Access to a Guanacastepene and Cortistatin-Related Skeleton via Ethynyl Lactone Ireland–Claisen Rearrangement and Transannular (4 + 3)-Cycloaddition of an Azatri­methylene­methane Diyl

Heating a 2,5-furanocyclic (2-azidoethyl)­allene initiates a cascade reaction comprising azide–allene cycloaddition, loss of nitrogen, and aza­trimethylene­methane (ATMM) diyl–furan transannular (4 + 3)-cycloaddition. The major product of this reaction contains the pentacyclic core common to guanacastepenes D and H and radianspenes J–L; in addition, the central oxa-bridged cycloheptene ring, flanked by two carbocyclic rings, is structurally related to the ABC-ring system found in the cortistatins. This is the first reported synthetic application of a “free” (nonconjugated) ATMM. The cyclization precursors were prepared via the first reported examples of the Ireland–Claisen rearrangement of an ethynyl lactone.