Total Synthesis of Ramonanins A–D

Abstract

The first total synthesis of the ramonanin family of lignan natural products is described. The short synthesis involves a 2,5-diaryl-3,4-dimethylene tetrahydrofuran intermediate, which participates in an unexpectedly facile Diels-Alder dimerization, generating all four natural products. Insights into the reactivity and stereoselectivity of the key dimerization are provided through computational studies employing B3LYP/6-31G(d) and M06-2X/6-31G(d) model chemistries.

Untitled

“…Subsequent low-temperature NMR studies not only identified the structure of the imine sulfonium I but also witnessed its robust formation and its extreme instability (Scheme 3). We further investigated the reaction by using DFT calculations and found that the reaction could be divided into three essential steps, including electrophilic assembling, deprotonation, and [3,3]-sigmatropic rearrangement. "…”