Novel diethylaluminum chloride promoted reactions of the azetidine ring: Efficient and stereocontrolled entry to functionalized olefins, pyrrolidines, and pyrroles

The first examples of Lewis acid promoted fragmentation or rearrangement of the azetidine ring are reported. Azetidine precursors, cis-beta-lactams, were easily available as single cis-diastereoisomers by the Staudinger reaction. Both cis- and trans-4-formyl-beta-lactams react with the appropriate dial or dithiol under acid catalysis to yield dioxolanes or dithiolanes, while reaction with trimethyl orthoformate or benzenethiol gave 4-acetal or thioacetal beta-lactams, respectively. Azetidines were smoothly obtained by reduction of easily available beta-lactams with monochloroalane (AlH2Cl), generated in situ from LiAlH4/AlCl3. The chemical reactivity of azetidines with AlEt2Cl was further investigated. Different substituted azetidines showed varied behavior on product formation during diethylaluminum chloride promoted reactions. Azetidines having 4-methoxyphenyl or 2-furyl groups at C2 and a benzyl or allyl substituent at nitrogen efficiently reacted with AlEt2Cl to give olefins stereoselectively through a fragmentation process, while acetal or thioacetal azetidines under the standard reaction conditions afforded in a stereocontrolled manner pyrrolidines as the sole product. Furthermore, thioacetal azetidines bearing a substituent at C3 on the azetidine ring that can promote aromatization (phenoxy or exocyclic double bond) gave pyrroles by reaction with AlEt2Cl.