Asymmetric reactions of 8-phenylmenthyl pyruvate with allyltrimethylsilane, silyl enol ethers and ketene silyl acetals

By mediation of TiCl4, allylsilane, silyl enol ethers and ketene silyl acetals attacked (–)phenylmenthyl pyruvate and (–)-phenylmenthyl phenylglyoxylate at their si-faces. The reactions are hypothesised to proceed with rigid cyclic transition states: anti aldol adducts 16a and 17a were favourably obtained from E-ketene silyl acetals 6 and 7 having E-configuration, whereas syn aldol adducts 18b–21b were predominantly obtained from Z-ketene silyl acetal 18 and Z-silyl enol ethers 19–21.