The heliannuols are a family of allelochemicals that have been isolated from the common sunflower (Helianthus annuus). The 8-membered cyclic ether moiety found in many of the heliannuols is a rare motif in nature and provides a challenging synthetic target. It has been found that conformational constraint in the form of a Z olefin helps promote the cyclization of aryl epoxides to the 8-membered cyclic ether. Hiyama couplings of aryl iodides and cyclic vinyl siloxanes...