Transformation of four-membered rings

The oxetane /-amides 222 undergo a ring expansion-contraction sequence in the presence of a Lewis acid to azetidine derivatives 223 (Equation 60) 2000JOC2253 . The overall reaction sequence has been described as double isomerization . The four-membered oxetane ring first enlarged to a [2.2.2]-dioxazabicycle, which in turn rearranged to the final azetidine derivatives. 

The /3-lactam carbonyl group transformations have been utilized successfully in the synthesis of azetidine derivatives. The reduction of the carbonyl group in azetidin-2-ones yielding azetidine derivatives is described in Section 2.01.2.8.10. Treatment of jV-BOC-protected 4-(trifluoromethyl)azetidin-2-one 229 with a stabilized Wittig reagent yielded the azetidines 28 and 230 (Equation 61) 20030L4101 . 

---