Table . Substrate scope of azetidine formation and deprotection (A) Cycloisomerization affording azetidine. Conditions: starting material 4f-4ac (0.2 mmol), C5 (3 mol%), N-fluoro-2,4,6-collidinium hexafluorophosphate (4.0 equiv), and 1,1,3,3-tetramethyldisiloxane (2.5 equiv) in toluene (1.0 mL) under argon atmosphere at 0°C for 30 min then room temperature for 3.5 h; isolation yields are given. Yields in parentheses are NMR yields using 1,4bis(trifluoromethyl)benzene as an internal standard. (B) Deprotection. Conditions of MHAT/RPC: starting material (0.1 or 1.0 mmol), catalyst (3 mol%), N-fluoro-2,4,6-collidinium hexafluorophosphate (4.0 equiv), and 1,1,3,3-tetramethyldisiloxane (2.5 equiv) in dichloromethane (0.5 mL) under argon atmosphere at 0°C for 30 min then room temperature for 3.5 h; isolation yields are given. Other reactions: see supporting information in detailed procedures To confirm the synthetic utility of the azetidine products, we attempted a further transformation (Table ). Fortunately, either of two differentiated protective groups, the tosyl or Cbz group, could be selectively removed from bicyclic diamine 5r by palladium-catalyzed hydrogenation (6r) or reduction with magnesium metal (7r), respectively, thereby achieving excellent yields. The same method was suitable for the synthesis of bicyclic products bearing two azetidines (6s and 7s). Note that the reduction with sodium naphthalenide resulted in a complex product mixture for the purpose of detosylation of 5r and 5s. We finally incorporated the azetidine unit into estrone, a biologically relevant complex compound. The cycloisomerization with C5 resulted in only 26% yield of 9, along with the formation of several isomeric byproducts (see supporting information). However, the yield improved slightly when C9 was used instead (48%), and the formation of the isomeric byproduct was somewhat suppressed. The structure of azetidine 9 was unambiguously determined after benzyl deprotection, followed by the X-ray analysis of 10. Full deprotection was completed by reduction with lithium aluminum hydride and palladium-catalyzed hydrogenation. The use of magnesium metal led to the full recovery of 9 after detosylation.