Keteniminium triflates

Ab initio calculations indicate that in the gas phase the reaction of ketene inline and formaldehyde is concerted but asynchronous whereas in dichloromethane it is a two-step zwitterionic reaction.38 The 2 + 2-cycloadditions of keteniminium triflates with imines yields 2-azetidiniminium salts with cis stereoselectivity.39 The intramolecular 2 + 2-cycloaddition of ketenimines with imines (24) provides a novel synthesis of azeto[2,l-Z>]quinazolines (25) (Scheme 9).40... 

The cycloaddition of keteniminium triflates 195, formed from tertiary amides by the action of triflic anhydride, with imines formed the azetidine iminium salts 196 (Equation 51) <1996JOC8480>. 

Intramolecular [2+2 cycloadditions ofketeniminium triflates. Two laboratories1,2 report that unsaturated keteniminium salts, generated from an unsaturated amide with 1 equiv. each of triflic anhydride and collidine, undergo intramolecular [2 + 2]cycloadditions. 

Acetyl-2-(methoxymethyl)-pyrrolidine is converted into 2-(methoxymethyl)-1-vinylidenepyrrolidinium triflate by treatment with 1 equiv of triflic anhydride (added over a period of 6 h) in CH2C12 at reflux in the presence of 1.2 equiv of collidine. The keteniminium salt 1 a is trapped in situ by 5 equiv of cyclopentene to give 2 and 3, after hydrolysis in water overnight yield 30% d.r. (2/3) 77 23. The enantiomeric excess (55%) and absolute configuration are determined by comparison with an authentic sample. 

---