The participation of the neighboring to the reaction center acyloxy group in nucleophilic substitution, isomerization, and othcr transfomaations has been discussed widely in the literature and has been used for the solution of various synthetic problems (in particular in the chemistry of carbohydrates [1]), for the isomerization of monosaccharides [2, 3], for thc synthesis of oligosaccharides [4, 5], and tbr the mutual transformation of the pyranosc and furanose forms of carbohydrates and related systems [6, 7]. Such reactions are usually pertbrmed undcr mild conditions in the presence of electrophilic catalysts and involve the formation of acyloxonium ions [1]. In the present work the possibility for uncatalyzed thermal isomerization of the dihydropyranone system to the corresponding furan system, accompanied by migration of the acyloxy group, was studied.2-Perfluoroalkyl-4H-pyran-4-ones, which are of interest as the precursors for substituted nitrogen-containing heterocyclic compounds [8, 9], can be obtained by dehydration of the correspondin~ 3-hydroxy-2,3-dihydro-4H-pyran-4-ones by the action of thionyl chloride in pyridine [10]. However, the 2,3-dialkyl-substituted hydroxydihydropyranones 1-3 are dehydrated with difficulty under these conditions, and this is due to the cis arrangement of the hydroxyl group and the vicinal hydrogen atom, which is unfavorable for elimination [ 11 ].In the present work the pyrolysis of the acetates of condensed hydroxypyranones 4a, 5a gives the spiroannellated 3(2H)-furanones 6a and 7 respectively with yields of 70 and 76% instead of the expected products from the svn elimination of acetic acid. The isomerization of the acetates 4a, 5a takes place during their single-stage distillation at atmospheric pressure and during external heating of the reaction mixture at 300-320~ In addition to the products 6a, 7a the distillate contains 15-20% of the initial acetates 4a, 5a, but this mixture is probably not an equilibrium mixture, since the formation of the acetoxydihydropyranone 4a is not observed as a result of distillation of the individual spiro compound 6a. Treatment of the pyranone 4a at the same temperature in a sealed tube in order to increase its conversion into the rearrangement product was accompanied by resinification of the reaction mass and did not lead to an increase in the yield of the furanone 6a.
