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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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(Z)-β-bromo-1-propenyl(pinacol)borane(4), recently made available in 85% yield as a ≥98% isomerically pure compound via bromoboration of 1-propyne, has been converted to β-alkyl-, aryl-, and alkenyl-substituted (Z)-2-methyl-1-alkenyl(pinacol)boranes(2a) in ca. 75% yield based on propyne via Pd-catalyzed Negishi alkenylation with suitable organozinc bromide. The previously sluggish and modest-yielding Suzuki alkenylation of β,β-disubstituted alkenylboranes has been significantly promoted by fluorides, especially nBu4NF(TBAF) or CsF to give trisubstituted alkenes, i.e., (Z)-β-Me-substituted 3-i—3-xi and (E)-β-Ph-substituted 2b-i and 2b-ii. In all cases, each alkene product was formed in a ≥98% seteoselectivity. The propyne-based protocol nicely complements the widely used Zr-catalyzed alkyne methylalumination—Pd-catalyzed alkenylation by providing a highly stereoselective(≥98%) route to (Z)-Me-substituted alkenes.

A modular and completely stereoselective approach for the construction of all-carbon tetrasubstituted alkenes is described. It is based on the three-fold, sequential metal-catalyzed, cross-coupling functionalization of simple enolphosphate dibromide templates with carbon nucleophiles, affording tetrasubstituted alkenes as single isomers.

[reaction: see text] Both 6-chloro-2-iodo-9-isopropylpurine (1) and 2-chloro-6-iodo-9-benzylpurine (4) undergo a selective I/Mg exchange reaction with iPrMgCl at -80 degrees C. The reaction course at 0 degrees C is different. Magnesiation of 1 proceeds with the migration of magnesium to the 8 position of the purine nuclei. In the case of 4, substitution of iodine with an alkyl group from the Grignard reagent accompanied with a Cl/Mg exchange reaction takes place, and 6-alkyl-2-magnesiated purines (9) are formed. Thus prepared Grignard reagents afford the corresponding alcohols by the reaction with aldehydes.