Carbocyclization of δ-acetylenic propargylic zinc reagents

Carbozincation Reactions of Carbon–Carbon Multiple Bonds

“…63 Such reactions are called zinc-yne-allene carbocyclization reactions. Metalation of 1,8-bis(trimethylsilyl)octa-1,7-diyne with s-BuLi in THF leads to a propargyl-allenyllithium that is transmetalated to the corresponding allenylzinc species (Scheme 24).…”

“…Subsequent electrophilic trapping with a proton, iodine, or allyl bromide in the presence of CuCN or an alkenyl iodide in the presence of a catalytic amount of Pd(PPh 3 ) 4 affords the corresponding functionalized five-membered ring products (Scheme 24). 63 This method has been extended to the intramolecular carbozincation of ω-acetylenic-metalated propargylic ethers to afford substituted tetrahydrofuran derivatives. 64 ω-Acetylenic-metalated propargylic ethers are also used for the preparation of functionalized exomethylene cyclopentanes (Scheme 25).…”

“…64 ω-Acetylenic-metalated propargylic ethers are also used for the preparation of functionalized exomethylene cyclopentanes (Scheme 25). 63 Metalation of the diyne with s-BuLi at −80 ∘ takes place selectively at the propargylic ether position. After addition of ZnBr 2 , the resulting allenylzinc cyclizes to the alkynylsilane moiety to afford the corresponding five-membered ring product as a single stereoisomer after protonation.…”

Tandem Zn‐Brook Rearrangement/Ene‐Allene Carbocyclization

“…13 General Procedure for the Brook Rearrangements: EtMgBr in Et 2 O (1 , 1 equiv.) was added dropwise to a cold (0°C) stirred solution of terminal alkyne in THF.…”

“…1 H NMR (CDCl 3 , 300 MHz): δ = 0.12 (s, 9 H), 1.68 (s, 9 H), 1.92-2.00 (m, 2 H), 2.73 (t, J = 7.8 Hz, 2 H), 4.30 (t, J = 6.6 Hz, 1 H), 7.14-7.26 (m, 5 H) ppm. 13 -1-(trimethylsilyl)pent-1-en-3-one (16a) General Procedure for Brook Rearrangements Followed by ZincEne-Allene Cyclization: A solution of EtMgBr in Et 2 O (1 , 1 equiv.) was added dropwise to a cold (0°C) stirred solution of terminal alkyne in THF.…”