Additions of allenyl/propargyl organometallic reagents to 4-oxoazetidine-2-carbaldehydes: novel palladium-catalyzed domino reactions in allenynes

Metal-mediated carbonyl allenylation and propargylation of 4-oxoazetidine-2-carbaldehydes were investigated in aqueous environment. Different propargyl bromide and metal promoters showed varied regio- and stereoselectivities on product formation. In addition, an unprecedented one-pot stereoselective synthesis of beta-chlorinated allylic alcohols, which can also be considered as functionalized allylsilanes, has been developed, which involves tin(IV) chloride-mediated reaction of propargyltrimethylsilane and 4-oxoazetidine-2-carbaldehydes. Some of the resulting coupling products were submitted to transition metal catalyzed reactions, such as the allenic Pauson-Khand and palladium-catalyzed reactions, leading to novel fused or bridged tricyclic beta-lactams. Remarkably, a novel domino process, namely the allene cyclization/intramolecular Heck reaction was found. A likely mechanism for the cascade reaction should involve an intramolecular cyclization on a (pi-allyl)palladium complex and a Heck-type reaction.