SN2-Selective allylic substitution of chiral γ-aryl substituted allylic picolinates with alkynylcopper reagents

Substitution of γ-aryl secondary allylic picolinates with alkynyl copper reagents was studied. The copper reagent, prepared from TMSCCMgBr and CuBr·Me2S in 2:1, was subjected to substitution of the picolinate derived from (E)-3-phenyl-1-methyl-2-propenyl alcohol at 0 °C for 1 h in THF to produce a mixture of α- and γ-products and the alcohol in 67:20:13, while the reagent in 3 or 4:1 ratio gave the α-product with 90–91% selectivity. On the contrary, reaction in CH2Cl2–THF (6–8:1) at 0 °C for 1 h furnished the α-product with 99% regioselectivity. The effect of CH2Cl2 was also demonstrated with eight more examples. Furthermore, 99% inversion was determined by transformation to the literature compound and by chiral HPLC.