Abstract
The allylic substitution of γ,γ-disubstituted secondary allylic picolinates (esters of 2-PyCO2H) with aryllithium-based copper reagents was carried out in order to construct quaternary carbons. Initially, 2-methyl-7-phenylhept-2-en-4-yl picolinate was reacted with phenyl copper reagents derived from phenyllithium with Cu(acac)2, Cu(OMe)2, CuBr·Me2S, and CuCN in 1–3:1 ratios in the presence of excess magnesium bromide. Although the SN2′ product with a quaternary carbon was formed, the regioselectivity was 90% at most. Instead, phenyllithium/copper(I) thiophene-2-carboxylate/magnesium bromide (Ph/Cu = 1.5–2:1, Mg/Li = >1) was found to produce >98% regioselectivity and sufficient reactivity. This system was successful with eight aryllithium based copper reagents possessing sterically congested, electron-donating, or electron-withdrawing substituents. The anti stereochemical course was established by using an enantiomerically enriched geranaldehyde-derived picolinate.
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