A convenient trans diastereoselective synthesis of 3-butadienylazetidinones and their Diels-Alder cycloaddition reactions

An efficient trans diastereoselective synthesis for azetidinones 3 having 3-dienyl functionalities was developed. The method first involved the [2 + 2] cycloaddition of butadienylketene with various imines 1. The 3-dienyl functionality of the resulting azetidinones 3 was then exploited in Diels-Alder cycloaddition reactions with dienophiles, viz. dimethyl acetylenedicarboxylate (DMAD), maleic anhydride (MA), N-phenylmaleimide (NPM), and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD). The reactions of 3 with DMAD and PTAD resulted in the diastereoselective synthesis of the corresponding Diels-Alder adducts 5a,b and 6. However, the reactions with MA and NPM yielded a mixture (2:1) of diastereoisomers 7/7' and 8/8'.