STEREOSELECTIVE SYNTHESIS OF ENEDIYNES AND ENYNE-ALLENES HAVING A TETRASUBSTITUTED CENTRAL CARBON-CARBON DOUBLE-BOND

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, CuBrSMe(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 l-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.