Cyclocarbonylation of Allenyl Glyoxylate Strategy to Build the Tricyclic Core of Cyclocalopin A

An approach for the construction of the tricyclic framework of naturally occurring cyclocalopin A is described. The establishment of the crucial intermediate α-methylene bis-γ,δ-lactone involves a [2 + 2 + 1]-cyclocarbonylation of newly introduced allenyl glyoxylate via direct methods using Mo(CO)6 or sequential reaction pathways. The sequential reaction route involved a stannylative cyclization by Pd(0) catalyst, bromination of an vinyl stannane moiety, and final cyclocarbonylation by palladium catalysis to provide the bis-γ,δ-lactone. The feasibility of forming the spiro-system by an exo-selective [4 + 2]-cycloaddition was accomplished.