Z-Selective Intramolecular Horner−Wadsworth−Emmons Reaction for the Synthesis of Macrocyclic Lactones

When the substrates (ArO)2P(O)CH2CO2---CHO (Ar = Ph, o-t-BuPh) were added to a THF solution containing 3 equiv of NaH at 0 °C or NaI−DBU at rt over 1−10 h, the intramolecular Horner−Wadsworth−Emmons reaction proceeded efficiently to give the 12−18-membered-ring lactones in 69−93% yields with 89−100% Z selectivity. On the other hand, (EtO)2P(O)CH2CO2---CHO gave the 13−18-membered-ring lactones in 52−82% yields with 89−99% E selectivity using LiCl−DBU in MeCN or THF.