Enantioselective aldol condensations: The use of ketone boron enolates with chiral ligands attached to boron.
✍ Scribed by Ian Paterson; M.Anne Lister; Cynthia K McClure
- Publisher
- Elsevier Science
- Year
- 1986
- Tongue
- French
- Weight
- 309 KB
- Volume
- 27
- Category
- Article
- ISSN
- 0040-4039
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✦ Synopsis
Aldol condensation between diethylketone and simple aldehydes using (lpc)2BOTfliPr2NEt in CH,CI, gives syn-adducts in good ee (66-90%) and with high diastereoselectivity (290%). Other chiral dialkylboron triflate reagents examined give lower ees.
The aldol condensation reaction of boron enolates, la-9 which affords high levels of relative stereocontrol (Z enolate + syn vs E enolate + anti aldol), has recently become important in the area of absolute stereocontrol through the use of specially designed chiral auxiliarieslbsC attached to the carbonyl carbon. The alternative use of readily-available chiral ligands on boron, as in enolates 1Z or lE, to directly promote enantioselective C-C bond formation between simple carbonyl compounds and aldehydes is an attractive idea which has not been nearly so well demonstrated.2 A particularly appealing application is to the synthesis of 6-hydroxyketones4 of high ee with predictable relative and absolute stereochemistry. We now report our initial results on the enantioselective synthesis of a-methyl+hydroxy ethylketones by aldol condensation of diethylketone with aldehydes using the chiral dialkylboron triflate reagents 2,3, and 4 (Scheme). Syn aldol adducts, 5 and 6,