Expeditious Entry to Enantiopure Mono- and Bis(Tricyclic) beta-Lactams by Single or Double [2+2] Cycloaddition of Allenynes

A thermal methodology for the expeditious preparation of structurally novel strained tricyclic beta-lactams containing a cyclobutene ring has been developed. Besides, the first examples accounting for the intramolecular double [2+2] cycloaddition of bis(allenyne)s have been achieved through thermolysis of C-2-symmetric or unsymmetric bis(beta-lactam-allenyne)s, which have been prepared by copper-promoted alkyne homo- or cross-coupling reactions. The bis(tricyclic) ring structures bearing a central seven-membered ring arise from the regioselective cyclization of the alkyne with the distal bond of the allene, most likely via a radical intermediate.