ISSN:
0018-019X
Keywords:
Chemistry
;
Organic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
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).
Additional Material:
5 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/hlca.19900730315
