The stereochemistry of the addition of titanium enolates of N-propionyl-oxazolidin-2-ones to 5- and 6-membered N-acyliminium ions

The stereochemistry of the addition of the N-propionyl titanium enolates 2a and 2b to 5- and 6-membered 2-ethoxycarbamates 1a-f was investigated. The addition proceeded stereoselectively to afford the corresponding (2S,1'S)-2-substituted pyrrolidines as the major diastereoisomer. Despite the lack of reactivity between 2-ethoxypiperidine 1b and N-propionyl titanium enolates 2a and 2b, less bulky carbamate groups on 2-ethoxypiperidines 1d and 1f restored reactivity and the corresponding 2-substituted piperidines were obtained in moderate to good yields although with poor diastereoselection (C) 1999 Elsevier Science Ltd. All rights reserved.