Abstract
We report the development of a stereoselective method for the allylation of ketones utilizing N-substituted allyl equivalents generated from a chiral allenamide. By choice of the appropriate ligand for the Cu-catalyst, high linear selectivity can be obtained with good diastereocontrol. This methodology allows access to chiral γ-hydroxyaldehyde equivalents that were applied in the synthesis of chiral γ-lactones and 2,5-disubstitued tetrahydrofurans.
Highlights
An elegant catalytic method for the allylation of ketone and imine electrophiles was developed by Buchwald employing hydrocupration of carbon-substituted allenes (2)[10] or 1,3-dienes[11] by a Cu−H catalyst to generate the reactive allylmetal reagent in situ (Figure 1A)
Our group became interested in developing a method to generate linear products utilizing this approach, which has not been reported with ketones
Organic Letters already determined that the turnover-limiting step is allylation of the ketone,10a so π−σ−π equilibration of 5 would be expected
Summary
A1a (0.25 mmol) and 14 (0.30 mmol) in 0.5 mL of toluene. See the Supporting Information for details. bTolman electronic parameter from ref 19. cLigand cone angle obtained from ref 19a. dDetermined by 1HNMR spectroscopy on the unpurified reaction mixture. edr of b-. When the electronrich ligand PCy3 is used, the branched product (b-19) is preferred when the unsubstituted allenamide 18 was used This may result from a shift in the equilibrium of the (allyl)Cu complexes to favor l-Z-20 because of the reduced coordinating ability of the oxazolidinone to the more electron-rich Cu atom. We have disclosed a strategy for the stereoselective reductive coupling of ketones and a chiral allenamide to selectively generate the linear reaction products providing useful chiral γ-hydroxyaldehyde equivalents This method employs simple starting materials and a readily available catalyst system to furnish chiral products with increased complexity in an efficient manner.
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