Synthesis and uses of azetidiniminium salts

Abstract

Abstract

Revisiting Keteniminium Salts: More than the Nitrogen Analogs of Ketenes

“…Noteworthy are the facts that (1) keteniminiums are more electrophilic and more reactive; [4,7,[12][13][14][15][16] (2) keteniminiums do not dimerize or polymerize (in contrast to some ketene derivatives) and can be stored in solution; [4,17] and (3) they offer an easy access to homochiral cyclobutanones as chiral substituents can be readily introduced on the nitrogen atom. [4,[13][14][15]18] In addition, the amide route to keteniminium ions (vide supra) employs inexpensive, stable, and readily available starting materials. [4,16,17] Keteniminium/olefin [2+2] cycloadditions are usually highly regioselective.…”

“…diene to give the Diels-Alder adducts 32 a/b as major products, resulting from cycloaddition across the C=C bond of the heterocumulene. [13,26,27] In principle, the reaction has two important advantages over the Staudinger reaction, which combines ketenes and imines. [2] In addition, "tetramethylketeniminium" salts can also combine with acetylenic compounds.…”

“…[2] In addition, "tetramethylketeniminium" salts can also combine with acetylenic compounds. [13] [7,2] Cycloaddition to Imines: The [2+2] cycloaddition between keteniminium salts and imines was discovered in 1974 by Ghosez et al, [25] and leads to the formation of 2-azetidinium salts (Scheme 10), which are valuable synthetic intermediates.…”

“…[13] In this specific case, high enantioselectivities could be obtained by employing (S)-2-(methoxymethyl)pyrrolidine as a chiral auxiliary (Scheme 16). [13] In this specific case, high enantioselectivities could be obtained by employing (S)-2-(methoxymethyl)pyrrolidine as a chiral auxiliary (Scheme 16).…”

“…While the ability of ketenes to undergo thermal [2+2] cycloadditions with alkenes was discovered as early as in 1908 by Chick and Wilsmore and by Staudinger and Klever, [11] it was only in the early 1970s that the group of Ghosez disclosed the importance of keteniminium ions, the nitrogen analogs of ketenes, as useful intermediates in a similar context. Noteworthy are the facts that (1) keteniminiums are more electrophilic and more reactive; [4,7,[12][13][14][15][16] (2) keteniminiums do not dimerize or polymerize (in contrast to some ketene derivatives) and can be stored in solution; [4,17] and (3) they offer an easy access to homochiral cyclobutanones as chiral substituents can be readily introduced on the nitrogen atom. [4,[13][14][15]18] In addition, the amide route to keteniminium ions (vide supra) employs inexpensive, stable, and readily available starting materials.…”