Over the past few years, Timothy Donohoe’s group at the University of Oxford has been thinking up new ways to make heteroaromatics using ring closing metathesis.1,2 They recently came up with a slick way to prepare substituted furans using cross-metathesis as well.3 There are no other reports on using cross metathesis for the construction of aromatic and heteroaromatic compounds probably due to the fact that cross metathesis yields predominantly trans double bonds. Starting with an allylic alcohol and an enone, tandem olefin cross metathesis – acid catalyzed isomerization provides a direct entry to differentially 2,5-substituted furans. The transformation was accomplished in one pot by sequential additions of the 2nd Generation Hoveyda-Grubbs catalyst and PPTS. Trisubstituted furans were more challenging due to an inefficient metathesis step with the catalysts tested. This problem was overcome by using a Heck reaction in the second step to invert the olefin geometry and to add the extra substituent.
This method was applicable for a wide variety of enones and allylic alcohols with yields ranging from 50 to 80%.
1 Furans: Donohoe, T. J.; Ironmonger, A.; Kershaw, N. M. Angew. Chem. Int. Ed. 2008, 47, 7314-7316, and references therein.
2 Pyridines & pyridazines: Donohoe, T. J.; Bower, J. F.; Basutto, J. A.; Fishlock, L. P.; Procopious, P. A.; Callens, C. K. A. Tetrahedron 2009, 65, 8969-8980.
3 Donohoe, T. J.; Bower, J. F. Proc. Natl. Acad. Sci. 2010, 107, 3373-3376.