Cyclocondensation of ketenes and imines

The reactions of AyV-dialkylhydrazoncs with benzoyloxyketene 2002AGE831 and aminoketene 2004OL2749 precursors appear to be a new general approach to the enantioselective synthesis of 4-substituted 3-alkoxyazetidin-2-ones and 3-aminoazetidin-2-ones. Such hydrazones 488, derived from formaldehyde, afforded 4-unsubstituted azetidin-2-ones 489 (Equation 198) in 80-96% yields and dr s up to 99 1 2004CEJ6111 . 

Imines with bulky groups at nitrogen give rise to the formation of fra r-azetidin-2-ones 2000TL6551 . Stereocontrolled syntheses have been carried out using chloral imines 2004TL6563 , tricarbonyl(7]6arene)chromium(0)complexed imines 

The past decade has witnessed considerable interest in solvent-free ketene-imine cycloaddition reactions employing microwave irradiation (Equation 203) C1995TL213, 1996TL6989, 1997H(44)405, 2005JOC334, 2005IJB2093 , 

R1 = Ph, 4-MeC6H4, 4-CIC6H4, 4-MeOC6H4, 4-N02C6H4, 4-EtOC6H4 R2 = Me, COCHPh2 

---