Abstract
The trimethyltin-substituted alkenylborane 2 was prepared by simply treating lithium 1-alkynyltrialkylborate 1, readily obtained from the corresponding trialkylborane and 1-lithio-1-alkyne, with trimethyltin chloride. The transformation is stereoselective with the boron and tin appendages cis to each other. Sequential treatment of 2 generated in situ with n-butyllithium, CuBr.SMe 2 , 1-bromo-1-alkyne, and iodine furnished the corresponding enynyl iodide 3 in a single operation. Similarly, ene-allenic iodide 7a was produced by coupling with methanesulfonate 6. The subsequent Pd(PPh 3 ) 4 -catalyzed cross-coupling between 3 or 7a with 1-alkynylzinc chloride or with 1-alkyne in the presence of CuI/n-BuNH 2 produced enediyne 4 or enyne-allene 8 having a tetrasubstituted central double bond. The use of propargyl alcohol for coupling with 7a resulted in the formation of 10, presumably because a facile cycloaromatization reaction of the anticipated enyne-allene 9 occurred
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