Yuan
Shuping
S
Si
Hongzong
H
Duan
Yunbo
Y
D000447:Aldehydes / Q000737:chemistry*; D000596:Amino Acids / Q000737:chemistry*; D002384:Catalysis; D006900:Hydroxylation; D007659:Ketones / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D013237:Stereoisomerism
D000447:Aldehydes; D000596:Amino Acids; D007659:Ketones; C116609:3-hydroxybutanal