SYNTHESIS OF 2-AZETIDINIMINIUM SALTS .1. DIASTEREOSELECTIVITY IN KETENIMINIUM TRIFLATE IMINE CYCLOADDITIONS/

A systematic study of the synthesis of 2-azetidiniminium triflates, by annulation of aldimines with iminium salts derived from tertiary carb oxamides and trifluoromethanesulfonic anhydride, has been carried out. The stereochemical output of a number of 2-azetidiniminium triflates is compared with that of the corresponding chloride salts synthesized by reaction of the same imines with a-chloro iminium chlorides. As a g eneral rule, the stereochemical output of the reactions involving a-ch loro iminium chlorides is in stark contrast to that of the correspondi ng triflates: while the chloride salts are trans stereoselective, the triflates show a preference for the cis products. The stereochemistry of the reactions involving the triflates has been examined in light of the structure of the reagents. Clear trends for a preferential format ion of cis or trans products with the steric and electronic demand of the imine have been observed. By contrast, no correlation of the produ ct distribution with the steric demand of the amide could be made. The transient formation of a keteniminium triflate intermediate has been suggested. According to this model, the annulation of the imine with t he keteniminium triflates occurs with a mechanism closely similar to t hat observed in the Staudinger reaction. A comparison between the reac tions involving the bona fide keteniminium triflates, and the correspo nding isoelectronic ketenes with the same, or structurally closely rel ated, imines has also been made. This comparison is performed in light of Georg's stereochemical rules that are used to explain, or predict, the stereochemical output of the Staudinger reaction.