Diastereoselective Synthesis of Axially Chiral Skipped Ene-Allenes Containing a Quaternary Stereocenter by Cyclopropene-Allene Metathesis.

Functionalized skipped ene-allenes (1,4-enallenes) bearing an all‑carbon quaternary stereocenter were prepared by a diastereoselective ring-opening metathesis of mesomeric cyclopropene and allene for the first time. A strategic pairing of a broad range of allenes with cyclopropenes revealed a new way to access high reactivity and chemoselectivity, and delivered products with excellent diastereoselectivity at low Ru catalyst loading. Mechanistic studies delineate the reactivity order of the metathesis partners, identify the key diastereoselectivity-determining step, and clarify the main factors governing the cross-/homo-metathesis ratio and diastereoselectivity. The presence of allenyl-Bpin, -silyl, and -germanyl groups allows highly chemoselective functionalization at the allene over the terminal alkene, demonstrating the synthetic utility of the products as versatile intermediates in organic synthesis.