Intramolecular cyclopropene-furan [2 + 4] cycloaddition followed by a cyclopropylcarbinyl rearrangement to synthesize the BCD rings of cortistatin A.

Synthesis of the BCD ring system of cortistatin A has been accomplished in 9 steps and 30% overall yield starting from commercially available 2-methylcyclopent-2-enone. Key transformations include the addition of cyclopropenyllithium 16 to aldehyde 15, an intramolecular cyclopropene-furan [2 + 4] cycloaddition leading to epimers 18/19, and a subsequent cyclopropylcarbinyl rearrangement to afford 24.