摘要：

Chloro(cyclopentadienyl)bis(1,2:5,6-di- O -isopropylidene-α- D -glucofuranos-3- O -yl)titanium ( 1 ) is used for the transmetallation of Li-enolates obtained from propionyl derivatives. While such Ti-enolates of ketones and hydrazones appear to be unreactive, the ( E )enolate 13 of 2,6-dimethylphenyl propionate ( 11 ) adds to the re -side of aldehydes, affording various syn -aldols 14 with high dia- and enantioselectivity (92–97% ds, 91–97% ee, cf . Scheme 2 and Table 1 ). Racemic syn -aldols (±)- 14 are obtained analogously from the achiral bis(2-propyloxy)-Ti-enolate 15 ( Scheme 2 and Table 2 ). In contrast to the unstable Li-enolate 10 , the Ti-enolates 13 and 15 isomerize at 30°, presumably to the thermodynamically more stable ( Z )-enolates ( Scheme 4 ), While the diastereoselectivity of the achiral enolate 15 is lost upon this equilibration, the chiral ( Z )-enolate 27 quite unexpectedly affords anti -aldols 12 of high optical purity (94–98% ec) and, in most cases, with acceptable-to-good diastereoselectivity (82–90% ds). Notable exceptions are branched unsaturated and aromatic aldehydes which form a greater proportion of synepimers of moderate optical purity ( Scheme 5 and Table 3). Consistent with these findings, re-facial-and ami -selective aldol-addition is also exhibited by the ( Z )-configurated Ti-enolate 22 of N -propionyl-oxazolidi-none 19 ( Scheme 3 ).

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