Synthesis of Seven-Membered Ring Glycals via endo-Selective Alkynol Cycloisomerization

Alkynyldiols 1 undergo cycloisomerization to the corresponding seven-membered cyclic enol ethers 2 under tungsten carbonyl catalysis. This novel transformation proceeds with good yields and virtually complete regioselectivity for all diastereomers of 1, favoring the product 2 resulting from endo-mode cyclization. The unexpected regioselectivity may be dependent on the presence of the dioxolane structure tethering the terminal alkyne and diol functional groups.