C. Gaul, D. Seebach
Mar 1, 2002
Citations
0
Influential Citations
18
Citations
Journal
Helvetica Chimica Acta
Abstract
The Li derivative of (S)-4-isopropyl-3-[(methylthio)methyl]-5,5-diphenyloxazolidin-2-one (Li-2; synthetically equivalent to a chiral formyl anion) adds to enones and enoates in a 1,4-fashion. Best results are obtained with 1,3-diarylpropenones (chalcones; Scheme 2), trityl enones, and 2,6-di(tert-butyl)-4-methoxyphenyl cinnamates (Scheme 3), with yields up to 80% and diastereoselectivities up to and above 99 : 1 of the products (5a–f and 8a,b,e) containing three stereogenic centers! X-Ray crystal-structure analysis reveals that the C,C-bond formation occurs preferentially with relative topicity ul (Re/Si; Fig. 2). The MeS group of the 1,4-adducts can be replaced by RO groups in Hg2+-assisted substitutions, with subsequent removal and facile recovery of the chiral auxiliary (Schemes 4–6). 4-Hydroxycarbonyl derivatives (‘homoaldols') and mono-, di-, and trisubstituted 1,4-diols are, thus, accessible in enantiomerically pure forms (cf.15, 16, and 18–20).