A Diastereoselective Sakurai-Metathesis Route to Dihydrobenzofurans

Significance

A short diastereoselective synthesis of cis- or trans-2,3-dihydrobenzofuran using a combined Sakurai-Grubbs metathesis route is described. Using readily available vinylsilane derivatives of phenols, the metathesis step leads to 2,3-dihydrobenzo[f]oxasilepin derivatives which, under typical Sakurai conditions, provide the dihydro­benzofuran products. The diastereoselectivity in the Sakurai process was found to be highly dependent on the electronic nature of the aldehyde. Thus, electron-poor aldehydes lead to cis products, whereas electron-rich aldehydes lead to the formation of the trans isomers. The stereo­chemistry was assigned by NMR studies and by X-ray crystal structure analysis on representative derivatives. A mechanistic proposal for the Sakurai reaction is given.