Allylic substitution on the pyran ring

Allylic substitution of pyrans 10 and 15 possessing the picolinoxy leaving group with several alkyl and aryl copper reagents derived from RMgBr and CuBr·Me2S was studied. First, reaction of 10 with three copper reagents derived from EtMgBr (2 equiv) and different equivalents of CuBr·Me2S (2, 1, and 0.5 equiv, respectively) was examined in THF at −20 °C for 1 h to afford anti SN2′ product 16a in high yields in all cases. Under these reaction conditions Me, i-Pr, Ph, o-MeC6H4, and o-MeOC6H4 were installed on the pyran ring of 10 successfully. Similar results were also obtained with pyran 15.