To establish
allylic substitution of secondary
allylic alcohol derivatives with
alkynyl copper reagents,
allylic esters bearing the (2-pyridine)CO2-, (2-pyrazine)CO2-, (EtO)2PO2-, C6F5CO2-,
o-(Ph2P)C6H4CO2-, MeOCO2-, or AcO- group were examined. First, picolinate (R1 = Me, R2 = CH2OPMB) was subjected to reaction with (TMS-C[triple bond]C)2CuLi.
LiBr at 0degreesC. Although no substitution took place, MgBr2 (3 equiv) was found to promote the reaction to produce the anti SN2' product in 93% yield with 94%
regioselectivity and 99% chirality transfer. In contrast, substitution of the other
esters with the
copper reagent in the presence of MgBr2 were less reactive ((2-pyrazine)CO2-) or marginally reactive (other cases). Generality of the substitution using picolinates was established with five picolinates (R1 = Me, Ph(
CH2)2, PMBO(
CH2)3; R2 = Me, CH2OPMB, CH2OTBS, C5H11, c-C6H11) and seven
alkynyl copper reagents (R3 = TMS, Ph, p-TBSOC6H4, p- and o-MeOC6H4, p-MeC6H4, p-FC6H4), furnishing anti SN2' products in 61-93% yields with high
regioselectivity (usually >90%) and high chirality transfer (usually >95%). In addition, transformation of the products was briefly studied.
