Azetidinium salt

In aqueous solution, azetidine (p/sTa 11.29) is slightly more basic than pyrrolidine and larger-ring cyclic amines and appreciably more basic than aziridine. It forms an addition compound (m.p. - 9 to -6 °C) with trimethylboron which is more stable than that formed by pyrrolidine (50JA2926, 64HC(l9-2)885). Azetidinium salts are well known (Section 5.09.2.2.7). 

Like their monocyclic analogs (see Section 5.09.2.2.7) bicyclic azetidinium salts are subject to nucleophilic ring opening. Quaternary salt (27) can react with a nucleophile either at C-5 or C-7 producing the ring expansion (28) or direct substitution products (29), respectively (64HCA745, 69AG(E)962. ... 

Reduction of A,(V-di bcnzy lam i noalkyl chloromethyl ketones 133, followed by spontaneous intramolecular ring closure of the resulting 7-chloroamines 134, afforded azetidinium salts 135, which were subsequently deprotected toward chiral 3-hydroxyazetidines 136 and 137 (Scheme 29) <1997JOC1815>. In a similar approach, ct-amino aldehydes 138 were converted into 3-hydroxyazetidinium salts 139 upon treatment with diiodomethane and samarium iodide, followed by stabilization using AgBF4. Ar-Dealkylation with Pd afforded enantiopure azetidines 140 (Scheme 30) <2000TL1231>. 

Addition of several lithium ester enolates to chiral 1-aminoalkyl choromethyl ketones gave the corresponding chlorohydrins which, when the solvent was completely evaporated to dryness at room temperature, underwent an intramolecular heterocyclization affording 3-hydroxyazetidinium salts 5 <02OL1299>. Azetidinium salts 5 were isolated in enantiomerically pure form, and with total or high diastereoselectivity. The degree of stereoselectivity was only moderately affected by the size of R1 in the a-amino-ketone and the substituents in the ester enolate. 

An intriguing route to ethylamino epoxides through a Darzens-type reaction has been reported <07CC2500>. Rather than the typical a-halo ester, this method uses an a-ammonium azetidinium salt 12 as the leaving group. This method provides the aminoethyl substituted epoxide 13 in excellent yield. 

The ijnatemieation of tertiary- azetidinea with alkyl bromides or iodides is uncomplicated, and a number of quatemsiy azetidinium salts have been obtained in this way in addition to those prepared by... 

Dehydrohalogenation of amide chlorides affords a-chloroenamines, which undergo cycloaddition with IV-diphenylmethylaldimines to furnish azetidinium salts (303 equation 161). From these salts the diphe-nylmethyl group can be removed by hydrogenation, subsequent deprotonation yields 2-amino-1-aze-tines. The addition of primary or secondary amines to nitrilium salts gives rise to formation of amidinium salts and amidines respectively, e.g. (304 equation 162). In a similar reaction from copper(I) imidazolide, r-butyl bromide and nitriles amidines (305 equation 163) were prepared. ... 

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