Olefin cross-metathesis-based approaches to furans: procedures for the preparation of di- and trisubstituted variants

Olefin cross-metathesis (CM)-based protocols enable short, flexible and regiocontrolled access to substituted furan derivatives. Specifically, CM of allylic alcohol and enone components provides gamma-hydroxyenone intermediates that are cycloaromatized to the final furan derivatives on exposure to either acid or a discrete Heck arylation step. This latter process concomitantly introduces an extra substituent onto the furan target with complete control of regiochemistry. The methodology described here serves as the basis for developing other CM-based entries to diverse heteroaromatic compounds. This protocol describes in detail the following stages of the furan procedures: (i) the tandem formation and acid-catalyzed cyclization of the gamma-hydroxyenone to afford a 2,5-disubstituted furan directly; (ii) CM of an allylic alcohol with an enone to provide an isolated gamma-hydroxyenone; and (iii) Heck arylation of this gamma-hydroxyenone to afford a 2,3,5-trisubstituted furan. The reaction procedure given for the formation of the 2,5-disubstituted furan (option A) takes similar to 26.5 h to complete. The procedure described for the formation of the 2,3,5-trisubstituted furan (option B) takes similar to 52.5 h.