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Contrary to all previous reports, bromoboration of propyne with BBr3 proceeds in ≥98% syn-selectivity to produce (Z)-2-bromo-1-propenyldibromoborane (1). Although 1 is readily prone to stereoisomerization, it can be converted to the pinacolboronate (2) of ≥98% isomeric purity by treatment with pinacol, which may then be subjected to Negishi coupling to give trisubstituted (Z)-alkenylpinacolboronates (3) containing various R groups in 73-90% yields. Iodinolysis of 3 affords alkenyl iodides (4) in 80-90% yields. All alkenes isolated and identified are ≥98% Z.

Dendralenes are simple alkenes with cross‐conjugated double bonds that are frequently synthesized due to being potentially valuable building blocks for the synthesis of more complex structures. The synthetic approaches to dendralenes are based on the cross‐coupling reactions of electrophilic and nucleophilic synthons derived from geminally substituted ethylene. Our novel methodology for the synthesis of only terminally substituted [3]‐ and [4]dendralenes, as well as 2,3‐disubstituted buta‐1,3‐dienes, involves the preparation of 1,2‐disubstituted cyclobutenes from readily available 2‐bromocyclobutanone and the subsequent thermal ring‐opening reactions.

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