Stereoselective allylation of 4-oxoazetidine-2-carbaldehydes. Application To the stereocontrolled synthesis of fused tricyclic beta-lactams via intramolecular diels-alder reaction of 2-azetidinone-tethered trienes

Allylation reactions of racemic and optically pure 4-oxoazetidine-2-carbaldehydes were investigated both under anhydrous conditions and in aqueous media. Different Lewis acid or metal mediators showed varied diastereoselectivities on product formation during allylation reactions of the above aldehydes with allyltrimethylsilane, allyltributylstannane, or allyl bromide. Under standard reaction conditions, tin(IV) chloride-promoted addition of allyltrimethylsilane to 4-oxoazetidine-2-carbaldehydes provided the highest diastereoselectivity and the best yield. Boron trifluoride-diethyl ether-promoted reaction of allyltributylstannane provided slightly lower diastereoselectivity with the same facial preference. Indium-promoted allylation showed a reverse diastereofacial preference, although the observed selectivity is not synthetically useful. The mesylates of these homoallylic alcohols were used for the stereoselective preparation of cis-4-butadienyl-2-azetidinones. Interestingly, mesylates having an extra alkene or alkyne tether at position 1 or 3 of the beta-lactam ring, on heating in a sealed tube with equimolecular amounts of DBU in toluene, yielded fused tricyclic 2-azetidinones through a tandem one-pot elimination-intramolecular Diels-Alder reaction.