Diastereoselective synthesis of bicyclic gamma-lactams via ring expansion of monocyclic beta-lactams.
cis-4-(1-Chloro-1-methylethyl)-1-(omega-hydroxyalkyl)azetidin-2-ones were diastereoselectively transformed into novel trans-1-aza-4-oxabicyclo[3.3.0]octan-8-ones and trans-1-aza-5-oxabicyclo[4.3.0]nonan-9-ones upon treatment with 1 equiv of AgBF4 and pyridine in toluene via intramolecular nucleophilic trapping of N-acyliminium intermediates by the hydroxyl moiety. Additionally, the corresponding aza-analogues of the aforementioned bicyclic gamma-lactams (i.e., trans-1,4-diazabicyclo[3.3.0]octan-8-ones and trans-1,5-diazabicyclo[4.3.0]nonan-9-ones) were prepared in a convenient way starting from cis-4-(1-chloro-1-methylethyl)-1-{omega-[(tert-butoxycarbonyl)amino]alkyl}azetidin-2-ones applying the same reaction conditions. The intermediate N-acyliminium ions were formed by ring expansion of the starting beta-lactams through generation of a transient silver(I)-induced carbenium ion.
Dekeukeleire
Stijn
S
Department of Organic Chemistry, Faculty of Bioscience Engineering, UGhent University, Coupure links 653, B-9000 UGhent, Belgium.
D'hooghe
Matthias
M
De Kimpe
Norbert
N
D001384:Azetidines / Q000737:chemistry; D002384:Catalysis; D013237:Stereoisomerism; D013379:Substrate Specificity; D047090:beta-Lactams / Q000138:chemical synthesis* / Q000737:chemistry
D001384:Azetidines; D047090:beta-Lactams; C082735:azetidine