Previous studies of 1,4-hydroxycarbonyls derived from alkanes have suggested that they can cyclize to saturated furans, which can subsequently eliminate water to form the corresponding dihydrofurans. CBS-QB3 and G3 studies of 5-hydroxy-2-pentanone and 2-hydroxypentanal show that both steps have activation barriers far too large for these reactions to occur as homogenous gas phase reactions. Similar results were obtained in CBS-QB3 studies of the analogous process leading from 2- and 3-methyl-4-hydroxy.-2-butenal (species posited to form in the degradation of isoprene) to 3-methylfuran. The latter two processes are much more favorable, thermodynamically, than the formation of dihydrofurans from the saturated 1,4-hydroxycarbonyls. (c) 2007 Elsevier B.V. All rights reserved.