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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