Highly diasteroselective and remarkably π-facially selective Lewis acid catalysed Diels-Alder cycloaddition reactions:: Access to novel 1,3,4-trisubstituted 2-azetidinones

Diastereoselective and pi-facially selective Lewis acid-catalysed intermolecular Diels-Alder reactions of 3-butadienylazetidin-2-ones with a variety of symmetrical dienophiles (maleic anhydride, N-phenylmaleimide, N-p-tolylmaleimide, benzoquinone and naphthoquinone) resulting in the synthesis of diastereomerically pure 1,3,4-trisubstituted 2-azetidinones are reported. The effects of different Lewis acids on the yields of the selectively formed diastereoisomers under different reaction conditions are also examined. Preferential chelation between the Lewis acid complexes of the different dienophiles and the carbonyl oxygens in the dienylazetidin-2-ones has been invoked to explain the observed pi-facial selectivity. The Lewis acid complexes of the dienophiles have been shown to approach preferentially from the si-faces of the dienes, resulting in the formation of pi-facially selective "endo" adducts. The structures of these "endo" adducts have even been supported by X-ray diffraction studies. (c) Wiley-VCH Verlag GmbH & Co.