Novel Conversions of Furandiols and Spiroacetal Enol Ethers into Cyclopentenones: Implications of the Isomerization Mechanism of 2-Furylcarbinois into Cyclopentenones

Two acid-catalyzed conversions of furandiols 8 and its dehydration spiroacetalized products 9 into oxabicyclic cyclopentenones 10 in good to excellent yields are reported. To disclose the mechanism of these conversions, the fact that H2O catalyzes the conversion of 9 into 10 is presented and intermediates 9k and 20i have been structurally verified. In addition, two other related conversions of spiroacetal enol ethers 11 and 14 derived from 8 into cyclopentenones are presented, for which an intramolecular aldol reaction is the key step. On the basis of these results, we propose that these conversions occur through an aldol condensation step instead of electrocyclization of the 4 pi-electron system, which was previously reported. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)