Synthesis of Forsythenethoside A, a Neuroprotective Macrocyclic Phenylethanoid Glycoside, and NMR Analysis of Conformers.

Forsythenethoside A (1) is a structurally unique macrocyclic phenylethanoid glycoside, which was isolated from Forsynthia suspensa and displayed considerable neuroprotective activities. Here, we report its first chemical synthesis via a longest linear sequence of 14 steps in 5% overall yield wherein intramolecular oxidative coupling was successfully employed to realize the pivotal macrocyclization. NMR analysis revealed the existence of an unexpected conformational interconversion of the congested macrocycles.